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Effects of suspended sediments on geochemical processes near the mouth of the Amazon River: examination of biological silica uptake and the fate of particle-reactive elements
Abstract
The effects of suspended-sediment concentration on the biological uptake of silica and the chemical behavior of particle-reactive elements are examined for the continental shelf waters near the mouth of the Amazon River. Dissolved silica and suspended-sediment concentrations measured during periods of high and low discharge indicate that uptake of dissolved silica begins where the suspended-sediment concentration falls below 10–20 mg l1. Near the mouth of the Amazon River the 10 mg l1 turbidity contour and the region of initial silica uptake both occur 20–40 km farther offshore during periods of high discharge (May and June) than during periods of low discharge (October and November). The location of initial silica uptake along an across-shelf transect near the river mouth varies by as much as 20 km on a time scale of days to weeks, which is comparable to the seasonal variation. Silica uptake, chlorophyll a concentration, and suspended-sediment concentration are poorly correlated along this transect (correlation coefficients range from +0.03 to −0.3). The poor correlations may be a result of grazing by zooplankton. Phaeopigment:chlorophyll a ratios increase offshore from 0 to 0.7, which supports the hypothesis that zooplankton grazing may be responsible for the low chlorophyll concentrations in the low-turbidity offshore waters exhibiting extensive silica removal.Excess210Pb inventories for box cores collected from an across-shelf transect just south of the river mouth range from 21 to 124 dpm cm2, which are 2–10 times greater than the inventory supported by atmospheric supply and in-situ production in the overlying water column. Riverine supply and lateral transport from offshore waters are the main sources of excess210Pb to the Amazon continental shelf.210Pb and234Th data indicate that the high particulate flux from the Amazon River scavenges particle-reactive species not only from river water and continental shelf water but also scavenges these elements from offshore open-ocean water which flows onto the shelf.
... In the more distal settings ripples, scour and fill, graded sand and silts interbedded with mud layers, and finally laminated mud which may be interpreted as tidalites are common (e.g. Johnson and Baldwin, 1986;Kuehl et al., 1986;Swift and Thorne, 1991;Miller and Eriksson, 1997;Nichols, 2009). Belderson et al., 1982). ...
... The term 'suspension' usually refers to material supported by turbulence in a boundary layer (e.g. McCave, 1984aMcCave, , 2005Nittrouer et al., 1986;Baas and Best, 2002;Lamb and Parsons, 2005). Numerous authors (e.g. ...
... Numerous authors (e.g. McCave, 1984aMcCave, , 2005Rine and Ginsburg, 1985;Nittrouer et al., 1986;Kineke et al., 1996;Wright et al., 1999;Wright et al., 2001;Rotondo and Bentley, 2002;Lamb and Parsons, 2005;Lamb et al., 2008) have studied the influence of waves on the deposition of shallow shelf sediments. They have demonstrated that with appropriate suspended sediment concentrations, deposition of mud may be dampened by wave action. ...
n this study the Cleveland Ironstone and Whitby Mudstone Formations have beeninvestigated to characterise: a) the evolving mudstone facies present in a basin that isgradually deepening and developing bottom water anoxia over time and b) what thefundamental geological controls were on this variability. Using detailed facies descriptions obtained from analyses of approximately 151 samplesobtained from combined optical, electron optical and geochemical methods 6 lithofacieshave been identified. These include: 1) sand and clay-bearing, silt-rich mudstones, 2) siltbearing, clay-rich mudstones, 3) clay-rich mudstones, 4) clay, calcareous nannoplankton-, and organic carbon-bearing mudstones, 5) fine-grained muddy sandstones, and 6) cementrichmudstones. Individually, the samples are highly heterogeneous and typically organisedinto thin beds
... Datong Station is the hydrological station nearest to the Changjiang Estuary, and the sediment load measured at this station is regarded as the sediment flux from the Changjiang catchment to the sea. The 210 Pb flux from the catchment can be estimated based on the amount of sediment discharged during different seasons and the corresponding 210 Pb activity in the suspended sediment [Key et al., 1985;DeMaster et al., 1986]. We collected samples of suspended sediment during the flood season in the Changjiang Therefore, these data were omitted when we calculated the deposition rates of the two sediment cores. ...
... Compared with other large rivers, such as the Amazon and Mississippi rivers [Scott et al., 1985;DeMaster et al., 1986;Smoak et al., 1996], the 210 Pb contribution of the Changjiang catchment to the Changjiang Estuary and the East China shelf area is not negligible. One plausible explanation is that the Changjiang catchment is located on the continent of Eurasia, which is the world's largest landmass, and the 210 Pb deposition flux from a catchment positively correlates with the continental land area where the drainage basin is located [Turekian et al., 1977]. ...
... Our calculations (Figure 5a) indicate that the total 210 Pb contribution from the Changjiang to the CSD was 1.33 AE 0.51 × 10 15 dpm yr À1 prior to 2003, and the retention index of 210 Pb (proportion retained in the CSD et al., 1986;Smoak et al., 1996], the Washington continental slope [Carpenter et al., 1981], and the Rhône Delta [Radakovitch et al., 1999], coastal currents and shelf circulation are the primary mechanisms leading to 210 Pb enrichment in inner shelf regions. However, due to the substantial 210 Pb contribution of the Changjiang catchment and the action of the cross-shelf plume, the inner shelf is not a sink but rather an important source of 210 Pb to the outer shelf of the East China Sea. ...
To investigate the impact of changes in the Changjiang catchment on the estuarine coast-shelf sedimentary system, variations in the transport, distribution, and budget of ²¹⁰Pb in the sediment of the Changjiang subaqueous delta (CSD) and the Zhejiang-Fujian coastal mud belt (ZFCMB) system were analyzed before and after the impounding of the Three Gorges Dam (TGD). The results indicate that the ²¹⁰Pbex activity in the surficial sediments of the CSD-ZFCMB decreased significantly after 2003 and that the surficial and vertical distribution of the ²¹⁰Pbex activity in the CSD changed substantially due to intensified sediment redistribution. The ²¹⁰Pb contribution of the Changjiang catchment to the CSD-ZFCMB is not negligible. Due to the trapping effect of the TGD, 76 ± 49% of the ²¹⁰Pb supply from the Changjiang upstream areas was retained in the TGD, resulting in the ²¹⁰Pb flux into the sea decreased substantially. Furthermore, our findings demonstrate that the cross-shelf plume provides a major conduit for the transport of ²¹⁰Pb over the East China Sea. The variation in the ²¹⁰Pb budget in the sediment of the CSD-ZFCMB also indicated that the influence of Changjiang catchment changes on the ²¹⁰Pbex and supported ²¹⁰Pb activities will become more significant in the future. In addition, the rate of ²¹⁰Pb burial in the sediments of the ZFCMB decreased, whereas the retention index of ²¹⁰Pb and the amount of ²¹⁰Pb retained in the CSD increased significantly, suggesting that after 2003, the material transfer function of the CSD weakened.
... Os sedimentos depositados na PCA são representados, principalmente, pelas argilas e siltes (Gibbs, 1967(Gibbs, , 1973Meade et al., 1985, entre outros), que se encontram sobrepostos a areias "relíquias" e aos sedimentos carbonatados da plataforma externa (Aller et al., 1986;Nittrouer & DeMaster, 1986;Ivo & Figueiredo, Jr. 1995. ...
... Os elevados percentuais da fração silte+argila dominam em 87,5% do total de amostras, sendo que 75% apresenta conteúdos acima de 60% desse material. Esses percentuais de silte+argila (tabela 1), variando de 34% (com 66% de areia, portanto) a praticamente 100%, ratifica as considerações emanadas da literatura científica sobre os sedimentos finos da PCA (Gibbs, 1967(Gibbs, , 1970(Gibbs, , 1975Milliman et al., l975;Gibbs & Konwar, 1986;Nittrouer & DeMaster, 1986;Kineke & Sternberg,1996;Ivo & Figueiredo, Jr., 1995, l996, entre outros). ...
... Como não se registrou nos difratogramas minerais específicos de Fe, como hematita ou goethita, resta a possibilidade desse componente químico encontrar-se incorporado à estrutura cristalina de outros minerais ou aderido ao sedimento como oxi-hidróxido de Fe amorfo aos raios-X. Tais possibilidades têm sido descritas para sedimentos da PCA por outros pesquisadores, entre os quais Gibbs (1967Gibbs ( ,1970Gibbs ( , 1973Gibbs ( ,1977Sholkovitz & Price, 1980;DeMaster et at., 1983DeMaster et at., , 1986Gibbs & Konwar, 1986;Aller et al., 1986;Mackin & Aller, 1986;Rude & Aller 1989;Patchineelam et al., l991;Souza & Patchineelam l993;Berner & Rao, 1994;Andrade & Patchineelam, 1999;Pereira et al., 1999;Pereira, 2000;Siqueira & Braga, 2000;Siqueira, 2003;Siqueira et al., 2005). ...
... Diversos trabalhos sobre as condições oceanográficas da Plataforma Continental do Amazonas podem ser encontrados na literatura, tais como (RYTHER; MENZEL; NATHANIEL, 1967;DIÉGUES, 1972;KNAPP;NITTROUER, 1983;KUEHL;NITTROUER, 1986;BRUNSKILL;FIGUEIREDO, 1995;SANTOS, 2004;ARAÚJO;BOURLÈS, 2005;MUNIZ, 2010;etc.). No entanto, observações de curto espaço de tempo das condições físicos e químicas em estações pontuais dos tipos nictemerais (atividade cíclicas de 24h) ou diurnas, na Plataforma Continental do Amazonas são desconhecidas. ...
... Diversos trabalhos sobre as condições oceanográficas da Plataforma Continental do Amazonas podem ser encontrados na literatura, tais como (RYTHER; MENZEL; NATHANIEL, 1967;DIÉGUES, 1972;KNAPP;NITTROUER, 1983;KUEHL;NITTROUER, 1986;BRUNSKILL;FIGUEIREDO, 1995;SANTOS, 2004;ARAÚJO;BOURLÈS, 2005;MUNIZ, 2010;etc.). No entanto, observações de curto espaço de tempo das condições físicos e químicas em estações pontuais dos tipos nictemerais (atividade cíclicas de 24h) ou diurnas, na Plataforma Continental do Amazonas são desconhecidas. ...
This study aims to determine the nycthemeral distribution of essential nutrients to marine organisms
(nitrate, nitrite, ammonia, phosphate and silicate) and physical and chemical parameters (temperature, salinity, pH,
dissolved oxygen and saturation rate in dissolved oxygen) on the Continental Shelf Amazon and its dependence
on the influence of the Amazon river in hydrochemistry spatial variability. Water samples were collected in 2 fixed
stations of 24 hours on the Continental Shelf, with sampling every 3 hours during the period of maximum discharge
of the Amazon (May 1999). The results show higher concentrations of nutrients, salinity and lower concentration
of dissolved oxygen in the collection station near the mouth of the Amazon River, indicating the contribution of
terrestrial inputs this river in to estuarine hydrodynamics that occurring in the Amazon Continental Shelf. At the
station farthest from the mouth, prevailed domain of oceanic forcings on the nycthemeral distribution of physical and
chemical parameters studied here, even though this study was conducted during the period of maximum discharge
of the Amazon River.
... The high chlorophyll-a concentrations in coastal areas is related mainly to the amount of nutrients that reaches those waters and to the high sunlight penetration. On the Amazon shelf, chlorophyll-a concentrations are generally considered to be an index of phytoplankton biomass and considerable algal blooms occur on these waters, and the source of the nutrients that sustain the algal blooms and the high turbidity have been widely discussed by Edmond et al. (1981); DeMaster et al. (1986); Geyer et al. (1996); Santos et al. (2008); amongst others. These authors studied the influence of the Amazon and Pará Rivers on the distribution of physical and chemical parameters, as well as on the phytoplankton biomass at the Amazon shelf and adjacent oceanic areas. ...
... The highest concentration of dissolved nutrients (Nittrouer and DeMaster, 1996;Santos et al., 2008) in the rainy season could be the principal factor responsible by the local phytoplankton density, which had a much greater biomass (chlorophyll-a concentrations) in March and June, 2009, especially at São Marcos and Calhau beaches when phytoplankton blooms were recorded. Edmond et al. (1981) and DeMaster et al. (1986) recorded algal blooms on the Amazon shelf during periods of high nutrient concentrations. ...
The present study focused on São Luís Island located between São José and São Marcos bays. It is part of the Brazilian Amazon coastal zone, which is dominated by a macrotidal regime, tidal currents, northeast winds, high rainfall rates and elevated river discharge. The present study aimed to evaluate the influence of climatic, hydrodynamic and hydrological variables on the concentration of chlorophyll-a in coastal waters of three beaches located in São Marcos Bay. Data were collected at three stations over a 12-hour period in December, 2008 (dry season, low river discharge), March, 2009 (rainy season, rising river discharge), June, 2009 (rainy season, high river discharge), and Sep-Oct, 2009 (dry season, falling river discharges). The highest chlorophyll-a concentrations were recorded at São Marcos beach and Calhau, and with the highest values being recorded in March and June, 2009 (high fluvial discharge). At São Marcos the average values ranged from 5.18 mg/m³ in December to 15.99 mg/m³ in March. At Calhau, concentrations varied from 2.86 mg/m³ (December) to 16.83 mg/m³ (June), and between 4.42 mg/m³ (December) and 12.38 mg/m³ (June) at Olho d'água. The turbidity does not appear to be a factor limiting chlorophyll-a concentrations, given that the highest values were recorded in the rainy season, and phytoplankton blooms were observed during this period at São Marcos and Calhau. The probable increase in the concentration of dissolved nutrients may have affected the local phytoplankton biomass, which had a much greater concentration in March and June.
... By comparing 210 Pb rain rates at the seafloor with the atmospheric 210 Pb flux, it is possible to estimate the proportions of local and lateral 210 Pb inputs (DeMaster et al., 1986;Biscaye and Anderson, 1994), where the atmospheric flux is considered as the local source for 210 Pb. Assuming a constant spatial and temporal atmospheric 210 Pb flux of 0.52 dpm cm -2 yr -1 (Peirson et al., 1966;Beks et al., 1998;Baskaran, 2011) across the Skagerrak, the 210 Pb input by the atmosphere was calculated to be 1.1 · 10 14 dpm yr -1 . ...
