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Mobile deltaic and continental shelf muds as suboxic, fluidized bed reactors
Abstract
The oft-cited general correlation between net sediment accumulation and preservation of organic matter, while revealing in many ways, can be a misleading indicator of general elemental cycling processes and controls on storage of biogenic material at the continental-ocean boundary. Deltaic environments are characterized by the highest rates of net sedimentation and are the single most important class of depocenters on Earth. Available data indicate that sedimentary organic C (Corg) of both terrestrial and marine origin is efficiently decomposed in deltaic areas, with decomposition percentages reaching ≥70% and ≥90%, respectively, the latter percentage (marine) being quite comparable to deep-sea, low sedimentation environments. Despite high primary productivity associated with most deltas and evidence of substantial deposition of fresh planktonic debris, patterns of SO4= reduction indicate that the reactivity of organic material being buried is low, and that a larger proportion of Corg is often degraded compared to other marine deposits of similar net accumulation rate. As indicated by properties of the surficial Amazon delta and downdrift coastal region of northeast South America (∼1600-km extent), the primary reasons for efficient remineralization are related to intense and massive physical reworking of sediment associated with estuarine fronts, upwelling, tidal oscillation, and wind-driven waves. Fluid muds and mobile surface material cause the seafloor and continental boundary to act as a massive, suboxic, fluidized bed reactor dominated in some cases by bacterial rather than macrofaunal biomass. Reoxidation, repetitive redox successions, metabolite exchange, and continual mixing-in of fresh planktonic debris with refractory terrestrial components, result in an efficient decomposition system largely decoupled from net accumulation. Similar processes occur on smaller scales in most estuarine-shelf systems, but appear to be most dramatically expressed off the major rivers forming deltas.
... A net reaction model was used to characterize net changes in OC composition during sediment transport, based on Aller andBlair (2004, 2006). If we assume that the composition of bulk OC is derived from the linear mixing of marine and terrestrial OC sources (OC marine and OC terrestrial ), which have distinctive isotopic values f (δ 13 C or Δ 14 C), their isotopic balances in sediments are as follows: ...
... OC bio-1997). Normalization of OC content to mineral SSA can eliminate grain size effects, and thus the variation of OC/SSA indicated OC addition or loss during sediment transport (Keil et al., 1997;Aller, 1998). In general, the OC/SSA ratio in non-deltaic regions ranged from 0.4 to 1 mg m − 2 (Blair and Aller, 2012;Mayer, 1994). ...
... This is somewhat counterintuitive considering that marine OC is generally more labile than terrestrially-derived OC and this nearshore depositional environment is conducive to OC decay. It appears that the replacement rate of terrestrial OC by marine OC occurs at a key time when rapid deposition and burial allow for OC to bypass decay -despite the high OC incinerating capacity of mobile muds (e.g., Aller, 1998). More expectedly, OC petro content and OC petro /SSA increase from Changjiang SPM to CEMM sediments but largely decrease from CEMM sediments to ZMMM sediments (Fig. 4a). ...
... Les systèmes de type III sont typiquement des systèmes deltaïques où une part significative (>10%) du dépôt des sédiments a lieu après le rebord du plateau continental, soit en raison de la progradation de la zone de l'embouchure du fleuve (IIIA-Mississippi-Balize), soit en raison de la proximité Chapitre 2: Structuration spatio-temporelle de la matière organique particulaire et de la composition de la macrofaune benthique dans la Vasière Ouest-Gironde Table 2.I. Location (WGS84, degrees, and decimal minutes) and depth of the 5 stations (1,2,3,8,4) Chapitre 3: Structuration spatio-temporelle de l'activité de la macrofaune benthique dans la Vasière Ouest-Gironde Table 3.I. Location (WGS84, degrees, and decimal minutes) and depth of the 5 stations (1,2,3,8,4) sampled in the WGMP during the present study and of the 5 stations sampled in the RRP (A, B, N, C, D) by ...
... Location (WGS84, degrees, and decimal minutes) and depth of the 5 stations (1,2,3,8,4) Chapitre 3: Structuration spatio-temporelle de l'activité de la macrofaune benthique dans la Vasière Ouest-Gironde Table 3.I. Location (WGS84, degrees, and decimal minutes) and depth of the 5 stations (1,2,3,8,4) sampled in the WGMP during the present study and of the 5 stations sampled in the RRP (A, B, N, C, D) by ...
... Continental margins are the interface between land and the open ocean, where 50 to 80% of continental Particulate Organic Carbon (POC) inputs are mineralized [1][2][3]. Less than 5% of these inputs are transferred to the deep ocean [4], the remaining part being buried in continental margin sediments. River-dominated ocean margins (RiOMars), defined as margins impacted by major rivers freshwater and/or sediment discharges (e.g., plumes of Amazon, Yangtze or Mississippi Rivers), are the main marine primary depositional areas of riverine particulate continental inputs [2,5]. ...
Les grands fleuves influencent fortement certaines régions côtières (i.e., les Riverdominated Ocean Margins, ou RiOMar), qui présentent de forts taux de sédimentation et auxquelles sont associés des communautés benthiques et des processus biogéochimiques dont le fonctionnement varie en fonction de la dynamique temporelle des flux particulaires et de leur interaction avec l'hydrodynamisme. La Vasière Ouest-Gironde (VOG) constitue un modèle pertinent pour ce type de systèmes puisqu’elle constitue la principale zone de dépôt primaire des particules issues de l'estuaire de la Gironde, et qu’elle est située dans un environnement hautement énergétique. Bien que sa dynamique sédimentaire ait fait l’objet de nombreux travaux, l’étude des caractéristiques (dont la matière organique associée) des sédiments superficiels, et de la macrofaune benthique y a été jusqu'à présent négligée. L’objectif de cette thèse consiste à mieux décrire la structuration spatio-temporelle de l’écosystème benthique de la VOG, via l’étude de la matière organique particulaire (MOP) sédimentée ainsi que de la composition de la macrofaune benthique et de son activité. Une comparaison a par ailleurs été effectuée avec le prodelta du Rhône qui a déjà beaucoup été étudié. Bien qu’également situé en zone tempérée, celui-ci diffère en effet de la VOG à la fois par la saisonnalité plus marquée des apports fluviaux ainsi que par la plus faible intensité de l'hydrodynamisme dans la zone réceptrice. Une campagne synoptique (juin 2018, 32 stations) et 4 campagnes saisonnières (5 stations le long d’un gradient côte-large, octobre 2016-avril 2018) ont été réalisées sur la VOG dans des conditions de débits et d’hydrodynamisme contrastées. Une large gamme de paramètres a été mesurée : (1) caractéristiques des sédiments superficiels (granulométrie, surfaces spécifiques, descripteurs quantitatifs et qualitatifs de la MOP), (2) composition de la macrofaune, et (3) traces d’activité biologique (imagerie de profils sédimentaires). Sur la base de l’analyse de la distribution spatiale de ces paramètres, les résultats obtenus confirment la subdivision de la VOG en une zone proximale et une zone distale qui avait déjà été mise en évidence par des études sédimentologiques. Ils montrent l’existence de gradients de profondeur (i.e., entre zones proximale et distale et à l’intérieur de la zone distale) marqués pour la plupart de ces paramètres. L’analyse des corrélations entre ces variations spatiales et celles de plusieurs facteurs de contrôle potentiels suggère le rôle prédominant de l’hydrodynamisme comparé à celui du débit de la Gironde et du chalutage de fond. Mes résultats montrent également l’existence de variations temporelles dont la composante saisonnière est liée à l’efflorescence printanière, et à laquelle se superpose une tendance interannuelle entre 2016 et 2018 pour la composition de la macrofaune benthique. Dans le cas de cette dernière, et pour les 3 stations déjà échantillonnées en 2010, mes résultats montrent enfin l’existence d’importants changements temporels entre 2010 et 2016-2018. Ces changements sont attribués à la succession de tempêtes exceptionnelles intervenues durant l’hiver 2013/2014, qui aurait profondément perturbé l’écosystème benthique de la VOG et initié une séquence de cicatrisation. De manière générale, une différence importante avec le prodelta du Rhône réside dans le rôle majeur joué par l’hydrodynamisme (i.e., par rapport aux apports fluviaux) dans le contrôle de la structuration spatio-temporelle des paramètres étudiés. Cette différence tend à valider la transposition aux zones tempérées de la typologie des RiOMar jusqu’ici établie sur des bases biogéochimiques et principalement à partir d’exemples tropicaux et subtropicaux.
... Continental margins are key areas for the marine component of major biogeochemical cycles, accounting for the mineralization of 50-80% of continental Particulate Organic Carbon (POC) inputs (Aller, 1998;Blair and Aller, 2012;Burdige, 2005). Continental margins impacted by major river freshwater and sediment discharges are defined as River-dominated Ocean Margins (RiOMar). ...
... nutrients cycling, habitats) ones (e.g. Aller, 1998;Levin et al., 2001;Lansard et al., 2009). Their benthic components constitute the main marine primary depositional areas of riverine particule inputs (Burdige, 2005;McKee et al., 2004) and it is estimated that RiOMar account for 40-50% of continental POC burial in continental margins (Blair and Aller, 2012;Burdige, 2005;Hedges and Keil, 1995). ...
... macrofauna) and biogeochemical fluxes (e.g. mineralization) are largely affected by a variety of natural and anthropogenic disturbances (Aller, 1998;Lansard et al., 2009;Lotze et al., 2006;Rhoads et al., 1985;Tesi et al., 2007;Ulses et al., 2008;Worm et al., 2006). ...
The benthic compartment of River-dominated Ocean Margins (RiOMar) is largely affected by sedimentary processes, as well as by natural and anthropogenic disturbances. Recent studies have confirmed the major importance of riverine inputs and local hydrodynamics in the spatial structuration of low- and high-energy temperate RiOMar, respectively. Differences in the nature of these structuring factors could also affect the temporal dynamics of these two types of systems. The present study is aiming at: (1) quantifying spatiotemporal changes in surface sediment and benthic macrofauna within the West Gironde Mud Patch (WGMP; high-energy system) over both short (2016–2018) and long (2010–2018) time scales, (2) identifying the main environmental factors controlling those changes, and (3) achieving a comparison with the Rhône River Prodelta (RRP; low-energy system) in view of further characterizing the functioning of the benthic components of these two temperate RiOMar. Surface sediment characteristics (grain size, quantitative and qualitative descriptors of particulate organic matter) and benthic macrofauna compositions were assessed based on 4 seasonal sampling of 5 stations located along a depth gradient within the WGMP. Results highlighted the existence of spatial patterns for both surface sediment and benthic macrofauna, which are cued by local hydrodynamics. Most variables presented seasonal changes. Benthic macrofauna compositions also showed pluri-annual changes, which were attributed to a cicatrization process following a major disturbance caused by the 2014 series of severe winter storms, which underlines the major role of local hydrodynamics in controlling long-term temporal changes in WGMP benthic macrofauna compositions. The comparison with the RRP highlighted major discrepancies between the two systems in the main processes (i.e., hydrodynamics versus river hydrological regime) controlling surface sediment characteristics and benthic macrofauna compositions, which further supports RiOMar typologies derived from meta-analyses mainly achieved on tropical and subtropical systems.
... Among this river-delivered OC, particulate organic carbon (POC) and dissolved organic carbon (DOC) each account for approximately 50% (Blair and Aller 2012;Bauer et al. 2013). Once delivered, up to 80-90% of the riverine POC can become buried in estuarine and adjacent coastal sediments (Aller 1998;Burdige 2006;Bianchi et al. 2018). ...
... A high sedimentation rate of organic matter (OM) in large estuaries and river-influenced marginal seas often results in rapid OM decomposition and carbon cycling in both the sediment and water column (Aller 1998;Goñi et al. 2005;Burdige 2006; Keil et al. 2015). The decomposition of OM is a driving force controlling not only diagenetic reaction sequences and nutrient cycling but also the sequestration efficiency of OM in marine sediment (Berner 1980;Burdige 2006). ...
... This result may suggest a rapid remineralization rate of SOC in the estuarine and coastal sediments. The observed submaxima of pore-water DOC concentrations in the East China Sea and Yellow Sea sediments (Fig. 3e) also support this finding, indicating that the most rapid degradation of SOC occurred in the upper 10 cm of the sediment, likely due to sufficient exposure to oxygen (Aller et al. 1996;Aller 1998;Goñi et al. 2005). The relatively low pore-water DOC concentrations in the upper 5 cm of the sediment were likely due to mixing from bioturbation or bottom currents, resulting in a rapid pore-water DOC flux to the overlying seawater, as suggested in previous studies (Alperin et al. 1999;Yao et al. 2014;Zhao et al. 2018). ...
Organic matter degradation and sequestration in marine sediments are important processes involved in carbon cycling in the ocean. Here, we present the results of carbon isotope (14C and 13C) and concentration measurements of sedimentary organic carbon (SOC), pore‐water dissolved organic carbon (DOC), and dissolved inorganic carbon (DIC) in sediments collected from the East China Sea, Yellow/Bohai Sea, and South China Sea. Our results indicated that selective degradation and preservation of organic matter occurred in these sediments, and marine‐derived young organic carbon degraded preferentially and rapidly, resulting in high concentrations of pore‐water DOC and DIC with distinct carbon isotopic signatures. The average 14C age of pore‐water DOC was thousands of years younger than that of SOC in the sediment, suggesting that DOC was newly produced and cycled much faster than SOC. Aged SOC was refractory and preserved in sediments. Using a dual‐isotope three‐end‐member model, the contributions of potential sources to SOC, DOC, and DIC were estimated. Marine‐derived biomass organic carbon contributed the most to DOC, and dissolution of biogenic carbonate contributed the most to DIC. Riverine inputs of pre‐aged soil and fossil organic carbon dominated the SOC pool. Our study demonstrated that marginal sea sediments are important sites of young DOC and DIC fluxes into the water column, thus acting as a major pathway for carbon and nutrient cycling in the ocean.
... However, mineral-associated protection of OC commonly occurs via different rates of decomposition and aging processes, which varies significantly across different sedimentary regimes (Bao et al., 2016;Blair & Aller, 2012;Zhao, Yao, Bianchi, Arellano, et al., 2018). For example, the entry points of sediments to marginal seas (e.g., rivers, estuaries and inner shelves) are highly dynamic sites where tidal oscillations, sediment resuspension and remobilization (mobile-muds), and active activities of benthic communities frequently occur, which can enhance OC decomposition in sediments, through increased oxygen exposure time and priming effects via fresh algal OC (Aller, 1998;Aller et al., 2019;Bianchi et al., 2011;Hartnett et al., 1998). Sedimentation rate, another well-established control of OC burial in coastal systems (e.g., Bao et al., 2018;Blair & Aller, 2012;Cui, Bianchi, Jaeger, et al., 2016), has been altered by many changes in the Anthropocene such as land-use, precipitation gradients, eutrophication, and damming. ...
... In addition, wave and tidal-induced sediment dynamic process (e.g., resuspension and redeposition) is a main factor controlling OC composition during sediment transport in these large-river sedimentary systems Blair & Aller, 2012). For example, the Amazon and Changjiang Estuaries have macrotidal and mesotidal environments, respectively (Rabouille et al., 2008;Wright & Nittrouer, 1995), likely resulting in intense OC remineralization (exponential decrease with distance) in estuarine mobile-muds due to tidal-induced frequent physical reworking (Figure 6a) (Aller et al., 1998;Bao et al., 2018;Yao et al., 2014). These macro-meso tidal systems in the ECMS are more important in sediment mixing compared to the microtidal system in the Mississippi River Estuary (Rabouille et al., 2008;Wright & Nittrouer, 1995). ...
