River-dominated Ocean Margins Research Articles

Molecular and radiocarbon constraints on the fate of sedimentary organic carbon in a human-impacted river-dominated ocean margin

Organic carbon (OC) burial in river-dominated ocean margins plays a pivotal role in the global carbon cycle, impacting atmospheric CO2 levels over the long term. Despite its significance, uncertainties persist regarding the influence of external environmental factors and intrinsic properties on sedimentary OC. In this study, we conducted a comprehensive analysis of surface sediments from the East China Sea, examining geochemical properties (including total OC content [TOC], Δ14C, δ13C, and C/N ratio), terrestrial biomarkers (n-alkanes), and mineral properties (such as specific surface area, Al/Si ratio, and mineral composition). Our aim was to shed light on the fate of sedimentary OC.The surface sediment's Δ14C values displayed significant spatial heterogeneity, delineating four distinct sub-regions. Strong positive correlations (all p < 0.01) were found between the ∆14C values and fine-grained sediments, specific surface area, and clay minerals, suggesting the potentially pivotal role of mineral protection in shaping the fate of sedimentary OC. The proportion of terrestrial OC gradually decreased towards the south, while marine OC proportion increased, corresponding to the enrichment of Δ14C. The co-variation of Δ14C values, mineral properties, and OC source proportions suggests that terrestrial OC may undergo progressive replacement by marine OC during southward transport. Temporal variations in ∆14C values indicated that seabed erosion led to a significant increase in ∆14C values (p < 0.01) in the coastal mud belt, a phenomenon likely common in river-dominated ocean margins globally due to the new sediment cycle during the Anthropocene.

First observations of seasonal bottom water deoxygenation off the Gironde estuary (Bay of Biscay, North East Atlantic)

Although not systematically considered as oxygen minimum zones, River-dominated Ocean Margins are sensitive to oxygen depletions. For example, the continental shelf off the Gironde, which flows into the Bay of Biscay, had not yet been studied from this perspective although recent simulations suggested that this area should already have experienced deoxygenations in recent decades. To fill this gap, profiles of temperature, salinity and dissolved oxygen were performed in the water column of the continental shelf off the Gironde during seven cruises distributed over the different seasons between 2016 and 2021. Turbidity, chlorophyll-a and pH were also measured during some of these cruises. In winter, the water column was slightly stratified due to high river flows. Then, a seasonal thermal stratification was present from spring to autumn. Similarly, dissolved oxygen showed a seasonal dynamic with: a well-oxygenated water column in winter, an oxygen oversaturation in the first 20 meters during the spring bloom, and then a progressive oxygen depletion in bottom waters until reaching an oxygen saturation minimum down to 45% in autumn. These deoxygenations are explained by the seasonal stratification that isolates the bottom waters from spring to autumn, and are likely enhanced by the advection of deoxygenated waters from the north of the Bay of Biscay and the settling of the organic matter produced in surface waters. A better understanding of these processes in the context of global warming undoubtedly requires better documentation of dissolved oxygen variability through the implementation of a long-term and continuous in situ monitoring.

Spatiotemporal dynamics of surface sediment characteristics and benthic macrofauna compositions in a temperate high-energy River-dominated Ocean Margin

