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Sedimentary organic matter preservation: An assessment and speculative synthesis
Abstract
Throughout Earth history, almost all preserved organic matter has been incorporated in marine sediments deposited under oxygenated waters along continental margins. Given modern oceanic productivity and sediment burial rates of 50 × 1015 and 0.16 × 1015 gC yr−1, respectively, organic preservation in the marine environment is < 0.5% efficient. Although correlative information is often used to suggest that productivity, sediment accumulation rate, bottom water oxicity, and organic matter source are key variables, the mechanisms governing sedimentary organic matter preservation have remained unclear.The factors which directly determine preservation vary with depositional regime, but have in common a critical interaction between organic and inorganic materials over locally variable time scales. More than 90% of total sedimentary organic matter from a wide variety of marine depositional environments cannot be physically separated from its mineral matrix. This strongly associated organic component varies directly in concentration with sediment surface area and thus appears to be sorbed to mineral grains. Sediments accumulating outside deltas along continental shelves and upper slopes characteristically exhibit mineral surface area loadings approximately equivalent to a single molecular covering. These monolayer-equivalent coatings include a fraction of reversibly bound organic molecules that are intrinsically labile, but resist appreciable mineralization as they pass rapidly through oxygenated surface sediments and are preserved within underlying anoxic deposits. The delivery of mineral surface area is the primary control on organic matter preservation within these expansive coastal margin regions where roughly 45% of all organic carbon accumulates.Deltaic sediments account for roughly another 45% of global carbon burial, but often exhibit much less than monolayer-equivalent organic coatings. This pattern is seen in periodically oxygenated sediments off the mouth of the Amazon River, even though the component clastic minerals are discharged by the river with monolayer coatings. Comparably extensive losses of organic matter, including distinct particles such as pollen grains, occur in the surfaces of deep-sea turbidites in which long term reaction with O2 is clearly the causative factor. Sub-monolayer organic coatings also are observed in continental rise and abyssal plain sediments where slower accumulation rates and deeper O2 penetration depths result in increased oxygen exposure times and little (~ 5% of the global total) organic matter preservation. A transition zone between monolayer and sub-monolayer organic coatings apparently occurs on lower continental slopes, and is marked along the Washington coast by parallel offshore decreases in total organic matter and pollen between 2000–3000 m water depth.Sediments underlying highly productive, low-oxygen coastal waters such as off Peru and western Mexico are characteristically rich in organic matter, but account for only ~ 5% of total organic carbon burial. These sediments show a direct relationship between organic matter content and mineral surface area, but at organic loadings 2–5 times a monolayer equivalent. Organic materials sorbed in excess of a monolayer thus also may be partially protected. Such high sedimentary organic contents may result from equilibration with DOM-rich porewaters, or very brief O2 exposure times which allow preservation of extremely oxygen-sensitive organic materials such as pigments and unsaturated lipids. Thus organic matter preservation throughout much of the ocean may be controlled largely by competition between sorption at different protective thresholds and oxic degradation.Future research strategies should be specifically directed at delineating the mechanisms for organic matter preservation in marine sediments. In particular, special effort is needed to determine the amounts and types of sorbed organic materials and the nature of their bonding to mineral surfaces. The extent and dynamics with which organic molecules are partitioned between porewaters and solid phases also should be determined, as well as the effects of these phase associations on their reactivities toward chemical and biological agents. In addition, processes for slow oxic (and suboxic) degradation of organic materials bear investigation in deep-sea sediments, as well as in other extreme environments such as oxidizing turbidites, weathering shales, and soils. Such studies should include characterizations of hydrolysis-resistant organic materials and emphasize the complementary use of biochemical compositions with readily separable particles such as pollen to calibrate and typify the mechanisms and stages of sedimentary organic degradation.
... Organic matter (OM) in marine sediments is a pivotal component of the global carbon cycle (1,2). Sedimentary OM predominantly originates from primary production in the sunlit layers of the ocean. ...
... Sedimentary OM predominantly originates from primary production in the sunlit layers of the ocean. The majority of the OM from primary production is consumed in the photic zone or degraded while settling through the water column, with the result that only a few percent eventually reaches the seafloor (1)(2)(3). After deposition, benthic animals or microbes consume and transform OM (3). ...
... In contrast to lipids, only 2.5 μmol (20 to 45 cmbsf) and 2.8 μmol (220 to 245 cmbsf) 13 C of algal crude proteins were mineralized, which equals 18 and 20% of the amount of 13 C introduced, respectively. This limited mineralization of algal protein may be due to its poor quality (nondissolvable black powder with 26% of the carbon being acid nonhydrolyzable) and interactions with the mineral surfaces (2,4,24). Using similar algal crude protein, Pelikan et al. (26), however, observed substantial protein degradation during incubation of subarctic sediments. ...
Organic matter (OM) transformations in marine sediments play a crucial role in the global carbon cycle. However, secondary production and priming have been ignored in marine biogeochemistry. By incubating shelf sediments with various ¹³ C-labeled algal substrates for 400 days, we show that ~65% of the lipids and ~20% of the proteins were mineralized by numerically minor heterotrophic bacteria as revealed by RNA stable isotope probing. Up to 11% of carbon from the algal lipids was transformed into the biomass of secondary producers as indicated by ¹³ C incorporation in amino acids. This biomass turned over throughout the experiment, corresponding to dynamic microbial shifts. Algal lipid addition accelerated indigenous OM degradation by 2.5 to 6 times. This priming was driven by diverse heterotrophic bacteria and sulfur- and iron-cycling bacteria and, in turn, resulted in extra secondary production, which exceeded that stimulated by added substrates. These interactions between degradation, secondary production, and priming govern the eventual fate of OM in marine sediments.
... Marine sediments store large amounts of carbon through the burial of detritus either sinking from above via the biological carbon pump or from river run-offs to coastal sediments . Globally, marine sediments are estimated to store between 1 500 and 2 300 Pg C Hedges & Keil, 1995). Once the sediment has been buried and becomes anoxic, the carbon is potentially stored for millennia, with very little or no degradation by microbes. ...
... The aim of the subgroup was to assess the evidence around potential impacts of fishing activities on seabed carbon burial and storage. Globally, marine sediments are estimated to store between 1500 and 2300 Pg C Hedges & Keil, 1995). As such, the sedimentation and burial of organic matter at the sea floor may represent an important pathway for carbon sequestration, providing a potential negative feedback against climate change. ...
... As such, the sedimentation and burial of organic matter at the sea floor may represent an important pathway for carbon sequestration, providing a potential negative feedback against climate change. In marine sediments, oxygen availability is a key limiting factor on the degradation of organic matter (Hedges & Keil, 1995). Fishing activity, for example bottom trawling, disturbs the seafloor which then results in the resuspension of significant sediment plumes. ...
... Up to 90% of the organic carbon (OC) burial in the world's oceans occurs in nearshore coastal sediments, in association with river deltas and continental shelves (Hedges & Keil, 1995). Upon sedimentation, this reduced carbon is sequestered for decades to millennia, effectively removing greenhouse-enhancing carbon dioxide from the atmosphere. ...
... Upon sedimentation, this reduced carbon is sequestered for decades to millennia, effectively removing greenhouse-enhancing carbon dioxide from the atmosphere. The cause of high OC burial rates in the coastal zone is believed to be a combination of high rates of productivity, high sediment accumulation rates, sorptive protection of organic matter on detrital mineral surfaces, low bottom water oxygen concentrations, and terrestrial sources for some of the organic matter (that may render it less accessible to marine microbes and macrobenthos) (Burdige, 2005;Hedges & Keil, 1995). high OC accumulation rates (15.3 g/m 2 /yr in BS-YS and 14.7 g/m 2 /yr in ECS) and OC burial rates (13.0 ...
The hydrogen isotope ratio (δ²H) of lipid biomarkers from phytoplankton and terrestrial plants were measured in surface sediments from Chinese marginal seas (CMS), in order to evaluate processes affecting their spatial distribution, and by extension, the controls on organic carbon cycling in these dynamic marginal seas. The dinoflagellate sterol dinosterol and the microalgal sterol brassicasterol had low δ²H values (<−300%) near the mouth of the Yangtze River. Near the mouth of the Yellow River, dinosterol again had low δ²H values but brassicasterol had intermediate (−292 to −281%) δ²H values. This discrepancy in the δ²H values of two phytoplankton sterols may be explained by the timing of dinoflagellate production relative to that of brassicasterol producers (e.g., diatoms). The δ²H values of C16:0 fatty acid, synthesized by all organisms, had intermediate δ²H values (−199 to −178%) near the Yangtze River and lowest δ²H C16:0 values (−216 to −206%) occurred near the old Yellow River delta. The lowest δ²H C28:0 values (<−184%) occurred adjacent to rivers, suggesting that leaf‐wax lipids produced on the loess plateau, which are enriched in ²H, may contribute less to the nearshore environment than the offshore regions. Higher δ²H C28:0 values offshore may be explained by a larger contribution of leaf wax fatty acids from aerosols relative to river‐borne suspended particles. Lipid biomarker δ²H fingerprinting thus provides a new tool for deciphering the controls on organic carbon cycling and accumulation in the dynamic CMS.
... Additionally, there is evidence that an increase in nutrient concentrations promotes an increase in marine Vibrio diversity . Sediments harbour higher organic matter content than seawater (Hedges & Keil, 1995;Riley & Chester, 2013), which may result in the highest diversity of marine Vibrio observed in the sediment. The NMDS results illustrated F I G U R E 7 Linear regressions for AVD associated with marine Vibrio alpha diversity, beta diversity, and gamma diversity. ...
... Additionally, PLS-PM indicated that RD indirectly affects community stability through diversity. Previous studies have shown that there are usually more biologically available resources (such as light energy and organic carbon) in surface water and sediment (Garel et al., 2019;Hedges & Keil, 1995;Riley & Chester, 2013;Skoog & Benner, 1997) and that the diversity of these resources may support the survival and reproduction of different biological species, thereby promoting community stability. Middle and bottom water resources are relatively scarce and experience greater changes in temperature and pressure (Garel et al., 2019), which may have different degrees of impact on species, leading to a decline in community stability. ...