Sediment fluxes to the seafloor govern the fate of elements and compounds in the ocean and serve as a prerequisite for research on elemental cycling, benthic processes and sediment management strategies. To quantify these fluxes over seafloor areas, it is necessary to scale up sediment mass accumulation rates (MAR) obtained from multiple sample stations. Conventional methods for spatial upscaling involve averaging of data or spatial interpolation. However, these approaches may not be sufficiently precise to account for spatial variations of MAR, leading to poorly constrained regional sediment budgets. Here, we utilize a machine learning approach to scale up porosity and ²¹⁰Pb data from 145 and 65 stations, respectively, in the Skagerrak. The models predict the spatial distributions by considering several predictor variables that are assumed to control porosity and ²¹⁰Pb rain rates. The spatial distribution of MAR is based on the predicted porosity and existing sedimentation rate data. Our findings reveal highest MAR and ²¹⁰Pb rain rates to occur in two parallel belt structures that align with the general circulation pattern in the Skagerrak. While high ²¹⁰Pb rain rates occur in intermediate water depths, the belt of high MAR is situated closer to the coastlines due to lower porosities at shallow water depths. Based on the spatial distributions, we calculate a total MAR of 34.7 Mt yr⁻¹ and a ²¹⁰Pb rain rate of 4.7 · 10¹⁴ dpm yr⁻¹. By comparing atmospheric to total ²¹⁰Pb rain rates, we further estimate that 24% of the ²¹⁰Pb originates from the local atmospheric input, with the remaining 76% being transported laterally into the Skagerrak. The updated MAR in the Skagerrak is combined with literature data on other major sediment sources and sinks to present a tentative sediment budget for the North Sea, which reveals an imbalance with sediment outputs exceeding the inputs. Substantial uncertainties in the revised Skagerrak MAR and the literature data might close this imbalance. However, we further hypothesize that previous estimates of suspended sediment inputs into the North Sea might have been underestimated, considering recently revised and elevated estimates on coastal erosion rates in the surrounding region of the North Sea.
... The continental input of chemical elements to the open ocean is significantly affected by the sedimentation and biogeochemical processes occurring in the estuaries. These processes include the following: (i) particle mixing of continental and marine origin (Turner et al. 1994;Turner and Millward 2000); (ii) particle resuspension due to dredging/tidal action (Martino et al. 2002); (iii) phase redistribution of metals under varying redox, pH, and salinity gradient (Martino et al. 2002); (iv) flocculation and coagulation of colloidal material (Sholkovitz 1978); (v) biological uptake/regeneration (DeMaster et al. 1986;Windom et al. 1991) and organic matter production by phytoplankton; and (vi) disposal of domestic and industrial wastes (Allen and Rae 1986). The monitoring of estuarine chemistry (particulate and dissolved) as well as the impact of geochemical processes on the metal fluxes are important aspects of marine chemistry to establish the mass balance of chemical elements. ...
The crude oil impurities have a constant detrimental effect on the environment and human health in the marine ecosystem. Thus, over several decades many safe and effective techniques have been used as a remediation method. The remediation method in combination with nanotechnology has been explored by researchers for the eradication of crude oil from the ocean matrices. Studies are going on to understand the interaction between the impurity, microbes, and nanoparticles to see the positive and negative impacts such as some nanoparticles act as toxic for the microbes while others act as stimulants. The marine microorganism used spilled oil as a primary source of energy in a technique known as bioremediation. Other methods such as in situ burning, skimmers, and sorbents can be problematic to use in extreme weather conditions. Nowadays nano-size particles have become an emerging method for the eradication of crude oil and other contaminants from the marine environment. Nanoparticles act as magnetic sorbents, and also show the qualities of emulsifiers by enhancing the bioaccessibility of the oil and giving the surface to the microorganism to which they can attach and ease the spread. The nano-enhanced bioremediation is the most prominent technique used for the eradication of crude oil from the marine environment and has less impact by weather change. The nanoparticle remediation technique gives a sustainable approach rather than traditional methods which are toxic and ineffective. This chapter deals to determine the principles of bioremediation aided by nano-size particles, and their interaction with the marine matrices. The researchers are looking for a balance between essential bioremediation methods in order to make marine life free from organic matter contamination and economic development of nano-sized particles remedy. Hence, this chapter deals with the effective method for the eradication of petroleum oil from marine life and its benefits to human well-being.KeywordsNanoparticleMarine ecosystemBioaugmentationBioremediationCrude oil
... The continental input of chemical elements to the open ocean is significantly affected by the sedimentation and biogeochemical processes occurring in the estuaries. These processes include the following: (i) particle mixing of continental and marine origin (Turner et al. 1994;Turner and Millward 2000); (ii) particle resuspension due to dredging/tidal action (Martino et al. 2002); (iii) phase redistribution of metals under varying redox, pH, and salinity gradient (Martino et al. 2002); (iv) flocculation and coagulation of colloidal material (Sholkovitz 1978); (v) biological uptake/regeneration (DeMaster et al. 1986;Windom et al. 1991) and organic matter production by phytoplankton; and (vi) disposal of domestic and industrial wastes (Allen and Rae 1986). The monitoring of estuarine chemistry (particulate and dissolved) as well as the impact of geochemical processes on the metal fluxes are important aspects of marine chemistry to establish the mass balance of chemical elements. ...
A marine and coastal ecosystem offers various services to human civilisation, such as provisioning, supporting, cultural, and regulating services. These services have a positive impact on human well-being both directly or indirectly through their commodities. However, rising extensive constraints like poaching, water contamination, destruction of coastal habitat, and overall deterioration of biodiversity and biogeochemical processes lead to marine species and their ecosystems being at risk. These threats mainly obstruct the flow of ecological services on a broad scale, whose long-term value lowers the current economic benefits to society. Hence, Sustainable Development Goals (SDGs) are implemented to uplift coastal and marine life for the betterment of society. This chapter deals with marine biota, ecological services, climatic effects on marine species, and SDGs for improving the marine environment.KeywordsMarine biota Marine and Coastal ecosystem services Climate impacts Sustainable Development Goals (SDGs).
... The continental input of chemical elements to the open ocean is significantly affected by the sedimentation and biogeochemical processes occurring in the estuaries. These processes include the following: (i) particle mixing of continental and marine origin (Turner et al. 1994;Turner and Millward 2000); (ii) particle resuspension due to dredging/tidal action (Martino et al. 2002); (iii) phase redistribution of metals under varying redox, pH, and salinity gradient (Martino et al. 2002); (iv) flocculation and coagulation of colloidal material (Sholkovitz 1978); (v) biological uptake/regeneration (DeMaster et al. 1986;Windom et al. 1991) and organic matter production by phytoplankton; and (vi) disposal of domestic and industrial wastes (Allen and Rae 1986). The monitoring of estuarine chemistry (particulate and dissolved) as well as the impact of geochemical processes on the metal fluxes are important aspects of marine chemistry to establish the mass balance of chemical elements. ...
This study addresses a bibliometric analysis of the global research literature and the temporal research record on coastal sediment pollution. Two thousand five hundred publications were exported from the Dimensions database and analyzed by the VOSviewer software tool. The results showed a notable increasing trend in scientific productivity related to coastal sediment pollution recorded since 2007, while the highest number of publications are in 2014 and 2013. Biological, environmental, and earth sciences were the top investigated subject categories. The Marine Pollution Bulletin journal was ranked as the lead in coastal sediment pollution research publications. The analysis based on term occurrences indicates the shift of research interest from studying heavy metals pollution in marine sediments to microplastic pollution. The bibliographic coupling analysis of countries indicated that China is the most active and productive country based on the publication weight (813), citations number (6365), and total link strength (583,176). These analytical investigations provide a global overview of academic publications related to marine sediments pollution, which will be helpful to the researchers in discerning the latest trends and progress in this research domain. However, future investigations are recommended to fill the knowledge gaps on coastal pollution management and its social and economic implications.KeywordsCoastal sediment pollutionGlobal research analysisHeavy metalsMicroplasticDimensionsVOSviewerBibliometric analysis
... Smith and Russel (1995) reported that phytoplankton growth is determined by three factors in the Amazon plume: transparency, stratification, and nutrient availability. Studies on the ACS have revealed the importance of suspended sediment concentrations as a factor limiting the penetration of light and nutrient concentrations (Edmond et al., 1981;DeMaster et al., 1986). The transparency of the water influences the penetration of light into the water column and thus phytoplankton growth. ...
The Amazon Continental Shelf (ACS) has a coastline of ~1000 km and is situated in a humid equatorial region with two distinct seasons, wet and dry, and two transitional climatic periods. A total of eight oceanographic cruises were conducted to investigate the dynamics of the inorganic nutrients dissolved in the surface layer of the water column of the ACS and their associations with Sargassum blooms. An excess of nitrogen-N* (NH4⁺ = 0.01–9.30 μmol l⁻¹) was verified through the salinity gradient from the continent to the ACS, mainly during the period of maximum discharge of the Amazon and Tocantins-Pará rivers. The highest NH4⁺ concentrations coincided with the occurrence of macrophyte (Sargassum sp.) blooms in the mesohaline and oceanic regions. During this period the high values of NH4⁺ accompany the low values of Sea Surface Salinity (SSS), indicating the advection of the Amazonian plume towards the Caribbean Sea. In the Sargassum belts we observed a N:P ratio = 60:1 during wet period, while outside this region, the N:P ratio was 34:1. According to DIN vs. AOU ratio, we observed that heterotrophic processes prevail over productive processes, mainly in the mesohaline and low salinity regions.
Nitrogen concentrations from runoff, mainly from hydrographic basins, showed significant increases in the last decade, mainly due to anthropogenic factors such as agriculture, deforestation, livestock, urban sewage (domestic and industrial), and demographic growth of the population in the Tocantins-Pará Basin. Additionally, associations of diatoms and cyanobacteria provide more nitrogen in the mesohaline and oceanic regions that nourishes Sargassum blooms. Seasonal comparisons (October and April) of Sargassum belts with other studies, indicated that currents play a fundamental role in the transport of these macrophytes from the east edge (Africa) to the west edge (Brazil and French Guiana) towards the Caribbean Sea in the tropical Atlantic.
... remobilized as stationary fluid muds, and/or resuspended and laterally transported as dense suspensions 11,12 . Particle reworking and remobilization can result in loss of chronological information [13][14][15] and is highly heterogeneous spatially and temporally. Thus, the distribution and thickness of the SML are important to improve global flux estimates of dissolved and particulate constituents of key biogeochemical cycles in Earth System Models 16 . ...
The sediment-water interface in the coastal ocean is a highly dynamic zone controlling biogeochemical fluxes of greenhouse gases, nutrients, and metals. Processes in the sediment mixed layer (SML) control the transfer and reactivity of both particulate and dissolved matter in coastal interfaces. Here we map the global distribution of the coastal SML based on excess 210Pb (210Pbex) profiles and then use a neural network model to upscale these observations. We show that highly dynamic regions such as large estuaries have thicker SMLs than most oceanic sediments. Organic carbon preservation and SMLs are inversely related as mixing stimulates oxidation in sediments which enhances organic matter decomposition. Sites with SML thickness >60 cm usually have lower organic carbon accumulation rates (<50 g C m−2 yr−1) and total organic carbon/specific surface area ratios (<0.4 mg m−2). Our global scale observations reveal that reworking can accelerate organic matter degradation and reduce carbon storage in coastal sediments. The authors map the global distribution of the mixed layer in coastal ocean sediments, based on a neural network model. These observations reveal that mixing can accelerate organic matter degradation and reduce carbon storage in the coastal ocean.
... In the upper core section, Pb is found in cluster-1 (Figs. 5(a) and (b)) of Mn and organic matter proxies (LOI 550 , TOC, TN, and P), which indicates a major anthropogenic source of Pb owing to its particle reactive nature (Demaster et al., 1986;Santschi et al., 1980). Previously, Ayyamperumal et al. (2006) demonstrated the incorporation of anthropogenic Pb in Mn oxide fractions of lacustrine sediment from the southeast coast of India. ...
Lacustrine sediment preserves high-resolution biogeochemical records of past variations in watershed processes controlling lake sedimentation. The current study explores historical variations in autochthonous and allochthonous sediment supplies to a large tropical freshwater lake system (Upper Lake, Bhopal) protected under the international Ramsar Convention of 2002 against anthropogenic pressures. For this purpose, multi-proxy biogeochemical data are presented for organic matter (total organic carbon, total nitrogen, phosphorous, and loss on ignition [LOI] at 550 °C), carbonate (LOI at 950 °C), lithic sediment (aluminum, titanium, iron, calcium, magnesium, sodium, potassium, manganese, zirconium, niobium, hafnium, tantalum, thorium, uranium, and rare earth elements), and anthropogenic inputs (lead) measured in a 38 cm long sediment core retrieved from the lake. In addition to the lacustrine sediment core, the samples of catchment bedrock, surface soils, major stream sediment, and eolian dust collected from the lake periphery also are analyzed. The systematic biogeochemical excursions in the upper core section (top ∼8 cm) indicate increased anthropogenic inputs, watershed denudation by agricultural activities, artificially reduced fluvial sediment supply, relatively increased dust inputs and lake eutrophication in the last few decades. The current study underscores the roles of anthropogenic land-use and wetland conservation practices in the rapid alteration of autochthonous and allochthonous sediment supplies to open aquatic ecosystems. Further, rising lake eutrophication levels despite a managed reduction in allochthonous sediment supplies seem challenging to control due to dissolved nutrient supply from urban sewage discharge and runoff from agricultural land in the watershed.
... The aggregation and settling down of frustules aggregate through the water column leads to the accumulation of silica in sediment (Passow et al., 2003). Such enrichment of silica at estuarine mouth have been reported in several previous studies (DeMaster et al., 1986;Nantke et al., 2019). Flocculation of particulate and colloidal SiO 2 in saline water and their settlement as bottom sediments may also be an explanation for its enrichment at the estuary mouth (Paudel et al., 2015). ...
Multi-element composition including rare earth elements (REE) of surface sediment from the Devi river estuary, eastern coast of India, have been analysed in order to study the weathering characteristics and provenance of sediment along with their behaviour under different physicochemical conditions. These sediments with dominantly felsic provenance have undergone low to moderate chemical alterations. Bulk chemical composition is mainly represented by SiO2, Al2O3, Fe2O3 and K2O. Concentrations of Ba, Nb, Pb, Rb, Th and Zr are above their respective upper crustal abundances. High LREE/HREE ratio, negative Eu anomalies, and (La/Yb)n and (Tb/Yb)n value confirm that sediments are dominantly derived from the Eastern Ghat Group of rocks. Upper estuary sediments show negative Eu anomalies which is similar to that of the source. However, positive Eu anomaly is mostly observed in lower estuary. Contrasting Eu anomalies between upper- and lower-estuarine sediments are uncharacteristic of previously studied major global estuaries. Strong negative correlation between Mn and Eu suggests control of redox conditions over distribution of Eu. Concentration of REEs, Sc, Fe, Mo, V, Zn, Zr, Nb, U, Ti, Na and P increases up to 20 ppt salinity, and followed by declining trend towards mouth. This is mostly due to removal through flocculation of colloidal particles from water column during fresh- and saline-water interaction. This could be the first report about coagulation-based behaviour of Mo in estuarine environment. There is gradual decline in concentration of Cr, Co, Ni, Cu, Rb, Sr, Sb, Cs, Ba, Pb, Al, Mn, Mg, Ca and K with increase in salinity which is attributed to saline induced desorption of elements from sediments. The SiO2 content shows increasing trend towards mouth. Findings of this study highlight the importance of intrinsic physicochemical parameters, mainly salinity and redox condition, on governing geochemical behaviour of different elements including REE in mangrove dominated estuarine sediment.
... Smith and Russel (1995) reported that phytoplankton growth is determined by three factors in the Amazon plume: transparency, stratification, and nutrient availability. Studies on the ACS have revealed the importance of suspended sediment concentrations as a factor limiting the penetration of light and nutrient concentrations (Edmond et al., 1981;DeMaster et al., 1986). The transparency of the water influences the penetration of light into the water column and thus phytoplankton growth. ...