... In addition, intense physical reworking accelerates iron redox cycling, which is regarded as the prevailing diagenetic pathway in mobile-muds of the East China Sea inner shelf (Zhao, Yao, Bianchi, Shields et al., 2018;Zhu et al., 2012Zhu et al., , 2016. This is because iron oxides help to shuttle electrons between O 2 and OC in marine environments with redox oscillations (Aller, 1998;Aller et al., 2004;Burdige, 1993;Canfield et al., 1993). Every time sediment is reworked, dissolved Fe in pore-waters is re-oxidized to iron oxides, but these newly formed iron oxides are rapidly reduced in sediments due to frequent redox oscillations in mobile-muds Burdige, 1993). ...
A regional synthesis of organic carbon (OC) burial was conducted using a comprehensive data set to reveal some of the key drivers and human multi‐stressors controlling OC burial and transport in the Eastern China Marginal Seas (ECMS). Both OC and Δ¹⁴C values of suspended particulate matter (SPM) in the Changjiang River, were significantly higher than estuarine mobile‐muds, suggesting selective decay of more labile younger OC from both marine and terrestrial sources and the accumulation of more recalcitrant older OC. Some of this decay is likely to be associated with iron‐redox cycling in mobile‐muds. In contrast, OC, δ¹³C, and Δ¹⁴C values increased along the Yellow River sediment dispersal pathway, indicating adding of young marine OC and less decay of terrestrial OC. OC burial efficiency in mud areas in the Bohai Sea (∼43%) was significantly higher than those in the Yellow (∼11%) and East China Seas (∼16%), owing to rapid deposition. Burial flux of biospheric OC in mud areas of the ECMS is 7.00 ± 0.79 Mt yr⁻¹, corresponding to atmospheric CO2 drawdown by silicate weathering in major river drainage basins of mainland China. The burial flux of petrogenic OC was estimated to be 0.81 ± 0.25 Mt yr⁻¹, accounting for >1.9% of total burial in the global ocean. While the ECMS is an important OC sink, river damming has greatly reduced OC burial. Thus, the overall impact on anthropogenically altered river‐dominated marginal seas remains an important and rapidly changing component of the coastal ocean carbon budget.
... Continental margins are the interface between land and the open ocean, where 50 to 80% of continental Particulate Organic Carbon (POC) inputs are mineralized [1][2][3]. Less than 5% of these inputs are transferred to the deep ocean [4], the remaining part being buried in continental margin sediments. River-dominated ocean margins (RiOMars), defined as margins impacted by major rivers freshwater and/or sediment discharges (e.g., plumes of Amazon, Yangtze or Mississippi Rivers), are the main marine primary depositional areas of riverine particulate continental inputs [2,5]. ...
... Burial is tightly cued by biogeochemical processes taking place at the water-sediment interface [3,5,7]. Therefore, assessments of the structuration and functioning of the benthic components of marine RiOMars are key issues to reach a comprehensive understanding of the marine contribution to global biogeochemical cycles [1,3]. ...
... The present study is aiming at (1) improving current knowledge regarding the spatial structuration of a potential temperate type 2 RiOMar, (2) disentangling the potential effects of hydrodynamics and bottom trawling in controlling this structuration, and (3) comparing this spatial structuration with the one of a type 1 temperate RiOMar (i.e., the Rhône River Prodelta [3]). It is based on the synoptic sampling of a large number of stations and the quantitative analysis of (1) surface sediment characteristics and (2) Sediment Profile Images (SPI). ...
The spatial distributions of (1) surface sediment characteristics (D0.5, Sediment Surface Area (SSA), Particulate Organic Carbon (POC), Chlorophyll-a (Chl-a), Phaeophytin-a (Phaeo-a), Total and Enzymatically Hydrolyzable Amino Acids (THAA, EHAA), δ13C) and (2) sediment profile image (apparent Redox Potential Discontinuity (aRPD), numbers and depths of biological traces) characteristics were quantified based on the sampling of 32 stations located within the West Gironde Mud Patch (Bay of Biscay, NE Atlantic) in view of (1) assessing the spatial structuration of a temperate river-dominated ocean margin located in a high-energy area, (2) disentangling the impacts of hydrodynamics and bottom trawling on this structuration, and (3) comparing the West Gironde Mud Patch with the Rhône River Prodelta (located in a low-energy area). Results support the subdivision of the West Gironde Mud Patch in a proximal and a distal part and show (1) the existence of depth gradients in surface sedimentary organics characteristics and bioturbation within the distal part; (2) no evidence for a significant effect of bottom trawling, as opposed to Bottom Shear Stress, on the West Gironde Mud Patch spatial structuration; and (3) major discrepancies between spatial structuration in the West Gironde Mud Patch and the Rhône River Prodelta, which were attributed to differences in tidal regimes, sedimentation processes, and local hydrodynamics, which is in agreement with current river-dominated ocean margin typologies.
... Large-river delta-front estuaries (LDEs) are "hot spots" for both sedimentary organic carbon (SOC) remineralization and burial in the coastal ocean . Fine-grained high porosity sediments, such as mobile muds, are commonly found in LDEs due to high sedimentation rates, tidal oscillations, and sediment resuspension/ remobilization (Aller, 1998). Intense physical reworking and sediment resuspension increases the oxygen exposure time (OET), thus enhancing SOC remineralization (Blair and Aller, 2012;Yao et al., 2014). ...
... Intense physical reworking and sediment resuspension increases the oxygen exposure time (OET), thus enhancing SOC remineralization (Blair and Aller, 2012;Yao et al., 2014). Most LDEs exhibit a seasonal sediment deposition-erosion cycle (Aller, 1998;Bianchi and Allison, 2009;DeMaster et al., 1985). During summer, high sedimentation rates, water stratification, and elevated primary production lead to the prevalence of anoxic remineralization of SOC in sediments, particularly in seasonally hypoxic estuaries (Glud, 2008;Rabouille et al., 2008;Zhou et al., 2021). ...
... However, only <10% of SOC is associated with Fe R in the sub-aqueous deltaic and mobile-muds sediments (Lalonde et al., 2012;Shields et al., 2016;Zhao et al., 2018a). Estuarine mobile muds are commonly located in the entry points of sediments to marginal seas (e.g., estuaries and inner shelves), and they are highly dynamic regions where tidal oscillations, sediment resuspension, and remobilization frequently occur, which can enhance SOC decomposition, through increased oxygen exposure time (e.g., reoxidation and repetitive redox successions) and priming effects via fresh algal OC (Aller, 1998;Bianchi, 2011;Blair and Aller, 2012;McKee et al., 2004). The composition of OC may also affect what types of OC-Fe R associations occur. ...
... The Changjiang sedimentary system is a typical LOAC, with a large river and a highly dynamic estuary, like the Mississippi and Amazon river sedimentary systems (Blair and Aller, 2012;Nittrouer et al., 2021). It is worth noting that each of these river systems is characterized by high Fe R and plant-derived OC concentrations in river particles, rapid Fe redox cycling in estuarine mobile muds, and selective retention of older terrestriallyderived OC during lateral sediment transport (Aller, 1998;Blair and Aller, 2012;Poulton and Raiswell, 2002;Zhao et al., 2021a). Protection of OC during the lateral transfer from land to ocean is a key process controlling the release of carbon back into the atmosphere in these LOAC systems (Regnier et al., 2022). ...
Reactive iron (FeR) plays an important role in the preservation of organic carbon (OC) in coastal sediments, yet changes in the OC bound to FeR (OC-FeR), during transport and deposition, remain poorly understood. The main goal of this work is to investigate the variation of the age and composition of OC-FeR from estuarine suspended particulate matter (SPM) to coastal sediments, to further understand the role of FeR in the preservation of terrestrial OC exported from large rivers into marginal seas. We examined OC and its carbon isotopic composition (Δ14Cbulk, δ13Cbulk), specific surface area (SSA), grain size composition, lignin phenols, FeR, Mössbauer spectroscopy, and isotopic signatures of OC-FeR (Δ14COC-FeR, δ13COC-FeR) in SPM and surface sediments of the Changjiang Estuary. Particulate OC (POC) and FeR concentrations in SPM are significantly higher than in surface sediments, with no significant differences between surface- and bottom-water SPM. This indicates that loss of OC and FeR largely occurs at the sediment-water interface due in part, to rapid Fe cycling. The percentage of OC-FeR (fOC-FeR) in SPM (6.6 ± 1.9%) is similar to that in mobile-mud sediment (8.8 ± 1.8%). There are no significant differences in OC-FeR content (p>0.05) from SPM to mobile-mud sediments, but non-OC-FeR largely decreases, suggesting that terrestrial OC-FeR has greater stability compared to terrestrial non-OC-FeR. Both δ13COC-FeR and Δ14COC-FeR are lower than bulk OC, indicating that FeR is mainly associated with pre-aged soil OC of terrestrial plant origin, especially in estuarine SPM and mobile-mud sediments. Taken together, binding with FeR is a potential long-term protection mechanism for terrestrial OC. Both Δ14Cbulk and Δ14COC-FeR decrease with an increase in the ratio of hematite to (super)paramagnetic Fe3+, indicating that high-crystallinity iron oxide is largely associated with pre-aged terrestrial OC, and there is a potential joint maturation mechanism between FeR and its associated OC. Based on literature comparisons of soils, estuarine SPM, and marine sediments, OC-FeR associations are controlled mainly by sedimentary regimes, FeR compositions, and OC sources. This work supports the notion that FeR plays an important role in the stabilization and transport of river-derived terrestrial OC.
... In contrast to the steep mountain rivers and narrow continental shelves, where the efficient transfer of sediments favors the rapid export of POC petro to marine depocenters with little or no oxidation (Hilton et al., 2011;Kao et al., 2014;Hilton and West, 2020), large river-dominated estuaries serve as regions for terrestrial OC processes and cycling where sediments are exposed to resuspension/ deposition loops and long-term oxygen exposure (Aller, 1998;Bianchi, 2011;Blair and Aller, 2012;Canuel and Hardison, 2016), which act to enhance the degradation of OC petro . As POC petro is mainly present in the form of inclusions or aggregates within minerals (Mayer, 1994;Galy et al., 2008;Bouchez et al., 2010), OCmineral interactions are thought to be the primary mechanism controlling OC petro preservation in continental margin sediments that physically protect OC petro against microbial and oxidative attack over million-year timescales (Arnarson and Keil, 2007;Blattmann et al., 2019;Hemingway et al., 2019). ...
... Similarly, high burial efficiencies of POC petro were found in some large riverdominated subaqueous deltaic clinoforms with high sediment accumulation rates (e.g., the Ganges-Brahmaputra (Bengal), Mackenzie, and Fly deltas) where riverine POC petro appears to be efficiently accumulated on the seabed without obvious oxidative losses (burial efficiency up to 100 %; Galy et al., 2007;Goñi et al., 2008Goñi et al., , 2014Vonk et al., 2015). In contrast, a relatively low burial efficiency of POC petro (46 ± 35 %) in the Yangtze River estuarine system is consistent with the efficient decomposition of terrestrial POC in the Amazon River delta (Aller, 1998;Aller and Blair, 2006), where repeated cycles of sediment reworking and deposition produced prolonged exposure of mineral-bound POC petro to O 2 biota facilitate the degradation of refractory OM. Our data further suggest that a substantial portion of riverine POC petro oxidization in estuarine-deltas may help explain a large ''missing" terrestrially derived OC in the coastal oceans (Hedges et al., 1997). ...
Riverine export of petrogenic organic carbon (OCpetro) from continents to coastal oceans is a dynamic component of the global carbon budget and affects the long-term atmospheric carbon reservoir. In large fluvial systems, oxidation of OCpetro during transit releases a large flux of carbon dioxide to the atmosphere, influencing climate changes; however, the transport and fate of OCpetro and their controls along the fluvial–marine transition remain poorly constrained. Here, we combined Raman spectral, radiocarbon activity (F¹⁴C), mineralogical, and sedimentological techniques with multiple geochemical analyses to characterize the dynamics of OCpetro in the water column and sediment particles from the Yangtze River channel–estuary–shelf continuum systems.
Our data show that much of the OCpetro present in suspended sediment (POCpetro) exported by the Yangtze River is “labile” fractions (mostly disordered materials) that can be degraded or lost during transport across the estuarine continuum, whereas the OCpetro deposited in seabed sediment are characterized by highly recalcitrant, unreactive, and graphitic carbon phases. As discrete “free” particles, coarse plant debris (>63 μm) with ages of several thousand years observed in the proximal delta of the Yangtze River exhibit nearly identical characteristics of disordered materials, and the presence of aged vascular plant detritus (carbon-rich and ¹⁴C-depleted materials) may lead to an overestimate of OCpetro in marine sediments. Using a Bayesian endmember mixing approach, a binary mixing model, and the ratio of fraction modern carbon (F¹⁴C) to Al/OC, we found a large decrease in POCpetro concentration and loading from the suspended sediments to seabed sediments, suggesting a loss of mineral-bound OCpetro fraction during sediment transport through the estuary and deposition on the shelf. We estimated that, on average, 46 ± 35% of the POCpetro initially present in suspended sediments delivered to the Yangtze River Estuary during a flood event was primarily oxidized at the sediment-water interface, leaving the most graphitic carbon components transported laterally and efficiently reburied in shelf sediments. We found that during estuarine mixing, flocculation process-induced microaggregates may provide transient physical protection for POCpetro in the form of inclusions/aggregates with carbonate minerals; however, when POCpetro is physically and chemically separated from its mineral matrix via disaggregation and dissolution, it may be easily oxidized by microbial activity. In contrast, OC-phyllosilicates interactions exert a first-order control on the preservation of OCpetro in marine sediments. Our findings suggest that the importance of POCpetro oxidation and loss in carbon cycling and budget assessments of estuaries may be underestimated.
... This explains why, in spite of numerous studies on RiOMars (e.g. Aller, 1998;Aller et al., 1996Aller et al., , 1986Aller and Blair, 2006;Blair and Aller, 2012;Deng et al., 2006;Kuzyk et al., 2017;McKee et al., 2004;Pastor et al., 2018Pastor et al., , 2011Yao et al., 2014;Zhu et al., 2013 and references therein), mechanisms controlling OC preservation in these environments as well as their carbon burial capabilities are not yet fully understood and quantified. ...
... This is consistent with the decrease of hydrodynamic intensity which controls the extent of sediment resuspension. The higher hydrodynamic intensity at proximal sites (i.e., 1 and 6) promotes thus sediment organic matter degradation (Aller, 1998;Aller and Blair, 2006;Yao et al., 2014) and results in a low OC storage efficiency (Table 2, Fig. 7). Conversely, OC storage efficiencies are the highest in the central and distal WGMP. ...
On the Bay of Biscay continental shelf, there are several mid-shelf mud patches including La Grande Vasière to the north, the West Gironde Mud Patch (WGMP) off the Gironde estuary and the Basque Mud Patch close to the Spanish border. In general, these deposits are several meters thick and cover coarser substrate. Questions remain about their storage capability for fine particles and carbon. This work investigates the sedimentation of the WGMP in order to develop a first estimate of organic carbon (OC) burial. Interface sediment cores were collected at nine stations along two cross-shelf transects in October–November 2016. X-radiograph imaging and grain-size analyses were used to characterize sedimentary structures. ²¹⁰Pbxs depth profiles were established to calculate sediment (SAR) and mass (MAR) accumulation rates. Sedimentary structures indicate episodic sandy inputs overlying older deposits at proximal sites, and relatively continuous sedimentation at seaward locations. On the outer-central portion of the northern transect, a maximum SAR (0.47 cm yr⁻¹) was observed, suggesting a depocenter. On the southern transect, excluding two stations where sedimentary inputs appear massive but sporadic, the SARs are lower (<0.3 cm yr⁻¹). Quantitative estimates of OC burial rates increase seaward with a maximum of 45 gC m⁻² yr⁻¹. To evaluate carbon loading independent of grain-size variability, OC values were normalized to surface area of sediments (SA). Interestingly, a qualitative comparison of OC burial efficiencies using the OC/SA ratio highlights three groups of sites (low, medium and relatively high OC burial efficiency) which are likely related both to different sedimentary environments and variable deposition conditions linked to local environmental conditions and depth. This work highlights the likely control of hydrodynamic intensity and sedimentary inputs on the amount of OC stored in the WGMP sediments.