Vibrio is a salt‐tolerant heterotrophic bacterium that occupies an important ecological niche in marine environments. However, little is known about the contribution of resource diversity to the marine Vibrio diversity and community stability. In this study, we investigated the association among resource diversity, taxonomic diversity, phylogenetic diversity, and community stability of marine Vibrio in the Beibu Gulf. V. campbellii and V. hangzhouensis were the dominant groups in seawater and sediments, respectively, in the Beibu Gulf. Higher alpha diversity was observed in the sediments than in the seawater. Marine Vibrio community assembly was dominated by deterministic processes. Pearson's correlation analysis showed that nitrite (‐N), dissolved inorganic nitrogen (DIN), ammonium (‐N), and pH were the main factors affecting marine Vibrio community stability in the surface, middle, and bottom layers of seawater and sediment, respectively. Partial least‐squares path models (PLS‐PM) demonstrated that resource diversity, water properties, nutrients, and geographical distance had important impacts on phylogenetic and taxonomic diversity. Regression analysis revealed that the impact of resource diversity on marine Vibrio diversity and community stability varied across different habitats, but loss of Vibrio diversity increases community stability. Overall, this study provided insights into the mechanisms underlying the maintenance of Vibrio diversity and community stability in marine environments.
... Despite only comprising 15% of the ocean area, continental margins are important sites of organic carbon (OC) storage since they receive organic matter (OM) from both terrestrial and marine sources, and account for the burial of ∼90% of sedimentary OC Harris et al., 2014;Hedges & Keil, 1995). The preservation of OC in marine sediments is highly dependent on its sources (i.e., recently synthesized biospheric marine or terrestrial OM, or petrogenic carbon from eroded bedrock), which often dictate its reactivity given their contrasting chemical composition: while fresh marine OM is more reactive than terrestrial biogenic organic residues derived from plants and soils, petrogenic OC has the most recalcitrant nature Blair & Aller, 2012). ...
... Hydrodynamic processes also influence the fate of OC since components of OM are preferentially associated with fine-grained sediments with the highest mineral-specific surface area Mayer, 1994) that may be prone to resuspension. In addition, this mineral binding can also serve a protective role by limiting the degradation of OC and promoting its aging during sediment redistribution (Ausín et al., 2021;Bao et al., 2016Bao et al., , 2018Hedges & Keil, 1995;Hemingway et al., 2019). Hence, combined analysis of the spatial distribution not only of OC content, but also characteristics such as its isotopic and elemental composition (δ 13 C, Δ 14 C, and C/N values) and mineral surface area, which can provide insight of the origin and quality of sedimentary OM, is crucial to understand the underlying processes that modulate the fate of OC in marine sediments (Ausín et al., 2023;Bao et al., 2018;Bröder et al., 2016;Goñi et al., 2013;Gordon & Goñi, 2003;Kim et al., 2022;Vonk et al., 2012). ...
As major sites of carbon burial and remineralization, continental margins are key components of the global carbon cycle. However, heterogeneous sources of organic matter (OM) and depositional environments lead to complex spatial patterns in sedimentary organic carbon (OC) content and composition. To better constrain the processes that control OM cycling, we focus on the East Asian marginal seas as a model system, where we compiled extensive data on the OC content, bulk isotopic composition (δ¹³C and Δ¹⁴C), total nitrogen, and mineral surface area of surficial sediments from previous studies and new measurements. We developed a spatial machine learning modeling framework to predict the spatial distribution of these parameters and identify regions where sediments with similar geochemical signatures drape the seafloor (i.e., “isodrapes”). We demonstrate that both provenance (44%–77%) and hydrodynamic processes (22%–53%) govern the fate of OM in this margin. Hydrodynamic processes can either promote the degradation of OM in mobile mud‐belts or preserve it in stable mud‐deposits. The distinct isotopic composition of OC sources from marine productivity and individual rivers regulates the age and reactivity of OM deposited on the sea‐floor. The East Asian marginal seas can be separated into three main isodrapes: hydrodynamically energetic shelves with coarser‐grained sediment depleted in OC, OM‐enriched mud deposits, and a deep basin with fine‐grained sediments and aged OC affected by long oxygen exposure times and petrogenic input from rivers. This study confirms that both hydrodynamic processes and provenance should be accounted for to understand the fate of OC in continental margins.
... The carbon burial in marine realm occurs by two different pathways (Cartapanis et al. 2018), namely, transformation of dissolved inorganic carbon (DIC) into C org through the utilization of DIC by photosynthetic organisms, and extraction of DIC from the surrounding water by calcareous organisms to produce carbonate minerals (CaCO 3 ) and their burial at the seafloor. The C org burial in marine sediments is modulated by the primary productivity in the surface water (Middelburg 2019; Thunell et al. 2000;Calvert and Pedersen 1992;Pedersen et al. 1992;Calvert et al. 1991), oxygen level in subsurface water (Middelburg 2019;Paropkari et al. 1993), sedimentation rate/mineral ballast (Hedges and Keil 1995;Ittekkot et al. 1991), texture of sediments (Calvert et al. 1995), and dilution by terrigenous material (Kandasamy and Nath 2016;Rullkotter 2006). However, the CaCO 3 content of the sediments is substantially modified by biological productivity in the overlying water column, dissolution in the water column as well as in sediments (Bhadra and Saraswat 2022) and dilution by terrigenous matter (Yadav et al. 2022;Govil et al. 2004). ...
... The low sedimentary organic matter despite high water column productivity means poor organic matter preservation in the sediments. The poor C org preservation was because of the coarse-grained texture of the sediments and the influence (Hedges and Keil 1995). The C org was also likely to be transported to the deeper depth with finer fraction of sediment due to the influence of riverine influx. ...
The unicellular calcareous planktic foraminifera sequester a significant portion of the carbon dioxide dissolved in the ocean, thus burying the carbon in sediments for millions of years. The global warming and associated processes are likely to affect the planktic foraminiferal abundance and diversity. Therefore, their baseline distribution has to be documented and correlated with ambient parameters to assess its fate under different climate change scenarios. Here, we report an exceptionally high abundance of planktic foraminifera and thus large carbon burial in the southwestern Bay of Bengal. The very high absolute abundance of planktic foraminifera in the Cauvery River basin is attributed to biannual productivity, warmer and saline waters. Globigerinita glutinata is the highest abundant species followed by Globigerinoides ruber and Globigerina bulloides. Globigerina bulloides is abundant on the shelf, where the upwelling is more frequent. The relative abundance of Globorotalia menardii is positively correlated with thermocline salinity and negatively correlated with thermocline temperature. Similarly, Neogloboquadrina dutertrei and Globoquadrina conglomerata are negatively correlated with mixed layer as well as thermocline temperature and mixed layer salinity. Both these species are positively correlated with thermocline salinity. Globigerina falconensis is more abundant in the southernmost transect influenced by intense winter monsoon precipitation. We report that G. ruber prefers high saline and warmer waters with the highest abundance in the southernmost transect. From the foraminiferal distribution, it is evident that the temperature and salinity of the mixed layer as well as thermocline, food availability, and monsoon-associated processes affect the planktic foraminiferal abundance and thus carbon burial in the southwestern Bay of Bengal. The changes in influx of southeastern Arabian Sea water will affect the planktic foraminiferal population and subsequent carbon burial in the southwestern Bay of Bengal.
... Increasing shares of acid-base to water-extractable organic matter, used as a dimensionless solid-liquid partitioning coefficient, support the findings that in downstream direction a higher proportion of organic matter is stabilized in organo-mineral associations and that aerobic and anaerobic SOM decay rates decrease accordingly (Zander et al. 2020. This share of organic matter is less accessible for microbial decay (for studies on terrestrial soils see Marschner et al. 2008;Six and Paustian 2014;Gao et al. 2019;Baldock and Skjemstad 2000) due to changes the steric conditions of SOM, reducing accessibility to degrading enzymes, as hypothesized by Hedges and Keil (1995) for marine sediments. Inversely, organic matter mineralization was highly correlated to water-extractable and hence not mineral-bound SOM (Table 1), which reflects the properties of natural dissolved organic matter (Olk et al. 2019). ...
Purpose
The share of microbially degradable sediment organic matter (SOM) and the degradation rate depend, among others, on the intrinsic properties of SOM as well as on the type and concentration of terminal electron acceptors (TEA). Next to its role as TEA, molecular oxygen enhances SOM decay by oxygenase-mediated breakdown of complex organic molecules. This research investigated long-term SOM decay (> 250 days) under aerobic and anaerobic conditions to (1) provide a basis for sediment carbon flux estimates from the River Elbe estuary and (2) assess the potential for carbon burial in relation to redox conditions and dredging interventions.
Methods
Long-term aerobic and anaerobic SOM decay in fluid mud, pre-consolidated and consolidated sediment layers was investigated over three years along a transect of ca. 20 km through the Port of Hamburg, starting at the first hydrodynamically determined hotspot of sedimentation after the weir in Geesthacht. Absolute differences between aerobic and anaerobic cumulative carbon mineralization were calculated, as well as their ratio. Findings were correlated to a suite of solids and pore water properties.
Results
SOM decay followed first order multi-phase exponential decay kinetics. The ratio between C release under aerobic and anaerobic conditions ranged around 4 in the short-term, converging to a value of 2 in the long term. Strong gradients in absolute C release along the upstream–downstream transect did not reflect in a corresponding gradient of the aerobic-anaerobic ratio. C release was most strongly correlated to the water-soluble organic matter, in particular humic acids. Contact of anaerobically stabilized sediment with the oxygenated water phase induced significant release of carbon.
Conclusion
SOM degradability in the study area exhibited strong spatial gradients in relation to the organic matter source gradient but was mainly limited by the high extent of organic matter stabilization. Under these conditions, molecular oxygen as TEA provides little thermodynamic advantage. Carbon-sensitive sediment management, considering SOM reactivity patterns in stratified depositional areas, is a powerful strategy to reduce environmental impacts of dredging measures.
... Although the analysis of the organic matter content of sediments in the stations of this study was not carried out, since the South China Sea is a marginal sea of the western Pacific Ocean, ocean margins are the repository for approximately 90% of the organic carbon buried in marine sediments. The Pearl River, which is the second largest river in China, discharges onto the northern shelf of the South China Sea (Hedges and Keil, 1995;Hu et al., 2006). Studies have shown that the organic matter content of sediments usually decreases with increasing water depth (Billett et al., 1993). ...
Knowledge about marine tardigrades from the South China Sea is very scarce, with only four species from shallow waters recorded to date. The present study investigated the structure and diversity of tardigrade communities from the deep sea (1517-1725 m) at 8 stations in a polymetallic nodule area of the northern South China Sea. A total of 151 arthrotardigrades were collected belonging to 11 genera (Angursa, Batillipes, Coronarctus, Euclavarctus, Exoclavarctus, Halechiniscus, Moebjergarctus, Raiarctus, Rhomboarctus, Tanarctus and Tholoarctus), representing 17 species. Two Angursa species (Angursa sp. 4 and Angursa sp. 3) were the most abundant (25.2% and 14.6%, respectively), followed by Moebjergarctus sp. (13.9%). Specimens were mostly (90.7%) distributed in the upper layer of the sandy-mud sediment (0-1 cm). The SIMPROF test showed that the composition of tardigrade communities at all stations was not significantly different. At different stations, the number of species, Shannon-Wiener diversity index and Pielou’s evenness index ranged from 4 to 10, 1.94 to 2.87, and 0.75 to 1.00, respectively. The average taxonomic distinctness (Δ+) ranged from 72.50 to 90.00, and the variation in taxonomic distinctness (Λ+) ranged from 316.67 to 1181.25. This study provides some basic information about the biodiversity of the marine tardigrade community in the South China Sea.