... In general, the concentration of SiO 4 is proportional to the amount of river discharge, sediment discharge, and SSC. For instance, earlier studies have reported that with the increase in SSC, SiO 4 concentration also increases and vice-versa (Demaster et al. 1986;Treguer and De La Rocha 2013). However, in contrast to the earlier reports, higher SiO 4 concentration with declining SSC levels was observed in MRE during the aforementioned period, which point towards the potential marine source. ...
Long-term water quality observations are mainly lacking in the Indian estuaries. Mahanadi River Estuary (MRE), formed at the mouth of India’s third-largest peninsular river, is experiencing anthropogenic stress. The present study evaluates the variability of water quality parameters (physical, chemical, and biological) over 23 years (1991–2013). This study also assessed the seasonality of water quality parameters with a tidal variation. The salinity level of the MRE was predominantly controlled by river discharge with higher salinity during high tide and vice versa. Dissolved oxygen and biochemical oxygen demand levels were within the permissible limit during the observation period. Inorganic macronutrients showed no clear trend. However, a gradual increase in silicate concentration was observed over the years. Variabilities in phosphate and ammonium concentration indicate point-source pollution from the nearby industries and townships. Phytoplankton and zooplankton did not show any specific trend over the study period in the MRE. Prominent variability in zooplankton abundances was observed over the tidal phase with a maximum spread in range during the high tide of monsoon and post-monsoon. Like zooplankton, all water quality parameters showed tidal variation that, in turn, varied across different seasons. Even though there is evidence of point-source pollution in the MRE, however, any substantial deterioration of water quality was not observed, possibly due to the high river runoff and quick flushing of water out of the estuary.
... This suggests potential variation in algal growth rates and export production unrelated to changes in stratification. At the Site 925 setting, the Amazon River plume represents another source of nutrients to the upper photic zone, which are distributed in the upper 50 m of water column (e.g., Boyle et al., 1977;DeMaster et al., 1986). The magnetic susceptibility at Site 925, inferred as an indicator of Amazon-derived detrital magnetic minerals (Francois and Bacon, 1991), is significantly correlated to N Gephyrocapsa (R = 0.68 / p ≤ 0.05; Fig. 6) and has a correspondence with the percentages of Gephyrocapsa (Fig. 3e and d). ...
The sensitivity of coccolithophores to changing CO2 and its role modulating cellular photosynthetic carbon isotopic fractionation (εp) is crucial to understand the future adaptation of these organisms to higher CO2 world and to assess the reliability of εp for past CO2 estimation. Here, we present εp measured on natural fossil samples across the glacial‐interglacial (G‐I) CO2 variations of marine isotope stages 12 to 9 interval (454–334 ka) at the western tropical Atlantic Ocean Drilling Program Site 925 together with a set of organic and inorganic geochemical, micropaleontological and morphometrical data from Gephyrocapsa coccoliths in the same samples. The ∼2‰ variation in εp is significantly correlated with the CO2[aq] concentrations calculated from assumption of air‐sea equilibrium with measured ice core pCO2 concentrations. The sensitivity of εp to CO2[aq] is similar to that derived from a multiple regression model of culture observations and is not well simulated with the classical purely diffusive model of algal CO2 acquisition. The measured range of Gephyrocapsa cell sizes is insufficient to explain the non‐CO2 effects on εp at this location, via either direct size effect or growth rate correlated to cell size. Primary productivity, potentially triggered by shifting growth rates and light levels, may also affect εp. Proposed productivity proxies % Florisphaera profunda and the ratio between the C37 to C38.et alkenone (C37/C38.et ratio) both correlates modestly with the non‐CO2 effects on εp. When the observed G‐I εp to CO2 sensitivity at this site is used to estimate pCO2 from εp since the Miocene, the inferred pCO2 declines are larger in amplitude compared to that calculated from a theoretical εp diffusive model. We find that oxygen and carbon stable isotope vital effects in the near monogeneric‐separated Gephyrocapsa coccoliths (respectively Δδ¹⁸OGephyrocapsa–Trilobatus sacculifer and εcoccolith) are coupled through the time series, but the origins of these vital effects are not readily explained by existing models.
... The optimal-growth zone of phytoplankton on the Amazon shelf occurs mainly seaward of the high-turbidity plume as well as shoreward of the nutrient-depleted offshore water. DeMaster et al. (1983) and DeMaster et al. (1986) reported that the nutrient uptake happens when the turbidity decreases, and the blooms occurred shoreward of the 33 isohaline. Smith and DeMaster (1996) showed that phytoplankton photosynthesis in waters influenced by the Amazon River is limited by low levels of available subsurface irradiance, particularly in the reaches of the plume closest to the river influence . ...
Estimations of the global carbon budget include a quantitative understanding of the evolving processes that occur along river-to-ocean gradients. However, high spatiotemporal resolution observations of these processes are limited. Here we present in situ measurements of the partial pressure of CO2 (pCO2) made through the Amazon River plume (ARP) during different discharge seasons, from 2010 to 2012. We evaluated the spatiotemporal distribution of pCO2 using Soil Moisture and Ocean Salinity (SMOS) satellite observations for each hydrologic period in the ARP. Regression models were used to estimate pCO2 at the ARP for the period of 2010–2014. From these distributions we calculated sea-air gas exchange of CO2 between the plume waters and the atmosphere (Fco2sea). Intra-annual variability of Fco2sea was related to discharge at the river mouth and ocean currents as well as trade winds in the plume. Climatic events during the study period had a significant impact on the Fco2sea. Including the plume area closer to the river mouth makes the ARP a net source of CO2 with an annual net sea-air flux of 8.6 ± 7.1 Tg C y⁻¹ from 2011 to 2014.
... In estuaries and coastal systems, river discharge containing nutrients and suspended sediments can either positively or negatively influence Chl-a. The positive and negative relationship between river discharge and Chl-a is largely dependent on the dominant influence of either nutrients or irradiance on phytoplankton growth [30][31][32][33][34][35]. ...
We investigated the spatio-temporal variability of chlorophyll-a (Chl-a) and total suspended matter (TSM) associated with spring-neap tidal cycles in the Ariake Sea, Japan. Our study relied on significantly improved, regionally-tuned datasets derived from the ocean color sensor Moderate Resolution Imaging Spectroradiometer (MODIS) Aqua over a 16-year period (2002-2017). The results revealed that spring-neap tidal variations in Chl-a and TSM within this macrotidal embayment (the Ariake Sea) are clearly different regionally and seasonally. Generally, the spring-neap tidal variability of Chl-a in the inner part of the Ariake Sea was controlled by TSM for seasons other than summer, whereas it was controlled by river discharge for summer. On the other hand, the contribution of TSM to the variability of Chl-a was not large for two areas in the middle of Ariake Sea where TSM was not abundant. This study demonstrates that ocean color satellite observations of Chl-a and TSM in the macrotidal embayment offer strong advantages for understanding the variations during the spring-neap tidal cycle.
... Chen et al. (2006a,b) conclude that tide-induced sediment resuspension ultimately controls the concentration of TSM. It has also been found that photosynthesis is enhanced when the concentration of TSM is < 10 mg L −1 in the plumes of the Changjiang and Amazon River (DeMaster et al., 1986;Ning et al., 1988;DeMaster and Pope, 1996). At our study site, we did not measure Chl a during 2010-2011. ...
... On the western side of the transect, between summer and late fall, an important part of the northward flowing North Brazil Current (NBC) is retroflected off the South American northeastern margin towards the east into the western tropical North Atlantic after crossing the equator (Philander, 2001), feeding into the NECC (Richardson and Walsh, 1986). The retroflection of the NBC then forces the Amazon River plume to be displaced eastwards in the uppermost~50 m of the water column (Boyle et al., 1977;De Master et al., 1986), carrying nutrientrich waters far into the central equatorial North Atlantic (Muller-Karger et al., 1988;Ffield, 2005;Molleri et al., 2010;Varona et al., 2019). During boreal winter, the southward migration of the ITCZ causes the weakening of the trade winds along the equator and the ceasing of the NBC retroflection (Johns et al., 2004;Philander, 2001;Varona et al., 2019). ...
Tropical oceans provide a benchmark for future primary productivity in increasingly warmer, stratified and nutrient-depleted waters. In this context, we assess the export fluxes of calcifying phytoplankton (coccolithophores) across the tropical North Atlantic, from upwelling affected NW Africa, via three ocean sites along 12°N to the Caribbean. Sampling was undertaken by means of a spatial array of four time-series sediment traps collecting particle fluxes in two-week intervals, from October 2012 to February 2014, allowing to track temporal changes along the southern margin of the North Atlantic central gyre. Species composition and seasonal export fluxes of coccolithophores show steep gradients in two groups. Upper photic zone (UPZ) and placolith-bearing species dominated by Emiliania huxleyi and Gephyrocapsa oceanica are most abundant in the mesotrophic surface waters of the Cape Blanc upwelling system off NW Africa. They decline gradually towards the Caribbean, paralleled by increasing surface temperatures and decreasing surface chlorophyll-a. Meanwhile the abundance of lower photic zone (LPZ) species Florisphaera profunda and Gladiolithus flabellatus increase in the same direction, reaching fluxes up to 3–5 times higher in the western end of the transect compared to the UPZ flora in mesotrophic waters. Adapted to low light conditions, the LPZ species follow the geostrophic wind-forced deepening of the thermocline/nutricline westward in ever lower species diversity towards the Caribbean.
Temporal changes were marked by weak seasonality in coccolith fluxes at all four sites, modulated by latitudinal migration of the Intertropical Convergence Zone (ITCZ) at the tropical sites M1, M2 and M4, and by spatiotemporal variation in wind-forced upwelling at site CB. A seasonal mismatch was observed between LPZ and UPZ-oligotrophic taxa (i.e. Umbellosphaera spp., Rhabdosphaera spp.) vs. UPZ-opportunistic species (E. huxleyi, G. oceanica) at the western site M4 contrasting with the more similar seasonal patterns amongst all species towards site CB. We interpret this as reflecting the entire photic zone becoming increasingly nutrient enriched towards east whenever wind-forced mixing occurs due to the eastward shoaling of the thermocline/nutricline. Our synoptic observations of seasonally resolved export fluxes at four sites contribute to the spatiotemporal understanding of coccolithophore fluxes across the entire tropical North Atlantic, urging for considering phytoplankton- and carbonate production across the entire photic zone when projecting the effects of ocean warming on future primary production.
... In contrast with the nitrogen and phosphorus cycles, which involve biological remineralization, the production of silicates in aquatic environments is basically a physical-chemical process. The Amazon River discharges approximately 10% of the total amount of silicates of fluvial origin deposited into the world's oceans (DeMaster et al. 1986), being other local rivers on the Amazon coast are also important sources of this material (Pereira et al. 2010;Pamplona et al. 2013). In general, a clear relationship is also found between high silicate concentrations and the increase in chlorophyll-a (Pereira et al. 2012). ...
Despite its ecological and economical importance, the Brazilian North Coast (BNC) or Amazonia Ecoregion sensu Spalding et al. (Bioscience 57:573–583, 2007), was poorly studied until the 1980s, when major interdisciplinary coastal research programs began. The Amazon and other major rivers strongly influence the BNC, causing seasonally reduced surface salinity and significant sediment deposition. From February to March, monthly accumulated rainfall is 400 mm or more and estuarine salinity varies between 0 and 10, but reaches over 40 in the dry season. Mangrove vegetation, with narrow seaward fringes of salt marsh, dominates the BNC. Macrotides between 4 and 7 m expose large areas of muddy to sandy sediments at low tide. Tidal amplitude is twice as large during spring tides, inundating large areas of mangrove. Tidal export, as well as riverine discharge, determines BNC dissolved nutrient profiles. Despite high turbidity, BNC estuaries have high phytoplankton biomasses and washout of benthic microalgae may also contribute to high chlorophyll-a concentrations. Though benthic diversity is low, secondary productivity in sediments is high, and important for nutrient cycling, especially in mangrove forest. The uçá-crab (Ucides cordatus) is economically important in the region. Strong gradients in salinity, along both the coast and individual estuaries, determine the relative abundance of freshwater and marine benthic taxa but abundance and diversity are lower in the wet season. Although relatively conserved, there is increasing pressure on the BNC through urban expansion, organic pollution, mangrove logging, and over-harvesting of coastal resources. With only 17 protected and especially managed areas along the BNC, the implementation of conservation policies is, so far, unsatisfactory.
... The western equatorial North Atlantic where station M4 was located is also seasonally influenced by the Amazon River, the world's largest river with respect to freshwater discharge into the open ocean (Mann and Lanzier, 2006). From August to December, when the retroflection of the NBC carries the river plume eastward in the uppermost ∼ 50 m of the water column, large amounts of nutrients (Boyle et al., 1977;Sholkovitz et al., 1978;De Master et al., 1986) are entrained into the NECC far into the central equatorial North Atlantic (Muller-Karger et al., 1988;Molleri et al., 2010;Ffield, 2005;Fig. 1). ...
Coccolithophores are calcifying phytoplankton and major contributors
to both the organic and inorganic oceanic carbon pumps. Their export
fluxes, species composition, and seasonal patterns were determined in
two sediment trap moorings (M4 at 12° N, 49° W and
M2 at 14° N, 37° W) collecting settling particles
synchronously from October 2012 to November 2013 at 1200 m
of water depth in the open equatorial North Atlantic.
The two trap locations showed a similar seasonal pattern in total
coccolith export fluxes and a predominantly tropical coccolithophore
settling assemblage. Species fluxes were dominated throughout the
year by lower photic zone (LPZ) taxa (Florisphaera
profunda, Gladiolithus flabellatus) but also included
upper photic zone (UPZ) taxa (Umbellosphaera spp.,
Rhabdosphaera spp., Umbilicosphaera spp.,
Helicosphaera spp.). The LPZ flora was most abundant during
fall 2012, whereas the UPZ flora was more important during
summer. In spite of these similarities, the western part of the
study area produced persistently higher fluxes, averaging 241×107 ± 76×107 coccoliths m−2 d−1 at station M4 compared to
only 66×107 ± 31×107 coccoliths m−2 d−1 at station M2. Higher fluxes
at M4 were mainly produced by the LPZ species, favoured by the
westward deepening of the thermocline and nutricline. Still, most
UPZ species also contributed to higher fluxes, reflecting enhanced
productivity in the western equatorial North Atlantic. Such was the
case of two marked flux peaks of the more opportunistic species
Gephyrocapsa muellerae and Emiliania huxleyi in
January and April 2013 at M4, indicating a fast response to
the nutrient enrichment of the UPZ, probably by wind-forced
mixing. Later, increased fluxes of G. oceanica and
E. huxleyi in October–November 2013 coincided with the
occurrence of Amazon-River-affected surface waters. Since the spring
and fall events of 2013 were also accompanied by two dust flux peaks,
we propose a scenario in which atmospheric dust also provided
fertilizing nutrients to this area. Enhanced surface buoyancy
associated with the river plume indicates that the Amazon acted not
only as a nutrient source, but also as a surface density retainer
for nutrients supplied from the atmosphere. Nevertheless, lower
total coccolith fluxes during these events compared to the maxima
recorded in November 2012 and July 2013 indicate that transient
productivity by opportunistic species was less important than
background tropical productivity in the equatorial North
Atlantic. This study illustrates how two apparently similar sites in
the tropical open ocean actually differ greatly in ecological and
oceanographic terms. The results presented here provide valuable
insights into the processes governing the ecological dynamics and
the downward export of coccolithophores in the tropical North
Atlantic.