The benthic compartment of River-dominated Ocean Margins (RiOMar) is largely affected by sedimentary processes, as well as by natural and anthropogenic disturbances. Recent studies have confirmed the major importance of riverine inputs and local hydrodynamics in the spatial structuration of low- and high-energy temperate RiOMar, respectively. Differences in the nature of these structuring factors could also affect the temporal dynamics of these two types of systems. The present study is aiming at: (1) quantifying spatiotemporal changes in surface sediment and benthic macrofauna within the West Gironde Mud Patch (WGMP; high-energy system) over both short (2016–2018) and longer (2010/2016–2018) time scales, (2) identifying the main environmental factors explaining those changes, and (3) achieving a comparison with the Rhône River Prodelta (RRP; low-energy system) in view of further characterizing the functioning of the benthic components of these two temperate RiOMar. Surface sediment characteristics (grain size, quantitative and qualitative descriptors of particulate organic matter) and benthic macrofauna compositions were assessed based on 4 seasonal sampling of 5 stations located along a depth gradient within the WGMP. Results highlighted the existence of spatial patterns for both surface sediment and benthic macrofauna, which are both better explained by local hydrodynamics. Most variables presented seasonal changes. Benthic macrofauna compositions also showed pluri-annual changes, which were attributed to a cicatrization process following a major disturbance caused by the 2013–2014 series of severe winter storms, and suggests the major role of local hydrodynamics in explaining 2010/2016–2018 temporal changes in WGMP benthic macrofauna compositions. The comparison with the RRP further highlighted major discrepancies between the two systems in the main processes (i.e., hydrodynamics versus river hydrological regime) explaining surface sediment characteristics and benthic macrofauna compositions, which supports current RiOMar typologies.

Anthropogenic perturbations to the fate of terrestrial organic matter in a river-dominated marginal sea

River-dominated ocean margins (RiOMar) are major terrestrial organic carbon (OCterr) repositories that play an important role in the global carbon cycle. However, riverine inputs of sediment and associated OCterr have decreased substantially due to intensifying human activity. Thus, studying OCterr processing during transport to and within RiOMar is crucial. The East China Sea (ECS) receives reduced sediment loads from the Changjiang River (CJR), which is compounded further by the impoundment of the Three Gorges Reservoir. Here, we examined the CJR-ECS source-to-sink system to understand the fate of OCterr in a typical RiOMar. We analyzed the sedimentary properties (grain size and the specific surface area), bulk organic carbon properties (C/N ratio and δ13C), and molecular biomarkers (n-alkanes and GDGTs) of riverine suspended particulate matter (SPM) and surficial marine sediment collected in 2006 and 2018. The C/N ratio, δ13C value, and short-chain n-alkanes abundances in the riverine SPM indicate that phytoplankton productivity could be an important source of OCterr in the basin due to the reduced sediment loads. Seabed erosion occurred in the estuarine–inner shelf areas of the ECS as a result of reduced riverine inputs, which resulted in a change in the distribution of sedimentary OCterr. The reduced total organic carbon content and n-alkanes abundance are indicative of the low sedimentary OCterr content. Variations in GDGT-based indices are further manifestation of reduced sediment load, as they show increased in situ production of GDGTs. Spearman correlations indicate that the biomarker abundances were positively correlated with silt, which is in accordance with silt being easily shaped by hydrodynamic sorting. Biomarker abundances along four estuarine–inner shelf transects also exhibited pronounced alongshore or cross-shelf OCterr transport. Furthermore, the relationship between the biomarkers and sedimentary properties indicates that mineral protection plays an important role in regulating the fate of OCterr on continental margins. Therefore, variations in riverine inputs and the sedimentary environment caused by anthropogenic perturbations have affected the abundance, distribution, composition, and transport of sedimentary OCterr in the study area. These findings can be applicable to other RiOMar, as humans continue to shape riverine inputs globally.

Seasonal Mixing and Biological Controls of the Carbonate System in a River-Dominated Continental Shelf Subject to Eutrophication and Hypoxia in the Northern Gulf of Mexico