... Coastal ecosystems are characterized by their remarkable carbon burial potential (Duarte et al., 2005;Hedges et al., 1997) and are often regarded as "hotspots" for mineralization (Middelburg et al., 2005). The deposition and preservation of organic matter are superior in coastal sediments compared to other sedimentary reservoirs (Romankevich, 1984;Hedges and Keil, 1995). The study of sediment organic matter dynamics is an important in understanding uctuations in environmental chemical compositions, exerting strong control over diagenetic transformations within sedimentary systems. ...
The tropic status and sediment quality of the mangrove forest has been assessed using biochemical indices. All the samples are collected from seven mangrove forest located at South west coast of India during pre-monsoon, monsoon and post-monsoon seasons. Biochemical composition of sedimentary organic matter from the selected mangrove ecosystem was characterized by the dominance of was dominated carbohydrate followed by proteins and lipids in all seasons and stations (CHO%>PRT%>LPD%). Comparatively higher concentration of carbohydrates in the sediments has been attributed to the accumulation of aged organic matter due to the faster utilization of proteins than carbohydrate by microorganisms. Tannin and lignin content was found at the selected mangrove stations for the study as the most common compound and crucial variable. The PRT/CHO ratio revealed the presence of aged organic matter at stations 1,2 and 3 (Northern Kerala mangroves) and newly created organic debris at stations 4,5 (Kochi) ,6 and 7 (Kollam). The LPD/CHO ratio also supported the same. The Bio Polymeric Carbon (BPC) values indicated that at all the samples, except at the station 6 showed eutrophic nature. In the statistical analysis talks about strong interrelationships prevailing between the biochemical constituents revealed their origin from a common source.
... In this scenario, a clear knowledge of the patterns of microbial taxonomical and functional diversity and their environmental drivers is still lacking, while assessing how microbial structure and function are modulated by freshwater, terrestrial and marine organic carbon inputs would be critical for understanding microbial role in the whole fjord ecosystems' functioning. Marine sediments act as the largest reservoirs of organic carbon on the planet [5]; many natural compounds produced by the living biota in the water column, as well as contaminants, are buried into the benthic compartment, stressing the role of sediments as environmental archives. ...
The benthic matrix acts as a key natural archive where the memory of long-term timescale environmental changes is recorded. Some ecological and chemical features of fjord sediments were explored during the AREX cruise carried out in the Svalbard archipelago in summer 2021. The activity rates of the enzymes leucine aminopeptidase (LAP), beta-glucosidase (GLU) and alkaline phosphatase (AP) and community-level physiological profiles (CLPP) were studied with the aim of determining the functional diversity of the benthic microbial community, while bacterial isolates were screened for their susceptibility to antibiotic molecules in order to explore the role of these extreme environments as potential reservoirs of antibiotic resistance. Enzyme activity rates were obtained by fluorogenic substrates, CLPP by Biolog Ecoplates; antibiotic susceptibility assays were performed through the standard disk diffusion method. Spatial trends observed in the functional profiles of the microbial community suggested the variability in the microbial community composition, presumably related to the patchy distribution of organic substrates. Complex Carbon sources, carbohydrates and amino acids were the organic polymers preferentially metabolized by the microbial community. Multiple resistance to ampicillin, clindamycin and gentamycin was detected in all the sampled sediments, excepting in the southern Hornsund area, stressing the role of sediments as a potential reservoir of chemical wastes ascribable to antibiotic residuals. This study provides new insights on the health status of fjord sediments of West Spitsbergen applying a dual ecological and biochemical approach. Microbial communities in the fjord sediments showed globally a good functional diversity, suggesting their versatility to rapidly react to changing conditions.
... More detailed analysis of the results of kNN and random forests are provided in the supplementary information. The correlation plot between measured and predicted data shows similar errors for are also consistent with the early work on TOC distributions by Berner (1982) and Emerson and Hedges (1988) marginal seas that promotes TOC preservation (Hedges and Keil, 1995). The map published by Lee et al. (2019) shows several large areas in the open Pacific that have unusually high TOC concentrations. ...
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.
... Margins are recognized as a significant source of organic carbon to the deep ocean, contributing over 50% of the particulate organic carbon (POC) in deep ocean sediments, with predominance of terrestrial POC (Berner, 1992;Buesseler et al., 2010;Danovaro et al., 1999;Hedges & Keil, 1995;Lamborg et al., 2008;Shen et al., 2020;Wheatcroft et al., 2010). While there is clear evidence for margins' contribution to offshore POC in areas with large river and drainage area systems (e.g., D. L. Cai et al., 1988;Li et al., 2017;Ludwig et al., 1996;Shen et al., 2020), less is known about offshore transport of POC in dryer areas, characterized by low discharge. ...
Sediment trap data set and ²³⁴Th profiles (deep water excesses and deficits) reveal that particulate organic carbon (POC) export at the highly oligotrophic Levantine Sea is dominated by lateral transport from the nearby margin. These intermediate nepheloid layers (INL) operate at multi‐depth, with the silt‐to‐clay size particulate matter (PM) fraction transported at water depths of about 100–500 m, while finer fraction arrives also at deeper depths. The shallow NIL is triggered by winter storms, manipulated by coastal flash floods and shelf resuspension and assisted by cross‐shore currents, which allow the arrival of PM at a distance of 50 km within about 10 days. The deeper INL could be related to sediments initially driven to depth by density currents. Our data show that inter‐annual differences in sediment trap fluxes were related to changes in both the intensity of coastal floods and current velocity. The frequent observation of deep‐water ²³⁴Th excesses during a (relatively) low export winter (2018) is related to lessened cleansing of the water column, that is, reduced removal of fine‐grained PM by sinking coarser‐grained material. These observations highlight the importance of winter storm intensity in the POC budget of marginal seas like the Levantine Basin (LB) even in areas with limited river discharge. This further suggests that the anticipated increase in extreme weather events due to the on‐going climate change should have an impact on this coastal‐deep sea conveyor and on POC export in the LB.
... Besides, the continental shelves receive huge amount of terrestrial OC in dissolved and particulate forms, as well as substantial nutrients exported from adjacent rivers. It has been estimated that large river-dominated marginal seas serve as hotspots to sequester terrestrial OC, with a burial flux of (58 ± 17) × 10 12 g C per year (Hedges and Keil 1995;Burdige 2005). Continental shelf regions, therefore, play a crucial role in the global carbon cycling and budget. ...
The use of ²³⁴Th–²³⁸U disequilibrium has been widely employed to estimate the sinking flux of particulate organic carbon (POC) from the upper sea and ocean. Here, the deficits of ²³⁴Th relative to ²³⁸U in the water column and the carbon isotope signature (δ¹³C) of POC in the East China Sea (ECS) Shelf were measured, which was used to distinguish the fraction of marine and terrestrial POC export fluxes. In the ECS Shelf, very strong deficits of ²³⁴Th relative to ²³⁸U were observed throughout the water column, with ²³⁴Th/²³⁸U activity ratios ranging from 0.158 ± 0.045 to 0.904 ± 0.068 (averaging 0.426 ± 0.159). The residence times of particle reactive radionuclide ²³⁴Th (τTh–T) in the ECS shelf water varied between 9 and 44 days, which is significantly shorter than that in the continental slope area or the basin area. This phenomenon indicates that there is a more rapid particle scavenging process in the ECS shelf water compared to the continental slope and basin upper water. By applying a two-end-member mixing model based on the δ¹³C, the fraction of terrestrial POC was estimated to be 0 to 74% (mean: 30 ± 22%) and the fraction of marine POC was in the range of 25% to 100% (mean: 70 ± 22%). Fluxes of marine and terrestrial POC settling to the seafloor exhibited significant spatial differences among different stations, ranging from 11 to 129 mmol C/m²/day and from 2.6 to 38 mmol C/m²/day, respectively. The averaged terrestrial POC fluxes in the southern and northern ECS Shelf were similar (~ 21 to 24 mmol C/m²/day), while the marine POC fluxes in the north (86 ± 37 mmol C/m²/day) were approximately four times higher than those in the south (26 ± 20 mmol C/m²/day). Interestingly, the estimated export flux of both marine and terrestrial POC were approximately one order of magnitude higher than the previously reported burial fluxes of POC (ranging from 1.1 ± 0.1 to 11.4 ± 1.1 mmol C/m²/day) in the underlying bottom sediments, indicating that the majority (> 90%) of both terrestrial and marine POC exported from the upper water column are degraded in the sediments of the ECS Shelf. This “carbon missing” phenomenon can greatly be attributed to rapid decomposition by other processes (including microbial reworking, cross-shelf transport, and possible consumption by benthic organisms). Our findings highlight the dynamic nature of carbon cycling in the continental shelf and the need for further research to understand these processes and improve carbon budget assessments.
... Additionally, organic matter degradation occurs over a broad spectrum of timescales, from minutes for biochemical breakdown in animal guts to 10 6 years for organic carbon mineralization in deep-sea sediments (Middelburg et al., 1993). This wide dynamic range underscores the significant influence of the characteristic timescale of experimental observations on measurable degradation rates (Hedges and Keil, 1995). Given that our model measures sediment organic carbon oxidation through oxygen consumption, it likely emphasizes the estimation of labile organic carbon degradation over semi-labile and refractory fractions due to their extended timescales. ...