... Riverine outflows enhance new primary production by providing nutrient inputs and by favoring the stratification of surface waters due to significant salinity gradients (Eppley and Peterson, 1979). DeMaster et al. (1986) showed that the high particle concentration within the Amazon River plume also scavenges particle-reactive species from open ocean waters. ...
Th-derived carbon export fluxes were measured in the Atlantic Ocean under the GEOTRACES framework to evaluate basin-scale export variability. Here, we present the results from the northern half of the GA02 transect, spanning from the equator to 64°N. As a result of limited site-specific C/²³⁴Th ratio measurements, we further combined our data with previous work to develop a basin wide C/²³⁴Th ratio depth curve. While the magnitude of organic carbon fluxes varied depending on the C/²³⁴Th ratio used, latitudinal trends were similar, with sizeable and variable organic carbon export fluxes occurring at high latitudes and low to negligible fluxes occurring in oligotrophic waters. Our results agree with previous studies, except at the boundaries between domains, where fluxes were relatively enhanced. Three different models were used to obtain satellite-derived net primary production (NPP). In general, NPP estimates had similar trends along the transect, but there were significant differences in the absolute magnitude depending on the model used. Nevertheless, organic carbon export efficiencies were generally < 25%, with the exception of a few stations located in the transition area between the riverine and the oligotrophic domains and between the oligotrophic and the temperate domains. Satellite-derived organic carbon export models from Dunne et al. (2005) (D05), Laws et al. (2011) (L11) and Henson et al. (2011) (H11) were also compared to our ²³⁴Th-derived carbon exports fluxes. D05 and L11 provided estimates closest to values obtained with the ²³⁴Th approach (within a 3-fold difference), but with no clear trends. The H11 model, on the other hand, consistently provided lower export estimates. The large increase in export data in the Atlantic Ocean derived from the GEOTRACES Program, combined with satellite observations and modeling efforts continue to improve the estimates of carbon export in this ocean basin and therefore reduce uncertainty in the global carbon budget. However, our results also suggest that tuning export models and including biological parameters at a regional scale is necessary for improving satellite-modeling efforts and providing export estimates that are more representative of in situ observations.
... The western equatorial North Atlantic where station M4 was located, is also seasonally influenced by the Amazon River, the world's largest river system which is known for discharging large volumes of fresh water into the open ocean each year (Mann and Lanzier, 2006). From August to December, when the retroflection of the NBC carries the river plume eastwards in the uppermost ~50 m of the water column, large amounts of nutrients (Boyle et al., 1977;Sholkovitz et al., 1978;De Master et al., 1986) are entrained into the NECC far into the central equatorial North Atlantic (Muller-Karger et al., 1988;Molleri et al., 5 2010;Ffield, 2005) (Fig. 1). From August to October, the Amazon plume was observed to reach speeds of ~90 cm/s near 45º W with a gradual slowdown to 30 cm/s while moving eastwards. ...
Coccolithophores are calcifying phytoplankton and major contributors to both the organic and inorganic oceanic carbon pumps. Their export fluxes, species composition and seasonal patterns were determined in two sediment trap moorings in the open equatorial North Atlantic (M4 at 12ºN 49ºW and M2 at 14ºN 37ºW), which collected settling particles synchronously in successive 16-day intervals from October 2012 to November 2013, at 1200 m water depth. The two trap locations show a similar seasonal pattern in total coccolith export fluxes and a predominantly tropical coccolithophore settling assemblage throughout the monitored year. Species fluxes were yearlong dominated by lower photic zone (LPZ) taxa (Florisphaera profunda, Gladiolithus flabellatus), but also included upper photic zone (UPZ) taxa (Umbellosphaera spp., Rhabdosphaera spp., Umbilicosphaera spp., Helicosphaera spp.). The LPZ flora was most abundant during fall 2012, whereas the UPZ flora was more important during summer. In spite of these similarities, the western part of the study area produced persistently higher fluxes, averaging 241x10 7 coccoliths m-2 d-1 (117x10 7 to 423 x10 7 coccoliths m-2 d-1) at station M4, compared to only 66x10 7 coccoliths m-2 d-1 (25x10 7 to 153x10 7 coccoliths m-2 d-1) at station M2. Higher fluxes at M4 were mainly produced by the LPZ species, although most UPZ species also contributed higher fluxes, reflecting enhanced productivity in the western equatorial North Atlantic. In addition, we found two marked flux peaks of the more opportunistic species Gephyrocapsa muellerae and Emiliania huxleyi indicating a fast response to nutrient-enrichment of the UPZ, probably by wind-forced mixing, whereas increased fluxes of G. oceanica and E. huxleyi in October/November 2013 coincided with the occurrence of Amazon River affected surface waters. Since the spring and fall events of 2013 were also 30 accompanied by two dust flux peaks we propose a scenario where atmospheric dust also provided fertilizing nutrients to this area. Enhanced surface buoyancy associated to the river plume indicates that the Amazon acted not only as a nutrient source, but also as a surface density retainer for nutrients supplied from the atmosphere. Still, lower total coccolith fluxes during these events compared to the maxima recorded in November 2012 and July 2013 indicate that transient productivity by opportunistic species was less important than " background " tropical productivity in the equatorial North Atlantic. This study illustrates how two seemingly similar sites in an open-ocean tropical setting actually differ greatly in ecological and oceanographic terms, and provides valuable insights into the processes governing the ecological dynamics and the downward export of coccolithophores in the tropical North Atlantic.
... The spatial patterns of phytoplankton biomass generally show a maximum at intermediate salinities due to declining turbidity in the presence of high nutrient levels ( Bode and Dortch, 1996;DeMaster et al., 1996;Dagg et al., 2004). This mid-salinity maximum in phytoplankton biomass has been reported for various large rivers ( DeMaster et al., 1986;Smith and DeMaster, 1996;Lohrenz et al., 1990;Lohrenz et al., 1999;Turner et al., 1990), including the Changjiang River ( Ning et al., 1988;Tian et al., 1993). The location of this maximum along the salinity gradient is determined by the salinity at which suspended matter concentrations decline to a level to allow sufficient light to enhance phytoplankton growth ( Edmond et al., 1985;Ning et al., 1988;Turner et al., 1990). ...
Harmful algal blooms (HABs) have been increasingly frequent in coastal waters around the world over the last several decades. Accelerated coastal eutrophication, resulting from the increased anthropogenic loadings of nutrients, is commonly assumed to be the primary cause of this increase. However, although important, accelerated coastal eutrophication may not be the only explanation for the increasing blooms or toxic outbreaks in estuarine waters. Changes in riverine material fluxes other than nutrients, such as sediment load, may significantly affect biological activities and HAB incidence in estuarine and coastal waters. A case study off the Changjiang (Yangtze River) Estuary indicated that with the increasing riverine loadings of nutrients, the sediment load from the Changjiang River has been reduced by 70% over the past four decades. A comparison of long-term data revealed that the phytoplankton biomass maximum has expanded to a region of much lower salinity due to the drastic reduction in riverine sediment load and the subsequent improvement in light penetration in the Changjiang River plume. Furthermore, there was an apparent mirror-image relationship between the sediment load from the Changjiang River and the HAB incidence off the Changjiang Estuary over the past four decades, and the number of HAB incidents was significantly negatively correlated with the sediment load. Therefore, it is argued that the drastic decline in sediment load from the Changjiang River reduced turbidity in the Changjiang Estuary and thus contributed to the increased frequency of HABs in the buoyant discharge plumes.
... Suspended riverine material has an excess 210 Pb activity ranging from 600 to 2100 ± 360 dpm kg −1 (10-35 ± 6 Bq kg −1 ), and activities of ex- cess 210 Pb from the seabed of the Amazon shelf range from 20,010 to 120,000 dpm kg −1 (333.5-2000 Bq kg −1 ) ( DeMaster et al., 1986;Smoak et al., 1996). ...
... In the coastal and shelf areas of River-Dominated Ocean Margins (RiOMars) system, ''mobile mud'' (MM), mainly comprised of sedimentary particles, with a size of 63 lm and high water content (50%), plays a major role in the transport, deposition, and eventual burial of river-derived sedimentary material and associated particle-reactive species [DeMaster et al., 1986;Milliman, 1991;Kineke and Sternberg, 1995;Aller, 1998]. Due to physical (e.g., waves and tide-generated currents) and biological (bioturbation) forcing, the sedimentary particulate matter may undergo several cycles of deposition-resuspension-transport processes in which the sorption/desorption of key micro and macronutrients and other redox-sensitive trace species could impact the biogeochemical cycling in the coastal and shelf areas [Aller, 1998;Aller et al., 2004]. ...
"Mobile mud" (MM), which has fine grain size distribution (>90% clay + silt, and <5% sand) and high porosity (≥0.50), plays an important role in the biogeochemical cycles in the estuarine areas and the inshore shelf. A suite of MM samples from the coastal area of the East China Sea (ECS) was collected in spring and summer of 2011 to observe their spatial and temporal distribution, grain size, and radionuclides concentrations. The MM thickness ranged from 0.5 to 11 cm (average: 2.2 cm (May) and 3.9 cm (August)). The thick mud layer is mainly distributed along the coast, with an area of 2.2 × 104 km2 in May and 1.5 × 104 km2 in August, with corresponding masses of 8.8 × 108 and 7.8 × 108 t, respectively. The estimated masses of MM are considerably larger than the annual sediment discharge mass of the Changjiang River. The distribution of 137Cs inventories in MM indicates that 137Cs can be effectively utilized as a transport tracer of MM in the river-dominated estuaries and coastal areas. The higher inventories of 7Be in MM in the river mouth in spring are attributed to higher depositional flux and higher sediment discharge. The ratio of the MM inventory of 234Thex/production in the overlying water column of >2.5 in south inshore indicates that the sediment focusing resulted in the increased mass flux. The residence time of MM is estimated as 3-6 years both by mass balance of MM and 210Pbex in MM.
... If the area-wide average activity of ''new'' sediments is about 2000 -2500 dpm kg À 1 , then the inventory of excess 210 Pb (f 40 -50 dpm cm À 2 in the upper 30 cm alone) exceeds what can be accounted for in fallout. DeMaster et al. (1986) determined that 210 Pb atmospheric fluxes on the nearby Amazon shelf support an excess inventory of about 10.5 dpm cm À 2 in the seabed. One source for the large excess 210 Pb in the Sinnamary sediments are particles delivered directly from the Amazon River via the Guianas Current. ...
... Carbon and nutrient dynamics in the Amazon system (e.g., Ertel et al., 1986;Hedges et al., 1986Hedges et al., , 1994Moreira-Turcq et al., 2013;Richey et al., 1990;Ward et al., 2013) and the adjacent coastal shelf (e.g., DeMaster et al., 1986Edmond et al., 1981;Sholkovitz et al., 1978;Subramaniam et al., 2008) have been studied extensively in the past, but surprisingly few studies have been performed across the Amazon River-to-ocean-continuum. Connecting the Amazon River with its plume is vital, however, to establish a comprehensive carbon budget across the terrestrial and marine ecosystems. ...
Abstract Coastal oceans link terrestrial and marine carbon cycles. Yet, carbon sources and sinks in these biomes remain poorly understood. Here, we explore the dynamics of dissolved organic matter (DOM) along the Amazon River-to-ocean continuum from the lower mainstem at Óbidos to the open ocean of the western tropical North Atlantic. We molecularly characterized DOM via ultrahigh-resolution Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR-MS), determined DOM stable carbon isotopes, and interpreted the data in the context of bacterial abundance and production, phytoplankton biomass and composition. Multivariate analysis revealed that the DOM molecular variability in the plume was mainly influenced by the input of terrigenous DOM. Incubation experiments with water from close to the river mouth showed that photo- and bio-degradation preferentially removed 13C-depleted and 13C-enriched terrigenous DOM, respectively. However, there was no significant quantitative change in the total amount of dissolved organic carbon (DOC) over five days. This result suggests that most of the reactive DOM had already been bio-degraded upstream within the river and that photo-degradation was diminished in the turbid plume close to the river mouth. Terrigenous DOM therefore appeared to be relatively non-reactive nearshore. In the less turbid offshore plume, enhanced light penetration stimulated growth of phytoplankton and increased bacterial production. Although marine DOM compounds became relatively enriched, bulk DOC concentrations were 9 to 30 % below levels expected from conservative mixing of river and ocean endmembers suggesting that quantitative removal of terrigenous DOM was not compensated by marine DOM production. We propose that removal of terrigenous DOM in the outer plume may be enhanced by (i) bio-degradation primed by reactive algal DOM, (ii) photo-degradation, which may further break down DOM into more bio-available forms, and possibly (iii) sorption of DOM to sinking particles.
... Over the last few decades researchers have begun to demonstrate that lithofacies belts can also be recognized in muddy depositional settings on shelves and that the interplay of a similarly complex suite of processes, including suspension settling (e.g. DeMaster et al. 1986;Nittrouer et al. 1986;Alldredge & Gotschalk 1990), bottom-water anoxia (e.g. Wignall & Hallam 1991;Wignall et al. 2005), waveenhanced gravity flows of fluid mud (e.g. ...
Few studies describe and compare the transport mechanisms operating to disperse mud in different parts of basins. Instead, the physical processes operating to disperse mud in offshore environments, where storm and tidal processes are interpreted to dominate, are generally considered in isolation from those occurring in basinal settings where changes in bottom-water anoxia and suspension settling from buoyant plumes are mostly interpreted to dominate. Using microtextural, mineralogical and geochemical data derived from the analyses of 151 thin sections obtained from the Lower Jurassic mudstone-dominated succession exposed on the coast of NE England, we investigate how varying sediment dispersal mechanisms, bioturbation and early diagenesis operated to produce the lithofacies variability observed. In particular, we consider the processes of sediment delivery while bottom waters were interpreted to be euxinic. Analyses of these samples reveal that the succession is highly variable at millimetre to centimetre scales. Six main lithofacies were observed: (1) sand- and clay-bearing, silt-rich mudstones; (2) silt-bearing, clay-rich mudstones; (3) clay-rich mudstones; (4) clay-, calcareous nannoplankton-, and organic carbon-bearing mudstones; (5) fine-grained muddy sandstones; (6) cement-rich mudstones. These units are organized typically into stacked successions of sharp-based, normally graded, thin (< 10 mm) beds. Single beds exhibit a variety of sedimentary structures. Specifically tempestites, wave-enhanced sediment gravity flows of fluid mud, ripples and gutter casts are common in the coarser-grained mudstone facies. In contrast, thin siltstone lags, compacted ripples and organo-mineralic aggregates are common in the finer grained mudstone facies and those with significant primary production-derived components. Bioturbation is common throughout. These data indicate that sediment was transported by density flows and traction currents operating at the sediment-water interface in all parts of the studied succession. Bioturbation has overprinted depositional fabrics in the majority of samples. The extent to which persistent bottom-water anoxia and low-energy suspension settling influenced lithofacies variability in the basinal parts of the succession has been overstated; these environments were more dynamic than most researchers have previously concluded.