Large rivers export a large amount of dissolved inorganic carbon (DIC) and nutrients to continental shelves; and subsequent river-to-sea mixing, eutrophication, and seasonal hypoxia (dissolved oxygen &amp;lt; 2 mg⋅L–1) can further modify DIC and nutrient distributions and fluxes. However, quantitative studies of seasonal carbonate variations on shelves are still insufficient. We collected total alkalinity (TA), DIC, and NO3– data from nine cruises conducted between 2006 and 2010 on the northern Gulf of Mexico continental shelf, an area strongly influenced by the Mississippi and Atchafalaya Rivers. We applied a three-end-member model (based on salinity and potential alkalinity) to our data to remove the contribution of physical mixing to DIC and nitrate distribution patterns and to derive the net in situ removal of DIC and nitrate (ΔDIC and ΔNO3–, respectively). Systematic analyses demonstrated that the seasonal net DIC removal in the near-surface water was strong during summer and weak in winter. The peak in net DIC production in the near-bottom, subsurface waters of the inner and middle sections of the shelf occurred between July and September; it was coupled, but with a time lag, to the peak in the net DIC removal that occurred in the near-surface waters in June. A similar 2-month delay (i.e., January vs. November) could also be observed between their minima. A detailed examination of the relationship between ΔDIC and ΔNO3– demonstrates that net biological activity was the dominant factor of DIC removal and addition. Other effects, such as air–sea CO2 gas exchange, wetland exports, CaCO3 precipitation, and a regional variation of the Redfield ratio, were relatively minor. We suggest that the delayed coupling between eutrophic surface and hypoxic bottom waters reported here may also be seen in the carbon and nutrient cycles of other nutrient-rich, river-dominated ocean margins worldwide.

River-dominated pCO2 dynamics in the northern South China Sea during summer: A modeling study

River-dominated ocean margins (RiOMars), characterized by river plumes and abundant riverine nutrient inputs, are especially critical in determining the oceanic uptake of atmospheric CO2. Using a well validated three-dimensional, coupled physical-biogeochemical model, we examined the dynamics of the carbonate system in the Pearl River Plume (PRP) during summer over a typical RiOMar in the northern South China Sea (NSCS). Sea surface pCO2 in the PRP was mainly influenced by a combination of physical processes, air-sea exchange, and biological activity. The interplay between these complex processes differed spatially and temporally depending on the evolution of the PRP. The latter was divided into three sub-regions: near-, mid- and far-field. In the near-field PRP, the evolution of surface pCO2 was primarily influenced by biological activity. Surface pCO2 decreased substantially at the initial stage as a result of phytoplankton blooms, and then increased due to the reduction in the phytoplankton and increase of zooplankton and detritus. In the mid-field, surface pCO2 was initially dominated by air-sea exchange. Subsequently, the rates of biological processes exceeded the rate of air-sea exchange, resulting in a strong drawdown of surface pCO2 or a strong sink for atmospheric CO2. The far-field of the PRP acted as a weak CO2 sink, where surface pCO2 was dominated by air-sea exchange as biological processes were fairly weak. In addition, given that the air-sea CO2 equilibrium time is much longer than water residence time of the PRP, the biologically-mediated low pCO2 surface water was enabled to be transported far away from estuary. Taken together, the combined effect of enhanced primary production, strong plume current and strong seawater carbonate buffering capacity were responsible for maintaining low surface pCO2 levels in this subtropical RiOMar system.

Influence of coastal Mediterranean rivers on the organic matter composition and reactivity of continental shelf sediments: The case of the Têt River (Gulf of Lions, France)

River-dominated ocean margins (RiOMars) are areas of high productivity, rapid sediment deposition, and intense recycling. The Gulf of Lions, located in the North occidental Mediterranean Sea, is no exception to this general rule and is under the prevalent influence of the Rhône River. One particularity of this continental shelf is that it is also fed by several small coastal rivers whose inputs actively contribute to the coastal dynamic and export to the deep-sea. Our objective was to gauge the influence of coastal rivers on the quality and reactivity of the organic matter delivered to the continental shelf as these properties control two important functions of coastal areas: benthic productivity and carbon sequestration. We chose for this study, the Têt River, which is typical of coastal Mediterranean rivers with a torrential regime. Sediment cores were sampled five times over a one-year period on four stations located along the dominant trajectory of the Têt River plume. The biogeochemical characteristics of the surface sediments were analysed to highlight spatio-temporal trends in the sedimentary organic matter composition and to identify the major environmental factors controlling its reactivity. Nutrient and dissolved oxygen fluxes at the water-sediment interface were furthermore measured at two stations using ex-situ whole core incubations.Results from this study highlight the seasonal influence of riverine versus autochthonous productions on the quality of sedimentary organic matter in the vicinity of the river mouth. Farther on the mid shelf, a terrestrial imprint is still visible, but the temporal signal is blurred as a consequence of strong mixing during frequent episodes of resuspension/deposition. Organic matter is efficiently remineralised in this system with total oxygen uptake (TOU) rates comparable to those measured off the Rhône River. Finally, organic matter quality and reactivity are also constrained in this system by physical forcings, which promote remineralisation and advection of sediments.