The Gaoping Submarine Canyon (GPSC) off southwest Taiwan has been extensively studied due to its unique geology, its role in transferring terrestrial material to the deep sea, and its diverse biological communities. However , there is a lack of understanding of carbon cycling across the sediment-water interface in the canyon. This study aims to fill the gap by utilizing the field data collected between 2014 and 2020 and a linear inverse model (LIM) to reconstruct the benthic food web (i.e., carbon flows through different stocks) in the head of GPSC and the upper Gaop-ing slope (GS). The biotic and abiotic organic carbon (OC) stocks were significantly higher on the slope than in the canyon, except for the bacteria stock. The sediment oxygen utilization was similar between the two habitats, but the magnitude and distribution of the OC flow in the food web were distinctively different. Despite a significant input flux of ∼ 2020 mg C m −2 d −1 in the canyon, 84 % of the carbon flux exited the system, while 12 % was buried. On the slope, 84 % of the OC input (∼ 109 mg C m −2 d −1) was buried, and only 7 % exited the system. Bacteria processes play a major role in the carbon fluxes within the canyon. In contrast, the food web in the upper slope exhibited stronger interactions among metazoans, indicated by higher fluxes between meio-fauna and macrofauna compartments. Network indices based on the LIM outputs showed that the canyon head had higher total system throughput (T ..) and total system throughflow (TST), indicating greater energy flowing through the system. In contrast, the slope had a significantly higher Finn cycling index (FCI), average mutual information (AMI), and longer OC turnover time, suggesting a relatively more stable ecosystem with higher energy recycling. Due to sampling limitations , the present study only represents the benthic food web during the "dry" period. By integrating the field data into a food web model, this study provides valuable insight into the fates of OC cycling in an active submarine canyon, focusing on the often overlooked benthic communities. Future studies should include "wet" period sampling to reveal the effects of typhoons and monsoon rainfalls on OC cycling.
... Major contributors to organic carbon production in coastal areas include large seaweed and seagrass beds, marsh plants, mangrove forests and coral reefs. Coastal ecosystems bury approximately 90 % of marine organic carbon (Hedges and Keil, 1995) and account for 20 % of the net absorption of CO 2 by oceans (Tao et al., 2016). Thus, previous studies have shown that increasing coastal carbon sinks is an effective approach to enhancing carbon sequestration. ...
... Additionally, organic matter degradation occurs over a broad spectrum of timescales, from minutes for biochemical breakdown in animal guts to 10 6 years for organic carbon mineralization in deep-sea sediments (Middelburg et al., 1993). This wide dynamic range underscores the significant influence of the characteristic timescale of experimental observations on measurable degradation rates (Hedges and Keil, 1995). Given that our model measures sediment organic carbon oxidation through oxygen consumption, it likely emphasizes the estimation of labile organic carbon degradation over semi-labile and refractory fractions due to their extended timescales. ...
The Gaoping Submarine Canyon (GPSC) off southwest Taiwan has been extensively studied due to its unique geology, its role in transferring terrestrial material to the deep sea, and its diverse biological communities. However, there is a lack of understanding of carbon cycling across the sediment–water interface in the canyon. This study aims to fill the gap by utilizing the field data collected between 2014 and 2020 and a linear inverse model (LIM) to reconstruct the benthic food web (i.e., carbon flows through different stocks) in the head of GPSC and the upper Gaoping slope (GS). The biotic and abiotic organic carbon (OC) stocks were significantly higher on the slope than in the canyon, except for the bacteria stock. The sediment oxygen utilization was similar between the two habitats, but the magnitude and distribution of the OC flow in the food web were distinctively different. Despite a significant input flux of ∼ 2020 mg C m−2 d−1 in the canyon, 84 % of the carbon flux exited the system, while 12 % was buried. On the slope, 84 % of the OC input (∼ 109 mg C m−2 d−1) was buried, and only 7 % exited the system. Bacteria processes play a major role in the carbon fluxes within the canyon. In contrast, the food web in the upper slope exhibited stronger interactions among metazoans, indicated by higher fluxes between meiofauna and macrofauna compartments. Network indices based on the LIM outputs showed that the canyon head had higher total system throughput (T..) and total system throughflow (TST), indicating greater energy flowing through the system. In contrast, the slope had a significantly higher Finn cycling index (FCI), average mutual information (AMI), and longer OC turnover time, suggesting a relatively more stable ecosystem with higher energy recycling. Due to sampling limitations, the present study only represents the benthic food web during the “dry” period. By integrating the field data into a food web model, this study provides valuable insight into the fates of OC cycling in an active submarine canyon, focusing on the often overlooked benthic communities. Future studies should include “wet” period sampling to reveal the effects of typhoons and monsoon rainfalls on OC cycling.
... The protective effect minerals have on OC is the product of a highly important stabilizing mechanism. More than 90% of organic matter (OM) in marine sediments cannot be separated from clay minerals and iron minerals, and this holds especially true for iron minerals that are closely associated with the long-term preservation of OC [5]. A previous study found that iron-bound OC (Fe-OC) in Arctic marine sediments persists below the uppermost oxygenated sediment layer over thousands of years [6]. ...
Although mangrove forests occupy only 0.5% of the global coastal area, they account for 10–15% of coastal organic carbon (OC) storage, and 49–98% of OC is stored in sediments. The biogeochemistry of iron minerals and OC in marine sediments is closely related. To better reveal the role of iron minerals in OC preservation in mangrove sediments, an established dithionite–citrate–bicarbonate (DCB) extraction method was used to extract iron-bound OC (Fe-OC), and then the parameters of OC, Fe-OC, iron content, carbon isotopes, infrared spectroscopy, and XRD diffractions of sediments at a 1 m depth in four typical mangrove communities in the Gaoqiao Mangrove Reserve, Guangdong, China, were systematically measured. XRD diffractograms showed that the iron minerals in mangrove sediments may mainly exist in the form of goethite, which is consistent with the predominant types of iron minerals in marine sediments. About 10% of OC is directly bound to iron, and it is further estimated that about 2.4 × 1012–3.8 × 1012 g OC is preserved in global mangrove forests each year based on the high burial rate of OC in mangrove sediments. Lower Fe-OC/OC molar ratios indicated that iron mainly binds to OC via adsorption mechanisms. More depleted δ13CFe-OC relative to δ13Cbulk indicated that iron minerals are mainly associated with terrigenous OM, and the infrared spectra also revealed that iron minerals preferentially bind to terrigenous aromatic carbon. This work supports the “giant rusty sponge” view, elucidating that iron plays an important role in the preservation of OC in mangrove sediments.
... Myriad factors are proposed to influence OC burial efficiency in marine sediments, including sedimentation rates, seawater chemistry, temperature, the biochemical reactivity of organic particles, and mineral protection (i.e., interactions between OC and mineral matrices, protecting OC from oxidation by means of sorption or inhibition of microbial decay) (11,(21)(22)(23)(24). Mineral protection, in particular, plays a strong role in shaping the fate of OC in modern seafloor sediments (25)(26)(27)(28), particularly via binding with clays (29)(30)(31). ...
The evolution of oxygen cycles on Earth’s surface has been regulated by the balance between molecular oxygen production and consumption. The Neoproterozoic–Paleozoic transition likely marks the second rise in atmospheric and oceanic oxygen levels, widely attributed to enhanced burial of organic carbon. However, it remains disputed how marine organic carbon production and burial respond to global environmental changes and whether these feedbacks trigger global oxygenation during this interval. Here, we report a large lithium isotopic and elemental dataset from marine mudstones spanning the upper Neoproterozoic to middle Cambrian [~660 million years ago (Ma) to 500 Ma]. These data indicate a dramatic increase in continental clay formation after ~525 Ma, likely linked to secular changes in global climate and compositions of the continental crust. Using a global biogeochemical model, we suggest that intensified continental weathering and clay delivery to the oceans could have notably increased the burial efficiency of organic carbon and facilitated greater oxygen accumulation in the earliest Paleozoic oceans.
... In the second section EF-V and EF-U exhibit negligible cumulative changes in comparison to EF-Mo ( Fig. 6c and d), perhaps due to the sorption of Fe-Mn oxyhydroxides or downwelling fluids during the period from ATS 524.1-521.1 Ma (Bennett and Canfield, 2020). There was low and non-variable OM In addition to the oxygen content in the sedimentary environment, the amount of OM preserved in sediments is also controlled by the biotic flux to the seafloor, and sedimentation rates (Hedges and Keil, 1995;Tucker, 1992). The other proxies of sedimentary chemical conditions (i. e., primary productivity, terrigenous detrital input, and paleoclimate) suggest that these factors had only a minor control on OM accumulation over this time (Section 4.7). ...
... The last two decades have seen a surge of interest in OM quality and composition 4-6 , with many studies offering insight into the molecular footprint of dissolved OM (DOM) and how this footprint is shaped and modified during transit from land to the oceans within aquatic networks 7-9 . In this regard, DOM composition is moderated by patterns of removal or persistence, that generate ambient DOM lacking fractions that are either reactive 10-12 or immobilized and protected [13][14][15][16] . Along the downstream network, the dynamics in molecular-level DOM complexity affect the composition of its microbial degrader community 17 , and shape their activity patterns 18 . ...
The aquatic networks that connect soils with oceans receive each year 5.1 Pg of terrestrial carbon to transport, bury and process. Stagnant sections of aquatic networks often become anoxic. Mineral surfaces attract specific components of organic carbon, which are released under anoxic conditions to the pool of dissolved organic matter (DOM). The impact of the anoxic release on DOM molecular composition and reactivity in inland waters is unknown. Here, we report concurrent release of iron and DOM in anoxic bottom waters of northern lakes, removing DOM from the protection of iron oxides and remobilizing previously buried carbon to the water column. The deprotected DOM appears to be highly reactive, terrestrially derived and molecularly distinct, generating an ambient DOM pool that relieves energetic constraints that are often assumed to limit carbon turnover in anoxic waters. The Fe-to-C stoichiometry during anoxic mobilization differs from that after oxic precipitation, suggesting that up to 21% of buried OM escapes a lake-internal release-precipitation cycle, and can instead be exported downstream. Although anoxic habitats are transient and comprise relatively small volumes of water on the landscape scale, our results show that they may play a major role in structuring the reactivity and molecular composition of DOM transiting through aquatic networks and reaching the oceans.
... The TOC contents of mudstones or shales reflect a relatively small fraction of the primary production due to photosynthesis in the photic zone of the surface ocean [31]. Most organic material generated by primary production sinks from the ocean surface into the thermocline and deep ocean, reaches the sediment-water interface, undergoes decomposition, and is lost [33,34]. Despite this decomposition and diagenesis, TOC contents can still be used to estimate the palaeo-primary productivity [5,22]. ...
In order to investigate the effect of primary productivity, organic matter dilution, and preservation on the accumulation of organic matter, geochemical data, and proxies of primary productivity, clastic influx, and redox conditions were obtained for organic-rich shales in the Cambrian Niutitang Formation. The primary productivity (total organic carbon [TOC], Mo, P, Ba, and Babio) and redox (Ni/Co, V/Cr, U/Al, and Th/U) proxies suggest the organic-rich shales were deposited in anoxic-euxinic conditions during periods of high primary productivity. Pyrite in the Niutitang Formation comprises spherical framboids, which also indicate that anoxic bottom waters were present during organic matter deposition. High primary productivity enhanced the organic C flux into the thermocline layer and bottom waters, which lead to the development of anoxic bottom waters owing to O2 consumption by microorganisms and organic matter degradation. The anoxic bottom waters were beneficial for the preservation of organic matter. In addition, Ti/Al ratios correlate well with TOC contents throughout the Niutitang Formation, indicating that clastic input increased the burial rate and prevented organic matter degradation during deposition. Therefore, the accumulation of organic matter in the Niutitang Formation was controlled mainly by primary productivity rather than bottom-water redox conditions.