... Coastal ocean plays an important role in global material cycling (Holligan and Boois, 1993). And the importance of suspended sediment transport in coastal environment has been widely recognized, since the dispersal and accumulation of suspended sediment have profound influences on physical, chemical and biological processes in the coastal ocean (DeMaster et al., 1986;Kuehl et al., 1996). The process of sediment transport in the coastal ocean is mostly dominated by both riverine and ocean dynamics that include river discharge, mixing of fresh and salt waters, and the tidal and wave forcings. ...
... Amazon coastal waters typically have high phytoplankton biomass (Edmond et al., 1981;DeMaster et al., 1986;Santos et al., 2008). In the present study, the highest chlorophyll a concentrations were recorded during the wet season, developed during the period of increasing fluvial discharge and elevated dissolved nutrient concentrations, peaking in March during the La Niña event. ...
... Although phytoplankton biomass is generally higher within the plume (defined as waters with salinity <35) than in the surrounding oceanic waters, the gradient of environmental conditions (light, nutrients, CDOM, salinity, temperature, mixed layer depths, etc.) that evolve (Berhane et al., 1997;Curtin and Legeckis, 1986;Del Vecchio and Subramaniam, 2004;DeMaster, 1996;DeMaster and Pope, 1996;Gibbs, 1976a;Lentz, 1995aLentz, , 1995bMolleri et al., 2010b) as the plume meanders northwards and mixes with the waters of the WTNA Ocean appears to have a profound impact on the magnitude and composition of phytoplankton communities of the Amazon River plume continuum (Carpenter et al., 1999;DeMaster et al., 1986;Santos et al., 2008;Shipe et al., 2006;Smith Jr. and DeMaster, 1996;Wood, 1966). Despite the large amount of nutrients discharged by the river, extremely high concentrations of riverine suspended particles as well as CDOM, restrict the amount of light available for phytoplankton photosynthesis (DeMaster et al., 1983;Smith Jr. and DeMaster, 1996;Smith and Russell, 1995;Teixeira and Tundisi, 1967). ...
... Upwelling areas on continental shelves or coastal areas with lateral transport of suspended particles typically have substantially higher sediment accumulation rates compared to deep sea oceanic rates (Anderson et al., 1983;Taguchi et al., 1989). Lateral transport of particle reactive species such as 210 Pb and 239 þ 240 Pu into regions of deposition leads to the increased flux of these radionuclide tracers (DeMaster et al., 1986;Dukat and Kuehl, 1995;Niggemann et al., 2007;Smoak et al., 1996). The 210 Pb ex inventories, shown in Table 2, give an integrated view of shelf dynamics, as the 210 Pb ex inventories vary greatly from one station to another, indicating a differing depositional flux along the shelf. ...
... These ETM are generated because the net flux of deep water upstream into the estuary allows negatively buoyant particulate matter to remain trapped in the estuary in dense clouds of turbidity (Crump et al. 1998). Estuarine turbidity maxima occur throughout many estuaries in the world, including (but not limited to) the Chesapeake (Schubel 1968), the San Francisco Bay (Jassby and Powell 1994), and the Delaware (Biggs et al. 1983) in the USA; the Yorkshire Ouse, Humber, and Tamar in the UK (Grabemann et al. 1997;Uncles et al. 1998); the Seine, Loire, and Gironde in France (Etcheber et al. 2007); the Elbe and Weser in Germany (Kappenberg and Grabemann 2001); the Guadiana in South Portugal (Garel et al. 2009); the Ariake Bay in Japan (Islam et al. 2006); the Changjiang in China (Li and Zhang 1998); and the Amazon in Brazil (Demaster et al. 1986;Nittrouer et al. 1986;Berhane et al. 1997), to name a few. ETM are hot spots of enzymatic activity and biogeochemical cycling and have been demonstrated to exhibit enhanced bacterial production rates compared to that outside of the ETM, with rates up to 5.6 μgL −1 h −1 in the Columbia River ETM as measured with tritiated thymidine (Crump and Baross 1996). ...
Dark CO2 fixation has been shown to rival the importance of oxygenic photosynthesis in the global carbon cycle, especially in stratified environments, such as salt wedge estuaries. We investigated this process in the Columbia River estuary using a variety of techniques including functional gene cloning of cbbL (the large subunit of form I RuBisCO), quantitative real-time PCR (qPCR) estimations of cbbL abundance, and analyses of stimulated 14C-bicarbonate assimilation. A diversity of red-type cbbL genes were retrieved from clone libraries, with 28 unique operational taxonomic units determined from 60 sequences. The majority of the sequences formed two clusters that were distinct from the major clusters typically found in soil environments, revealing the presence of a unique community of autotrophic or facultatively autotrophic/mixotrophic microorganisms in the Columbia River estuary. qPCR estimates indicated that roughly 0.03–0.15 % of the microbial population harbored the cbbL gene, with greater numbers of total bacteria and cbbL gene copies found in the estuarine turbidity maxima (ETM) compared to non-ETM events. In vitro incubations with radiolabeled bicarbonate indicated maximum stimulation by thiosulfate and also suggested that a diversity of other potential electron donors may stimulate CO2 fixation, including nitrite, ammonium, and Mn(II). Taken together, these results highlight the diversity of the microbial metabolic strategies employed and emphasize the importance of dark CO2 fixation in the dynamic waters of the Columbia River estuary despite the abundance of organic material.
... These depositional systems store an abundance of geological information (e.g., the materials flux, the depositional cycles, and their variability, which are influenced by the marine dynamics and climatic changes as well as intense human activities), which act as important media for the study of land-ocean interaction, oceanic mass flux, and biogeochemical cycles (including the global carbon cycle). The sediment supplies and marine dynamic conditions are the two key environmental factors in the formation of these depositional systems (see, e.g., DeMaster et al. 1986;Kuehl et al. 1986Kuehl et al. , 1997Allison et al. 2000;Su and Huh 2002;Cattaneo et al. 2003;McKee et al. 2004;Liu et al. 2006Liu et al. , 2007Liu et al. , 2009Xu et al. , 2012. ...
Two surveys were conducted in December, 2008, and August, 2009, in the mud depo-center off the Zhejiang-Fujian coast (MDZFC) in the inner shelf of East China Sea to depict the seasonal variation of the water column structure and analyze the factors responsible for the variation. The results were also used to discuss the sediment transport process and formation mechanism of the MDZFC. The water column structures varied significantly between the two surveys, with respect to the temperature, salinity, and turbidity. The summer water body, with relatively high temperatures and salinities, was evidently stratified with respect to the temperature, whereas the salinity remained constant throughout the water column. The stratification restricts sediment resuspension and transport. From the north to the south, the temperature in the middle-bottom water layer slightly increased, whereas the salinity remained mostly constant. In winter, the water body, with relatively low temperatures and salinities, was well mixed vertically. The temperature and salinity both increased from the surface to the bottom toward the east (deep water) and the south. A wedge-shaped water mass, which appears as a coastal upwelling, with relatively low temperature and high salinity in summer and relatively high temperature and high salinity in winter, spread landward along the sea floor, from the sea deeper than 50 m, whereas the extension was relatively stronger in winter. The water turbidity in winter was clearly higher than in summer. In the surface layer, the turbidity was generally greater than 5 FTU in winter and less than 1 FTU in summer. In the bottom layer, the turbidity was much greater than 200 FTU in winter and slightly greater than 50 FTU in summer. Moreover, the turbid water layer close to the sea floor in winter can reach into an area deeper than 50 m with a thickness of over 10 m; however, it was only limited to only 30-m-deep water with a thickness of 5 m in summer. The differences of marine sedimentary environment in the MDZFC were attributed to the seasonal variations of hydrodynamics environment, weather conditions, sediment supplies, and seasonal circulations. The results suggest that winter is the key season for particle transportation and deposition. The bottom turbid layer is the primarily channel of sediment transport, and the upwelling currents and the oceanic front systems play an important role in the sediment deposit processes and the formation of the MDZFC.
... ........ ' 'Fig. 3 Detailed coring has been performed along the Open-Shelf Transect from the 40-m to the 70-m isobaths, ampb measurements of surface sediment samples reveal an increase in activity with increasing water depth along the transect, consistent with DeMaster et al. (1986) . Examination of xradiographs has revealed some evidence of mass movement. ...
Estuaries are the major conduits of material transfer from the continental river basins to the marine environments. In this study, suspended particulate matter (SPM) was collected from the estuarine region of Nethravati River, southwest India, over a one-year period on a seasonal basis to assess the metal fluxes to the eastern Arabian Sea. The focus was to understand the effect of secondary fluvial geochemical processes on metal chemistry as well as the role of Fe and Mn particles in heavy metal transport along a tropical micro-tidal estuary. The SPM content and its metal concentrations in the Nethravati estuary showed strong seasonal and spatial variability. The Fe–Mn oxyhydroxides formed the important carriers of metals in the estuarine region; however, it depended on the seasonality. The heavy metals displayed higher concentrations at a low Fe/Mn ratio in the estuary, which suggests a dominant role of Mn-oxyhydroxides as the metal carrier phase. Thus, the geochemical assessment of SPM in the Nethravati estuary demonstrates redox cycling of metals coupled with adsorption–desorption of heavy metals onto the metal oxyhydroxides. Higher concentrations of heavy metals were observed in the estuarine bed sediments than in the suspended sediments. However, from the pollution point of view, the heavy metal concentrations were not enriched and were found to be within the limits of the National Oceanic and Atmospheric Administration (NOAA) guidelines.KeywordsSuspended particulate matter (SPM)BiogeochemistryFe-Mn oxyhydroxidesHeavy metalsTropical estuaryArabian Sea
Activity profiles of excess ²¹⁰Pb determined in three sediment cores from Amazon River mouth, Amapá State, Brazil, provide the evaluation of sedimentation rates, contributing to a better knowledge of the hydrological conditions. Chemical data on the sediments allowed calculation of the Geoaccumulation Index (Igeo) for each constituent analyzed. Significant direct relationships among Fe2O3 and major/minor constituents Al2O3 (r = 0.86), K2O (r = 0.94), MgO (r = 0.89), MnO (r = 0.92), BaO (r = 0.64), P2O5 (r = 0.62), and TiO2 (r = 0.54), inverse correlation between Fe2O3 and SiO2 (r = −0.93) and the direct correlation between SiO2 and LOI (loss on ignition) (r = 0.88) suggest competitive sorption of Si to Fe oxides and organic compounds. Igeo values between −3.75 and 0.22 for Si, Al, Na, K, Ca, Mg, Fe, Mn, Ti, P, S, Sr, Ba, Rb, Zn, Cr, Nb, Cu, Ni, As, V, Ga, Tb, Y, and Ce indicate practically unpolluted environments despite several mining activities in the Amazon basin and its vicinity. The apparent sediment mass accumulation rates between 0.5 and 2.4 gcm⁻²yr⁻¹ are compatible with the results of others studies. The ²¹⁰Pb activity at one sampling point (P1) exhibited an abrupt fluctuation, similar to findings from previous studies held at the northern channel of the Amazon River discharge. On the other hand, in two sediment profiles (P2 and P3), the excess ²¹⁰Pb activity data validate the use of the CF:CS method, yielding unique straight lines and increasing the sedimentation rates database.
The seasonal variation in material exchange through the Bohai Strait (BS) was examined using the climatological mean outputs from the Regional Ocean Modelling Systems and in-situ observations from two acoustic Doppler current profilers at two typical stations in the Strait from July 2017 to June 2018. The suspended sediment concentration (SSC) was retrieved from ocean color remote sensing data. The long-term mean numerical results and the one-year observations suggest that the volume and suspended sediment flux in the BS section exhibit distinct seasonal variations. The annual mean net outflow of the suspended sediment through the BS section is estimated to be 15.8 Mt/yr in the climatological state. The observations further reveal the circulation structure, with inflow in the north and outflow in the south. Freshwater from the Yellow Sea discharged sediment into the Bohai Sea in the deep northern channel at a rate of 0.4 Mt/yr, whereas turbid water discharged sediment into the Yellow Sea in the shallow southern channel at a rate of 0.19 Mt/yr. Several factors determine the seasonal variation in the sediment flux. In the winter half-year, variation in SSC plays a dominant role, while in summer, the primary influence on sediment flux is current velocity. Moreover, SSC significantly influences the southern shallow and deep northern channels, while the current velocity is dominant in the central channel. In addition, the meridional wind displays a close relationship with the seasonal variation of the sediment exchange through the BS by dominating both the current and SSC.
Small pigmented eukaryotes (⩽ 5 µm) are an important, but overlooked component of global marine phytoplankton. The Amazon River plume delivers nutrients into the oligotrophic western tropical North Atlantic, shades the deeper waters, and drives the structure of microphytoplankton (> 20 µm) communities. For small pigmented eukaryotes, however, diversity and distribution in the region remain unknown, despite their significant contribution to open ocean primary production and other biogeochemical processes. To investigate how habitats created by the Amazon river plume shape small pigmented eukaryote communities, we used high-throughput sequencing of the 18S ribosomal RNA genes from up to five distinct small pigmented eukaryote cell populations, identified and sorted by flow cytometry. Small pigmented eukaryotes dominated small phytoplankton biomass across all habitat types, but the population abundances varied among stations resulting in a random distribution. Small pigmented eukaryote communities were consistently dominated by Chloropicophyceae (0.8–2 µm) and Bacillariophyceae (0.8–3.5 µm), accompanied by MOCH-5 at the surface or by Dinophyceae at the chlorophyll maximum. Taxonomic composition only displayed differences in the old plume core and at one of the plume margin stations. Such results reflect the dynamic interactions of the plume and offshore oceanic waters and suggest that the resident small pigmented eukaryote diversity was not strongly affected by habitat types at this time of the year.
Sediment transfer from land to ocean begins in coastal settings and, for large rivers such as the Amazon, has dramatic impacts over thousands of kilometers covering diverse environmental conditions. In the relatively natural Amazon tidal river, combinations of fluvial and marine processes transition toward the ocean, affecting the transport and accumulation of sediment in floodplains and tributary mouths. The enormous discharge of Amazon fresh water causes estuarine processes to occur on the continental shelf, where much sediment accumulation creates a large clinoform structure and where additional sediment accumulates along its shoreward boundary in tidal flats and mangrove forests. Some remaining Amazon sediment is transported beyond the region near the river mouth, and fluvial forces on it diminish. Numerous perturbations to Amazon sediment transport and accumulation occur naturally, but human actions will likely dominate future change, and now is the time to document, understand, and mitigate their impacts.
Expected final online publication date for the Annual Review of Marine Science, Volume 13 is January 3, 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.