The important role of submarine groundwater discharge (SGD) to derive nutrient fluxes into River dominated Ocean Margins – The East China Sea

Abstract Submarine groundwater discharge (SGD), which has been recognized as an important pathway for the transport of terrestrial chemical components (i.e., nutrients, trace elements and other contaminants) to the ocean, plays an important role on the biogeochemical cycling in marine environment, especially coastal/marginal seas. In the present work, the initial results on the amount of SGD into the continental shelf of the East China Sea (ECS), one of the marginal seas with large riverine input from the Changjiang River (Yangtze River) are evaluated. According to the locations and features of water masses in the ECS, radium (Ra) isotopes mass balance model was built and the conservative and non-conservative (excess) components of Ra were evaluated. Using the inventories of excess 228Ra and 226Ra, the residence time of water in the ECS was estimated as 1.30 ± 0.27 years. Then the SGD flux was estimated to be (5.42 ± 0.14) × 1011 m3 yr−1, which were 47 ± 1% of the total river discharge into the ECS along the coast. Nutrient fluxes driven by SGD were estimated to be (7.32 ± 0.19) × 1010, (1.79 ± 0.05) × 109 and (1.59 ± 0.04) × 1011 mol yr−1 for dissolved inorganic nitrogen (DIN), dissolved inorganic phosphorus (DIP) and dissolved inorganic silicate (DSi), respectively, which were about 0.7, 2.2 and 1.4 times, respectively, of the riverine inputs. Furthermore, SGD-driven nutrient had obviously high DIN/DIP ratios, which could lead to a number of large-scale environmental problems to the ECS, such as the frequent harmful algal blooms and hypoxia especially in the estuary and coastal area.

Diversity, Abundance, and Niche Differentiation of Ammonia-Oxidizing Prokaryotes in Mud Deposits of the Eastern China Marginal Seas.

The eastern China marginal seas (ECMS) are prominent examples of river-dominated ocean margins, whose most characteristic feature is the existence of isolated mud patches on sandy sediments. Ammonia-oxidizing prokaryotes play a crucial role in the nitrogen cycles of many marine environments, including marginal seas. However, few studies have attempted to address the distribution patterns of ammonia-oxidizing prokaryotes in mud deposits of these seas. The horizontal and vertical community composition and abundance of ammonia-oxidizing archaea (AOA) and bacteria (AOB) were investigated in mud deposits of the South Yellow Sea (SYS) and the East China Sea (ECS) by using amoA clone libraries and quantitative PCR. The diversity of AOB was comparable or higher in the mud zone of SYS and lower in ECS when compared with AOA. Vertically, surface sediments had generally higher diversity of AOA and AOB than middle and bottom layers. Diversity of AOA and AOB showed significant correlation with latitude. Nitrosopumilus and Nitrosospira lineages dominated AOA and AOB communities, respectively. Both AOA and AOB assemblages exhibited greater variations across different sites than those among various depths at one site. The abundance of bacterial amoA was generally higher than that of archaeal amoA, and both of them decreased with depth. Niche differentiation, which was affected by dissolved oxygen, salinity, ammonia, and silicate (SiO), was observed between AOA and AOB and among different groups of them. The spatial distribution of AOA and AOB was significantly correlated with δ15NTN and SiO, and nitrate and δ13C, respectively. Both archaeal and bacterial amoA abundance correlated strongly with SiO. This study improves our understanding of spatial distribution of AOA and AOB in ecosystems featuring oceanic mud deposits.