... Globally, it is estimated that within surficial sediments 87,000 ± 43,000 Mt is held within the top 5 cm (Lee et al., 2019), with potentially up to 3,117,000 Mt OC held within the top 1 m (Atwood et al., 2020). Each year these stores grow by an estimated 156 Mt OC through the accumulation and burial of OC at the seabed (Berner, 1982;Hedges and Keil, 1995;Smith et al., 2015). However, anthropogenic pressures which increasingly disturb the seabed have the potential to not only disrupt benthic ecosystems but may also affect the OC stored within these sediments and could release carbon dioxide (CO2) (Duplisea et al., 2001), further contributing to global anthropogenic GHG emissions. ...
This briefing is the product of a research agreement between Danmarks Naturfredningsforening (DN) and the University of St Andrews (UStA), Department of Geography and Sustainable Development, with the aim to provide an estimate of the potential effects of bottom trawling on the OC stored within Denmark's surficial marine sediments.The focus of this briefing are the sub-tidal marine sediments within Denmark's Exclusive Economic Zones (EEZ). Additionally, the effect of bottom trawling on the sedimentary OC stocks within the MPAs that are composed of the Natura 2000 areas (N2000) and areas protected according to the Marine Strategy Framework Directive (MSFD) and within the 3, 6 and 12-nautical mile boundaries will be assessed to potentially allow new recommendations to be made for the development of appropriate management and policy interventions.
Based on an integration of borehole and reflection seismic data, the opal-A to opal-CT transformation front was identified at all the 68 Ocean Drilling Program and Deep Sea Drilling Project sites analyzed. The silica front is represented by a sub-bottom depth interval of various thicknesses where the physical properties of the hosting sediment unit, including bulk density, porosity, resistivity, and P-wave velocity, drastically change. The boundary of sharp physical-property changes is tied to the exhibition of a high-amplitude high-impedance reflection on seismic profiles that commonly cross-cuts the host lithology. The objective and downhole criteria for differentiating between the active and relict silica phase-change transitions were extended. The association with rootless polygonal faults and domal contractional folds, intersecting relationship with the encompassing stratigraphy, the development of irregular morphologies, and wide ranges of thickness, age, and temperature indicated that a greater number of the examined opal-A to opal-CT conversion boundaries (66 out of 68) are currently fossilized. The upward advancement and arrest of fossilized silica transitions is believed to be controlled by an episode of increased palao heat flow whose mechanism is presently unknown. The temperature is known to be the main control for silica-front thickness. The hosting lithology, age, pore geochemistry, and test surface area play a secondary role in thickness variations. The opal-A to opal-CT conversion time-temperature stability zone proposed by Hein et al. (1978) was modified based on the fossilization age and temperature of the reaction fronts. The older reaction boundaries (> 25 Ma in age) were developed as a response to the low-temperature (< 30 °C) active-diagenesis under shallow burial while the formation of younger transitions (< 25 Ma in age) at higher temperatures (< 30 °C), according to the opal-CT formation stability field defined by the present work. Revealed through the decreased trend of geothermal evolution in time, three time spans of different thermal histories were introduced for the silica transitions examined: high thermal-evolution records (> 15 °C/Ma) of the silica fronts migrated upwards since the Early Pliocene and arrested in the Late Pliocene, transitions of thermal-history values between 5 and 15 °C/Ma being active since the Late Oligocene and fossilized in the Late Miocene, and those with low thermal-history records (< 5 °C/Ma) advanced in the Early Paleocene and arrested from the Late Oligocene to Early Miocene.
Radiocarbon (Δ¹⁴C) serves as an effective tracer for identifying the origin and cycling of carbon in aquatic ecosystems. Global patterns of organic carbon (OC) Δ¹⁴C values in riverine particles and coastal sediments are essential for understanding the contemporary carbon cycle, but are poorly constrained due to under-sampling. This hinders our understanding of OC transfer and accumulation across the land–ocean continuum worldwide. Here, using machine learning approaches and >3,800 observations, we construct a high-spatial resolution global atlas of Δ¹⁴C values in river–ocean continuums and show that Δ¹⁴C values of river particles and corresponding coastal sediments can be similar or different. Specifically, four characteristic OC transfer and accumulation modes are recognized: the old–young mode for systems with low river and high coastal sediment Δ¹⁴C values; the young–old and old–old modes for coastal systems with old OC accumulation receiving riverine particles with high and low Δ¹⁴C values, respectively; and the young–young mode with young OC for both riverine and coastal deposited particles. Distinguishing these modes and their spatial patterns is critical to furthering our understanding of the global carbon system. Specifically, among coastal areas with high OC contents worldwide, old–old systems are largely neutral to slightly negative to contemporary atmospheric carbon dioxide (CO2) removal, whereas young–old and old–young systems represent CO2 sources and sinks, respectively. These spatial patterns of OC content and isotope composition constrain the local potential for blue carbon solutions.
Marine sediments in glacially-carved fjords at high latitudes feature high organic carbon (OC) burial rates, but there are fewer data on the role of glacial activity on high-latitude OC burial rates outside of fjords. Here, we investigate the relationship between sediment OC burial rates in the deep troughs and basins of the southwest Greenland shelf and Holocene glacial dynamics. Since the onset of prominent Neoglacial advances ~2500 years ago, the nature of the OC buried in the deep troughs and basins of the shelf was influenced by the glacier-driven increase in sediment accumulation rates (SAR), reactive iron (oxyhydr)oxide concentrations and fine-grain sediment, while OC burial rates were primarily enhanced by increasing SAR. Peak OC burial rates (~18.5 ± 5.7 g m⁻² a⁻¹) in the deep troughs and basins of the shelf during the past ~1300 years are comparable to those of many high-latitude fjords, and the inferred total annual OC burial in these trough and basin areas is equivalent to ~5% of the annual CO2 uptake by the Labrador Sea deep convection.
The role of continental margin sediments in the carbon cycle and the associated management potential for climate mitigation are currently poorly understood. Previous work has indicated that margin sediments store significant amounts of organic carbon, but few studies have quantified the rates at which organic carbon is accumulated. Here, we use machine learning to make spatial predictions of the organic carbon stocks and accumulation rates of sediments on the Norwegian continental margin. We show that surface sediments (upper 10 cm) store 814 Tg and accumulate 6 Tg yr⁻¹ of organic carbon. Shelf-incised glacial troughs account for 39% of the stocks and 48% of the accumulation, with the main accumulation hotspot located in the Skagerrak. Continental margin sediments accumulate organic carbon at scales much larger than vegetated coastal ecosystems in Norway because of their larger extent. Future studies should explore to what extent management interventions could increase accumulation rates, e.g., by minimising anthropogenic disturbance of seafloor sediments.
The biological pump, defined as the marine biological production and sedimentation of particulate organic carbon (Corg), is a fundamental process to fix atmospheric carbon dioxide in the oceans, transfer carbon away from the atmosphere to the deep ocean, and maintain the CO2 level of the atmosphere. The level of carbon sequestration by the biological pump has varied throughout the last 50 million years, from particularly weak in the warm Eocene to much stronger in the Holocene. However, persistently warm climates in the more recent past, e.g., the Miocene Climate Optimum (MCO; 17 million years ago [Ma] to 13.8 Ma) also have affected the biological sequestration of carbon. A series of scientific ocean drill sites from the equatorial Pacific contain very low sedimentary Corg % in the period prior to 14 Ma but higher and much more variable Corg % afterward. Although lower absolute productivity may have contributed to the lower Corg burial at the MCO, higher relative Corg degradation also occurred. Ratios of Corg to other productivity indicators indicate higher relative loss of Corg. Temperature records imply that the higher Corg degradation occurred in the upper water column, and global cooling strengthened the biological pump but led to more variability in burial. Similar records of low Corg at the MCO can be found in the North Pacific, which suggest this was a global—rather than regional—change. A weakened biological pump during warm climate intervals helps to sustain periods of global warmth.
Studying monsoon dynamics during past warm time periods such as the Miocene Climatic Optimum (MCO; ∼16.9–14.5 Ma) could greatly aid in better projecting monsoon intensity, in the context of future greenhouse warming. However, studies on regional MCO temperature change and its effect on the monsoons during this time period are lacking. Here, we present the first high‐resolution, low‐latitude record of sea surface temperature (SST) and paleoceanographic change covering the Miocene Climatic Optimum, in the eastern equatorial Atlantic, at Ocean Drilling Program Site 959, based on TEX86 paleothermometry. SSTs were ∼1.5°C warmer at the onset of the MCO (16.9 Ma) relative to the pre‐MCO (∼18.3–17.7 Ma). This warming was accompanied by a transient increase in %total organic carbon. Prior to the MCO, sediment composition, geochemical proxy data as well as dinoflagellate cyst assemblages imply a productive surface ocean at Site 959. Immediately following the MCO onset (∼16.9–16.5 Ma), we record an intensification of the West African Monsoon (WAM) characterized by higher amplitude variability in all proxy records on precession to obliquity timescales. We interpret increased orbital‐scale SST, biogenic Ba and dinocyst assemblage variability to represent intensification of equatorial upwelling, forced by the WAM strength. Furthermore, higher SSTs during eccentricity maxima correlate to increased relative abundances of the warm and stratification‐favoring dinocyst Polysphaeridium zoharyi, during periods of low WAM intensity. Finally, while long‐term SSTs decline toward the middle Miocene, maximum SSTs and Polysphaeridium zoharyi abundances occur during MCO peak warming at ∼15.6 Ma.
The field of marine metal stable isotope geochemistry has expanded dramatically since the last edition of the Treatise on
Geochemistry. This chapter examines the marine stable isotope cycling of nine transition metals: V, Cr, Fe, Ni, Cu, Zn, Mo,
Cd and W. Classifying the metals according to their oceanic residence time and degree of internal cycling enables comparison
between systems and, in each case, to draw out the importance of one or more key controlling processes. Isotopic variability
internal to the ocean is driven by a range of biogeochemical processes and their interaction with the physical ocean
circulation. Mean whole ocean isotopic compositions are controlled by the isotopic composition of oceanic sources and
fractionation into sedimentary sinks. The isotopic oceanic mass balance of each metal is reviewed and, in some cases,
updated, providing revised estimates of their oceanic residence times.