The grain-size distribution of surface sediments in the Bohai Sea (BS) and the northern Yellow Sea (NYS), and its relationship with sediment supply and hydrodynamic environment were investigated based on grain-size compositions of surface sediments and modern sedimentation rates. The results showed that the surface sediments in the BS and the NYS were primarily composed of silty sand and clayey silt with a dominant size of silt. In addition, the Yellow River delivered high amount of water and sediments to the BS, and they are dominated in surface sediments (mainly silt) in the Bohai Bay, the Yellow River mouth, the center of the BS, and the north coast of Shandong Peninsula. The coarse-grained sediments were mainly deposited at the river mouth due to the estuarine filtration and physical sorting. Meanwhile, there was a significant relationship among the modern sedimentation rate, the surface sediment grain size distribution and sediment transport pattern. The areas with coarser surface sediments generally corresponded low sedimentation rates because of strong erosion; whereas the sedimentation rate was relatively high at the place that the surface sediments were fine-grained. Furthermore, the grain-size trend analysis showed that the areas with fine-grained surface sediments such as the mud area in the central BS and the upper Liaodong Bay were the convergent centers of surface sediments, except for the Bohai Bay and the subaqueous Yellow River Delta where offshore sediment transport was evident.
In the Amazon region, few data are available on the impacts caused by the urban
settlements found in the proximity of estuaries. In the estuary of the Caeté River, the
focus of the present study, the nutrient input is controlled by both natural features and
anthropogenic disturbances generated by local communities. In this context, the
principal aim of the study was to analyze the quality of the water of the Caeté estuary,
and the relative contribution of natural and anthropogenic forcings. To this end,
climatological, hydrodynamic and hydrological features were monitored, and potential
sources of pollution were identified in the different sectors of the Caeté estuary.
Potential future scenarios for the estuary are also described, based on the analysis of
anthropogenic and natural processes, which may contribute to the quality of its waters.
The results indicate higher levels of nutrient input in the upper sector of the estuary,
where 90% of the local population is concentrated, and most of the region’s commercial
activities (e.g., public markets, ice factories, and docking facilities) are found. As a
consequence, eutrophic waters with high concentrations of faecal coliforms (up to 1100
MPN/100 ml) were observed during spring tides in the dry season when the transport
and dilution of the estuary’s waters are less effective. Eutrophication also occurred to a
lesser extent in the other (middle and lower) estuary sectors, although in this case, the
results indicate the influence of natural processes, reflecting the high nutrient
concentrations of this Amazonian region. During neap tides, eutrophication was less
pronounced, and water quality was improved in both dry and rainy seasons. A
comparative analysis showed that, under similar conditions of the flood cycle, the
trophic status of the estuary varied little between spring and neap tides. As the
population of the region surrounding the Caeté estuary is increasing by 10–20% per
decade, resulting in a significant increase in human pressures and impacts on the study
area. The current eutrophication status of the estuary may have permanent effects,
which may be aggravated during the dry season or drought events, when the estuary is
more vulnerable to the retention of nutrients. The water quality of the Caeté Estuary can be improved by the implementation of the following measures: (i) urban planning to
control the discharge of sewage, (ii) the construction of water treatment plants to reduce
the input of untreated effluents, and (iii) the introduction of regulations for the use of
water based on its current quality.
Nutrient over-enrichment in many areas around the world is having pervasive ecological effects on coastal ecosystems. These effects include reduced dissolved oxygen in aquatic systems and subsequent impacts on living resources. The largest zone of oxygen-depleted coastal waters in the United States, and the entire western Atlantic Ocean, is found in the northern Gulf of Mexico on the Louisiana/Texas continental shelf influenced by the freshwater discharge and nutrient load of the Mississippi River system. The mid-summer bottom areal extent of hypoxic waters (< 2 mg l -1 O2) in 1985-1992 averaged 8,000 to 9,000 km 2 but increased to up to 16,000 to 20,000 km 2 in 1993-2000. Hypoxic waters are most prevalent from late spring through late summer, and hypoxia is more widespread and persistent in some years than in others. Hypoxic waters are distributed from shallow depths near shore (4 to 5 m) to as deep as 60 m water depth but more typically between 5 and 30 m. Hypoxia occurs mostly in the lower water column but encompasses as much as the lower half to two -thirds of the water column. The Mississippi River system is the dominant source of fresh water and nutrients to the northern Gulf of Mexico. Mississippi River nutrient concentrations and loading to the adjacent continental shelf have changed in the last half of the 20 th century. The average annual nitrate concentration doubled, and the mean silicate concentration was reduced by 50%. There is no doubt that the average concentration and flux of nitrogen (per unit volume discharge) increased from the 1950s to 1980s, especially in the spring. There is considerable evidence that nutrient enhanced primary production in the northern Gulf of Mexico is causally related to the oxygen depletion in the lower water column. Evidence from long-term data sets and the sedimentary record demonstrate that historic increases in riverine dissolved inorganic nitrogen concentration and loads over the last 50 years are highly correlated with indicators of increased productivity in the overlying water column, i.e., eutrophication of the continental shelf waters, and subsequent worsening of oxygen stress in the bottom waters. Evidence associates increased coastal ocean productivity and worsening oxygen depletion with changes in landscape use and nutrient management that resulted in nutrient enrichment of receiving waters. Thus, nutrient flux to coastal systems has increased over time due to anthropogenic activities and has led to broad -scale degradation of the marine environment.
Gold exploration has been intensive in Brazilian Amazon over the last 40 years, where the use of mercury as an amalgam has caused abnormal Hg concentrations in water bodies. Special attention has been directed to Madeira River due to fact it is a major tributary of Amazon River and that since 1986, gold exploration has been officially permitted along a 350km sector of the river. The (21)(0)Pb method has been used to date sediments taken from nine lakes situated in Madeira River basin, Rondônia State, and to verify where anthropogenic Hg might exist due to gold exploitation in Madeira River. Activity profiles of excess (21)(0)Pb determined in the sediment cores provided a means to evaluate the sedimentation rates using a Constant Flux: Constant Sedimentation (CF:CS) and Constant Rate of Supply (CRS) of unsupported/excess (21)(0)Pb models. A significant relationship was found between the CF:CS sedimentation rates and the mean values of the CRS sedimentation rates (Pearson correlation coefficient r=0.59). Chemical data were also determined in the sediments for identifying possible relationships with Hg occurring in the area. Significant values were found in statistical correlation tests realized among the Hg, major oxides and Total Organic Carbon (TOC) content in the sediments. The TOC increased in the sediment cores accompanied by a loss on ignition (LOI) increment, whereas silica decreased following a specific surface area raising associated to the TOC increase. The CRS model always provided ages within the permitted range of the (21)(0)Pb-method in the studied lakes, whereas the CF:CS model predicted two values above 140 years.
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Tropical cyclone impacts on wetland, terrestrial, and shelf systems have been previously studied and reasonably delineated, but little is known about the response of coastal lakes to storm events. For the first time tropical cyclone impacts on a shallow coastal lake in the Louisiana coastal plain have been studied using direct lines of evidence. Using side-scan sonar, CHIRP subbottom, and echo sounder bathymetric profiles, the lake bottom and shallow subsurface of Sister Lake was imaged pre- and post-Hurricanes Katrina and Rita to provide a geologic framework for assessing storm effects. Box cores were collected to provide site-specific "ground truth" data to further evaluate the accretion or erosion of sediment over the short storm period between synoptic geophysical surveys. X-ray radiographs of box cores showed clear increments of recent event sedimentation (1–10 cm in thickness), corroborated with radionuclide dating as being products of the storm period. High percentages of approximately 40% fine sand in the storm layer and its thickness relative to an average long-term sedimentation rate of 2.0 mm/y suggest that transport of storm-related sediments from the inner shelf is a large factor in Sister Lake sedimentation. This study provides a framework and fundamental understanding of lake bottom characteristics and impacts of storm-related physical processes on erosion, sediment resuspension, and deposition. For a general case of subsidence in the Louisiana coastal plain of 6–8 mm/y and future sea-level rise rate of at least 3 mm/y, the sediment deficit for Sister Lake is 7–9 mm/y, which suggests that Sister Lake will deepen and widen with time.
The mesoscale variability of dimethyl sulfide (DMS) and ocean color is explored to determine the feasibility of a predictive relationship. During NASA`s Global Tropospheric Experiment/Chemical Instrumentation Test and Evaluation (GTE/CITE 3), simultaneous shipboard and aircraft studies were carried out in the North Atlantic, followed by aircraft studies in the South Atlantic. Surface concentrations of chlorophyll alpha were measured with an airborne spectroradiometer, the Ocean Data Acquisition System (ODAS), with simultaneous determinations of tropospheric DMS. Shipboard measurements of DMS in air and water as well as in situ chlorophyll alpha were taken in the North Atlantic. No relation was observed between shipboard aquatic DMS and chlorophyll alpha or primary productivity. Higher levels of aqueous DMS were not always reflected by atmospheric DMS, although shipboard and aircraft measurements of atmospheric DMS agreed very well. A significant relationship between atmospheric DMS and ocean color was seen once at low altitudes in both the North and South Atlantic only under clean air conditions. Atmospheric DMS levels during the North Atlantic experiment were probably lowered by the presence of mostly polluted air masses in the study area and were, overall, probably not representative of the in situ sea-to-air flux of DMS. Changes in concentration of aircraft-sensed chlorophyllous pigments were not reflected by atmospheric DMS. If a predictive algorithm is to be found, phytoplankton blooms should probably be the first place to study an ocean color-DMS relationship.
Reveals deficiencies relative to equilibrium with 238U that are related to uptake onto sinking particles. The deficiencies are generally enhanced in the upper 50 m of the water column (Polar Water), a zone of high suspended particle, particulate organic carbon, and pigment concentrations. Removal of 234Th from the water column is balanced, within the uncertainties, by inventories of excess 234Th in bottom sediments, with both water column deficiences and sediment inventories greater in 1993 than 1992. This indicates that on the timescale of several half-lives of 234Th (~100 days) the polynya is in balance with respect to 234Th (and probably the particles with which it is associated). -from Authors
The Amazon River has built a subaqueous feature which stretches for hundreds of kilometres offshore and alongshore from its mouth. The feature is prograding seaward and accreting upward, and it contains fine-scale stratification typical of classic deltas. The feature forming at the mouth of the Amazon is a subaqueous delta; it differs from classic deltas primarily in its lack of subaerial expression.-from Authors
In the lower Amazon River, suspended sediment is stored during rising stages of the river and resuspended during falling river stages. The storage and resuspension in the reach are related to the mean slope of the flood wave on the river surface; this slope is smaller during rising river stages than during falling stages. The pattern of storage and resuspension damps out the extreme values of high and low sediment discharge and tends to keep them near the mean value between 3.0 x 10(6) and 3.5 x 10(6) metric tons per day. Mean annual discharge of suspended sediment in the lower Amazon is between 1.1 x 10(9) and 1.3 x 10(9) metric tons per year.
The naturally occurring radioactive isotope of lead, Pb-210, provides a valuable tracer for the behavior of heavy metals in the soil-stream-estuary system. Since it is continuously produced only as a member of the U-238 decay series, it is free from the problems of environmental or analytical contamination so often encountered in stable heavy metal studies. In addition, because of its half-life of about 22 years it is useful not only as a tracer but also as a dating tool to monitor events of the past 100 years in various repositories.
The half removal time of228Th from the surface waters by settling particles,tc, does not change much with season, except in the winter when regenerated228Th as well as210Pb and210Po were transported back to the surface water from the bottom water and/or near-shore sediments. The removal of228Th and210Pb from the surface waters of New York Bight by phytoplankton-zooplankton-fecal pellet route is not important in the shelf but is important in the slope areas. The removal of210Po is almost entirely associated with the phytoplankton-zooplankton-fecal pellet pathway throughout the New York Bight.
Experiments using radioactive tracers in microcosms of 150 l and 13 m3 volumes, which are designed to mimic Narragansett Bay, indicate that Th isotopes are good analogues for studying the removal behavior of “particle-reactive” pollutants such as Am, Pb, Po, Hg and Cr(III) in coastal environments. The removal of Th isotopes and Fe has been found to be closely linked in microcosms and Narragansett Bay. In addition, the settling velocities of tracer microspheres in the experimental tanks were found to be faster in spring and summer than expected from Stokes' law and similar to those of234Th-containing particles in Narragansett Bay [5]. It is concluded that aggregation of particles is important in accelerating the removal of surface-reactive elements during the warm season.
Estimates of the atmospheric /sup 210/Pb flux can be made from old (approx.100 years) soil profiles because of the strong retention of this nuclide in the organic rich surface layers. Correction for radon loss can be made by modeling the observed /sup 210/Pb and /sup 226/Ra profiles. This is shown to have only a small effect for most of the soils studied. Comparison of soil-based /sup 210/Pb fluxes with other estimates indicates good agreement except in the forested montane region of eastern New Mexico; the high /sup 210/Pb content of soils there remains to be explained.
A two-dimensional radiative-convective climate model has been developed, which includes meridional heat transport, surface albedo feedback and lapse rate feedback. The radiative part of the model is based upon the one-dimensional radiative-convective model of Ramanathan (1976). The two -dimensional model is designed to calculate a mean annual zonally averaged temperature profile for a symmetric model earth. The two-dimensional model calculates the vertical temperature profile for six latitudinal belts of 15(DEGREES) width. These six belts serve as a basis set from which the temperature at any latitude can be calculated. The heat transport is parameterized in terms of a diffuse transport mechanism. The surface albedo feedback for latitudes above 45(DEGREES)N is derived from seasonal data for the present atmosphere. The lapse rate feedback is based upon the suggestions for Stone and Carlson (1979). Three climatic studies have been carried out with the model: increased CO(,2), variation of the solar constant, and variation of cloud amount and height. These studies suggest that the lapse rate parameterization of Stone and Carlson (1979) is extremely sensitive to climatic perturbations. The model proves to be very stable to variations in the solar constant when lapse rate feedback is removed. A great advantage to this model is its flexibility. The model is also computationally efficient compared to the three-dimensional models. Because of the flexibility and computational efficiency, this model can serve as a useful tool in studying the importance of various atmospheric processes.
Aliquots of suspended matter collected from Rhine river water were resuspended in distilled water, in diluted artificial sea water (1 : 1), in artificial sea water, and in ‘nitrate sea water’, respectively at pH's 7.5 and 8.0. The desorption of heavy metals into these solutions was calculated in relation to the concentrations as determined by extraction in 4 M hydrochloric acid. The heavy metal concentrations after resuspension and the relative desorption percentages were compared with calculated inorganic metal ion speciation. These comparisons, though quantitatively not quite satisfactory, demonstrated that complex formation is important in desorption processes during estuarine mixing. The order of decreasing desorption of metals into 1 : 1 diluted sea water and sea water in the experiments is : Cd > Zn > Mn > Ni > Co > Cu > Cr; fo for Fe and Pb no desorption was found.Analyses of samples collected at a tidal station at Hoek van Holland show that both suspended matter and filtered water are depleted in most heavy metals. Certainly, in this tidal area suspended matter and adsorbed and dissolved heavy metals do not show a conservative mixing behaviour. In order to correlate experimental results and field obsevartions, large scale flocculation and sedimentation of suspended matter must be accepted at an early stage of estuarine mixing. It is not certain whether this can be proved; future research should include more chemical indicators for testing these estuarine processes.