Occurrence of aragonite corrosive water in the North Yellow Sea, near the Yalu River estuary, during a summer flood

To understand seawater CaCO3 saturation state (Ωarag for aragonite and Ωcal for calcite) dynamics in river-dominated ocean margins, we investigated the carbonate system and ancillary parameters in the North Yellow Sea, near the Yalu River estuary, during and after a summer flood (August and September 2013, respectively). During the flood period, nearshore Ωarag ranged from 0.76 to 1.61 and Ωcal ranged from 1.22 to 2.53. Aragonite undersaturation values were associated with quite low salinity of 14.58–25.18 and a broad dissolved oxygen saturation range from 58% to 102%. A week after the flood period, aragonite undersaturated waters together with the estuarine plumes disappeared. Based on field data analyses and simplified model simulation, we suggest that net community respiration/remineralization and the dilution of riverine water discharge both contributed to the low CaCO3 saturation states.

Sources and preservation of sedimentary organic matter in the Southern Bohai Sea and the Yellow Sea: Evidence from lipid biomarkers

The region of Southern Bohai Sea (SBS) and Yellow Sea (YS) represents one of the best examples of large river-dominated ocean margins. The transport and accumulation of sediments played a primary role in distribution of the lipids in the region and could lead to a preferential accumulation of these more refractory land-based lipids. The spatial distribution of total organic carbon (TOC) matched with the lipid distributions over the shelf, and high TOC presented in the central YS, but TOC burial fluxes decreased from 110ton/km2/yr of SBS to 25–32ton/km2/yr of YS. Good correlations were observed between terrestrial n-alkanes and TOC in the sediments of SBS and northern YS, but they were poor for southern YS, meanwhile higher ratios of nC27+nC29+nC31/nC15+nC17+nC19 (12±6) were observed in SBS, and lower in YS (2.2±1.0), suggesting the Yellow River discharge was an important source of terrestrial organic matter to the shelf.

The fate of river organic carbon in coastal areas: A study in the Rhône River delta using multiple isotopic (δ13C, Δ14C) and organic tracers

A significant fraction of the global carbon flux to the ocean occurs in River-dominated Ocean Margins (RiOMar) although large uncertainties remain in the cycle of organic matter (OM) in these systems. In particular, the OM sources and residence time have not been well clarified. Surface (0–1cm) and sub-surface (3–4cm) sediments and water column particles (bottom and intermediate depth) from the Rhône River delta system were collected in June 2005 and in April 2007 for a multi-proxy study. Lignin phenols, black carbon (BC), proto-kerogen/BC mixture, polycyclic aromatic hydrocarbons (PAHs), carbon stable isotope (δ13COC), and radiocarbon measurements (Δ14COC) were carried out to characterize the source of sedimentary organic material and to address degradation and transport processes. The bulk OM in the prodelta sediment appears to have a predominantly modern terrigenous origin with a significant contribution of modern vascular C3 plant detritus (Δ14COC=27.9‰, δ13COC=−27.4‰). In contrast, the adjacent continental shelf, below the river plume, seems to be dominated by aged OM (Δ14COC=−400‰, δ13COC=−24.2‰), and shows no evidence of dilution and/or replacement by freshly produced marine carbon. Our data suggest an important contribution of black carbon (50% of OC) in the continental shelf sediments. Selective degradation processes occur along the main dispersal sediment system, promoting the loss of a modern terrestrial OM but also proto-kerogen-like OM. In addition, we hypothesize that during the transport across the shelf, a long term resuspension/deposition loop induces efficient long term degradation processes able to rework such refractory-like material until the OC is protected by the mineral matrix of particles.