Dissimilatory iron reduction (DIR) can drive the release of organic carbon (OC) as carbon dioxide (CO 2 ) by mediating electron transfer between organic compounds and microbes. However, DIR is also crucial for carbon sequestration, which can affect inorganic‐carbon redistribution via iron abiotic–phase transformation. The formation conditions of modern carbonate‐bearing iron minerals (IC Fe ) and their potential as a CO 2 sink are still unclear. A natural environment with modern IC Fe , such as karst lake sediment, could be a good analog to explore the regulation of microbial iron reduction and sequential mineral formation. We find that high porosity is conducive to electron transport and dissimilatory iron‐reducing bacteria activity, which can increase the iron reduction rate. The iron‐rich environment with high calcium and OC can form a large sediment pore structure to support rapid DIR, which is conducive to the formation and growth of IC Fe . Our results further demonstrate that the minimum DIR threshold suitable for IC Fe formation is 6.65 μmol g ⁻¹ dw day ⁻¹ . DIR is the dominant pathway (average 66.93%) of organic anaerobic mineralization, and the abiotic‐phase transformation of Fe ²⁺ reduces CO 2 emissions by ~41.79%. Our findings indicate that as part of the carbon cycle, DIR not only drives mineralization reactions but also traps carbon, increasing the stability of carbon sinks. Considering the wide geographic distribution of DIR and IC Fe , our findings suggest that the “iron mesh” effect may become an increasingly important vector of carbon sequestration.
This chapter addresses controls on the stratigraphic record: the mechanisms, processes, and contingencies affecting sediment supply and accommodation and the resulting stratal surfaces and units. Although it is not necessary to know the forcing mechanisms of sequence formation to construct a sequence-stratigraphic framework and map the distribution of rock properties, it is commonly useful to incorporate one’s understanding of key processes to provide predictive capabilities away from sample control.
Many factors influence the development and expression of parasequences and depositional sequences. These factors can be grouped usefully into two main categories: processes (sediment supply and accommodation) and contingencies (inherited and coeval factors that condition the effects of those processes). The main components of sediment supply include detrital, biogenic, and authigenic processes as well as lateral and temporal changes thereof; the components of accommodation include those processes that affect the upper and lower boundaries of sediment accumulation. Contingencies exert a significant influence on the expression of sequence-stratigraphic surfaces and units because they affect when, where, and how the processes of sediment supply and accommodation operate. Four main contingencies affect all depositional settings and generally do not change significantly during a depositional sequence: (1) geological age, (2) plate-tectonic setting, (3) paleolatitude, and (4) paleogeography at the continental and basinal scale. Other contingencies tend to be specific to particular types of depositional settings or change significantly during accumulation; these include (1) inherited and evolving bathymetry, (2) climate mode, and (3) ocean chemistry.
Ultimately, it is difficult to uniquely identify causal mechanisms because of the many influences on accommodation and sediment supply and the commonly convergent effects of those influences (i.e., similar stratal patterns can result from various combinations of influences). Knowledge of mechanisms is not, however, an essential part of the sequence-stratigraphic approach (and is potentially not possible in many circumstances—especially not from the stratal patterns alone). Sequence stratigraphy allows construction of a comprehensive and useful stratigraphic framework based on a single criterion—the physical relations of the strata themselves—that reveals genetically related rocks.
The Monterey Formation illustrates the expression of sequence-stratigraphic surfaces and units in continental slope and basin settings that are quite unlike those of most mudstone units considered thus far in this book. These strata span a wide variety of siliceous, calcareous, argillaceous, phosphatic, and kerogenous composition, detrital, biogenic, and authigenic origins, and clay to cobble grain size.
The Monterey Formation poses particular challenges with its deposition of dominantly biogenic sediments in a variety of deep-basinal environs in an active tectonic region. This chapter shows how applying the sequence-stratigraphic method and approach (from first principles), even to a relatively unfamiliar setting, can provide insights into the accumulation of rocks enriched in organic matter and biogenic material as well as the complex distribution of its time-transgressive facies.
The dominant sediment influx was biogenic, composed mostly of diatoms and controlled by primary organic production. The dominant organic-matter input was almost exclusively from diatoms, also controlled by primary organic production. However, intervals with highest sediment accumulation rates do not have the highest organic-matter content but are dominantly biosiliceous, with little organic matter. This dominantly biogenic distal setting offers an opportunity to examine sequence stratigraphy and its relation to organic-matter content and biosilica accumulation where sediment accumulation rates are strongly related to organic-production rates under relatively constant preservational conditions and input of uniform organic matter with relatively low input of siliciclastic detritus and variable downslope influx of reworked biogenic material. At the macro to meso scale, sequence boundaries are marked by truncation below and onlap above, and maximum flooding surfaces are marked by downlap above and conformable strata below, even in this relatively distal, deep-water setting. Parasequence boundaries are particularly well highlighted by early diagenetic evidence of significant pauses in sediment accumulation—most notably, dolomite cement and phosphorite nodules.
This chapter illustrates the systematic approach we used for developing a stratigraphic framework consistent with observations that range from seismic to geochemistry. This framework and a few basic concepts enable one to develop predictive capabilities in continental margins settings.
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.
This paper reviews and critically evaluates the recent literature pertaining to biodegradation in reservoirs, by referring to the author's experience in that field. It includes a discussion of unpublished case histories. The aim is to carry out a survey of several examples which can provide a sequence illustrating the step-by-step biodegradation of crude oil. Degrees of alteration ranging from incipient to drastic changes of both alkanes and aromatics are presented.
The organic carbon contents and textural composition of a total of 166 surficial sediment samples (from 10 to 1226 m water depths) together with data on primary productivity rates and dissolved oxygen concentrations have been studied to investigate the main controls on the distribution of organic carbon buried within the modern sediments across the Sea of Marmara.The distribution of average annual primary production rates in the Sea of Marmara exhibits great lateral variations; the highest values are calculated for the southern shelf (161 gCm−2 year−1), the areas with high terrigenous input supplied by the southerly major rivers, and on the northeastern shelf (104 gCm−2 year−1) where organic- and nutrient-rich surface inflow from the Black Sea is prominent. The low primary productivities estimated for the southwestern shelfof the Sea of Marmara (64 gCm−2 year−1) suggest influences from the relatively organic- and nutrient-poor subsurface inflow from the Aegean or Mediterranean.Organic carbon contents in sediments from the northeastern (0.37–2.16%), northern (0.57–1.64%), southern (0.44–1.90%) and southwestern shelf regions (0.37–1.51%) all appear to be within the same range and show no direct relationship with surface productivity and oxygen deficiency in the Sea of Marmara. Production and accumulation of organic matter in the Sea of Marmara are believed to have been mostly affected by the inflow of relatively organic-rich Black Sea waters, by the southerly major rivers, and by inflow of organic-poor Aegean or Mediterranean waters. Lateral offshore transport in surface waters must have resulted in the decrease of organic carbon fluxes to the sediments.
Sediment from the FOAM (Friends of Anoxic Mud) site in Long Island Sound, Connecticut, was partitioned into several different size fractions (and magnetite), and the concentration and isotopic composition of sulfur in each fraction was determined. Size fractions were chosen to represent different associations of pyrite with iron. Demonstrates that calculated values of sediment DOP (degree of pyritization) cannot be used to demonstrate uniquely iron or sulfur limitation in pyrite formation. Pyrite formation depends on the relative rates of sulfate reduction compared to rates of sulfide reaction with iron minerals; DOP does not yield information on these factors. -from Authors
Anoxic and oxic degradation pathways of sedimentary chloropigments were examined by spiking marine sediment with ¹⁴ C‐labeled algal cells and purified chloropigments from the diatom Skeletonema costatum. These experiments suggest that Chl a degrades through multiple pathways. Under oxic conditions, most bulk sedimentary Chl a degraded to various colorless compounds and only a minor fraction degraded to pheophytin a ; added ¹⁴ C‐labeled Chl a also degraded quickly, but 30–40% of this Chl a was converted to pheophytin a. Under anoxic conditions, only a small fraction of bulk Chl a degraded, but added ¹⁴ C‐labeled Chl a continuously degraded and ~30–40% of it was converted to pheophytin a. Pheophytin a is relatively stable under anoxic conditions but degrades under oxic conditions, thus it is a potential end product of chloropigment degradation in anoxic environments.
Degradation pathways are likely dependent on the relative proportion of unassociated Chl a to chlorophyll complexes present in the sediment. Only unassociated Chl a appears to be available for anoxic decomposition. Under oxic conditions, some colorless products were further degraded and solubilized; none of the ¹⁴ C label added as purified pigments was lost under anoxic conditions during the 1‐month incubation. About 80% of the acetone‐extractable ¹⁴ C in labeled cells was lost in 1 month from sediments under oxic conditions and ~30% under anoxic conditions.
This chapter discusses the biogeochemical cycling of primarily toxic and persistent compounds, such as Pu isotopes, 210Po, 210Pb, As, Hg, and polynuclear aromatic hydrocarbons. It describes the major improvements—such as sample collection, preparation, and analytical chemistry capabilities for the determination of trace organic compounds, stable metals, and their speciation and radionuclides in environmental samples—in studies of chemical cycling off the Washington coast. The chapter discusses various processes such as chemical inputs from rivers, advecting seawater, air–sea exchange, and hydrothermal activity as well as outputs to the major sedimentary and biological reservoirs via uptake and exchange of chemicals with water column. The chapter describes the cycling of chemicals on the open Washington coast north of the Columbia River mouth. Related research in the Strait of Juan de Fuca and in Puget Sound is compared and contrasted in the chapter.
Discusses the available methods for rate measurements and selected results of rate measurements between the sediment - water interface and about 1-m depth in the sediment. Focuses on both freshwater and marine sediments, but there are significant differences in anaerobic organic matter decomposition processes. The most significant are that little sulfate is present in freshwater sediments, whereas it is a major oxidant in marine sediments. Lignin and structural polysaccharides, such as cellulose, are important constituents of the organic matter supplied to most freshwater sediments, but are minor constituents in nearly all marine sediments. -from Author
It is generally accepted that the formation of kerogens is the result of the so-called depolymerization-recondensation pathway (Tissot and Welte, 1984; Durand, 1980), Thus, naturally occurring macromolecular substances such as polysaccharides and proteins are enzymatically depolymerized to oligo- and monomers, which for the most part are mineralized. However, a small part of them are thought to condense with other substances such as low-molecular-weight lipids in a random way (Fig. 1). During diagenesis, the “geopolymers” thus formed continuously undergo chemical transformations by which they become more and more insoluble and resistant. Kerogens are being formed, which—depending on the nature of the original organic matter contributions—can generate various amounts and sorts of oil under thermal stress.
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.