... Suboxic conditions resulting from frequent resuspension in dynamic environments allow efficient degradation of OM (Aller, 1998). However, dissolved PO 4 3− content in superficial sediments remained low while OM and ASOP are respectively 4.3 ± 0.2% and 3.48 ± 0.21 μmol g − 1 (Fig. 4a). ...
Coastal marine sediments can be either major scrubbers or eutrophication contributors to surface waters. Standard methods for direct measurement of nutrient fluxes at the sediment-water interface do not consider hydrodynamic forcing although several ex-situ studies suggest that sediment resuspension can dramatically increase dissolved fluxes. We provide a new model to quantify dissolved phosphate (PO43-) resuspension flux (JR) based on physical representation of its identified components: diffusion stimulation by exposure of deeper sediment layer with higher PO43- concentration in the porewater (JD), pore water mixing with overlying water (JM) and net adsorption/desorption from suspended sediments (JK). This approach was applied to field data from a Seine intertidal mudflat periodically submitted to millimetric erosion. On a tidal scale, the model output reveals a JR of 272.3 ± 360.0 µmol m-2 h-1 (± 52 % from parameter uncertainty), well above flux calculated by application of Fick's first law (0.15 ± 0.85 µmol m-2 h-1) or by ex situ core incubation (40.8 µmol m-2 h-1). Iron bound phosphorus within suboxic layers buffers PO43- concentrations in superficial sediments leading to negligible contributions of JD and JM to total fluxes. Conversely, JK appears to be the main exchange pathway, even though improvements in turbidity measurement would allow this term to be defined more precisely. Correction required to enhance and control model robustness are described. These results show the importance of considering the dissolved PO43- resuspension flux in dynamic environments.
... Intense sediment reworking/irrigation introduces large amounts of labile OC into sediments, exposing anoxic sediments and buried recalcitrant OC. This process produces and accumulates bacterial biomass, whose remineralization would prime high anaerobic OC decomposition rates and the reduction of metal oxides and sulfate 34,36,52,[54][55][56] . Under this circumstance, OC decomposition triggers anoxic conditions conducive to the reduction of Re and U and shoals the depth of their reductive removal 4,44,57,58 Fig. 1 and Supplementary Fig. 2). ...
Rhenium (Re) and uranium (U) are essential proxies in reconstructing past oceanic oxygenation evolution. However, their removal in continental shelf sediments, hotspots of early diagenesis, were previously treated as quantitatively unimportant sinks in the ocean. Here we examine the sedimentary reductive removal of Re and U and their coupling with organic carbon decomposition, utilizing the ²²⁴Ra/²²⁸Th disequilibria within the East China Sea shelf. We identified positive correlations between their removal fluxes and the rates of sediment oxygen consumption or organic carbon decomposition. These correlations enable an evaluation of global shelf reductive sinks that are comparable to (for Re) or higher than (~4-fold for U) previously established suboxic/anoxic sinks. These findings suggest potential imbalances in the modern budgets of Re and U, or perhaps a substantial underestimation of their sources. Our study thus highlights shelf sedimentary reductive removal as critical yet overlooked sinks for Re and U in the modern ocean.
... atéria orgânica, onde pode ter uma mistura de matérias marinha e terrígenas e funcionam como um fator controlador nos fluxos de carbono orgânico dissolvido e particulado (DOC e POC, respectivamente) para a região costeira do oceano, assim como os fluxos de CO2 para a atmosfera(Hedges e Keil, 1999; Middelburg e Herman, 2007;Canuel e Hardison, 2016).Aller (1998), Komada e Reimers (2001) e Middelburg e Herman (2007), em um estudo realizado em estuáriosna Europa ocidental, mostraram que estuários dominados por maré tendem a ser caracterizador por uma alta concentração de partículas de matéria orgânicas suspensas por longos períodos, fazendo com que a matéria orgânica entre em repetidos ciclos de ...
The Devonian Period marked a time of significant transformation of the Earth's surface: from barren landscapes and/or those with little vegetation to densely vegetated wetlands. As a consequence of these changes, global events of ocean anoxia occurred at the end of the Devonian, marked by mass extinctions and the deposition of black shales rich in organic matter (OM). The Pimenteiras Formation, in the Parnaíba Basin, is characterized by dark shales rich in organic matter deposited in a context of an epicontinental sea laterally adjacent to a significant deltaic system (Cabeças Formation). Sedimentary successions of the Pimenteiras Formation were studied in the area of the Parque dos Gaviões. Geophysical profiles and core samples from three exploratory wells (1-OGX-93-MA, 1-OGX-101-MA, and 1-OGX-110-MA) were used, along with chemical analyses of carbon, nitrogen, and sulfur. The chemical analyses were conducted on the core samples, following a methodological flowchart aimed at better sample processing. The samples were dried, weighed, and analyzed for total carbon (TC), total nitrogen (TN), 13Corg, 15N, total organic carbon (TOC), and total sulfur (TS). The shales were divided into five intervals (shales A, B, C, D, and E) and three depositional sequences (Sequence 1, 2, and 3). In the shale A interval, associated with Sequence 1, a predominantly marine and anoxic environment was observed. Sequence 2, encompassing the B and C shale intervals, showed a strong influence of terrestrial OM and sub-oxic conditions. The influence of terrestrial OM decreased during the transition from shales B to C. Sequence 3, intervals of shales D and E, revealed more reducing conditions, with the influence of marine OM. The data showed that during the Eifelian, there was a predominance of a transitional environment from proximal marine with a strong influence of continental OM (closed marine) to a more distal marine environment with the influence of marine OM (semi-open marine).
Keywords: DEVONIAN, PIMENTEIRAS FORMATION, EPICONTINENTAL SEA, SOURCE ROCK, TOTAL ORGANIC CARBON
... Together, these rivers deliver around 1,600 Mt yr 1 of particulate load to the CMS, accounting for ∼10% of global riverine sediment fluxes (Milliman & Farnsworth, 2011). Under the influence of coastal currents, fluvially derived sediments are transported parallel to the coastline forming extensive mud-belts in the CMS (Liu et al., 2007;Qiao et al., 2017) that comprise hotspots for both OC burial and remineralization (Aller, 1998;Jiao et al., 2018). Multiple geochemical approaches based on bulk and molecular carbon isotopes have been applied to constrain specific components of OC terr (e.g., pre-aged soil OC, fossil OC), to assess controls on its spatiotemporal variation in both riverine and estuarine suspended particulate matter (SPM) and underlying sediments, and to evaluate OC terr burial efficiencies from a source to sink perspective (Bao, van der Voort, et al., 2018;Wei et al., 2021;Wu et al., 2013;Yu et al., 2019bYu et al., , 2021. ...
Plain Language Summary
Determining the factors driving degradation of terrestrial organic carbon (OCterr) is important for understanding the fate of OCterr in marginal seas and its impact on the global carbon cycle. In this study, we use bulk and molecular‐level carbon isotopic measurements as well as sedimentological data to investigate how OCterr characteristics evolve along the river‐estuary‐coastal ocean continuum for three different river systems discharging into the Chinese marginal seas. Marked decreases in the OCterr abundance and ¹⁴C contents reveal marked aging associated with degradation processes within estuaries over relatively short transport distances, likely due to degradation of younger and labile OCterr. Both aging and apparent “rejuvenation” of OCterr after estuarine transport was observed in different shelf systems, the latter likely resulting from inputs of fresh OCterr from proximal sources. Our findings suggest both sources and transport processes influence terrestrial OC and biomarker signatures in marginal sea sediments, highlighting the complexity of OCterr dynamics in coastal systems.
... This discrepancy might be partially related to temporal and spatial variabilities of flux in coastal sediments. In this respect, we must emphasize that our flux estimates were obtained in one of the largest mobile mud systems in the world where the decomposition of sedimentary organic matter can be highly efficient (Aller, 1998;Wei et al., 2022). As such, the true flux of DIC in such a sedimentary environment should be particularly high. ...
... Large-river delta-front estuaries (LDEs) are important interfaces between terrestrial and marine environments that receive huge amounts of sediments and organic carbon (OC) input from both sides (Bianchi and Allison, 2009). LDEs are also active regions where suspended particulate matter is actively deposited, transported, and transformed (Aller, 1998;Syvitski et al., 2009;Galy et al., 2015). Based on Stokes' Law, the particle sedimentation velocity is proportional to the density difference between the solid phase and the liquid phase, and proportional to the square of particle diameter (Lamb, 1994). ...
... Rivers play a key role as conduits in the carbon cycle, moving OC eroded from rock, soil and vegetation from the terrestrial to marine carbon reservoir if the transported carbon is buried in a sedimentary basin (e.g., Schlünz and Schneider, 2000). The long spatial and temporal scales of this 45 transport allow for carbon transformation during transfer and intermittent storage (Blattmann et al., 2019;Galy et al., 2008), for instance in floodplains, estuaries, and coastal mud belts (Repasch et al., 2022;Scheingross et al., 2021;Canuel and Hardison, 2016;Aller, 1998). ...
Fluvial transport of organic carbon from the terrestrial biosphere to the oceans is an important term in the global carbon cycle. Traditionally, the long-term burial flux of fluvial particulate organic carbon (POC) is estimated using river suspended sediment flux; however, organic carbon can also travel in river bedload as coarse particulate organic matter (POMBed). Estimates of fluvial POC export to the ocean are highly uncertain because few studies document POMbed sources, flux and evolution during long-range fluvial transport from uplands to ocean basins. This knowledge gap limits our ability to determine the global terrestrial organic carbon burial flux. In this study we investigate the flux, sources and transformations of POMBed during fluvial transport over a ~1300 km long reach of the Rio Bermejo, Argentina, which has no tributary inputs. To constrain sourcing of POMBed, we analysed the composition and stable hydrogen and carbon isotope ratios (δ2H, δ13C) of plant wax biomarkers from POMBed at six locations along the Rio Bermejo, and compared this to samples of suspended sediment, soil, leaf litter and floating organic debris (POMfloat) from both the lowland and headwater river system. Across all samples, we found no discernible differences in n-alkane average chain length or nC29 δ13C values, indicating a common origin for all sampled POMBed. Leaf litter and POMfloat nC29 δ2H values decrease with elevation, making it a useful proxy for POMBed source elevation. Biomarker δ2H values suggest that POMBed is a mix of distally-derived headwater and locally-recruited floodplain sources at all sampling locations. These results indicate that POMBed can be preserved during transport through lowland rivers for hundreds of kilometres. However, the POMBed flux decreases with increasing transport distance, suggesting mechanical comminution of these coarse organic particles, and progressive transfer into the suspended load. Our provisional estimates suggest that the carbon flux from POMBed comprises less than 1 percent of the suspended load POC flux in the Rio Bermejo. While this represents a small portion of the river POC flux, this coarse and high density material likely has a higher probability of deposition and burial in sedimentary basins, potentially allowing it to be more effective in long-term CO2 drawdown relative to fine suspended particles. Because the rate and ratio of POMBed transport versus comminution likely varies across tectonic and climatic settings, additional research is needed to determine the importance of POMBed in the global carbon cycle.
... They play a key role in marine nutrient and carbon cycles (McKee et al. 2004;Cai 2011;Bauer et al. 2013;Bianchi et al. 2018). These dynamic environments are known to have high riverine input and sedimentation rate (Aller 1998). Furthermore, coastal sediments account for 85% of long-term organic carbon burial in the ocean, with deltaic environments accounting for the majority 40 (Burdige 2005), but they are also powerful biogeochemical reactors (Aller et al. 1996;Rassmann et al. 2016). ...
At the land-sea interface, the benthic carbon cycle is strongly influenced by the export of terrigenous particulate material across the river-ocean continuum. Episodic flood events delivering massive sedimentary materials can occur, but their short-term impact on carbon cycling is poorly understood. In this paper, we use a coupled data-model approach to estimate the temporal variations of sediment-water fluxes, biogeochemical pathways and their reaction rates during these abrupt phenomena. We studied one episodic depositional event in the vicinity of the Rhône River mouth (NW Mediterranean Sea) during the fall-winter of 2021–2022. The distribution of dissolved inorganic carbon (DIC), sulfate (SO42−) and methane (CH4) were measured in sediment porewater collected every 2 weeks before and after the deposition of a 25 cm sediment layer during the main winter flood event. Significant changes in the distribution of DIC, SO42− and CH4, concentrations were observed in the sediment porewaters. The use of an early diagenetic model (FESDIA) to calculate biogeochemical reaction rates and fluxes revealed that this type of flooding event can increase the total organic carbon mineralization rate in the sediment by 75 % a few days after deposition, essentially by increasing the sulfate reduction contribution to total mineralization relative to non-flood depositional period. It predicts a short-term decrease of the DIC flux out of the sediment from 100 to 55 mmol m−2 d−1 after the deposition of the new sediment layer with a longer-term increase by 4 %, therefore implying an initial internal storage of DIC in the newly deposited layer and a slow release over relaxation of the system. Furthermore, examination of the stoichiometric ratios of DIC and SO42− as well as model output over this five-months window shows a decoupling between the two modes of sulfate reduction following the deposition – organoclastic sulfate reduction (OSR) intensified in the newly deposited layer below the sediment surface, whereas anaerobic oxidation of methane (AOM) intensified at depth below the former buried surface. This depth-wise bifurcation of both pathways of sulfate reduction in the sediment column is clearly related to the deepening of the sulfate-methane transition zone (SMTZ) by 25 cm after the flood deposition. Our findings highlight the significance of short-term transient biogeochemical processes at the seafloor and provide new insights on the benthic carbon cycle in the coastal ocean.
... Suspended particulate matter transport is faster in small rivers usually associated with mountains, narrow continental shelves, or active continental margins, resulting in relatively low OM remineralization rates along the continent-ocean gradient (Blair et al., 2003). In contrast, SPM is subject to deposition and resuspension cycles in large rivers, resulting in increased OM remineralization due to long residence times (Aller, 1998). Additionally, OM (and BC) transported alongside particles can be replaced downstream by OM produced at lower elevations (Aller et al., 1996;Burdige, 2007). ...
This study assessed black carbon (BC) dynamics, concentrations, and the organic matter (OM) isotopic carbon composition in northeastern South America drainage basin coastal sediments. Paraíba do Sul (PSR; Atlantic Rainforest, Brazil) coastal sediments displayed more 13C-enriched values (-22.6 ± 1.3 ‰ [n = 13]) than Amazon and Sinnamary (Amazon Rainforest in French Guiana and Brazil) sediments (-25.0 ± 3.1 ‰ [n = 14] and - 26.1 ± 1.0 ‰ [n = 6], respectively), indicating that local land-use basin changes have altered the OM composition, i.e., from natural C3 plant to C4 plants contributions. BC contents normalized to total organic carbon (TOC) content were 0.32 ± 0.24 (n = 8), 0.73 ± 0.67 (n = 6), and 0.95 ± 0.74 (n = 13) mg g-1 TOC for Amazon, Sinnamary and PSR samples, respectively, with BC sources appearing to differ according to different drainage basin vegetation covers. With increasing distance from the river mouths, BC contents exhibited different trends between the coastal zones, with values increasing for the PSR and decreasing values for the Amazon samples. BC distribution in Sinnamary coastal sediments did not display specific patterns. Regarding the Amazon coastal zone, BC contents decreased while the B6CA:B5CA ratios did not show a pattern, which could indicate that BC in the area originates from river transport (aged BC) and that the hydrophobic component of dissolved BC is removed. The BC content mostly increased in the PSR coastal zone, while the B6CA:B5CA ratios were not altered for the entire gradient, indicating the BC stability and possible atmospheric deposition of soot. Our findings indicate that different sources, transformation processes, and hydrological conditions affect BC contents within coastal zones. Continuous land cover changes in both the Amazon and Atlantic Rainforests may result in large-scale marine carbon cycling impacts.