Modeling biogeochemical processes in sediments from the Rhône River prodelta area (NW Mediterranean Sea)

Abstract. In situ oxygen microprofiles, sediment organic carbon content, and pore-water concentrations of nitrate, ammonium, iron, manganese, and sulfides obtained in sediments from the Rhône River prodelta and its adjacent continental shelf were used to constrain a numerical diagenetic model. Results showed that (1) the organic matter from the Rhône River is composed of a fraction of fresh material associated to high first-order degradation rate constants (11–33 yr−1); (2) the burial efficiency (burial/input ratio) in the Rhône prodelta (within 3 km of the river outlet) can be up to 80 %, and decreases to ~20 % on the adjacent continental shelf 10–15 km further offshore; (3) there is a large contribution of anoxic processes to total mineralization in sediments near the river mouth, certainly due to large inputs of fresh organic material combined with high sedimentation rates; (4) diagenetic by-products originally produced during anoxic organic matter mineralization are almost entirely precipitated (&gt;97 %) and buried in the sediment, which leads to (5) a low contribution of the re-oxidation of reduced products to total oxygen consumption. Consequently, total carbon mineralization rates as based on oxygen consumption rates and using Redfield stoichiometry can be largely underestimated in such River-dominated Ocean Margins (RiOMar) environments.

U.S. and Chinese Scientists Discuss the Ocean's Response to Climate Change: Climate Change and Coastal Oceans Workshop; Qingdao, China, 26–28 October 2008

A 3‐day workshop was held in China to discuss coastal ocean processes, the biogeochemistry of large river‐dominated ocean margins (RiOMars), and climate change and variability studies; to formulate a strategy for a joint venture to assess how climate change has affected coastal oceans; and to predict the ocean's response to future change scenarios. The workshop, which brought together experts from Texas A&amp;M University (TAMU) and five Chinese universities and institutes—Ocean University of China (OUC), Institute of Oceanology of the Chinese Academy of Sciences (IOCAS), Xiamen University (XU), Tianjin University of Science and Technology (TUST), and East China Normal University (ECNU), highlighted the similarities in topics important to North America and Asia.

Diagenetic state and source characterization of marine sediments from the inner continental shelf of the Gulf of Cádiz (SW Spain), constrained by terrigenous biomarkers

Surface sediments from the Gulf of Cádiz (GoC) were analyzed by alkaline CuO oxidation, in order to estimate the contribution of terrigenous organic matter (TOM) to the inner continental shelf of the southwest Iberian Peninsula. The parallel analysis of sediment samples from the two most important rivers draining to this coastal area (i.e. Guadiana River and Tinto–Odiel fluvial system) provided fundamental information regarding local terrestrial sources. Relatively constant intensive lignin parameters (S:V=1.0±0.1 and C:V=0.22±0.04) and high values of the lignin phenol vegetation index (LPVI=155±43) indicated that non-woody angiosperm tissues constitute the dominant component of vascular plant material reaching the shelf sediments. The NW to SE decreasing isotopic (13C) and molecular (Λ8) signatures found among the sediments, coinciding with the Guadiana delivery plume, suggest that this river is the main terrestrial source in the inner GoC shelf. Slightly elevated values of degradation indicative ratios ([Ad:Al]V=0.41±0.10; [Ad:Al])S=0.34±0.07; [3,5-Bd:V]=0.14±0.05; P:[V+S]=0.24±0.09) suggested the alteration state of the shelf sediments. The two fold higher ratios of the river sediments (Guadiana: [Ad:Al]V=0.82±0.08; [Ad:Al]S=0.84±0.03; Tinto–Odiel: [Ad:Al]V=0.86±0.12; [Ad:Al]S=0.83±0.013) and the increasing degradation trend observed outward in the shelf, lead us to consider preferential sorption processes, instead of in situ diagenesis, to affect the degradation signature of the shelf sediments. Preferentially solubilized degraded OM is more likely to be sorbed and stabilized prior to transport to the marine system, showing an apparently more advanced degradation state. The use of the 3,5-Bd:V ratio in conjunction with (Ad:Al)V revealed a composition continuum of the sedimentary OM ranging from fresh plant materials to highly altered soil humic constituents. Elemental and molecular analyses show a land to sea gradient by a NW to SE decrease of the terrestrial influence, accounting for larger terrestrial inputs (TOM: 71–98%) in those sediments near the Guadiana mouth, and predominantly autochthonous composition (TOM: 42–50%) in those located offshore. This work utilizes lignin derived biomarkers to determine the contribution of terrigenous OM delivered to this poorly described coastal area from regional rivers. Within a context of increasing international efforts to better understand the global C cycling, this study illustrates the relevance of using the alkaline CuO oxidation approach to evaluate C budgets and continental influence in river dominated ocean margins.