On the basis of data from seven cruises in the Amazon River-Atlantic Ocean area, and with 14 anchored stations extending over complete tidal cycles, the currents, suspended sediment concentration, temperature, and salinity were measured at various depths. The physical circulation of water shows Amazon River water thrusting out across the continental shelf and over the sea water with some entrainment and mixing with the sea water. The brackish plume from the Amazon is then turned northwestward along the outer shelf by the Guiana current and longshore currents to parallel the coastline for 500 to 700 km. Sea water upwells and flows landward under this plume. The sediments of the Amazon River are thrust out onto the outer shelf, where a gradual depositional process occurs: the sand is deposited first, followed by the silt, and most of the mud is carried shoreward by the landward-moving bottom waters. This combination of processes results in the existing depositional facies pattern; modern mud deposits along the shoreline grade outward into silt deposits and finally into modern sand. The depositional process occurring is contrary to the classic model with sand along the shoreline.
Estimates of the atmospheric ²¹°Pb flux can be made from old (approx.100 years) soil profiles because of the strong retention of this nuclide in the organic rich surface layers. Correction for radon loss can be made by modeling the observed ²¹°Pb and ²²â¶Ra profiles. This is shown to have only a small effect for most of the soils studied. Comparison of soil-based ²¹°Pb fluxes with other estimates indicates good agreement except in the forested montane region of eastern New Mexico; the high ²¹°Pb content of soils there remains to be explained.
Evolution of the silica dissolved in the Scheldt estuary shows that the reduction in silica content observed is much greater than that predicted by the dilution curve computed from the mixture of fresh water rich in silica and surface sea water. According to the results reported in the literature and our own experiments, the removal of silica from the solution is not due to a reaction with clay minerals in suspension, but to biological activity, essentially by diatoms that live preferentially in saline waters. After death, diatoms that become part of the sediments return silica to the interstitial waters, which can then combine with disordered clays of continental origin to give rise to new aluminosilicates richer in silica and in alkaline or alkaline-earth ions.
A transformed formulation of the theory, which appears to converge
rapidly at low temperatures, has been developed. It has been shown that
renormalization corresponds to a transformation from a particle
representation to a quasiparticle representation of the theory, in which
the boson and fermion quasiparticle energies and momentum distribution
functions appear as a natural consequence of the renormalization. The
vertex functions appearing in the diagrammatic expansions employed in
the theory have been expressed in terms of matrix elements of the
two-particle interaction. Calculations of the entropy and total energy
of the system, as well as the boson and fermion quasiparticle energies
and momentum distribution functions, have been made through second order
in matrix elements of the two-particle interaction. The boson
quasiparticle energy has been shown to be phonon-like at low
temperatures and to approach a zero momentum limit of zero, through
second order.
Measurements of 226Ra and 228Ra in the Amazon River estuary show that desorption from riverborne suspended particulate matter in the estuary increases the riverine flux of both isotopes to the ocean by a factor of approximately 5 over the flux attributable to radium dissolved in the river water alone. The total Amazon flux supplies approximately 0.20% of the 226Ra and approximately 2.6% of the 228Ra standing crops in the near-surface Atlantic (0-200 m). Diffusive flux from estuarine and shelf sediments and desorption from resuspended sediments in the region of the estuary approximately double the estuarine 226Ra concentration and quadruple the estuarine 228Ra concentration above that caused by the dissolved and desorbed river components alone.
Reactions involving clay minerals may exert a major control on some aspects of marine water and sediment chemistry. The potential of clay mineral reactions in this regard was investigated in Amazon continental shelf muds and overlying waters using highly sensitive dissolved Al analyses. Data are restricted to low-Fe pore waters from undisturbed and incubated (4–11 days, T = 28 ± 1 °C) surface sediment at 9 stations, a surface water transect through the Amazon River plume, and water column profiles determined at coring sites. Approximately constant relations between dissolved Al, Si, and H+ in pore waters imply that aluminous authigenic clays (Si/Al = 0.83, H+/Al = 0.43) are forming in muddy regions of the Amazon shelf. Equilibrium models based upon the pore water data also predict the correct magnitude of dissolved A1 concentrations in the Amazon River plume in the absence of high biological productivity, indicating that authigenic clays control some characteristics of overlying water chemistry. In water column profiles, dissolved Al increases with depth at high salinities apparently because of sediment resuspension into low dissolved Si waters and subsequent clay dissolution. The results of this study confirm predictions based upon previous laboratory and field studies of dissolved Al behavior. They also point out some of the possible complexities of clay reconstitution reactions where aluminous authigenic clays form from more siliceous precursors in nearshore sediments and waters.
Measurements of234Th (t1/2 = 24.1d) in water column and seabed samples from an area near the mouth of the Yangtze River (People's Republic of China) provide the following information:(1)|Most of the234Th in waters near the mouth of the Yangtze River is in particulate form, and horizontal transport and sedimentation of234Th-bearing particles dominate the234Th budget in this area. A model incorporating processes in at least two dimensions is required to understand the fate of particle-reactive species on the Yangtze continental shelf.(2)|The residence times of particle-reactive species (with respect to scavenging) range from 0.3 days (nearshore) to 4.0 days (offshore). The residence times (with respect to removal to the seabed) range from 0.5 day (nearshore) to 11.0 days (offshore).(3)|Scavenging residence times for234Th decrease as the concentration of suspended particles increases. The time for removal of234Th to the seabed increases with distance from shore (a variable which integrates such factors as scavenging rate, bathymetry, and frequency of resuspension).
The vertical distribution of chlorophyll and phaeo-pigments were studied off Baja California. Concentrations of both increased with depth until a maximum was reached below which lower levels were observed. The chlorophyll maximum was usually near the base of the euphotic zone, in the thermocline. The phaeo-pigment maximum either coincided with the chlorophyll maximum or was at a slightly greater depth.The phaeo-pigment maximum coincided with that portion of the water column where nutrients increased in concentration. This layer possibly represents an accumulation of detritus, and an area where remineralization and lower re-utilization of nutrients occur.Phaeo-pigments are readily produced by zooplankters grazing on phytoplankton. A relationship exists between the number of zooplankters in the water column and the quantity of phaeo-pigments in the euphotic zone. Phaeo-pigments in the euphotic zone may be indicative of the past history of grazing activity in the water column, and may also be a measure of nutrient turnover.
The optimum conditions for the determination of silicate in natural waters by a molybdenum blue method have been investigated p-Methylaminophenol sulphate has a number of advantages over other reducing agents for the reduction of silicomolybdic acid to molybdenum blue. Beer's law is obeyed up to concentrations of at least 3 mg/l.The interference of several inorganic ions has been investigated; none of those encountered in natural waters is likely to cause error. The method has been applied to the analysis of sea-water and the “salt error” correction evaluated.RésuméLes conditions optima de dosage de silicate dans les eaux normales par la méthode au bleu de molybdène ont été examinées. Le sulfate de p-méthylaminophénol présente un certain nombre d'avantages sur les autres réducteurs utilisés pour la réduction de l'acide silicomolybdique en bleu de molybdène. La loi de Beer est suivie pour des concentrations allant au moins jusqu'à, 3 mg/1. L'influence de divers ions inorganiques a été examinée, aucun de ceux présents dans les eaux naturelles ne gêne. Cette méthode a été appliquée a l'analyse de l'eau de mer.ZusammenfassungEs wurden die optimalen Bedingungen gesucht fur die Bestimmung von Silikat im natürlichen Wasser mit der Molybdenblaumethode. Das Sulfat von p-methylaminophenol weist eine gewisse Anzahl Vorteile auf gegenüber den andern Reduktionsmitteln, welche für die Reduktion der Silicomolybdänsäure zu Molybdänblau verwendet werden. Das Beer'sche Gesetz findet Anwendung für Konzentrationen, die mindestens bis zu 3 mg/l gehen. Der Einfluss von verschiedenen anorganischen Ionen wurde geprüft. Keines der im naturlichen Wasser vorkommenden Ionen stört die Bestimmung. Diese Methode wurde verwendet für die Analyse des Meerwassers.
ESTIMATES published in the past 20 years of the sediment load delivered to the sea by the Amazon River range from 4 to 10 × 108 tonnes yr−1 Estimates published around 1960, when few data were available on either the sediment concentration or the river discharge, were in the range 9–10 × 108 tonnes yr−1 (refs 1, 2). Estimates published in 1967 and 1968, after significant new data had been collected, were in the lower range of 4–5 × 108 tonnes yr−1 (refs 3, 4). Our more comprehensive data collected mostly since 1970 (and especially in 1977) suggest that the earlier higher estimates may have been more nearly correct. We show here that the mean annual load of suspended sediment at Óbidos, Brazil is between 8 and 9 × 108 tonnes yr−1. Most of this sediment is discharged onto the continental shelf.
On the basis of wet- and dry-season sampling along the Amazon River, at its mouth, and from its 16 large tributaries, the factors that control the salinity and the composition and concentration of the suspended solids of the Amazon River were determined. The multiple regression analysis of the analytical results of 74 river-water samples and numerous in situ measurements revealed: (1) of the 92 percent of the variability of the salinity accounted for, 85 percent was accounted for by relief - 86 percent of the total dissolved salts discharged by the Amazon was supplied from the 12 percent of the total area of the Basin comprising the mountainous-environment type; (2) of the 62 percent of the variability of the concentration of the suspended solids accounted for, 43 percent was accounted for by relief - 82 percent of the total suspended solids discharged by the Amazon was supplied from the mountainous environment-type. The multiple regression analysis of the analytical results obtained by X-ray diffraction from 50 suspended solids samples revealed: (1) increased-relief physical weathering controlled the percentages of quartz, plagioclase, K-feldspar, mica, chlorite, and amphibole; (2) the percentage of "calcic" rocks in the upper portions of tributary basins controlled montmorillonite concentrations; and (3) a combination of low-relief chemical weathering and the percentage of igneous and metamorphic rocks controlled the percentages kaolinite and gibbsite. The increased-relief physical weathering dominant in the Andean mountainous environment controls the geochemistry of the Amazon River, indicated by the following observations: (1) the over-all composition of the suspended solids discharged by the Amazon differs only slightly from that of the suspended solids eroded from the mountainous environment; and (2) 84 percent of the total amount of dissolved salts and suspended solids discharged is eroded from the 12 percent of the total area of the Amazon Basin comprising the mountainous Andean environment-type.
Current measurements from an array of moored current meters at 15 and 32 m water depths in the Amazon plume region at 4°N during the period of peak discharge are described. Principal variability occurred at semi-diurnal (tidal), 1–2 day (event), and week to 10 day periods. Interpretation within the context of observed hydrographic gradients and identified water masses indicates an offshore flow regime with a mean current along-shelf northwestward and an inshore flow regime with a mean current component across-shelf seaward. Surface plume mixing events and minimal tidal modulation are characteristic of the inshore regime and the upper water column offshore. Upwelling and significant tidal modulation are associated with the lower water column offshore. Current meandering and/or development effect longer period (week to 10 day) fluctuations in both regimes. In the frontal zone between regimes, interleaving and mixing are important processes.
Formation of fine-scale () strata on the Amazon continental shelf was investigated using radiographic and radiochemical (e.g. 210Pb) techniques. Three distinct sedimentary environments are found within the inner shelf ( water depth) mud deposit: (1) interbedded mud and sand in a corridor extending from the river mouth across the inner shelf; (2) faintly laminated muds on both flanks of the corridor; and (3) bioturbated muds comprising the remainder of the deposit. High rates of sediment accumulation () are found in the region of interbedded mud and sand, and in the regions of faintly laminated muds. Accumulation rates in the region of bioturbated muds are relatively low (0.1–2 cm/yr), and an inverse relationship is observed between accumulation rate and extent of sedimentary structure alteration by benthic fauna. A first-order sediment budget indicates that sediment accumulation on the Amazon shelf (between the Brazil-French Guiana border and the Para River) is nearly equal to estimates of sediment discharged by the Amazon River ().The model of Amazon shelf sedimentation most consistent with the radiographic and radiochemical data suggests that the inner shelf mud deposit is a subaqueous delta prograding seaward over relict basal sands and that a turbulent jet emanates from the river mouth with competence (during some flood periods) to carry sand as bedload across the inner shelf. The presence of cross-bedded sands in the corridor of interbedded mud and sand supports the hypothesis of a turbulent jet. The faintly laminated muds probably represent suspended sediment rapidly deposited on both sides of the jet.
Based on temperature, salinity, and light transmission properties within the Amazon plume region (500 km along-shelf and 200 km across-shelf from the river mouth) during peak discharge, four water masses are defined: River Water, High Temperature Surface Water, Subsurface Ocean Water, and Bottom Boundary Layer Water. The primary frontal zone for water mass interaction is located between the 10 and 30 m isobaths, with an across-shelf scale of 80–100 km. Within this frontal zone, vertical scales for water mass interleaving are 5–10 m. Seven mixing sites within the across-shelf structure are identified, and their role in the interaction process is discussed.
The distribution and movement of suspended sediment has been examined on the continental shelf near the mouth of the Amazon River, during peak discharge of the river. Observations included: transmissometer/CTD profiles widely distributed on the shelf, and combined with current meter profiles at two anchor stations near the river mouth; drogue deployments northwest of the river mouth; and transmissometer/current meter moorings near the northern boundary (4°N) of the study area. Suspended-sediment concentration (SSC) becomes significant (10 mg l−1) landward of about the 30-m isobath, where it characteristically increases exponentially toward the seabed. A high SSC (generally 20 mgl−1 and often 100 mgl−1) is present in a turbid bottom layer which increases in thickness toward shore, probably as a result of shoaling surface waves and strong tidal currents. Landward of the 10-m isobath, a high SSC occupies most of the water column. On time scales less than a month, the predominant variations in SSC occur as a result of resuspension-deposition and of onshore-offshore advection associated with semidiurnal tides. Instantaneous fluxes of suspended sediment adjacent to the river mouth can exceed 50 g cm−1 s−1, and the net along-shelf flux (northwestward) associated with the North Brazilian Coastal Current may reach 3 g cm−1 s−1 between the 10- and 30-m isobaths. Estimates of annual sediment transport northwestward out of the study area, combined with estimates of sediment supply and accumulation, indicate that a much higher along-shelf flux must occur landward of the 10-m isobath and that coastal accretion is probably occurring.
Suspended matter from the surface waters of the Amazon Estuary were collected during May and June 1976 on the ‘R/V Alpha Helix’, and their major-element compositions (Al, Si, Ti, K, Mg, Ca, P, Fe and Mn) were measured.Between salinities of 0 and 10%. the suspended material, predominantly terrigenous in derivation, decreases in load from 500 to 3 mg/l, but has a chemical composition which remains essentially constant. With the onset of a large amount of biological productivity at approximately 10%. salinity, there are large increases in the ratios of , , , , and which are maintained at higher salinities. Calculations of “excess” concentrations of elements held in the non-terrigenous components of the suspended material further support our main conclusion that Si, P, Ca, Mg, Ti and Mn are incorporated into the skeletal and organic phases of marine phytoplankton (predominately diatoms) of the Amazon Estuary. The data suggest, but with less certainty, that Fe and K follow the above elements.This study has demonstrated that the chemical composition of river-introduced suspended matter can be significantly altered by biological activity within estuarine waters as can be the geochemical cycle of inorganic elements.