... Here, solute transport is driven by diffusion along concentration gradients, and solid-phase electron acceptors and Corg are deposited to the sediment surface. In contrast, vertical particle transport in muds only occurs by 100 macrofaunal activity or by strong physical forcing, for example in mobile deltaic muds (Aller, 1998;Song, et al., 2022). ...
Multiple investigators have suggested that the benthic flux of dissolved iron (Fed) from continental shelf sediments represents an important source of this micronutrient to ocean waters. The magnitude, biogeochemical controls, and seasonal dynamics of Fed fluxes to date, however, have mostly been studied for muddy cohesive sediments dominated by molecular diffusion. Data from these studies have been included in global biogeochemical models to determine the contribution of this Fe source to the ocean. Fed fluxes from sandy advective sediments have received little consideration, although these sediments cover 50–60 % of the continental shelves. Sandy permeable deposits function as dynamic catalytic filters characterized by the rapid exchange of solutes and infiltration of particles —including labile Corg and reactive metal oxides— and high biogeochemical reaction rates. In this article, we discuss how the fundamentally different modes of solute and particle transport in sands affect the sedimentary Fe cycle and Fed flux. We present a case study in which we simulate bioirrigation in sands in summer and winter. In our experiments, Fed fluxes from non-irrigated sediments under diffusive conditions did not exceed 6 and 13 μmol Fe m-2 d-1 in winter and summer, respectively. Fluxes from irrigated cores reached values of 150 μmol Fe m-2 d-1 (winter) and 115 μmol Fe m-2 d-1 (summer). The results indicate that the pumping activity of the benthic macrofauna plays a key role in controlling the extent of the benthic Fed flux from permeable sediments, and that both biogenic and physical advection enhance fluxes. We argue that bioturbated sandy advective sediments constitute an important benthic Fe source to coastal waters and advocate for a more differentiated treatment of sediment type (muddy diffusive vs. sandy advective) and macrofaunal activity -reflecting different functional groups of the macrobenthos- in global biogeochemical Fe models. A better understanding of the benthic Fe cycle in sandy advective sediments is particularly important to help predict how anthropogenic effects such as changes in the deposition patterns of Corg and metals, the expansion of oxygen minimum zones, and changes in benthic biodiversity will affect the tightly coupled benthic-pelagic ecosystem along continental shelves.
... The offsets between the TOC and carbonate 14 C ages observed in the GoM have implications for organic matter aging and cross shelf transport as well as TOC sources (primary productivity versus terrigenous input). Essentially, as terrigenous organic material is degraded upon deposition on the shelf, only refractory material remains (Aller, 1998;Aller and Blair, 2004) and the remaining refractory material has the lowest 14 C. Santschi and Rowe (2008) found that subsurface (4-26 cm below seafloor) sedimentary organic matter collected in slope areas from six sites throughout the northern GoM had stable carbon isotope (δ 13 C) values ranging from − 24‰ to − 26.5‰, which indicated a terrigenous origin likely deposited by mass wasting. The range of TOC stable carbon isotope values from this study ranged from − 19.8‰ to − 22.2‰, which is consistent with a higher admixture of marine algal sources than those found by Santschi and Rowe (2008). ...
... Us- ing a simple metric for estimating relaxation timescale of the perturbation, our calculations for the first end-member scenario (EM1) show that the upper bound of the timescale of relaxation for oxygen is 5±3 d, whereas it was approximately 2 ± 2 d for the second end-member scenario (EM2). This reflects the property of oxygen, which quickly approaches a steady-state situation after an event (Aller, 1998). This viewpoint is supported by an ex situ controlled laboratory setup. ...
Episodic events of flood deposit in coastal environments are characterized by deposition of large quantities of sediment containing reactive organic matter within short periods of time. While steady-state modelling is common in sediment biogeochemical modelling, the inclusion of these events in current early diagenesis models has yet to be demonstrated. We adapted an existing model of early diagenetic processes to include the ability to mimic an immediate organic carbon deposition. The new model version (FESDIA) written in Fortran and R programming language was able to reproduce the basic trends from field sediment porewater data affected by the November 2008 flood event in the Rhône River prodelta. Simulation experiments on two end-member scenarios of sediment characteristics dictated by field observation (1–high thickness deposit, with low TOC (total organic carbon) and 2–low thickness, with high TOC), reveal contrasting evolutions of post-depositional profiles. A first-order approximation of the differences between subsequent profiles was used to characterize the timing of recovery (i.e. relaxation time) from this alteration. Our results indicate a longer relaxation time of approximately 4 months for SO42- and 5 months for DIC (dissolved inorganic carbon) in the first scenario, and less than 3 months for the second scenario which agreed with timescale observed in the field. A sensitivity analysis across a spectrum of these end-member cases for the organic carbon content (described as the enrichment factor α) and for sediment thickness indicates that the relaxation time for oxygen, sulfate, and DIC decreases with increasing organic enrichment for a sediment deposition that is less than 5 cm. However, for larger deposits (>14 cm), the relaxation time for oxygen, sulfate, and DIC increases with α. This can be related to the depth-dependent availability of oxidant and the diffusion of species. This study emphasizes the significance of these sediment characteristics in determining the sediment's short-term response in the presence of an episodic event. Furthermore, the model described here provides a useful tool to better understand the magnitude and dynamics of flooding event on biogeochemical reactions on the seafloor.
... Active Fe (hydr)oxide in offshore sediments is mainly consumed by dissimilatory microorganism reduction in early diagenesis and by reaction with the product of sulfate reduction to produce sulfide precipitation (Aller, 1998;Borch et al., 2010). When the oxygen in the sediment is exhausted, Fe (hydr)oxide reduction begins. ...
The release of trace metals caused by industrial effluents and anthropogenic activities has been recorded in the Xixi River estuary, southern China. However, a thorough understanding of the behavior of trace heavy metals in Xixi River sediments is lacking. A total of 12 sediment cores were collected in June and December in the upper estuary section and mouth of the estuary. Here, an in situ high-resolution sampling technique, namely, diffusive gradients in thin films (DGT), was employed to acquire profiles of trace element concentrations and the release of bioavailable metals from sediments in different seasons. A three-step Community Bureau of Reference (BCR) sequential extraction method was used to explore the chemical speciation of trace metals in different seasons and to thereby assess the release potential of trace elements in sediments. The BCR sequential extraction results showed that the trace metals Fe, Mn, Co and Pb were mainly in the residual fraction, which rarely influences living organisms. The total mobile fractions (F1 + F2 + F3) of all trace metals were higher in winter than in summer, suggesting that accumulation occurred from summer to winter. DGT measurements showed that the intensity of sulfate reduction was higher in summer than in winter because of the high temperatures and high organic matter in summer. The intensity of sulfate and Mn(III/IV) reduction increased from the upper estuary section to the lower estuary. Fe(III) reduction decreased in summer but increased slowly in winter. The Pearson correlation results showed that the release of DGT-labile Co in pore water was related to Mn(III/IV) reduction, while the release of DGT-labile Pb was basically not controlled by the Fe-Mn-S redox transition. Abnormally high DGT-labile Pb concentrations were observed at the sampling station (XR3) closest to the estuary in winter, which might have been caused by the high Pb content in the local micro-sediments.
... It may be counterintuitive that erosion increases OC burial efficiency as postdepositional mobilization of sediments is considered to promote OC degradation (Aller, 1998;Aller & Blair, 2006). However, this discrepancy may result from a difference in the scale of the system under consideration. ...
The burial of organic carbon (OC) preserves paleoenvironmental archives and drives the evolution of Earth's biogeochemical cycles. While the impact of kinetic heterogeneity on OC preservation has been studied, the effects of sedimentation dynamics remain largely unknown. Here, we incorporate the expected stochastic variability in sedimentation rates into a reactive‐transport model, generate predictions for stratigraphic variability of OC burial efficiency, and compare the model outputs to field observations. We find that internal sedimentation dynamics profoundly influence OC preservation efficiencies and create autogenic signals that may obscure signals originated from external forcings. Simulations match observations in terms of power spectra when our model considers transient periods of erosion during net deposition, implying that the manifestation of sedimentation dynamics and their interactions with biogeochemistry in chemostratigraphic records are prevalent in nature. As sedimentary OC records reflect both internal variability and external forcing, they may be used as proxies for past depositional conditions.
... The seabed within the shallow Gulf of Martaban contains a thick mixed layer (0.25-1.2 m thick) which is evidence of intense resuspension, but exhibits relatively low accumulation rates (Kuehl et al., 2019). It has been characterized as a "fluid mud reactor" (i.e., Aller, 1998), because the frequent resuspension and apparent trapping within the Gulf likely impact geochemical cycling of organic matter there (Kuehl et al., 2019;Flynn et al., in press). Accumulation rates generally increase offshore in the Gulf of Martaban, and Flynn et al. (in press) note a general transition from the mud blanket to a zone of accumulation at about the mouth of the Gulf (Figure 1). ...
The Ayeyarwady and Thanlwin Rivers, which drain Myanmar, together form one of the largest point sources of freshwater and sediment to the global ocean. Combined, these rivers annually deliver an estimated 485 Mt of sediment to the northern Andaman Sea. This sediment contributes to a perennially muddy zone within the macro-tidal Gulf of Martaban, but little is known about the processes that dominate dispersal and trapping of sediment there, as very few water column observations are available. A research cruise in December 2017 provided a rare opportunity to obtain Acoustic Doppler Current Profiler (ADCP) data along transects from the Gulf of Martaban and adjacent continental shelf. Two transects were obtained from the outer portion of the Gulf of Martaban in water depths that ranged from about 20–35 m. These showed very fast currents, especially during flood tide conditions, exceeding 1.5 m/s. The backscatter record from the ADCP indicated asymmetries in distribution of suspended sediment during the ebb versus flood phase of the tide. During ebb tidal conditions, the backscatter record indicated that sediment was transported in either a surface advected layer, or fairly well-mixed throughout the water column. In contrast, during flood tidal conditions, sediment was confined to the bottom boundary layer, even though the velocities were faster during flood than the ebb conditions. The vertical structure of the currents during flood tide conditions indicated the presence of sediment–induced stratification because currents within the near-bed turbid layers were relatively slow, but speeds increased markedly above these layers. This albeit limited dataset provides an exciting glimpse into the dynamics of sediment transport within the muddy, macrotidal Gulf of Martaban, and implies the importance of tidal straining and bottom nepheloid layer formation there.
... Using a simple metric for estimating relaxation timescale of the pertubation, our calculations for the first end-member scenario (EM1) show that the upper bound of the timescale of relaxation for oxygen is 5 ± 3 days, whereas it was approximately 2 ± 2 days for the second end-member scenario (EM2). This reflects the property of oxygen, which quickly approaches a steady state situation after an event (Aller, 1998). This viewpoint is supported by an 555 ex situ controlled laboratory setup. ...
Episodic events of flood deposit in coastal environments are characterized by deposition of large quantities of sediment containing reactive organic matter within short periods of time. While steady-state modelling is common in sediment biogeochemical modelling, the inclusion of these events in current early diagenesis models has yet to be demonstrated. We adapted an existing model of early diagenetic processes to include the ability to mimic an immediate organic carbon deposition. The new model version was able to reproduce the basic trends from field sediment porewater data affected by the November 2008 flood event in the Rhone River prodelta. Simulation experiments on two end-member scenarios of sediment characteristics dictated by field observation, (1-high thickness deposit, with low TOC and 2-low thickness, with high TOC), reveal contrasting evolutions of post-depositional profiles. A first-order approximation of the differences between subsequent profiles was used to characterize the timing of recovery (i.e relaxation time) from this alteration. Our results indicate a longer relaxation time of approximately 4 months for SO42- and 5 months for DIC in the first scenario and less than 3 months for the second scenario which agreed with timescale observed in the field. A sensitivity analysis across a spectrum of these end-member cases for the organic carbon content (described as the enrichment factor α) and for sediment thickness – indicates that the relaxation time for oxygen, sulfate, and DIC decreases with increasing organic enrichment for a sediment deposition that is less 5 cm. However, for larger deposits (> 14 cm), the relaxation time for oxygen, sulfate and DIC increases with α. This can be related to the depth dependent availability of oxidant and the diffusion of species. This study emphasizes the significance of these sediment characteristics in determining the sediment’s short-term response in the presence of an episodic event. Furthermore, the model described here provides a useful tool to better understand the magnitude and dynamics of flooding event on biogeochemical reactions on the seafloor.
... Sediment trapping and intense mixing within fluid muds can also result in the efficient oxidation of OC (Blair and Aller, 2012;Bianchi et al., 2018). Nearly 1/3 of TerrOC within fluid muds offshore of the Amazon has been found to remineralize prior to deposition within the Amazon subaqueous delta (Aller et al., 1996;Aller, 1998;Showers and Angle, 1986). This may be comparable to the Ayeyarwady considering the evidence of extensive mixing and fluid muds present within the Gulf of Martaban (Kuehl et al., 2020;Harris et al., 2022), and the potential for linkage between the Gulf and the clinoform, as is found within the Amazon (Fig. 8). ...
Large river deltas serve as globally important archives of terrestrial and shallow marine biogeochemical signatures and because of rapid sedimentation have the potential to impact global biogeochemical cycling. The Ayeyarwady Delta in Myanmar ranks as the world's third largest river delta in terms of sediment supply; however, modern increases in regional anthropogenic impacts risk severe alteration to sediment and TerrOC loads within this major system. By investigating modern sediment and terrestrial organic carbon (TerrOC) accumulation within the offshore Ayeyarwady Delta this study estimates baseline sediment and TerrOC budgets for this understudied mega-delta. Using ²¹⁰Pb geochronology of 27 sediment cores collected from the continental shelf, we estimate that 405 ⁺⁵² Mt of sediment, or ~70–80% of fluvial sediment discharged from the Ayeyarwady and Thanlwin rivers (the main inputs to the delta), accumulates there annually. Sediment not retained on the shelf is likely partitioned between the Ayeyarwady floodplain, shoreline accretion, and minor deep-sea export. Estimates of TerrOC (based on δ¹³C mixing models) were coupled with modern sediment accumulation rates to determine an annual burial of 1.93 +1.09 Mt C on the shelf, with TerrOC burial fluxes being highest in the foreset beds of the subaqueous delta, coincident with the area of highest sediment accumulation rate. Based on estimates of the Ayeyarwady and Thanlwin rivers' TerrOC delivery, an apparent ~100% of TerrOC input is preserved on the continental shelf. However, an across shelf trend of increasing TerrOC degradation with distance offshore is also observed, indicating that while the shelf has high apparent TerrOC sequestration, carbon remineralization is also occurring prior to deposition within the subaqueous delta. Based on these conflicting outcomes, we suggest that input of TerrOC from additional sources other than the Ayeyarwady and Thanlwin rivers roughly balance the observed carbon remineralization. Main additional sources of TerrOC include the Sittang and several smaller rivers, and the Ayeyarwady delta plain below the river gauging station. As anthropogenic development within the Ayeyarwady and Thanlwin watersheds continues to increase, these sediment and TerrOC budgets provide a baseline from which future changes within the offshore Ayeyarwady Delta can be monitored.
... Overall, despite C losses to the atmosphere during stream transport due to metabolization, terrestrial organic matter represents approximately one-third of all buried organic matter within marine sediments globally and is preferentially remineralized and buried in continental shelf sediments (Burdige, 2005). Carbon burial efficiency increases with a sedimentation rate (Aller, 1998), which causes coastal mountains that exhibit high marine shelf sedimentation due to landslides to be prime locations for the burial of landslide-mobilized carbon, especially when the frequency or magnitude of carbon mobilizing events is high (Frith et al., 2018;Hilton et al., 2011;West et al., 2011). ...