Use of lipid biomarker patterns as a proxy of environmental variability in the coastal sedimentary record from the Gulf of Cádiz (SW Spain)

Surface sediments from the Gulf of Cádiz were analyzed using gas chromatography–mass spectrometry in order to estimate the origin, spatial distribution and degradation extent of the sedimentary organic matter (OM) from the inner continental shelf of the southwest Iberian Peninsula. A wide variety of lipid assemblages ( n-alkanes, n-alkan-2-ones, n-aldehydes, n-fatty acids, α, ω-alkanedioic acids, diterpene resin acids and isoprenoids) were extracted with organic solvent and exhibited local variations depending on OM sources. Whereas the sediments located under the Guadiana river’s influence showed dominant continental derived, long chain (>C 20) distributions, typically marine-derived short chain (<C 20) alkyl compounds were detected in the offshore sediments. Miscellaneous OM source signatures, derived from algae, sea grasses, microbial biomass and higher plants, were displayed by those sediments of intermediate location, indicating a NW-to-SE transition from terrigenous to marine dominant composition. The relative abundance of unsaturated C 16:1 and C 18:1 fatty acids, the presence of the strong reductive nature of the n-aldehydes, the occurrence of molecular sulfur in most of the extracts and the low pristane/phytane ratio values detected appear to suggest little drastic alteration in the sedimentary OM in the area. The study illustrates the relevance of using molecular biomarkers combined with bulk geochemical proxies to evaluate OM sources and continental influence in river dominated ocean margins.

Comparison of hypoxia among four river-dominated ocean margins: The Changjiang (Yangtze), Mississippi, Pearl, and Rhône rivers

We examined the occurrence of seasonal hypoxia (O 2<2 mg l −1) in the bottom waters of four river-dominated ocean margins (off the Changjiang, Mississippi, Pearl and Rhône Rivers) and compared the processes leading to the depletion of oxygen. Consumption of oxygen in bottom waters is linked to biological oxygen demand fueled by organic matter from primary production in the nutrient-rich river plume and perhaps terrigenous inputs. Hypoxia occurs when this consumption exceeds replenishment by diffusion, turbulent mixing or lateral advection of oxygenated water. The margins off the Mississippi and Changjiang are affected the most by summer hypoxia, while the margins off the Rhône and the Pearl rivers systems are less affected, although nutrient concentrations in the river water are very similar in the four systems. Spring and summer primary production is high overall for the shelves adjacent to the Mississippi, Changjiang and Pearl (1–10 g C m −2 d −1), and lower off the Rhône River (<1 g C m −2 d −1), which could be one of the reasons of the absence of hypoxia on the Rhône shelf. The residence time of the bottom water is also related to the occurrence of hypoxia, with the Mississippi margin showing a long residence time and frequent occurrences of hypoxia during summer over very large spatial scales, whereas the East China Sea (ECS)/Changjiang displays hypoxia less regularly due to a shorter residence time of the bottom water. Physical stratification plays an important role with both the Changjiang and Mississippi shelf showing strong thermohaline stratification during summer over extended periods of time, whereas summer stratification is less prominent for the Pearl and Rhône partly due to the wind effect on mixing. The shape of the shelf is the last important factor since hypoxia occurs at intermediate depths (between 5 and 50 m) on broad shelves (Gulf of Mexico and ECS). Shallow estuaries with low residence time such as the Pearl River estuary during the summer wet season when mixing and flushing are dominant features, or deeper shelves, such as the Gulf of Lion off the Rhône show little or no hypoxia.