Sediment accumulation on the Brazilian continental shelf near the Amazon River is investigated using radiochemical (e.g.210Pb,14C) techniques to provide a better understanding of this major dispersal system of fine-grained sediment.210Pb profiles from 57 cores collected during 1983 reveal the distribution of modern (100-y time scale) accumulation rates on the Amazon subaqueous delta. Accumulation rates increase from <0.1 cm y1 (0.1 g cm2 y1) nearshore, to rates as high as ∼ 10 cm y−1 (6.9 g cm2 y−1) on the outer topset and the foreset regions (30–50 m water depth). Reduced upward accretion nearshore (<15 m water depth), which is reflected in the limited subaerial expression of the Amazon delta, probably results from the intense activity of surface waves and tidal currents. A thick (as much as 2 m) depth). This layer probably is reworked by waves and currents, and most of the sediment is eventually transported to other parts of the dispersal system.14C dating of an anomalous area of relict (age >100 y) sediment in the northwestern portion of the subaqueous delta indicates that this sediment was deposited <1000 y ago. The absence of modern sediment in this area is not understood. A sediment budget for the Amazon shelf indicates that 6.3 ± 2.0 × 108 tons of sediment accumulate annually. Much of the remainder of Amazon River sediment (∼6 × 108tons y−1) probably is transported northwestward beyond the Brazilian shelf and/or is accumulating landward of the shelf as coastal accretion.
We present a model of estuarine mixing, removal, and input for dissolved constituents, and apply the model to 39 nutrient (P, N, Si) profiles collected over a 14-month period in a pristine river/ estuary: Ochlockonee Bay, Florida. Each profile is deconvolved into three component functions: linear mixing (conservative) first-order removal (biological productivity), and parabolic input (regeneration). After correction for temporal variations in the fluvial end-members, the model provides quantitative estimates of total estuarine primary production, net regeneration, and subsequent fluxes to the ocean over a year-long period. The modeled data set is internally self-consistent: virtually perfect mass balances are obtained for P and Si. All biological P-uptake is regenerated within the estuary so that virtually 100% of the fluvial reactive-P enters the ocean. One-third of the fluvial reactive-P enters the estuary as particles whose phosphate is released after deposition in estuarine sediments. About 20% of the dissolved fluvial silica flux is removed biologically; all of this biogenic silica dissolves in the estuary and enters the ocean. N cannot be mass balanced, probably because it enters and escapes the bay in unmeasured forms (as NH4 or via denitrification to N2 and N2O). In the Ochlockonee, biological productivity removes nutrients in the ratios N:P ≅ 9:1 and Si:P ≅ 20:1.
The distribution of 234Th(t1/2 = 24.1days) in excess of its parent238U in the upper layers of near-shore sediment makes possible the evaluation of short-term sediment reworking and diagenetic rates.234Th has a maximum residence time in Long Island Sound water of 1.4 days. Seasonal measurement of234Th/238U disequilibrium in sediment at a single station in central Long Island Sound demonstrates rapid particle reworking and high234Thxs (>1 dpm/g) in the upper 4 cm of sediment with slower, irregular reworking and low234Thxs to at least 12 cm. The rate of rapid particle reworking varies seasonally and is highest in the fall. The rapidly mixed zone is characterized by steep gradients in sediment chemistry implying fast reactions spanned by234Th decay time scales.238U is depleted in the upper mixed zone and shows addition in reducing sediment at depth.
In May and June 1976, the Amazon plume was dominated by a diatom blood centered on the 10 x 10−3 isohaline. The bloom was apparently initiated by the increased transparency produced by the rapid settling of the fluvial detrial load. The bloom removes the nitrate and phosphate from the surficial layer. The underlying salt wedge was enriched by remineralisation of planktonic debris. A balance between the dissolved and particulate material shows that the regeneration was essentially complete for carbon and phosphorus; but about 50% of the nitrogen was unaccounted for, having been solubilised to species other than nitrate and nitrite. Only 20% of the silica removed as diatoms (∼25% of the river dissoled flux) could be accounted for as salt wedge enrichment. The rest must have been incorporated in the sediment. The composition of the river waters was strongly affected, before entry to the mixing zone, by remineralisation of a large fraction of the fluvial particulate organic material in the broad area of multi-channel flow above the mouth.
Trace element samples spanning the salinity range were collected in the Amazon plume in June 1974 and 1976. In 1976, laboratory mixing experiments using unfiltered river water and seawater were undertaken. The studies show that copper and nickel are unreactive in the Amazon plume on a time scale of a few days: in both field and laboratory studies the elements are related linearly to salinity, so that neither adsorption and precipitation not desorption reactions significantly alter the net flux of these elements. The 1974 field data indicate there may be up to 25% removal of copper, probably biologically, although a conservative interpretation could be entertained if systematic deviations from the resulting copper-salinity plot are ignored. Cadmium behavior was not clearly defined. There is some indication of desorption, and the estuarine data can be used to set an upper limit on the net flux. The net effective contribution of the elements to the ocean from the Amazon is copper, 24 nmol kg−1; nickel, 4 to 5 nmol kg−1; and cadmium, <0.1 nmol kg−1.
The concentrations of total lead were determined in both ordinary coastal seawater and in coastal waters highly polluted with sewage. The average concentration of total lead ranged from about 25 ng/kg in surface samples of the former type to an average of 150 ng/kg in the latter type. 0.4−μm filtrate lead was found to be equivalent to lead extracted from untreated seawater by dithizone in chloroform. In highly polluted waters, particle lead ranged from 56 to 39% of the total lead, with the proportion increasing as the amount of sewage in the seawater increased. Virtually all of the lead in sewage is contained in the particle phase before it enters the ocean but about 11% is made freely available by cation exchange when the sewage is mixed with seawater. In ordinary coastal waters that are not highly polluted with sewage lead, particle lead ranged from 90 to > 20%, and most of this lead was associated with plankton. About one-third of total lead, or one-half of particle lead, in untreated seawater adheres to the walls of the water container within the first day. Upon prolonged standing (> 3 months) most of the remaining particle lead can be collected on the surface of algal colonies growing on the walls of the water container. The concentrations of freely available lead in seawater do not seem to be significantly affected by wall adsorption. The 206Pb/207Pb ratio of total lead in coastal surface seawater collected near Los Angeles was 1.194, and a day earlier was 1.188 near La Jolla. It is believed that this shows lack of mixing of contributions from two different sources of lead pollution: a pulse of rain-storm runoff of gasoline lead(206Pb/207Pb ∼ 1.197) from paved surfaces added to Los Angeles waters; and sewage lead (206Pb/207Pb ∼ 1.188) from San Diego added to La Jolla waters.
Satellite imagery in the 0.4−1.1 μm band from GOES, NOAA-7 and NIMBUS-7 and shipboard measurements of suspended sediment concentration and chlorophyll are synthesized. Five main surface features are delineated in the plume region of the Amazon during peak discharge: a River Zone (RZ), Interaction Zones A, B and C (IZA, IZB, IZC), and a Nearshore Zone (NZ). The loci, temporal and spatial scales, and magnitudes of suspended sediment concentration for each zone are documented. IZA appears to act as an along-shelf conduit for northwestward transport of suspended particulates. IZB is associated with a diatom bloom and is important in determining the nutrient mass balance. IZC contains a buoyant, near-surface lens sensitive to fluctuations in wind forcing and barotropic currents. The role of NZ is not clear. A possible circulation scheme, consistent with the surface fields, includes surface divergences within IZA and IZC and a surface convergence within IZB. Wind forcing may control the extent of the surface lens in IZC and the transport of surface suspended sediment away from IZA.
The activity ratio of228Th/228Ra in the open surface ocean averages 0.21. This suggests that thorium is removed from surface water in about 0.7 yr. As plant matter is cycled within the surface sea on a similar time scale, the suggestion is made that highly reactive compounds are inadvertently removed by plants in their quest for the critical nutrients nitrate and phosphate. Combined with the coefficient for horizontal eddy diffusivity obtained from the distribution of228Ra in surface sea water this result provides a basis for the prediction of the distribution of “reactive” pollutants released to the surface sea from coastal areas.
A survey of U.S. east coast estuaries confirms that large-scale rapid removal of iron from river water is a general phenomenon during estuarine mixing. The river-borne ‘dissolved’ iron consists almost entirely of mixed iron oxide-organic matter colloids, of diameter less than 0.45 μm, stabilized by the dissolved organic matter. Precipitation occurs on mixing because the seawater cations neutralize the negatively charged iron-bearing colloids allowing flocculation. The process has been duplicated in laboratory experiments using both natural filtered and unfiltered river water and a synthetic colloidal goethite in 0.05 μm filtered water. The colloidal nature of the iron has been further confirmed by ultracentrifugation and ultrafiltration. A major consequence of the precipitation phenomena is to reduce the effective input of ‘dissolved’ iron to the ocean by about 90% of the primary river value, equivalent to a concentration of less than 1 μmol per liter of river water.
The estuarine chemistry of dissolved humic acids was determined by carrying out both field and laboratory studies. These approaches were combined in an investigation of the Amazon estuary while laboratory mixing experiments were performed using filtered (0.45−0.001 μm) river water fractions of the Water of Luce (Scotland).The results demonstrate that a small fraction of river dissolved organic matter is preferentially and rapidly flocculated during estuarine mixing. This fraction is the high molecular weight component of dissolved humic acids (0.45−0.1 μm filtered). Approximately 60–80% of the dissolved humic acid in these rivers flocculates during estuarine mixing. This represents a removal of only 3–6% of river dissolved organic matter and is responsible for the non-conservative behaviour of dissolved humic acid in the Amazon estuary even though total dissolved organic carbon appears conservative.The salinity dependence with which humic acid flocculates in estuaries is similar to that of iron. This implies that both constituents may be removed from river water by a common mechanism of colloid flocculation.
The cation-exchange characteristics of Amazon River suspended sediment have been studied in order to determine the contribution of exchangeable cations to the geochemical fluxes from the river. Sediment samples were obtained throughout most of the Amazon Basin. The range of exchangeable cation compositions is very narrow in the river and in seawater as well. In river water, the exchangeable cation complement (equivalent basis, exclusive of H+) is 80% Ca2+, 17% Mg2+, 3% Na+ plus K+. In seawater Na+ and Mg2+ are about equal (38%) while Ca2+ ~ 15% and K+ ~ 9%.On reaction with seawater, river suspended sediment took up an amount of Na+ equal to nearly one-third of the dissolved river load, as well as amounts corresponding to 15–20% of the dissolved fluvial K+ and Mg2+. These estimates reflect an unusually high suspended-sediment:dissolved-solids ratio of 6.4 at the time of sampling. At a more representative world average ratio of four, the uptake of Na+ would be 20% of the dissolved fluvial load, and that for K+ and Mg2+ about 10%. Over the annual cycle of the Amazon, it is estimated that ion exchange has a still smaller effect, as a consequence of the low average suspended-solids:dissolved-solids ratio of 1.7.Variations in the ratio , the equivalent fraction of exchangeable Ca2+ and Mg2+, throughout the river, can be described by a single isotherm. This same isotherm accurately describes the distribution of exchangeable Ca2+ and Mg2+ on sediment equilibrated with seawater, despite that a high proportion of exchange sites is occupied by Na+ and K+.
During June 1983, surface- and bottom-water samples were collected (over a 38-h period) at two anchor stations just seaward of the Amazon River mouth. Samples were analyzed for dissolved and particulate234Th activities.234Th distribution coefficients (Kd; ratio of particulate to dissolved234Th activities) exhibited significant temporal variability, ranging from 7.4 × 103 to 1.0 × 105ml g−1 in surface waters and from 8.8 × 102 to 3.4 × 104ml g−1 in bottom waters. Environmental parameters (e.g. salinity, suspended-sediment concentration) also exhibited a high degree of temporal variability as a result of tidal processes. Based on correlation with Kd, suspended-sediment concentration and particle characteristics were the major environmental parameters controlling the scavenging of thorium. Thorium distribution coefficients were never observed to reach equilibrium values in the study area, indicating that the scavenging of thorium was limited by reaction kinetics. A kinetic (non equilibrium) approach to scavenging reveals that suspended particles (because of their short residence time in the water column) had only 1–2 days to equilibrate with any particular hydrographic regime within the anchor-station area. The residence time of dissolved234Th (with respect to scavenging) ranged from 1.8 to 3.8 days, depending on suspended-sediment concentrations and particle characteristics. The total residence time of234Th (with respect to removal from the water column) was nearly uniform (4–5 days) throughout the anchor-station area, because locations with the shortest scavenging residence times (as a result of high turbidity) had the longest particle residence times (as a result of resuspension).
Water column and seabed samples were obtained from 92 stations on the Amazon continental shelf during October of 1979. Uptake of silica near and southeast of the river mouth began at a salinity of 8%. and accounted for 17% of the riverine silica flux to this region. Uptake northwest of the river mouth began at a salinity of 20%. and resulted in 33% removal of the riverine silica flux. Examination of filtered suspended solids revealed abundant diatoms in the surface waters, including Coscinodiscus. Skeletonema, Synedra. and Thalassiosira. The biological uptake of silica appears to be dependent on three factors: turbidity, turbulence, and nutrient availability. There was no evidence of abiological removal of silica in the Amazon estuary. 75 to 88% of the silica removed from surface waters by diatoms dissolves prior to accumulation in the seabed. Based on the mean biogenic silica content of shelf sediment (0.25%) and estimates of rates of sediment accumulation, the biogenic silica accumulation rate on the shelf is 2 × 1012 g/yr, which represents only 4% of the dissolved silica supplied by the Amazon River. Biological uptake of silica in estuarine surface waters may not accurately reflect permanent removal of biogenic silica to the seabed because of dissolution which occurs in bottom waters and near the sediment-water interface.
A general model is presented for mixing processes between river and ocean water in which are established criteria for the identification of any non-conservative behavior of the dissolved constituents involved. A review of previous data shows that in no case has removal of silica been demonstrated unambiguously in estuarine regimes. New data for iron which show highly non-conservative behavior are used in an example of the application of the model.
Thesis (Ph. D.)--Yale University, 1976. Includes bibliographical references (leaves 137-151). Photocopy.
The Connecticut River, its estuary, and the Amazon River estuary were studied to elucidate some of the processes which control river water chemistry and the flux of elements to the sea. The approach taken was to identify inputs to the Connecticut River and to investigate geochemical processes which modify the dissolved load. The form and quantity of nuclides which are in turn supplied to the estuary are altered by processes unique to that transition zone to the ocean. The Connecticut River estuary was sampled on a seasonal basis to investigate the role of the estuary in controlling the flux of elements to the sea. The knowledge gained from the Connecticut River study was applied to the quantitatively more significant Amazon River estuary. There a variety of samples were analyzed to understand the processes controlling the single greatest flux of elements to the Atlantic Ocean. The results indicate that estimates of the total flux of nuclides to the oceans can best be calculated based on groundwater inputs. Unless significant repositories for nuclides exist in the river-estuarine system, the groundwater flux of dissolved nuclides is that which will eventually be delivered to the ocean despite the reactions which were shown to occur in both rivers and estuaries. 153 references, 63 figures, 28 tables.
Approximately 25 percent of the dissolved silica carried by the Amazon River is depleted through diatom production in the inner estuary. Annual production of opaline frustules is estimated to be 15 million tons. However, few diatoms accumulate in modern shelf sediments and chemical recycling appears to be slight. Instead, many frustules apparently are transported landward into the river system, where they deposit in dunes and layers on and within mud and sand bars.