Landslides, a forest disturbance, mobilize carbon (C) sequestered in vegetation and soils. Mobilized C is deposited either onto hillslopes or into the water, sequestering C from and releasing C to the atmosphere at different time scales. The C‐dense old‐growth temperate forests of SE Alaska are a unique location to quantify C mobilization rate by frequent landslides that often evolve into saturated moving masses known as debris flows. In this study, the amount of C mobilized by debris flows over historic time scales was estimated by combining a landslide inventory with maps of modeled biomass and soil carbon. We analyzed SE Alaskan landslides over a 55‐year period where a total of 4.69 ± 0.21 MtC was mobilized, an average rate of 2.5 tC km⁻² yr⁻¹. A single event in August 2015 mobilized 57,651 ± 3,266 tC, an average of 63 tC km⁻². Depositional fate was inferred using two methods, a standard stream intersection analysis and a second novel approach using simulated debris flow deposition modeling calibrated to the study area. Approximately 60% of debris flow deposits intersected the stream network (9% into mainstem channels, 91% into small tributaries), consistent with long‐term modeled connectivity, suggesting that debris flows are likely to contribute to globally significant amounts of C buried in local fjord sediments. Our results are consistent with an emerging consensus that landslide disturbances that mobilize organic carbon may play an important role in the global carbon cycle over geologic time, with coastal temperate forests being hotspots of potential carbon sequestration.
... These studies suggest that this terrOM was extensively degraded and lost as CO 2 and H 2 O before reaching kerogen-forming depocenters (Sugimura and Suzuki, 1988;Kirchman and Suzuki, 1991;Middelburg et al., 1993;Hedges et al., 1994;Hedges and Oades, 1997a) however, more recent studies have shown this not to be the case (Sampere et al., 2008;Kuliń ski et al., 2016;Wu et al., 2019). Much of the evidence for degradation in deltaic large-river regions occurs in mobile-muds, a process that has been referred to as ''incineration" of organic matter because of the highly oxidative conditions in these sedimentary deposits (Aller, 1998;Blair and Aller, 2011;Bianchi et al., 2016). Resuspension of sediments through storm events, particularly hurricanes and tropical storms which are prevalent in this region, can also cause extensive oxidation of the sediments to occur (Bianucci et al., 2018;Moriarty et al., 2018). ...
Sediment samples along a transect extending from the Mississippi River Birdsfoot Delta to the Mississippi Canyon on the Louisiana continental shelf were examined, by advanced analytical techniques, electrospray ionization coupled to a 12T Fourier transform ion cyclotron resonance mass spectrometer (ESI-FTICR-MS) and quantitative solid-state multiple cross polarization magic angle spinning (multi-CPMAS) ¹³C NMR, in an effort to understand the source and export of terrigenous organic matter to the Gulf of Mexico. Both NMR and mass spectral data indicate that condensed aromatics (CA) and carboxyl-containing aliphatic molecules (CCAM) are present at the mouth of the river, reflective of high contributions from terrigenous soil-like organic matter. Mass spectral peak magnitudes of CA diminish by 15%, with increasing distance offshore, and represent 30% of integrated NMR peak areas. In contrast, mass spectral and NMR CCAM peaks increase by 7 and 13%, respectively. These trends in humic acid extracts,provide novel molecular evidence of terrigenous organic matter deposition in offshore sediments.
... Contrairement au flux diffusif qui ne concerne que des substances dissoutes, la bioturbation affecte à la fois la fraction dissoute et solide. La bioturbation augmente sensiblement le transport des espèces dissoutes en fonction des sédiments étudiés (Aller, 1998a;Fenchel and Glud, 1998;Glud et al., 1995). La bioturbation impacte également la séquence diagénétique en maintenant le fer et le manganèse sous forme oxydées et améliore ainsi l'oxydation potentielle des sulfures en sulfates (Aller, 1994a(Aller, , 1994b. ...
Les éléments traces métalliques arrivant dans le milieu marin s’accumulent naturellement dans les sédiments. En zone portuaire, ces sédiments peuvent être remis en suspension dans la colonne d’eau par des phénomènes naturels (houles, tempêtes…) et/ou anthropiques (trafic maritime, activité de dragage). En particulier, la Méditerranée Nord-occidentale représente un lieu d’échanges maritimes conséquents, engendrant des évènements de remise en suspension d’origine anthropique fréquents, tout en étant peu soumise à l’influence des phénomènes naturels liés aux marées. Dans ce contexte, les objectifs de cette thèse consistaient à étudier le potentiel de remobilisation des éléments traces lors d’évènements de remise en suspension afin d’apporter des connaissances contribuant à mieux prédire et ainsi mieux gérer le risque associé. Les cinétiques de transferts des ETMMs, étudiés lors d’expérimentations en laboratoire, ont été précisées à la lumière de la variabilité des contextes portuaires. Le transfert des ETMMs durant les 5 premiers jours de remise en suspension est apparu déterminé principalement par des processus abiotiques. La compréhension de ces processus a permis d’expliquer partiellement les observations de terrain réalisées lors d’une étude d’impact d’une opération de dragage.
Spatial predictions of total organic carbon (TOC) concentrations and stocks are crucial for understanding marine sediments’ role as a significant carbon sink in the global carbon cycle. In this study, we present a geospatial prediction of TOC concentrations and stocks at a 5 x 5 arc minute grid scale, using a deep learning model — a novel machine learning approach based on a new compilation of over 22,000 global TOC measurements and a new set of predictors, such as seafloor lithologies, grain size distribution, and an alpha-chlorophyll satellite data. In our study, we compared the predictions and discuss the limitations from various machine learning methods. Our findings reveal that the neural network approach outperforms methods such as k Nearest Neighbors and random forests, which tend to overfit to the training data, especially in highly heterogeneous and complex geological settings. We provide estimates of mean TOC concentrations and total carbon stock in both continental shelves and deep sea settings across various marine regions and oceans. Our model suggests that the upper 10 cm of oceanic sediments harbors approximately 171 Pg of TOC stock and has a mean TOC concentration of 0.68 %. Furthermore, we introduce a standardized methodology for quantifying predictive uncertainty using Monte Carlo dropout and present a map of information gain, that measures the expected increase in model knowledge achieved through in-situ sampling at specific locations which is pivotal for sampling strategy planning.
This chapter reviews and synthesizes key advances in our understanding of organic carbon (OC) cycling in estuaries over the past decade, combining a discussion of both particulate and dissolved OC (POC and DOC). Estuaries receive OC from terrestrial and oceanic sources as well as internal production, and this OC is highly diverse in chemical composition and biogeochemical reactivity. We review methodological advances as well as key established techniques that are used to quantify OC, distinguish OC sources, and track OC transformations. We also examine the different sources themselves and how their OC fluxes are controlled. Estuaries have high rates of OC transformation, and we examine the effects of flocculation, sorption to minerals, and microbial and photochemical degradation. We discuss how advances in our knowledge are applied in biogeochemical ocean models of estuaries and coastal zones, and examine the challenges in modeling OC across the interface from land to ocean. We conclude by evaluating how our understanding of the carbon budget for the coastal zone has evolved, highlighting key challenges for the future.
Submarine turbidity currents form the largest sediment accumulations on Earth, raising the question of their role in global carbon cycles. It was previously inferred that terrestrial organic carbon was primarily incinerated on shelves and that most turbidity current systems are presently inactive. Turbidity currents were thus not considered in global carbon cycles, and the burial efficiency of global terrestrial organic carbon was considered low to moderate (∼10–44%). However, recent work has shown that burial of terrestrial organic carbon by turbidity currents is highly efficient (>60–100%) in a range of settings and that flows occur more frequently than once thought, although they were far more active at sea-level lowstands. This leads to revised global estimates for mass flux (∼62–90 Mt C/year) and burial efficiency (∼31–45%) of terrestrial organic carbon in marine sediments. Greatly increased burial fluxes during sea-level lowstands are also likely underestimated; thus, organic carbon cycling by turbidity currents could play a role in long-term changes in atmospheric CO 2 and climate.
Expected final online publication date for the Annual Review of Marine Science, Volume 16 is January 2024. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.
This volume provides a state-of-the-art summary of biogeochemical dynamics at major river-coastal interfaces for advanced students and researchers. River systems play an important role (via the carbon cycle) in the natural self-regulation of Earth's surface conditions by serving as a major sink for anthropogenic CO2. Approximately 90 percent of global carbon burial occurs in ocean margins, with the majority of this thought to be buried in large delta-front estuaries (LDEs). This book provides information on how humans have altered carbon cycling, sediment dynamics, CO2 budgets, wetland dynamics, and nutrients and trace element cycling at the land-margin interface. Many of the globally important LDEs are discussed across a range of latitudes, elevation and climate in the drainage basin, coastal oceanographic setting, and nature and degree of human alteration. It is this breadth of examination that provides the reader with a comprehensive understanding of the overarching controls on major river biogeochemistry.
This volume provides a state-of-the-art summary of biogeochemical dynamics at major river-coastal interfaces for advanced students and researchers. River systems play an important role (via the carbon cycle) in the natural self-regulation of Earth's surface conditions by serving as a major sink for anthropogenic CO2. Approximately 90 percent of global carbon burial occurs in ocean margins, with the majority of this thought to be buried in large delta-front estuaries (LDEs). This book provides information on how humans have altered carbon cycling, sediment dynamics, CO2 budgets, wetland dynamics, and nutrients and trace element cycling at the land-margin interface. Many of the globally important LDEs are discussed across a range of latitudes, elevation and climate in the drainage basin, coastal oceanographic setting, and nature and degree of human alteration. It is this breadth of examination that provides the reader with a comprehensive understanding of the overarching controls on major river biogeochemistry.
This volume provides a state-of-the-art summary of biogeochemical dynamics at major river-coastal interfaces for advanced students and researchers. River systems play an important role (via the carbon cycle) in the natural self-regulation of Earth's surface conditions by serving as a major sink for anthropogenic CO2. Approximately 90 percent of global carbon burial occurs in ocean margins, with the majority of this thought to be buried in large delta-front estuaries (LDEs). This book provides information on how humans have altered carbon cycling, sediment dynamics, CO2 budgets, wetland dynamics, and nutrients and trace element cycling at the land-margin interface. Many of the globally important LDEs are discussed across a range of latitudes, elevation and climate in the drainage basin, coastal oceanographic setting, and nature and degree of human alteration. It is this breadth of examination that provides the reader with a comprehensive understanding of the overarching controls on major river biogeochemistry.
This volume provides a state-of-the-art summary of biogeochemical dynamics at major river-coastal interfaces for advanced students and researchers. River systems play an important role (via the carbon cycle) in the natural self-regulation of Earth's surface conditions by serving as a major sink for anthropogenic CO2. Approximately 90 percent of global carbon burial occurs in ocean margins, with the majority of this thought to be buried in large delta-front estuaries (LDEs). This book provides information on how humans have altered carbon cycling, sediment dynamics, CO2 budgets, wetland dynamics, and nutrients and trace element cycling at the land-margin interface. Many of the globally important LDEs are discussed across a range of latitudes, elevation and climate in the drainage basin, coastal oceanographic setting, and nature and degree of human alteration. It is this breadth of examination that provides the reader with a comprehensive understanding of the overarching controls on major river biogeochemistry.
This volume provides a state-of-the-art summary of biogeochemical dynamics at major river-coastal interfaces for advanced students and researchers. River systems play an important role (via the carbon cycle) in the natural self-regulation of Earth's surface conditions by serving as a major sink for anthropogenic CO2. Approximately 90 percent of global carbon burial occurs in ocean margins, with the majority of this thought to be buried in large delta-front estuaries (LDEs). This book provides information on how humans have altered carbon cycling, sediment dynamics, CO2 budgets, wetland dynamics, and nutrients and trace element cycling at the land-margin interface. Many of the globally important LDEs are discussed across a range of latitudes, elevation and climate in the drainage basin, coastal oceanographic setting, and nature and degree of human alteration. It is this breadth of examination that provides the reader with a comprehensive understanding of the overarching controls on major river biogeochemistry.
This volume provides a state-of-the-art summary of biogeochemical dynamics at major river-coastal interfaces for advanced students and researchers. River systems play an important role (via the carbon cycle) in the natural self-regulation of Earth's surface conditions by serving as a major sink for anthropogenic CO2. Approximately 90 percent of global carbon burial occurs in ocean margins, with the majority of this thought to be buried in large delta-front estuaries (LDEs). This book provides information on how humans have altered carbon cycling, sediment dynamics, CO2 budgets, wetland dynamics, and nutrients and trace element cycling at the land-margin interface. Many of the globally important LDEs are discussed across a range of latitudes, elevation and climate in the drainage basin, coastal oceanographic setting, and nature and degree of human alteration. It is this breadth of examination that provides the reader with a comprehensive understanding of the overarching controls on major river biogeochemistry.
This volume provides a state-of-the-art summary of biogeochemical dynamics at major river-coastal interfaces for advanced students and researchers. River systems play an important role (via the carbon cycle) in the natural self-regulation of Earth's surface conditions by serving as a major sink for anthropogenic CO2. Approximately 90 percent of global carbon burial occurs in ocean margins, with the majority of this thought to be buried in large delta-front estuaries (LDEs). This book provides information on how humans have altered carbon cycling, sediment dynamics, CO2 budgets, wetland dynamics, and nutrients and trace element cycling at the land-margin interface. Many of the globally important LDEs are discussed across a range of latitudes, elevation and climate in the drainage basin, coastal oceanographic setting, and nature and degree of human alteration. It is this breadth of examination that provides the reader with a comprehensive understanding of the overarching controls on major river biogeochemistry.
This volume provides a state-of-the-art summary of biogeochemical dynamics at major river-coastal interfaces for advanced students and researchers. River systems play an important role (via the carbon cycle) in the natural self-regulation of Earth's surface conditions by serving as a major sink for anthropogenic CO2. Approximately 90 percent of global carbon burial occurs in ocean margins, with the majority of this thought to be buried in large delta-front estuaries (LDEs). This book provides information on how humans have altered carbon cycling, sediment dynamics, CO2 budgets, wetland dynamics, and nutrients and trace element cycling at the land-margin interface. Many of the globally important LDEs are discussed across a range of latitudes, elevation and climate in the drainage basin, coastal oceanographic setting, and nature and degree of human alteration. It is this breadth of examination that provides the reader with a comprehensive understanding of the overarching controls on major river biogeochemistry.
The sediment-water interface in the coastal ocean is a highly dynamic zone controlling biogeochemical fluxes of greenhouse gases, nutrients, and metals. Processes in the sediment mixed layer (SML) control the transfer and reactivity of both particulate and dissolved matter in coastal interfaces. Here we map the global distribution of the coastal SML based on excess 210Pb (210Pbex) profiles and then use a neural network model to upscale these observations. We show that highly dynamic regions such as large estuaries have thicker SMLs than most oceanic sediments. Organic carbon preservation and SMLs are inversely related as mixing stimulates oxidation in sediments which enhances organic matter decomposition. Sites with SML thickness >60 cm usually have lower organic carbon accumulation rates (<50 g C m−2 yr−1) and total organic carbon/specific surface area ratios (<0.4 mg m−2). Our global scale observations reveal that reworking can accelerate organic matter degradation and reduce carbon storage in coastal sediments. The authors map the global distribution of the mixed layer in coastal ocean sediments, based on a neural network model. These observations reveal that mixing can accelerate organic matter degradation and reduce carbon storage in the coastal ocean.
The transport and deposition of mud in rivers are key processes in fluvial geomorphology and biogeochemical cycles. Recent work indicates that flocculation might regulate fluvial mud transport by increasing mud settling velocities, but we lack a calibrated mechanistic model for flocculation in freshwater rivers. Here, we developed and calibrated a semi‐empirical model for floc diameter and settling velocity in rivers. We compiled a global data set of river suspended sediment concentration‐depth profiles and inverted them for in situ settling velocity using the Rouse‐Vanoni equation. On average, clay and silt (diameters <39 μm) are flocculated with settling velocity of 1.8 mm s⁻¹ and floc diameter of 130 μm. Among model variables, Kolmogorov microscale has the strongest positive correlation with floc diameter, supporting the idea that turbulent shear limits floc size. Sediment Al/Si (a mineralogy proxy) has the strongest negative correlation with floc diameter and settling velocity, indicating the importance of clay abundance and composition for flocculation. Floc settling velocity increases with greater mud and organic matter concentrations, consistent with flocculation driven by particle collisions and binding by organic matter which is often concentrated in mud. Relative charge density (a salinity proxy) correlates with smaller floc settling velocities, a finding that might reflect the primary particle size distribution and physical hosting of organic matter. The calibrated model explains river floc settling velocity data within a factor of about two. Results highlight that flocculation can impact the fate of mud and particulate organic carbon, holding implications for global biogeochemical cycles.