Comparison of hypoxia among four river-dominated ocean margins: The Changjiang (Yangtze), Mississippi, Pearl, and Rhône rivers

Comparison of hypoxia among four river-dominated ocean margins: The Changjiang (Yangtze), Mississippi, Pearl, and Rhône rivers

An evaluation of mobile mud dynamics in the Mississippi River deltaic region

Large rivers are the primary interface between terrestrial and ocean environments. A relatively small number of rivers account for a disproportionate amount of the freshwater and suspended materials that are delivered to the coastal ocean. Sediment delivery to these coastal systems plays a key role in the global carbon cycle since deltas and continental shelves are considered to be the main repositories of organic carbon in marine sediments. Particulate material in these environments are typically deposited and resuspended several times before permanent accumulation or transport off the shelf. This sediment cycling is an important component influencing biogeochemical processes that occur in coastal environments. During two cruises in April and October 2000 on the shelf adjacent to the Mississippi River, water and sediment samples were collected for analysis of suspended solids and particle reactive radionuclides ( 210Pb, 137Cs, 7Be and 234Th) to evaluate the transport and fate of terrestrial and marine material. A comparison of the distribution of these tracers provides insight about the pathways and residence times of particulate materials on the shelf. Inventories of these short-lived radiotracers showed variations of more than two orders of magnitude, indicating dramatic variations in sediment deposition between sampling events. Short-lived radiotracers indicate that river-borne materials are transported less than ∼30 km from the river mouth before initial deposition. However, seasonal variations in 7Be and 137Cs indicate significant remobilization of sediment and potential export of sediment out of the study area during the high energy (e.g., wind/wave) winter months. In addition, depth profiles of 7Be and excess 234Th indicate sediment deposition rates between 0.8 and 3.9 cm month −1 (0.4–2.1 g cm −2 month −1) at two shelf locations (near river and open shelf environments). These rates are much greater than those observed on decadal time scales (1.3–2.0 cm year −1 or 0.6–1.5 g cm −2 year −1) via 210Pb at the same sites. This further substantiates active sediment reworking and potential export off the shelf as has been observed in other river-dominated ocean margins.

Transport and transformation of dissolved and particulate materials on continental margins influenced by major rivers: benthic boundary layer and seabed processes

Within the benthic boundary layer (BBL) and seabed of river-dominated ocean margins (RiOMars), the timing, kinetics and extent of important biogeochemical processes are greatly influenced by large riverine inputs of dissolved and particulate terrestrial materials. An examination of our current state of knowledge reveals that the rates of primary productivity, sediment deposition, remineralization and burial in these margins are among the highest of all marine systems. Transport and transformation processes within the benthic region of these RiOMar areas are highly variable (temporally and spatially). As a result, measurement and modeling of these processes are very challenging. A more quantitative understanding of these systems will require coordinated interdisciplinary studies that: (a) better define the quantity and composition of riverine inputs; (b) greatly improve our current knowledge of transport and transformation within the BBL of these systems; (c) focus on the sequential timing of physical forcings (riverine discharge, high energy events); (d) develop new nonclassical diagenetic models; (e) further characterize and delineate differences between sub-environments within a RiOMar and between RiOMar “types”; and, (f) provide a better mechanistic understanding of what controls the net retention of terrestrial materials (diagenetic transformation vs. burial) within RiOMar systems.