Late Quaternary sediments of the Madeira Abyssal Plain (MAP) consist of alternations of metre-thick distal turbidites and thin (centimetre to decimetre) pelagic clays, marls and oozes. The geochemical characteristics of 29 turbidites are described using major- and trace-element data from more than 350 samples. These were obtained from two representative piston cores located in the NE and W central parts of the MAP. Three separate groups of turbidites are defined by the geochemical data: (a) organic-rich, (b) 'volcanic' and © calcareous turbidites. 'Organic' turbidites contain 0.3-2% organic carbon. Their sediment originated from the lower continental slope of NW Africa N of 20°N, although one turbidite may have been derived from S of this latitude. 'Volcanic' turbidites contain a large proportion of volcaniclastic material and are distinguished by their high TiO 2 contents (about 1.5% on a carbonate-free basis). Much of their sediment originated from the oceanic islands of the Canaries and Madeira to the E. Calcareous turbidites are composed predominantly of pelagic carbonate and are defined by high CaCO 3 values (more than 75%). Their sediment was derived from the Great Meteor-Cruiser Seamount Chain, W of the MAP.
Factors controlling the provenance, concentrations, and nature of sedimentary organic matter (SOM), particularly the nitrogenous fraction, were examined for sites throughout the Gulf of Maine and two of its estuaries. Stable nitrogen isotope data corroborate previously reported Br: C ratios in indicating that macroalgae are important contributors to organic nitrogen pools in estuarine sediments. Significant contributions of terrigenous organic carbon in both the estuaries and the deep basins of the open Gulf are not accompanied by significant contributions of terrigenous organic nitrogen. Variations in concentrations of organic carbon, nitrogen, chlorophyll, and enzymatically hydrolyzable protein throughout the Gulf and its estuaries can be explained largely by variations in the delivery rate of organic matter to the benthos and the specific surface area of minerals in the sediments. Chlorophyll and protein concentrations represent relatively fresh organic matter and exhibit greater sensitivity to organic delivery variations than do total organic carbon or nitrogen. A strong, linear relationship between organic carbon and surface area is consistent with monolayer adsorption of SOM on mineral surfaces, which appears to inhibit degradation of the SOM. Amino acid compositions are remarkably constant throughout the Gulf, with large differences seen only with elevated tyrosine in estuaries and elevated basic amino acids and methionine in the open Gulf. Leucine and glutamic acid correlate with chlorophyll concentrations in open Gulf sites. Enzymatically hydrolyzable protein comprises small to major fractions of total hydrolyzable amino acids. Amino acid-nitrogen: total nitrogen correlates inversely with total nitrogen.
Radiocarbon measurements sequentially with depth in a diver-obtained core from Long Island Sound has been interpreted in terms of sediment accumulation rates and sources of organic carbon in the sediment. A sediment accumulation rate of 0·075 ± 0·013 cm year−1 is determined from the data below 10 cm in the core. An age of 2320 year BP is obtained for the sediment-water interface by extrapolation. This means that the dominant carbon component preserved in the core is soil-derived. The contributions of fossil fuel carbon and bomb radiocarbon associated with plankton are also evaluated.
The oxidation-reduction status of Gulf Coast wetland sediment greatly influenced hydrocarbon breakdown. Degradation of 14C-labelled octadecane and 14C-labelled naphthalene was faster in oxidized (+500 mV) sediment compared with reduced (−200 mV) sediment. Naphthalene showed no breakdown in reduced anaerobic sediment. However, a shift from strict anaerobic to aerobic conditions markedly increased napthalene degradation. An oxidized layer of sediment overlying reduced sediment was identified in several zones in a Louisiana salt marsh. Hydrocarbon degradation is likely to be greater in the oxidized surface layer.
This book presents the papers given at a conference on the atmospheric chemistry of carbon dioxide. Topics considered at the conference included the carbon cycle, the tropospheric methane cycle, the last deglaciation, reef growth, climatic change, carbon deposition rates in the Atlantic Ocean, low-latitude biomass, carbon isotopes, geochemistry, charcoal fluxes, volcanism, geologic ages, tectonics, carbonate rocks, marine surveys, and biogeochemistry.
The depth variation of total organic carbon (TOC), organic matter composition, and porewater composition in marine sediments suggests that different components of the organic matter undergo decomposition at widely different rates. The decomposition of 14C-labeled organic substances was followed in sediment microcosms in the laboratory. The substances used were chosen to simulate a portion of material settling to the sediment-water interface (a marine diatom) or hypothesized components of refractory sediment organic matter (melanoidins and a bacterial polymer). The microcosms were found to be good models of the sediment-water interface in terms of how well they mimicked sediment decomposition rates and processes. The decomposition of the labeled material and the natural sediment TOC were monitored over 1 month: the water overlying the sediment remained oxic, and net consumption of nitrate was small. There was no detectable sulfate reduction. The algae and the bacterial polymer were decomposed on average 9 x faster than the melanoidins and 90 x faster than the natural sediment TOC. The soluble fraction of the algae was decomposed more rapidly than the particulate material.
We have developed sampling methods and an analytical system to determine the concentration of dissolved organic C (DOC) in marine pore waters. Our analytical approach is a modification of recently developed high-temperature, Pt-catalyzed oxidation methods; it uses Chromatographic trapping of the DOC-derived CO,, followed by reduction to CH, and flame ionization detection. Sampling experiments with nearshore sediments indicate that pore-water separation by whole-core squeezing causes artificially elevated DOC concentrations, while pore-water recovery by sectioning and centrifugation does not appear to introduce DOC artifacts. Results from a set of northwestern Atlantic continental slope cores suggest that net DOC production accounts for >50% of the organic C that is recycled at the sediment-water interface.
In situ microelectrode, box-core pore water gradient, and in situ benthic chamber estimates of organic carbon degradation and CaCO3 dissolution are combined with organic-C and carbonate-C accumulation rates to approximate the total carbon flux to the seafloor along two transects of the continental slope and rise off central California. Microelectrode profiles of dissolved O2 demonstrate that sediments at 13 sites, ranging in water depth from 580 to 4080 m, become anoxic below the uppermost 0.4–3 cm of the sediment column. If a current-swept area of nondeposition on the upper slope is excluded, we find total organic-C and carbonate-C fluxes to the seafloor vary from 40 to 100 μmol C cm−2yr−1 and from 32 to 91 μmol C cm−2yr−1, respectively. From the distribution of these fluxes there is no indication that total fluxes or remineralization rates of organic or carbonate carbon are influenced markedly by conditions in the oxygen minimum zone. Instead, the upper continental rise with its system of submarine valleys and fans stands out as the most important locus for carbon deposition and remineralization. When benthic fluxes and burial rates are extrapolated over the whole slope and rise of the region, aerobic respiration is the major mechanism of organic matter oxidation, and organic-C and carbonate-C recycling are on average 87% and 98% efficient, respectively. These results suggest that modern sediments on the outer regions of continental margins are important sources of CO2 that is injected directly into ocean deep water. However, if benthic carbon fluxes on the central California margin are typical of margins globally, this injection rate is less than 0.7 Gt C yr−1, which does not indicate a significant anthropogenic enhancement of carbon export to continental slopes and rises.
A new method for determining organic carbon and total nitrogen in calcitic sediments is described and was employed in a study
of Black Sea sediment samples from a transect of five proximal stations which crossed the depth where the water column oxycline
impinged on the shelf. Bottom waters ranged from anoxic and sulfidic to fully oxic, with a suboxic zone between. The samples
were collected with the aim of clarifying the relative importance of oxygen availability to organic matter preservation. Surface
fluff layer organic carbon and total nitrogen levels both dropped steadily from anoxic to oxic stations, but carbon to nitrogen
ratios increased due to preferential loss of a nitrogen-rich component. The source of organic matter appears constant across
the transect, and both the bulk organic matter and diagenetic substrate are predominantly marine. Core profiles from oxic
and anoxic stations displayed major differences in shape and in the depth extension of diagenesis. Distinct midcore transitions
in carbon and nitrogen levels at two suboxic stations appear to be linked to changes from oxic to current suboxic bottom water
conditions, arising from a shoaling water column oxycline. Profile differences result from degradation of a substrate which
becomes progressively more nitrogen-depleted. Collectively, the results suggest that oxygen availability may play an independent
role in organic matter preservation, but remain equivocal due to spatial heterogeneity of sediment accumulation rates and
bioturbation.
Descibed the fluxes of total CO2, O2, NO3-+NO2-,NH4+, dissolved organic nitrogen and HS- in Aarhus Bight, Denmark. -from Authors
Overall sediment accumulation on the abyssal plain E of Great Meteor Seamount is dominated by distal turbidite deposition. A colour contrast is seen at the tops of many individual turbidites, so that a single turbidite has two distinct colour tones. Porewater data from the uppermost turbidite are used to demonstrate that these colour contrasts are consequences of an oxidation-front phenomenon. Distributions of organic carbon, uranium, vanadium and copper are proposed as diagnostic solid-phase indicators in both active and fossil types. A generalized description of the features of such turbidites is presented.
The sediments of the Madeira Abyssal Plain consist of thick turbidites (up to 5 m) separated by thin pelagic layers of clay, marl or ooze. Over most of the area the turbidites consist of ungraded massive silts and silty clays, sometimes with coarser bases. The mineralogy of these sediments suggests that they derive from the NW African margin. Isopach maps of individual turbidites show several of them thickening towards the W; their silty bases, however, coarsen towards the E suggesting transport from the NW African margin, followed by ponding on the plain. Some of the smaller turbidities are concentrated in particular areas of the plain. One group is concentrated in the N and NE, whereas a group of pure CaCO 3 turbidites are limited to the SW, suggesting an origin from the nearby seamounts.
This paper attempts to demonstrate that the multi-disciplinary philosophy behind palynofacies investigations is of great value to the meaningful interpretation of the origins and palaeoenvironment of marine petroleum source rocks (MPSRs).
The key control on the formation of MPSRs is identified as bottom water oxygenation (correlated with the location of the Eh interface and the intensity of macrobenthic activity). Particular emphasis is placed on the necessity for greater accuracy in the terminology used to describe levels of oxygenation. Aerobic environments are characterized by organically lean sediments (0–3.0% TOC) with Type III kerogen assemblages composed of relatively refractory land plant debris or highly degraded, marine-derived, amorphous organic matter (AOM). Their organic richness is largely dependent on sediment accumulation rate and proximity to sources of terrestrial organic matter supply. They produce mainly gas at maturation. Dysaerobic to anoxic environments are characterized by MPSR facies with high TOC values and Type II kerogen assemblages dominated by relatively lipid-rich AOM. These represent the classic ‘black shale’ source rocks and are the main source of petroleum in marine basins.