Deltaic systems are characterized by the highest sedimentation rates in the globe. Meanwhile, sedimentary organic matter therein can be efficiently decomposed so that these depositional systems may deviate substantially from the oft-quoted correlation between net sediment accumulation and preservation of organic matter. The exact mechanisms that cause such a deviation in any given case, however, remain poorly understood. In this study, we utilize a novel ²²⁴Ra/²²⁸Th disequilibrium method to examine sediment oxygen consumption and the release of diagenetic products of organic matter along the major mud wedge system in the inner shelf of the East China Sea. Our sampling campaign was carried out in two contrasting seasons: the summer when seasonal hypoxia was at its peak and physical conditions were relatively quiescent, and the winter when the water column was well oxygenated by intense winter mixing and underlying deposits were subjected to reworking. Unexpectedly, during summer 2017 when the seafloor received the annual maximum supply of organic matter, sediment oxygen consumption rates and benthic fluxes of NH4⁺ were relatively low, ranging from 6 to 59 mmol O2 m⁻² d⁻¹ and from 1.6 to 13 mmol N m⁻² d⁻¹, respectively. In contrast, during winter 2018 sediment oxygen consumption rates and benthic fluxes of NH4⁺ surged to 44-690 mmol O2 m⁻² d⁻¹ and 22-58 mmol N m⁻² d⁻¹, respectively. We have also identified an exponential relationship between amplification factor of sediment surface area and oxygen concentration in the bottom water. This relationship suggests that kinetic energy dissipation in the water column not only controlled air-sea exchange and seawater mixing, but also intensified sediment-water interaction. Importantly, sediment oxygen consumption rates (FO2) in the mud wedge can be empirically described using a modified form of Michaelis-Menten kinetics, suggesting that FO2and the associated benthic consumption and production of chemicals are controlled by both the transport and reaction processes. We have further demonstrated that a large portion of the organic matter deposited over the seafloor in summer is likely decomposed in winter. Overall, this study highlights intense winter mixing as an important mechanism that causes the highly efficient decomposition of sedimentary organic matter in coastal seas.
Marine sediments are a sink for trace metals but also a potential source during sediment resuspension events. Understanding the factor that regulates this uptake or release of contaminant is of prime importance. While the impact of abiotic processes has been widely studied, the quantitative influence of microbial activities on metal cycling during sediment resuspension events is still largely unknown. This study was designed to quantify such microbial contributions on the cycling of a suite of metals (Al, As, Ba, Co, Cr, Cs, Cu, Fe, Li, Mn, Mo, Ni, Pb, Rb, Sb, Sr, Ti, Tl, U, V and Zn) and evaluate the specific contributions of heterotrophic micro-organisms originating from either the seawater or the sediment. For that purpose, the sediment and seawater were selectively sterilized using either autoclave (sediment) or filtration under 0.2 µm (seawater) prior to be mixed for 5 days in darkness. Dissolved concentration in trace elements were measured over time, along with physical-chemical parameters. The pH in the different conditions decreased all along the experiments while the redox potential decreased during up to 4 days before increasing back to its initial value. Three groups of trace metals were identified: metals whose dissolved concentrations (1) increased (Al, Ba, Co, Cs, Cu, Mn, Mo, Ni, Pb, Sb, Tl, U and Zn) as a consequence of the transfer from sediment, (2) decreased (Cr, Fe and Ti) as a consequence of transfer onto sediment, and (3) remained unchanged over time (As, Li, Rb, Sr and V). The sterilization of either sediment and/or seawater did not have a statistically significant impact onto the dynamics of the physical-chemical parameters, nor onto the metals’ behavior (except Mn). Our results demonstrate (i) that marine sediment autoclaving prior to mixing with seawater did not disrupt the behavior of metals in the seawater / sediment mixing over the 5 days of experiments and (ii) that the microbial activity had a negligible influence on the variation of physical-chemical parameters or metals’ transfers over the mixing time.
The present-day regional pattern of organic carbon on the Peru continental margin is characterized by two areas of preferential accumulation: the outer shelf-upper slope at about 100-450 m of water depth between about 11°S and 16°5, and the lower continental slope (>2000 m). The middle slope lacks significant recent organic-rich sediments. Deposition on the upper slope originates predominately from high biological production in response to persistent coastal upwelling. Two organic carbon concentration maxima (>10% dry weight) are found on the slope between 11° S and 14°5, where the shelf is only apprx 15 km wide below or on the fringes of centers of maximum annual primary production (l000 gCm-2y-1). At 7°-10° S, where a similarly productive upwelling center is located, the wide shelf (~30 km) and shallow water depths promote continuous reworking by bottom currents. Interaction of the Peru Current system with shelf-slope morphology is the controlling factor for the present distribution of organic matter on the continental margin. Between about 11° and 14° S, bottom currents that fluctuate in both strength and direction appear responsible
for the non-deposition on the mid- slope and the subsequent downslope accumulation of resuspended particulate organic matter. At 15° S, the subsurface current that flows predominantly poleward relaxes at slope depths, and, as a consequence, organic carbon and
fine-grained clays accumulate here preferentially (>9 gC g Cm-21000 y-1). This accumulation maximum is not reflected in the carbon content of the sediments (<10%) due to dilution by fine-grained terrigenous debris. Organic carbon accumulation is further enhanced here by intensification of the O2-minimum from north to south. The temporal pattern of organic carbon accumulation between 11° and 14° S is recorded in a series of sediment sequences separated by hiatuses. The present mode and magnitude of organic carbon accumulation is higher at the northern end of the upper-slope mud lens facies
(11° S) than in the south (13° S), and has been active for no longer than 500 years. A second record (1500-3000 years ago) is similar in magnitude to the present and separated from a third period of organic carbon accumulation by a major hiatus lasting from about 4000 to 10000 years ago. Prior to that time organic carbon accumulation rates were about 1/6 of the present accumulation at the 11° S sites, but nearly equal to the present rates at the 13° S sites. The timing of the hiatuses and the regional changes in organic carbon accumulation suggest a climatic control whereby both the intensities and the positions of currents and primary productivity centers must have responded to global warming and sea level changes which have occurred since the Late Pleistocene.
Abbau organischer Substanz in rezenten marinen Sedimenten
Zusammenfassung
Im Porenwasser dreier Sedimentkerne aus dem Bereich des westafrikanischen Kontinentalrandes wurden Kohlendioxid, Ammoniak und reaktives Phosphat bestimmt. Sie sind die Oxydationsprodukte der organischen Substanz im Sediment als Folge der bakteriellen Sulfatreduktion. Für diesen Abbau wird ein von RICHARDS (1965) ursprünglich für anoxische Gewässer entwickeltes Modell in abgewandelter
Form vorgeschlagen :
(CH20)106 (NH3)8 (H3PO4)(0.7-0.2) + 53 SO42- =
106 CO2 + 106 H2O + 8 NH3 + (0.7 - 0.2) H3PO4 + 53 S2-
In diese Oxydations-Reduktions-Reaktion gehen die Menge an reduziertem Porenwassersulfat und das atomare Verhaltnis von Corg : Norg : Porg in der
organischen Substanz der Sedimente ebenso mit ein, wie ein kleinerer Teilbetrag an Kohlendioxid, der durch Ausfällung von Kalziumkarbonat dem Porenwasser verloren gegangen ist. Porenwasser wie auch die Sedimentzusammensetzung lassen einen bevorzugten Verlust an Stickstoff wie auch an Phosphor während der Zersetzung
der organischen Substanz erkennen. ·Außerdem scheint es so, als stünden in den tieferen Kernabschnitten nur noch organische Verbindungen mit einer gegenüber der angegebenen Formel niedrigeren Oxydationsstufe für den Abbau zur Verfügung. Aus dem Defizit des Porenwassers an Sulfat läßt sich abschätzen, daß nach dem vollständigen Abschluß des Sediments vom freien Wasser durch ausreichende Überdeckung nur noch etwa 2 % der organischen Substanz abgebaut werden; dagegen ist dieser Betrag mit etwa 15 bis 20% wesentlich großer, legt man die Menge entstandenen Sulfidschwefels zugrunde. Dies gilt für die Kernabschnitte pleistozänen Alters (unterhalb 50-61 cm), die nur eine sehr dünne Oxydationshaut besessen haben dürften, sodaßdie Zersetzung mit Hilfe der bakteriellen Sulfatreduktion schon sehr fruh einsetzen und noch ein fast ungehinderter Nachschub an Sulfat aus dem freien Wasser erfolgen konnte.
Abstract
Carbon dioxide, ammonia, and reactive phosphate in the interstitial water of three sediment cores of the West African continental margin result from oxidation of sedimentary organic matter by bacterial sulfate reduction. The proposed model is a modification of one initially suggested by RICHARDS (1965) for processes in anoxic waters:
(CH20)106 (NH3)8 (H3PO4)(0.7-0.2) + 53 SO42- =
106 CO2 + 106 H2O + 8 NH3 + (0.7 - 0.2) H3PO4 + 53 S2-
The amount of reduced interstitial sulfate, the carbon-to-nitrogen-to-phosphorus atomic ratio of the sedimentary organic matter, as well as small amounts of carbon dioxide, which precipitated as interstitial calcium carbonate, are included in the general
oxidation-reduction reaction. Preferential loss of nitrogen and phosphorus from organic matter close to the surface was recorded in both the interstitial water and sediment composition. It appeared that in deeper sections of the core organic carbon compounds were oxidized which were probably in an even lower oxidation state than that indicated by the proposed model. An estimated 2 % of the amount of organic matter still present was oxidized after it became incorporated into the sediment; whereas sulfide sulfur contents indicate that a much larger percentage (15-20%) seemed to have been subject to bacterial oxidation during the Pleistocene period, when a very thin oxidizing layer on the sediment allowed the above decomposition process to start relatively early favoured by almost fresh organic matter, and by almost unrestricted exchange of sulfate with the overlying water.
In order to examine the transport and burial of terrigenous organic matter along the coastal zones of large river systems, we assessed organic matter dynamics in coupled river/delta systems using mineral surface area as a conservative tracer for discharged riverine particulate organic matter (POM). Most POM in the rivers studied (n = 6) is tightly associated with suspended mineral material; e.g., it is sorbed to mineral surfaces. Average organic loadings in the Amazon River (0.67 ± 0.14 mg C m-2), the river for which we have the largest dataset, are approximately twice that of sedimentary minerals from the Amazon Delta (∼0.35 mg C m-2). Stable carbon isotope analysis indicate that approximately two-thirds of the total carbon on the deltaic particles is terrestrial. The combined surface-normalized, isotope-distinguished estimate is that >70% of the Amazon fluvial POM is not buried in the delta, consistent with other independent evidence (Aller et al., 1996). Losses of terrestrial POM have also been quantified for the river/delta systems of Columbia in the USA, Fly in New Guinea, and Huange-He in China. If the losses of riverine POM observed in these river/delta systems are representative of rivers worldwide, then the surface-constrained analyses point toward a global loss of fluvial POM in delta regions of ∼0.1 X 1015 g C y-1.
Net organic metabolism (that is, the difference between primary production and respiration of organic matter) in the coastal ocean may be a significant term in the oceanic carbon budget. Historical change in the rate of this net metabolism determines the importance of the coastal ocean relative to anthropogenic perturbations of the global carbon cycle. Consideration of long-term rates of river loading of organic carbon, organic burial, chemical reactivity of land-derived organic matter, and rates of community metabolism in the coastal zone leads us to estimate that the coastal zone oxidizes about 7 × 1012 moles C/yr. The open ocean is apparently also a site of net organic oxidation (∼16 × 1012 moles C/yr). Thus organic metabolism in the ocean appears to be a source of CO2 release to the atmosphere rather than being a sink for atmospheric carbon dioxide. The small area of the coastal ocean accounts for about 30% of the net oceanic oxidation. Oxidation in the coastal zone (especially in bays and estuaries) takes on particular importance, because the input rate is likely to have been altered substantially by human activities on land.
Analysis of data from 280 rivers discharging to the ocean indicates that sediment loads/yields are a log-linear function of basin area and maximum elevation of the river basin. Other factors controlling sediment discharge (e.g., climate, runoff) appear to have secondary importance. A notable exception is the influence of human activity, climate, and geology on the rivers draining southern Asia and Oceania. Sediment fluxes from small mountainous rivers, many of which discharge directly onto active margins (e.g., western South and North America and most high-standing oceanic islands), have been greatly underestimated in previous global sediment budgets, perhaps by as much as a factor of three. In contrast, sediment fluxes to the ocean from large rivers (nearly all of which discharge onto passive margins or marginal seas) have been overestimated, as some of the sediment load is subaerially sequestered in subsiding deltas. Before the proliferation of dam construction in the latter half of this century, riv
The decay rate of particulate organic carbon (POC) and nitrogen (PON) was followed during 94 days in three homogenized sediment microcosms: 1. With a natural density of the polychaete Nereis virens (NOx-cores); 2. Defaunated, with an aerobic water phase (Ox-cores); and 3. Defaunated, with an anaerobic water phase (An-cores). In all cores there was a marked preferential mineralization of PON compared to POC. The presence of Nereis increased the net decomposition of POC and PON 2.6 and 1.6 times relative to Ox-cores. Ventilation of burrow structures by the worms increased the flux of O2, TCO2 and DIN across the sediment-water interface 2.5-3.5 times. This significantly decreased the pore water concentrations of TCO2 and DIN. Similarly, nitrification and denitrification were stimulated 2.3-2.4 times due to nereid activity. Oxygen did not increase organic degradation: in fact, the decay of POC and PON was faster in An- than in Ox-cores, 1.5-1.6 and 1.2 times, respectively. Sulfate reduction, measured at the end of experiment, was surprisingly low in the aerobic NOx- and Ox-cores relative to An-cores. Net ammonium production measured at the end of the experiment agreed with the mean loss of PON for Ox- and An-cores, but was low for NOx-cores, suggesting that a high C:N substrate was being degraded in these cores at the end. An empirical model describing the temporal decay pattern of POC and PON is presented: the detritus in all cores were initially composed of two fractions (similar C:N); a readily degradable (∼43%) and a low degradable (∼57%) fraction. A substantial part of the degradable fraction in NOx-cores was used during the experiment, with nitrogen being mineralized preferentially. The mean C:N molar ratio of detritus used was 5.9, compared to a value of 15.5 determined at the end. The Ox- and An-cores, however, showed similar C:N ratios for the detritus used during the experiment (3.7 and 4.8) and that measured at the end (4.2 and 4.6). Presumably not all the low C:N detritus had yet been mineralized in these cores at the end of experiment.
Widespread occurrence of several different minor sedimentary structures in shallow-water Gulf of Mexico sediments has been shown by more than 2,000 cores and bottom samples. Minor internal structures in two areas, one off the east side of the Mississippi River Delta and the other along the central Texas coast, were found to be similar. Deposits in these two areas were subdivided into those containing: (1) regular layers (thin beds or laminations), (2) irregular layers (rough or crude layers and lenses), (3) mottles (discontinuous lumps, tubes, and pockets), and (4) structureless homogeneous sediments, depending on the principal type that occurs. Areal distributions of these different structures in the surficial sediments are mappable. Similar sedimentary structures and sequences of structures occur in the open gulf and in the bays and sounds. In the open gulf the structures occur in wide bands, and the sequences are developed through wide ranges of water depths; in the bays and sounds the bands generally are narrower, and the sequences are compressed into much smaller ranges of water depths. The different types of structures are formed on or near the depositional surface, contemporaneous or nearly contemporaneous with deposition. The differences between depositional environments which produce the various structures are differences in: (1) sediment sources, (2) in physical processes and their intensities, and (3) rates of deposition. Regular layers are characteristic of areas of rapid deposition and/or relatively few bottom-living animals; they are either primary--formed by fluctuations of sediment, or secondary--formed by wave or current winnowing. Irregular layers and mottles are mostly secondary features, formed mainly by bottom-living animals altering existing sediments. Homogeneous deposits are either primary or secondary; they form by extremely rapid deposition, unif rm deposition, or by complete secondary reworking. The appearance of many normal marine sediments is due in large measure to burrowing and crawling organisms. Structures similar to those in presently forming sediments of the Gulf of Mexico apparently were developed in rocks of all ages when environmental conditions were proper. Knowledge of how these structures form, why they vary, and the relationships between them furnishes a valuable key for interpreting deposition of ancient rocks.