Except where they are redeposited, dinocysts are characteristically absent or rare in marine ‘black shales’. They are mainly produced in unstable, seasonally mixed water masses and may consequently be regarded as indices of hydrographic stability. Prasinophycean phycomas, which differ from dinocysts in their function, are often the dominant, or most conspicuous, marine palynomorphs of pelagic sediments and stably stratified ‘black shale’ basins.
The combined results of many physical oceanographic, geochemical, and geological investigations provide a coherent picture of the sedimentology of the Washington continental shelf. The major source of modern sediment is the Columbia River. Sediment on the shelf is transported by bottom currents resulting from surface wind stress and gravity waves which occur as individual storm or wave events lasting one to several days, primarily during the winter months.
Field measurements of the sediment response to wind- and wave-generated bottom currents suggest that the modern sediment is transported as suspended load and disperses primarily northward parallel to the isobaths and is associated with a well-developed mid-shelf silt deposit. Smaller quantities of sediment are transported seaward over the shelf edge and into the numerous submarine canyons that incise the shelf. Analysis of available wave data suggests that thresholds of sediment motion were exceeded on the order of 22%, 16%, and 1.5% of the time annually on the inner, central, and outer shelf respectively. Bottom current measurements suggest that threshold conditions were exceeded 13% and 6% of the time annually on the inner and central shelf respectively.
Measurements of accumulation rates using ²¹⁰ Pb geochronology show that about 67% of the annual sediment discharge from the Columbia River is being deposited in the mid-shelf silt deposit; 6% is being deposited over the shelf edge on the open slope, and 11% is accumulating in the four major submarine canyons.
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.
Sediment cores up to 35 m long have been removed from turbidite sequences in the Madiera Abyssal Plain and the Southern Nares Abyssal Plain. Detailed geochemical analyses of these sediments (Si, Al, Fe, Mn, Ca, organic and inorganic carbon) and associate pore waters (nitrate, ammonia, silicate, Fe{sup 2+}, Mn{sup 2+} and alkalinity) provide representative profiles of both steady-state and nonsteady-state diagenetic reactions initiated by oxidation and reduction processes. Models demonstrate that chemical transport in the pore water system is diffusion dominated. Over periods of a few thousands of years to several hundreds of thousands of years Mn and Fe mobilized in the subsurface mildly reducing sediments sediments are precipitated as easily reducible oxyhydroxides in the near-surface oxidized sediments. Paleo-oxidation zones can be recognized in old turbidites by distinct changes in color between lower and upper portions of the deposit. Depletion of carbonate content in the oxidized zones in the Southern Nares Abyssal Plain is particularly significant in that as much as 80% of the initial carbonates have been lost. Also the relative amounts of weak acid soluble Fe and Mn are generally less in the paleo-oxidized zones, due to the initial oxidation of metal-carbonate and metal hydroxide forms to easily reducible oxide forms, which are then subsequently solubilized under the mildly reducing conditions at depth in the sediment column.
The distributions of steranes, pentacyclic triterpanes and aromatic steroid hydrocarbons in core sections of a recent intertidal sediment (Bridgwater Bay, Severn Estuary, U.K.) have been examined by gas chromatography-mass spectrometry. Depending on sample depth, the distributions are explicable in terms of 1. (i) a reworked origin for these compounds from a geological source, probably as a result of erosion of nearby organic-rich sediments of Lower Liassic age, or 2. (ii) a dual origin from both (i) and a more mature source, probably an anthropogenic input of petroleum origin. These results show that not all of these steroid and triterpenoid hydrocarbons in surface sediments necessarily result from fossil fuel pollution. They indicate also that attention should be paid to the possibility of contributions from reworked components when using the distributions of these compounds in geochemical studies of sediments that are low in extractable organic matter.
Weight percent total organic carbon (TOC) is one of the most commonly used descriptors for both Recent and ancient sediments.
It is commonly considered to be approximately proportional to organic carbon accumulation rates and, therefore, a gauge of
those rates. Weight percent TOC is also used to judge hydrocarbon source rock potential and primary productivity of the overlying
water column. On the basis of data from both the Recent and the entire Phanerozoic, we conclude that weight percent TOC is
an unreliable measure of the rate of deposition of organic carbon and that arguments and conclusions based on that relationship
are suspect.
A procedure is described that makes it possible to separately measure and analyze isotopically two types of carbon in sedimentary rocks: the elemental carbon (charcoal, soot, metamorphic carbon) and kerogen. The procedure is based on oxidation of the sample dissolved in HF-HCl with 0.1 M K2Cr2O7 in 2 M H2SO4 at 50 C, upon which kerogen, which reacts with a half-life of 6-180 hrs, depending on maturity, can be quantitatively removed from elemental C, which reacts with half-life of 600-2000 hrs. The rate of the oxidation reaction was measured for carbon black and for carbonaceous residues from two K-T boundary clays, as well as for 12 samples of charcoal and kerogen. It was found that most materials obey first-order kinetics, but that carbon black and K-T soot (both consisting of spherules) show a sudden 1.7-fold increase in the rate after a mass loss of 23 or 82 percent, respectively, indicating that the interiors of soot spherules are more reactive than the exteriors.
MANY bacteria oxidize thermodynamically unstable manganese(II) to Mn oxides
and deposit the oxides on their surfaces1,2, a process that appears to
account for most Mn oxidation in natural waters3–5 and
sediments6. Among the reasons that have been proposed for the evolutionary
selection of this process are protection from damage by toxic metals and oxygen species,
protection from ultraviolet light, and strengthening of the bacterial sheath or
capsule1,7. Mn oxides may promote harmful free radical reactions, however,
and marine Mn-oxidizing bacteria are particularly susceptible to
photoinhibition8. Here we report that Mn oxides lyse complex humic
substances, which in general cannot be used by microorganisms
directly9–11, to form low-molecular-weight organic compounds that
can be used as substrates for microbial growth. Mn-oxidizing bacteria may thus be able to
use the carbon pool in humic substances, which represent one of the largest organic
reservoirs in natural waters, sediments and soils.
Sediment trap and net plankton samples were collected monthly for a year at three depths in a marine bay (Dabob Bay, Washington). These materials and subsamples from a sediment box core were analyzed for lignin oxidation products as well as elemental and stable carbon isotope compositions. The sediment core was compositionally uniform over its entire 50‐cm length. The elemental and lignin compositions of the sediment trap and core samples indicate nitrogen‐rich (atomic C : N ≅ 7.5) plankton‐derived organic matter mixed with vascular plant debris.
At most, vascular plant debris accounts for 10% (nonwinter months) to 35% (winter months) of the total organic carbon in the upper water column (30 m) sediment trap samples and consists predominantly of gymnosperm wood along with some nonwoody gymnosperm tissues and angiosperm woods. Bulk land‐derived organic matter in Dabob Bay contains a maximum of 50% vascular plant debris and comprises an average of one‐third of the total organic carbon in the sediment trap samples and two‐thirds of the total organic carbon in the underlying sediments. Lignin in the sediment trap and core samples shows evidence (from elevated vanillic acid:vanillin ratios) of white‐rot fungal degradation before (but not after) introduction to the water column at the study site. Vascular plant debris introduced to the bay has already lost almost half of its initial bulk polysaccharide. Glucose yields are particularly low whereas rhamnose and fucose are obtained in excess of expected yields and must have additional sources.
Lignin and neutral sugars together account for <20% of the total organic carbon in the sediment trap and core samples. Overall, the sediments of Dabob Bay compositionally resemble the gymnosperm wood‐rich particulate material introduced to the overlying water column during winter and poorly record the input of plankton and other types of vascular plant debris during nonwinter months.
Laboratory study of the bacterial decomposition of Long Island Sound plankton in oxygenated seawater over a period of 2 years shows that the organic material undergoes decomposition via first‐order kinetics and can be divided into two decomposable fractions, of considerably different reactivity, and a nonmetabolizable fraction. This planktonic material, after undergoing varying degrees of oxic degradation, was added in the laboratory to anoxic sediment taken from a depth of 1 m at the NWC site of Long Island Sound and the rate of bacterial sulfate reduction in the sediment measured by the ³⁵ S radiotracer technique. The stimulated rate of sulfate reduction was in direct proportion to the amount of planktonic carbon added. This provides direct confirmation of the first‐order decomposition, or G model, for marine sediments and proves that the in situ rate of sulfate reduction is organic‐matter limited. Slower sulfate reduction rates resulted when oxically degraded plankton rather than fresh plankton was added, and the results confirm the presence of the same two fractions of organic matter deduced from the oxic degradation studies.
Near‐surface Long Island Sound sediment, which already contains abundant readily decomposable organic matter, was also subjected to anoxic decomposition by bacterial sulfate reduction. The decrease in sulfate reduction rate with time parallels decreases in the amount of organic matter, and these results also indicate the presence of two fractions of organic carbon of distinctly different reactivity. From plots of the log of reduction rate vs. time two first‐order rate constants were obtained that agree well with those derived from the plankton addition experiment. Together, the two experiments confirm the use of a simple multi‐first‐order rate law for organic matter decomposition in marine sediments.
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.
Surface area measurements as well as organic carbon, nitrogen and phosphorus analyses on various grain size fractions of carbonate mud samples confirm that in natural environments of carbonate deposition, surface sorption processes take place which are similar to those described earlier for dissolved organics and artificially suspended calcite particles in both seawater and synthetic solutions. The specific surface area of the sediment increases from 1.8m 2 /g for the coarse-grained fraction to 12.5 m 2 /g for the fine material; likewise organic carbon, nitrogen and phosphorus increase with increasing surface area so that there are 1.20 mg C, 0.175 mg N and 0.06-0.20 mg P associated with every square meter of carbonate surface irrespective of the mineralogy of the sediment particles. It appears that the organic matter in these sediments is similar in composition, structure and quantity to the organic layers produced in sorption experiments. With their apparently defined structure and ubiquitous nature, these layers could determine the mineralogy and orientation of submarine carbonate cement or could even be a prerequisite to calcification in general.
The surface areas, densities, and pore structures of a representative selection of marine sediments have been investigated by BET gas adsorption methods, mercury porosimetry, and related techniques. It was found that all sediments examined were porous to varying degrees, with measured surface areas ranging from 1-100 m2/g. Removal of organic matter increased the available surface by exposing previously blocked pores. Pore-size distribution covered a wide range, but a pronounced maximum was commonly observed in the region of radius 15-25 Å. Total pore volumes and average pore radii were calculated, and it is shown that sorption could account for the content of organic matter normally associated with sediments. Bulk and true densities were determined experimentally, the latter by displacement of helium. It appears that particle-size distributions of marine sediments obtained with the Coulter counter are not completely equivalent to those found by sedimentation techniques.