This paper summarizes the results of detailed field studies of C,N,P and S cycling in the anoxic sediments of a small coastal lagoon, Cape Lookout Bight (CLB), located on the Outer Banks barrier island chain of North Carolina (U.S.A.). These results are utilized to illustrate the efficiency of the principal processes controlling the biogeochemical cycling of each element, including factors leading to their differential release to overlying waters and preservation via burial. The Cape Lookout site is particularly well suited for studies of elemental cycling in organic-rich sediments because of the steady-state nature of annual sediment accumulation occurring since the mid-1960’s and the relative lack of physical disturbances via bioturbation or resuspension of the sediment column. Comparison of Cape Lookout results with those from other sedimentary environments leads to important insights concerning how the biogeochemical cycles of C, N, P and S interact.
The dynamic processes involved in the circulation of estuarine waters are probably the most important influences on the transport and deposition of sediments in estuaries. In moderately stratified or vertically homogeneous estuaries such as the Savannah (Georgia) and Thames (England) Rivers, sediments are moved progressively landward along the bottom and they accumulate near the limit of net landward flow. In highly stratified estuaries such as Southwest Pass of the Mississippi River, coarse sediments are trapped near the toe of the salt wedge while fine sediments are carried seaward over the salt wedge with the outflowing river water. On the basis of present information, however, we cannot always distinguish between the results of natural dynamic processes and the results that are induced by the activities of man. Other possible influences on estuarine sedimentation are the inorganic and organic processes that tend to aggregate sedimentary particles, and the properties of the particles themselves. The effects of salt flocculation have been amply demonstrated by laboratory experiments, but no field evidence is available to show their importance in complex natural estuaries. The agglomeration of sediments by organisms likewise has been investigated in the laboratory, but its importance in nature needs to be evaluated. Our knowledge of the composition and size distribution of the sediment particles contributed to estuaries from different sources is seriously deficient.
Benthic-pelagic coupling is described on a community level reviewing examples from Kiel Bight (Baltic Sea) and the Norwegian-Greenland Sea. An energy flow equation for marine sediments is developed, which includes processes such as biodeposition, sedimentation, as well as lateral advection, all types of bioturbation, and physical transport mechanisms through the sediment-water interface. The fast response and deep-reaching effects of sedimentation events, the budget problems, and the importance of lateral advection as well as resuspension for the understanding of a marine soft-bottom ecosystem are discussed. -Author
A model of coupled nitrification/denitrification was developed for continental shelf sediments to estimate the spatial distribution of denitrification throughout shelf regions in the North Atlantic basin. Using data from a wide range of continental shelf regions, we found a linear relationship between denitrification and sediment oxygen uptake. This relationship was applied to specific continental shelf regions by combining it with a second regression relating sediment oxygen uptake to primary production in the overlying water. The combined equation was: denitrification (mmol N m⁻² d⁻¹) = 0.019* phytoplankton production (mmol C m⁻²d⁻¹). This relationship suggests that approximately 13% of the N incorporated into phytoplankton in shelf waters is eventually denitrified in the sediments via coupled nitrification/denitrification, assuming a C:N ratio of 6.625:1 for phytoplankton. The model calculated denitrification rates compare favorably with rates reported for several shelf regions in the North Atlantic
Immense volumes of argillaceous muds are being deposited along the world's longest continuous mud coastline which faces the open equatorial Atlantic. These are a mud analogue to nearshore and innershelf sand deposits. Transition zones of both mud and sand systems reflect a transition from offshore to shoreface processes and sediments. Both mud and sand shoreface/foreshore systems reflect sediment input and wave, current, and tidal processes that act on them. Mud banks of Suriname resemble linear sand ridges on the continental shelf of the eastern USA, in shape, oblique orientation to the coastline, and orientation with respect to dominant direction of transport processes. The sequence of massive and laminated muds with discontinuity features resembles laminated to burrowed sequences found in ancient and Recent nearshore and shallow-marine sands. -from Authors
ORGANIC matter preserved in marine sediments provides a molecular record of marine biological processes(1), accounts for approximately 20% of all carbon burial(2) and plays a key role in balancing the long-term flux of oxygen to the atmosphere(3). Only recently has it been appreciated that more than 90% of the organic matter preserved in most marine sediments is intimately associated with mineral surfaces(4). Little is known, however, of the effect that sorption to mineral surfaces might have in controlling either the lability or-quantity of-organic matter in the marine sedimentary record. The preserved organic material could be either intrinsically stable, or stabilized through interactions with mineral matrices. We show here that sorption of organic matter to mineral surfaces in marine sediments stabilizes the component molecules, slowing remineralization rates by up to five orders of magnitude. Sorptive protection can therefore account for the enigmatic preservation of intrinsically labile molecules such as amino acids and simple sugars in marine deposits(5,6) and links the preservation of organic carbon in marine sediments to the deposition of mineral surfaces.
Results of pore waters sulfate analysis from 26 cores from the Gulf of California were used to calculate initial gradients of sulfate. The values range from 0.05 to 0.72 mM cm-1. The highest values are from the continental shelf; the intermediate and lowest values are from deep basins. Regression analysis between sedimentation rates and sulfate gradients gives an equation of the form w = -B(c/x)o, with an empirical constant B = 1.49. This constant differs from that obtained for marine sediments from other areas. This difference may be due to the fact that Gulf of California sediments apparently contain less reactive organic matter than do other marine sed ments with similar sedimentation rates.
Onshore-offshore trends in phosphorus (P), organic carbon (OC), and total nitrogen (TN) concentration, P distribution, elemental organic C:N:P ratios, and stable carbon isotopic composition of OC (δ13Coc) of surficial sediments, are presented from three river-dominated coastal regimes: the Mackenzie River/Beaufort shelf in the Canadian Arctic; the Mississippi Delta and Louisiana shelf in the temperate Gulf of Mexico; and the tropical Amazon shelf. These parameters, measured in surficial sediments from the three sites, are used to assess changes in the importance of terrestrial and marine organic matter sources to sediments as a function of distance from the locus of riverine discharge.
We demonstrate the fast transfer of suspended particles from the boundary layer into the upper strata (z <
4 cm) of permeable sediments with topography-related interfacial water flows. The transport is driven by
pressure gradients (A P I 3 Pa) generated when bottom flows (u I 10 cm s-l) are deflected by small surface
structures (z < 3 cm) of hydrodynamical or biological origin. Acrylic pigment grains of 1 - and 1 O-pm diameter
traced the intrusion of particulate matter into sandy sediment (k > 2 x 10-l l m2) incubated in a laboratory
flume. Increased pressure up- and downstream of small mounds (z = 2.5 cm) drove water 5.5 cm into the
core, carrying suspended particles (1 pm) to 2.2-cm sediment depth within 10 h. Simultaneously, decreased
pressure at the downstream slope of the protrusions drew pore fluid from deeper layers (z 5 10 cm) to the
surface. In the sediment, friction reduced the velocity of the particulate tracers, resulting in size fractionation
and layers of increased particle concentration. Ripple topography (0.8-2.8 cm high) enhanced interfacial
particle (1 pm) flux by a factor 2 .3 when compared to a level control core. The pathways of the particle and
solute tracers below a sediment ripple are explained with a source-sink model that describes the pore flow
velocity field. Our results suggest that bedform-induced interfacial flows are important for the uptake of
particulate organic matter into permeable shelf sediments.
Radiochemical measurements of 234Th (t1/2 = 24days), 137Cs (bomb-produced), and 210Pb (t1/2 = 22y) have been used to characterize rates of mixing, deposition, and accumulation on 100-day and 100-y time scales in East China Sea sediments. In the inner-shelf mud deposit near the mouth of the Changjiang (Yangtze River), 234Th data indicate deposition rates as rapid as 4.4 cm month-1 on a 100-day time scale. 210Pb data indicate that on a 100-y time scale accumulation rates are an order of magnitude slower (typically 1 to 5 cm y-1) than the short-term deposition rates. Most of the sediment deposited near the mouth of the Changjiang on a 100-day time scale is transported southward along the dispersal system on a 100-y time scale, probably as a result of winter storms and a strong coastal current.210Pb accumulation rates from the inner-shelf mud deposit indicate that approximately 40% of the sediment discharged by the Changjiang can be accounted for in the sediments north of 30°N. The offshore mud deposit in the East China Sea is associated with the Huanghe (Yellow River) dispersal system. The dominant process affecting radionuclide profiles within this deposit is particle mixing (not sediment accumulation). In this area the upper 5 cm of the seabed are intensely reworked (Db = 26cm2 y-1) relative to the zone between 5 and 25 cm (Db = 2cm2 y-1). The maximum accumulation rate in the offshore mud deposit is 0.3 cm y-1. Less than 2% of the sediment discharged by the Huanghe can be accounted for in the offshore mud deposit. The relative intensity of mixing and accumulation is different for the proximal deposits of the Changjiang (where accumulation dominates) relative to the distal deposits of the Huanghe (where mixing dominates). Near the Changjiang mouth radiographs show distinct horizontal stratification, and the value of G (mixing rate/accumulation rate) is near zero. In the off-shore mud deposit radiographs reveal nearly homogeneous sedimentary structure, and the G value is =>18.
The sediments off the mouth of the Amazon River are unique in terms of modern depositional environments because Fe reduction dominates redox properties for several meters into the seabed. This unique character offers the opportunity to investigate the effects of Fe reduction on reactive solutes like iodine and boron. Also, direct measurements of solute distributions and reaction rates, reported in this paper for iodine and NH4+, constrain the timing of major physical disturbances, which are partly responsible for poising the sediments in the Fe reduction stage. Major enrichments of dissolved iodine and boron occur in 1 3 m cores from the region, in the absence of high organic matter decomposition rates. Coherent relationships between boron and iodine, high dissolved I/N and I/Br production ratios, and the lack of a strong correlation of iodine production with dissolved NH4+ all indicate that Fe reduction exerts the major control on iodine and boron distributions. Nonsteady-state models of dissolved iodine and NH4+ and solid-phase iodine distributions in long cores, using measured surface (0 20 cm) reaction rates, indicate that the surface mixed layer (as defined by210Pb; which includes the upper 1.5 m of sediment in some areas) was formed within 50 250 days prior to our sampling (May June 1983; high Amazon River flow). These data imply that physical sediment reworking on the Amazon shelf has a strong seasonal character, with greatest disturbances occurring during rising river flow. The full implications of these reworking events, for the persistence of the unique authigenic mineral assemblages found in the sediments and for the shelf sediment budget, will require future seasonal studies.
In coastal muds downdrift from the Amazon River mouth, marine diatom frustules are rapidly converted to various forms of authigenic aluminosilicate phases during burial. The dominant neoformed crystalline phases have a composition similar to K-smectite, yield electron diffraction patterns characteristic of clay minerals, and exhibit a range of crystal sizes and morphologies, including euhedral pseudohexagonal crystals and anhedral flakes replacing biogenic silica. A poorly crystalline or microcrystalline K-rich and Fe-rich aluminosilicate material also replaces the siliceous frustules. The conversion process is not always complete, leaving relics of the original frustule in the resulting authigenic aluminosilicate phases. Laboratory incubation experiments with cultured diatoms demonstrate that the conversion process occurs in 20 23 months. The conversion of biogenic silica to authigenic clays occurs throughout the Amazon deltaic deposits and presumably takes place in other comparable depositional settings. Biogenic silica alteration provides proof for a direct link between the biogeochemical cycle of silica in nearshore environments and the neoformation of cation-rich aluminosilicate phases, and it may prove to be important for oceanic geochemical cycles as a sink for Si, K, and other elements incorporated in the authigenic aluminosilicates. Rapid formation of authigenic K-smectite may also represent a reaction stage leading to eventual formation of illitic clays during later diagenesis.
Transport of participate material across continental shelves is well demonstrated by the distributions on the seabed and in the water column of geological, chemical, or biological components, whose sources are found farther landward or farther seaward. This paper addresses passive (incapable of swimming) particles and their transport across (not necessarily off) continental shelves during high stands of sea level. Among the general factors that influence across-shelf transport are shelf geometry, latitudinal constraints, and the timescale of interest. Research studies have investigated the physical mechanisms of transport and have made quantitative estimates of mass flux across continental shelves. Important mechanisms include wind-driven flows, internal waves, wave-orbital flows, infragravity phenomena, buoyant plumes, and surf zone processes. Most particulate transport occurs in the portion of the water column closest to the seabed. Therefore physical processes are effective where and when they influence the bottom boundary layer, causing shear stresses sufficient to erode and transport particulate material. Biological and geological processes at the seabed play important roles within the boundary layer. The coupling of hydrodynamic forces from currents and surface gravity waves has a particularly strong influence on across-shelf transport; during storm events, the combined effect can transport particles tens of kilometers seaward. Several important mechanisms can cause bidirectional (seaward and landward) transport, and estimates of the net flux are difficult to obtain. Also, measurements of across-shelf transport are made difficult by the dominance of along-shelf transport. Geological parameters are often the best indicators of net across-shelf transport integrated over time scales longer than a month. For example, fluvially discharged particles with distinct composition commonly accumulate in the midshelf region. Across-shelf transport of particulate material has important implications for basic and applied oceanographic research (e.g., dispersal of planktonic larvae and particle-reactive pollutants). Continued research is needed to understand the salient mechanisms and to monitor them over a range of timescales.
Denitrification rates in sediments within the oxygen deficient waters off Mexico and from the Gulf of Maine were investigated on the basis of interstitial nutrient profiles. Nitrate fluxes into the sediments were calculated from gradients across the sediment-water interface and vertical molecular diffusion coefficients and averaged 0.151 (Mexico) and 0.0920 (Gulf of Maine) pmol NO−3 cm−2 s−1. These are minimum values, since these gradients may have been underestimated. In the Gulf of Maine, bottom water irrigation by macrobenthos increases the nitrate supply well above this estimate. In addition, only 15-22% of the expected ammonium is present in Gulf of Maine sediments perhaps because of removal by a rapid coupling of nitrification with denitrification. This large apparent loss of the regenerated ammonium appears to be ubiquitous in shelf sediments with oxygenated bottom water. The global denitrification rate in continental shelf sediments was reassessed to be >50 Tg N yr−1 (1 Tg = 1012 g), demonstrating that sediments are an important sink for oceanic nitrogen. Globally, current nitrogen losses from the oceans may exceed inputs by 60-90 Tg N yr−1. Over the glacial-interglacial cycle the global sedimentary denitrification rate probably varied commensurately with the changing continental shelf area. An oscillating oceanic nitrogen budget over these time scales could occur given the sequence of (1) scouring and dumping of terrestrial nitrogen into the oceans during glacial advance, (2) removal of oceanic combined nitrogen to the atmosphere by denitrification following glacial retreat, and (3) reincorporation of this N into terrestrial biomass during the interglacial period.
The Amazon river mouth provides a dynamic setting for studying the formation of sedimentary strata under conditions where fluvial and marine processes merge. River-mouth anchor stations were occupied for diurnal tidal cycles during three stages of river flow, and reoccupied for consecutive spring and neap tides during two stages of river flow. At each anchor station, box cores were collected every two hours and complementary time-series measurements were made of water-column suspended-sediment concentrations, salinity, and current velocities.During high-energy periods in the fortnightly cycle (i.e. most spring tides), a cross-laminated sand layer is present at the seabed surface that exhibits varying degrees of bi-directional current structure and contains low porewater salinities (10–15‰). During low-energy periods in the fortnightly cycle (i.e., most neap tides), a mud bed forms at the seabed surface in association with fluid muds in the water column. This mud layer ranges in thickness from 2 to 15 cm, and exhibits higher radionuclide activities, higher porewater salinities (15–25‰), and lower saturated bulk densities (1.18–1.30 g/cm3) than the sand beds (1.30–1.60 g/cm3). The mud beds are subject to resuspension and deposition by semidiurnal tidal currents that form thin sandy interlaminations.Interlamination and interbedding of sand and mud result from the combination of estuarine and tidal processes at the river mouth. Interlaminations (alternating layers of sand and mud 1 cm in thickness) muds and sands. The thickness of the fortnightly beds is dependent upon monthly variations in spring/neap amplitudes.The processes active near the Amazon river mouth that form interlaminated and interbedded sediments operate in other fluvial-marine settings, and produce similar types of interlayered sediments due to the presence of estuarine circulation, high suspended-sediment concentrations, and tidal energy.
210Pb and 234Th activity profiles in sediment cores from underconsolidated mudflats 300 km downdrift of the Amazon river mouth record an ephemeral surface layer of fine-grained sediment up to 1.5 m thick. This layer contains about l.5 × 108 tons of Amazon sediment deposited rapidly (~1 cm/d) from a fluid-mud suspension (10–400 g/l) during the months between January and June. Virtually the entire layer is remobilized in July–December and the sediment is advected alongshore to the northwest. Seasonal variations in trade-wind strength and in supply of Amazon shelf sediment are thought to control emplacement, and removal of this ephemeral deposit. Solitary surface gravity waves characteristic of this setting generate a net landward sediment flux, which, with shore-normal tidal currents, controls spatial geometry of the surface layer. The resultant lens-shaped deposit dissipates incident wave energy and provides a substrate above mean high water for mangrove colonization and irregular shoreline progradation of meters per year. Macroscale (sand/silt laminations) and microscale (plasmic fabric) sedimentary structures in the ephemeral layer record diverse temporal variations (e.g., tidal and wave-induced) in bottom shear stress and sediment supply. Ephemeral deposition of 108 tons is inferred to be common in coastal areas associated with large and energetic river dispersal systems.
This paper addresses three related questions: (1) What factors control the efficiency of carbon burial in sediments? (2) Are rates of anaerobic organic matter degradation intrinsically lower than aerobic rates? (3) How important are anaerobic processes in the global marine sediment carbon economy?Carbon burial efficiency (the ratio of the carbon burial rate and the carbon flux to the sediment surface) was estimated from literature data for a range of environments and was shown to be a function of sedimentation rate. No difference independent of sedimentation rate was found between aerobic and anaerobic sediments.A review of recent microcosm and laboratory studies shows that anaerobic rates are not intrinsically lower than aerobic rates; fresh organic matter degrades at similar rates under oxic and anoxic conditions. Aerobic decomposition rates near the sediment surface are typically greater than anaerobic rates at depth because the most labile carbon is consumed before it can be buried in the anoxic zone.A model approach was taken in estimating the importance of anaerobic processes in the global marine sediment economy, instead of extrapolating measured rates as done previously. The result, 150 Tg C yr, is two to nine times lower than previous estimates. This rate is about 9% of the global aerobic carbon oxidation rate and is about equal to the rate of long‐term carbon burial. The importance of anaerobic processes in marine sediments lies in their role in determining the amount of carbon preserved, not in the amount of carbon remineralized overall.
Export of organic C to the intermediate and deep water on the continental slope may constitute a significant sink. Preservation of organic C in the coastal sediments is of less importance although this flux has been strongly affected recently by the increase of the river particulate load and enhanced sedimentation rates on the continental shelves. The increase of the riverine nitrogen flux has only small effects on the coastal productivity on a global scale. -from Author
Most sediment and organic material transported from rivers of southern Papua New Guinea enters into the Gulf of Papua, depositing on the inner shelf as either laminated or bioturbated, silt-dominated mud. These facies are the major trawling grounds for a growing penaeid prawn fishery. In contrast to most other terrigenous shelf deposits, decomposition processes in the upper 20 cm of these Papuan silts are apparently dominated by oxic and suboxic diagenesis. Rates of surface oxygen consumption were high (mean = 26.9; range = 17.8–46.8 mmol O2 m⁻² d⁻¹) as were bacterial numbers (range: 1–4 × 10¹⁰ cells g⁻¹ DW) and rates of bacterial carbon production (tritiated thymidine uptake; range: 3-f0 gC m⁻² d⁻¹). Rates of sulfate reduction were low (range: 3.6-6.8 mmol S m⁻² d⁻¹) with little (18–25%) of the total reduced³⁵S04 recovered as acid-volatile sulfide. Free sulfides were not detected in porewaters. Total solid-phase S concentrations were low (0.15-0.20% DW) indicating low net S precipitation in the upper 20 cm. Concentrations of dissolved Fe and Mn were elevated in porewaters in the laminated silts. Solid-phase Fe concentrations were moderately high (range: 4.6-5.3% DW) and measured dissolved metal and nutrient fluxes suggest active Fe and Mn reduction (at some stations) and generally high turnover of the porewater N pools.
The composition of any environment or object is determined by a particular balance between material transport processes and chemical reactions within and around it. In the case of marine sedimentary deposits, the dominant agents of mass transport are often large bottom-dwelling animals that move particles and fluids during feeding, burrowing, tube construction, and irrigation. Such biogenic material transport has major direct and indirect effects on the composition of sediments and their overlying waters. In this chapter I review some of what is presently known about these effects, their implications for both chemical and biological properties of a deposit, and how they can be conceptualized in quantitative models.
Stable carbon isotope ratios have been used to study the sources and transport of particulate organic carbon (POC) in the Changjiang estuary and the East China Sea. δ13C values of POC collected during the period of high water discharge (June 1980) show a range of −25·4 to −19·7 ppt, suggesting mixing of riverine and marine organic carbon. Samples collected during the low water discharge period (November 1981) have a δ13C range of −26·6 to −23·7 ppt, indicative of dominant terrestrial organic carbon contribution. The sedimentary organic carbon in the East China Sea exhibits a relatively narrow δ13C range (−22·9 to −20·8 ppt) suggesting a dominant marine source.
The nature and spatial relationships of sedimentary structures in the Amazon subaqueous delta are determined from radiographs of widely distributed sediment cores, and are contrasted with observations of sedimentary structures from other major deltas. Five distinct facies are identified in modern (accumulated within the past 100 y) Amazon sediments, including: (a) physically stratified sand, (b) interbedded mud and sand, (c) proximal-shelf sandy silt, (d) faintly laminated mud, and (e) mottled mud. In the outer topset and foreset regions of the delta, organic-rich laminae are observed in cores from several of these facies. Physical sedimentary structures are preserved in the landward portion of the topset region (<30 m water depth) because deep scouring and subsequent redeposition places the structures below the depth of effective biological mixing. Physical reworking is reduced in the outer delta (30–70 m water depth), and the preservation of primary structures depends on the relative magnitudes of biological mixing rate and sediment accumulation rate. The amount of sand decreases along the dispersal system to the northwest, and interbedded mud and sand displays a northwestward gradation into faintly laminated mud. Longer-term (>100 years) variations of sedimentary structures are observed in piston cores. Thick (>1 m) sandy-silt beds are present in proximal areas below 3 m depth in the seabed, and could have formed as a result of lateral migration of the proximal-shelf sandy-silt facies.
The interaction between photochemical and biological processes in the degradation of marine dissolved organic matter (DOM) was investigated with seawater from a coastal southeastern U.S. salt marsh. Seawater supplemented with humic substances was exposed to alternating cycles of sunlight (equivalent to 8 h of midday sun) and dark incubations with natural bacterial populations (l-2 weeks in length). Photochemical degradation of the DOM was monitored during sunlight exposure by direct measurements of dissolved inorganic carbon (DIC) and carbon mon- oxide (CO) formation in 0.2-Frn filtered seawater. Bacterial degradation was monitored during dark incubations by tritiated leucine uptake and changes in bacterial numbers in bacterivore-free incubations and by direct measurements of DOM loss. The alternating cycles of sunlight and microbial activity resulted in more complete degradation of bulk DOM and marine humic substances than was found for nonirradiated controls (i.e. with microbial activity alone) by a factor of up to 3-fold. Increased decomposition was due both to direct losses of carbon gas photoproducts (DIC and CO in a 15 : 1 ratio) and to enhanced microbial degradation of photodegraded DOM, with approximately equal contributions from each pathway. Mass balance calculations indicated that low-molecular-weight carbon pho- toproducts, currently considered to be the compounds responsible for stimulating bacterial activity following pho- todegradation of DOM, were insufficient to account for the enhanced bacterial production observed. Thus, higher molecular weight, chemically uncharacterized fractions of DOM may also be modified to more biologically available forms during exposure to natural sunlight.
Seasonal sampling of Amazon shelf deposits revealed that calanoid copepods as well as other pelagic zooplankton were often buried along with benthic infauna throughout the upper ∼25 cm of the sea-bed. During February through March, the period of rising to peak riverine discharge and maximum trade wind stress, shelf-wide maxima occur in numbers and depth of burial. Buried copepods were present in all stations with abundances reaching 5168 copepods m−3in the 10–25-cm depth interval off the river mouth in 18 m of water (Station RMT-2). The intact nature of the buried copepods and presence of phytoplankton within the digestive tracts of many, supports the notion that burial was sudden and rapid (∼hours). In contrast, from August to October, the period of falling to low riverine discharge and minimum wind stress, burial of zooplankters was restricted to stations along a southern transect (ST) and at the innermost river mouth station (RMT-1). Burrowing macroinfauna, meiofauna and bacterial inventories increase dramatically at all shelf stations during the time of minimum zooplankton burial. Successful recolonization by benthos and lack of entrainment of water-column organisms show that the bottom is most stable during seasons of falling to low river flow. Sedimentologic and chemical, as well as biological, evidence indicates rapid turnover of ∼20–30 cm of the sea-bed during rising- and peak-flow periods when the sea-bed is most unstable.
Lignin, elemental, and stable carbon isotope compositions are reported for local plants and for coarse (>63 µ m) and fine (<63 µ m) suspended particulate materials collected along a 1,950‐km reach of the lower Amazon River during four contrasting stages of the 1982–1983 hydrograph. Fluxes of chemically recognizable lignin in the two size classes generally parallel each other along the mainstem with the fine fraction usually predominating. Particulate organic matter transported in the coarse size fraction of the mainstem and its major tributaries is composed of recently formed and well preserved tree leaf debris along with some wood. Organic matter in the fine size fraction is comparatively old, degraded, and rich in immobilized nitrogen and derives primarily from soils. C‐4 grasses, which are abundant in the mainstem floodplain (várzea), are not major components of either the coarse or fine particulate material in the river.
Particulate organic matter in both size fractions is introduced largely from upstream sources within the Rio Solimões and Rio Madeira drainage basins. Most of this organic matter is unreactive and is transported conservatively with mineral particles along the Amazon mainstem. However, some downstream compositional trends are seen in both size fractions which reflect the addition or exchange of highly degraded, ¹³ C‐depleted, and lignin‐poor organic materials from lower basin sources.
Aldose, amino acid, and elemental compositions were determined for flux-weighted samples of coarse (> 63 pm) and fine (< 63 pm) particulate organic material and ultrafiltered (> 1,000 Daltons) dissolved organic matter collected at three sites along the Brazilian Amazon River and six of its major tributaries. Concentrations of total organic C (TOC) were relatively uniform (55Ok 100 PM) at all sites, with DOC comprising the major (50-100%) component. An average of 77% of the total DOC was isolated by ultrafiltration. The greatest compositional differences observed in the Amazon River system were among the coarse, fine, and dissolved organic fractions. All coarse particulate fractions were nitrogen-poor (atomic C : N = 21) and exhibited stable carbon isotope, aldose, and amino acid compositions similar to those of angio- sperm tree leaves. Coarse particulate organic materials, although the least degraded of the three fractions, had lost appreciable carbohydrate and had immobilized excess nitrogen of apparent bacterial origin. Fine particulate materials were more nitrogen-rich (C : N = 9) than coarse counterparts and had lower total aldose yields and glucose percentages. Fine particles gave greater total yields of amino acids, characterized by high ratios of basic vs. acidic components. Coexisting dissolved organic materials recovered by ultra- filtration were nitrogen-poor (C: N = 27-52) and yielded the lowest amounts of aldoses, among which deoxy sugars were concentrated. Dissolved fractions gave extremely low yields of amino acids in mixtures that were enriched in nonprotein components and in acidic vs. basic molecules. These yield and com- position patterns are consistent with a "regional chromatography" model in which highly degraded leaf material is solubilized and then partitioned between soil minerals and water during transport to the river, resulting in suspended fine particulate organic materials of soil origin that are nitrogen-rich and coexisting dissolved organic substances that are nitrogen-poor.
Provides: (1) a glossary of terms used in biochemical engineering; (2) a list of key developments in the field; and (3) emphases placed in 15 topic areas in a course restructured on the basis of these developments. Topic areas include enzyme kinetics/applications, genetics and microbial control, transport phenomena, and others. (JN)
Seafloor recycling of organic materials in Santa Monica Basin, California was examined through in situ benthic chamber experiments, shipboard whole-core incubations and pore water studies. Mass balance calculations indicate that the data are internally consistent and that the estimated benthic exchange rates compare well with those derived from deep, moored conical sediment traps and hydrographic modeling. Pore water and benthic flux observations indicate that the metabolizable organic matter at the seafloor must be composed of at least two fractions of very different reactivities. While the majority of reactive organic compounds degrade quickly, with a half-life of <6.5 years, ¼ of the total metabolizable organic matter appears to react more slowly, with a half-life on the order of 1700 years. Down-core changes in pore water sulfate and titration alkalinity are not explained by stoichiometric models of organic matter diagenesis and suggest that reactions not considered previously must be influencing the pore water concentrations.Measured recycling and burial rates indicate that 43% of the organic carbon reaching the basin seafloor is permanently buried. The results for Santa Monica Basin are compared to those reported for other California Borderland Basins that differ in sedimentation rate and bottom water oxygen content. Organic carbon burial rates for the Borderland Basins are strongly correlated with total organic carbon deposition rate and bulk sedimentation rate. No significant correlation is observed between carbon burial and bottom water oxygen, extent of oxic mineralization and sediment mixing. Thus, for the California Borderlands, it appears that carbon burial rates are primarily controlled by input rates and not by variations in preservation.
Use of sedimentary organic carbon concentrations as a record of
paleoceanographic conditions is complicated by an insufficient
understanding of the mechanisms controlling present-day variations in
the organic matter content of surface open ocean sediments. This paper
is a review of organic carbon distributions in marine sediments, the
global marine balance of particulate and dissolved organic carbon and
the processes controlling organic matter diagenesis. The discussion
focuses on the last topic with the intention of bringing together mass
balance and organic chemical evidence for mechanisms that control the
preservation of organic matter in open ocean sediments.
Published sediment trap experiment data are used in a model coupling organic carbon and oxygen reactions in marine sediments to investigate the mechanisms controlling the preservation of organic carbon in the ocean. These data indicate that the degradation rate constant for organic material oxidation in marine sediments increases with increasing particulate organic carbon flux to the bottom. The recent suggestion that organic carbon burial on the continental shelf is an effective sink of anthropogenic fossil fuel CO2 is noted to require 50 percent of the particulate algal carbon raining from the photic zone of the shelves to escape degradation and undergo burial; the present calculations suggest that the organic matter degradation rate is too rapid for this to occur, so that this potential sink is overestimated by a factor of 5.