River-dominated Continental Margin Research Articles

Sediments as a Source of Iron, Manganese, Cobalt and Nickel to Continental Shelf Waters (Louisiana, Gulf of Mexico)

Continental shelf sediments are a key source of trace metals to the ocean. In this study, we investigate the impact of sedimentary processes on water column concentrations of iron (Fe), manganese (Mn), cobalt (Co), and nickel (Ni) at five stations on the Louisiana continental shelf and slope, Gulf of Mexico. The highest trace metal concentrations were observed close to the seafloor at the most nearshore shelf station (water depth of 16 m), with most of the metals present in particulate form. This enrichment in the bottom water is likely the combined effect of input of trace metals in suspended matter from the Mississippi/Atchafalaya Rivers and, for Mn, Co, and Ni, benthic release from the shelf sediments. While particulate matter was the dominant carrier of Fe and Mn in bottom waters in the shelf and slope regions, Co and Ni were nearly exclusively present in dissolved form. Hence, lateral transport of Co and Ni in shelf waters is decoupled from that of Fe and Mn. Concentrations of particulate and dissolved trace metals in the water column generally decreased from the shelf to the slope, while those in the sediment increased. This suggests an increased retention of metals deposited on the sediment with distance from the coast, linked to the decrease in organic matter input and associated reductive sediment processes. The offshore decline in sediment trace metal mobilization is likely typical for river-dominated continental margins where most organic matter is deposited close to the coast.

Roles of sediment supply, geochemical composition and monsoon on organic matter burial along the longitudinal mud belt in the East China Sea in modern times

Terrestrial sediment supply and its composition, aquatic productivity and coastal hydrodynamics together control the nature of organic matter (OM) that buried in ocean margins. Distinguishing their individual roles in river-dominated continental margins is challenging, but crucial for understanding (i) the regional carbon cycle in modern times and (ii) the OM burial-climate link on shorter timescales. Here we investigate the contents of total organic carbon (TOC) and total nitrogen (TN) and stable carbon isotopic composition of TOC (δ13C), along with grain size and calcium carbonate, in three short, 210Pb- and 137Cs-dated, sediment cores (DH1-1, DH3-3 and DH5-1) from the Yangtze distal mud belt in the East China Sea to delineate the spatiotemporal distribution, origin and burial flux of sedimentary OM for the last ca. 130 years. We also use selected inorganic elements (Si, Al, Ca, Na, K, Rb and Sr) and geochemical proxies derived from them (i.e. chemical index of alteration-CIA, molar Al/Si, molar K/Na, K/Rb and Rb/Sr) to elucidate the importance of detrital composition and chemical weathering on the supply, transport and burial of OM.Bulk organic geochemical and grain size data suggest a longer transport of fine sediments-laden OM, indicating the role of China Coastal Current on hydrodynamic sorting in the study area. Spatial distribution of Al/Si, K/Na, Rb/Sr and CIA in all cores corroborates the selective transport of fine-grained minerals from the Yangtze estuary to the distal mud belt. Less variable molar C/N ratios (5.6–7.7) and δ13C values (–22.4 to –21.2‰) in our cores and the results of δ13C mixing model indicate the predominance of marine OM burial over the last century in the study area. Relationships of TOC, δ13C and mean grain size and spatial evolution of δ13C along the river-mud belt continuum show that adsorption of marine OM on the Yangtze-derived fine sediments during their southward transport. Estimated mean burial fluxes of terrestrial and marine OC (0.73 ± 0.21 and 1.88 ± 0.29 Tg C yr−1) indicate that the Yangtze distal mud belt, despite covering ~18% of the shelf area, acts as a key OM sequestration center (>50%) in the East China Sea. Comparison of records of riverine OC burial and geochemical weathering and grain size proxies of our cores with East Asian winter monsoon index, and sediment and water discharges in the Yangtze River confirms that hydrodynamics associated with the winter monsoon winds-driven China Coastal Current predominantly controlled the sediment accumulation and OM burial along the mud belt before 1970s. Nonetheless, human activities especially the construction of Three Gorges Dam have reduced the burial flux of terrestrial OC from 0.93 to 0.54 Tg yr−1, significantly affecting the supply and burial of OM and thus the regional carbon cycle since 1970s.

Variation in sedimentary 210Pb over the last 60 years in the Yangtze River Estuary: New insight to the sedimentary processes

Anthropogenic activities have decreased the sediment transport from catchment to the river-dominated continental margin mainly through river damming that controlled the estuarine and associated shelf modern sedimentation. In this study, radioisotopes 210Pb (210Pbex and 226Ra) and 137Cs, grain size, and X-radiograph were combined to evaluate the modern sedimentary processes in the Yangtze River Estuary (YRE). We found that seabed cores collected on the estuary and associated shelf show a direct temporal correlation between decreases in initial activity and burial flux of 210Pbex and suspended sediment load in relation to river damming and soil conservation from 1950 to 2016. Most of sediment cores are characterized by low, uniform and reverse 210Pb patterns at surface and negligible sediment accumulation rates (SARs), indicating that due to the limited sediment supply following the construction of the Three Gorges Dam in 2003, erosion and remobilization of fine-grained sediments by energetic tidal currents and biological mixing tends to have become determinant forces in controlling the fate of modern sediment burial in the YRE. Moreover, these results indicated that decline in sediment load and additional input of eroded materials will largely violate the basic assumption of constant 210Pb flux, raising a substantial challenge for 210Pb chronology in the river-dominated continental margins. However, we suggest that radiotracer 210Pb could provide critical information for understanding the complex the sedimentary processes (e.g., terrestrial sediment supply and redistribution by tidal currents) and assessing past environmental status on the river- and/or tidal-dominated estuaries subject to human modification.

The fate of organic carbon burial in the river-dominated East China Sea: Evidence from sediment geochemical records of the last 70 years

The accumulation of organic carbon (OC) in the sediments of river-dominated continental margins plays a critical role in the global carbon budget and is subject to human modification. However, the relationships between human activities in drainage basins and carbon burial in modern sediments are still poorly understood. Here, a well preserved sediment core from a mud zone in the East China Sea (ECS) was analyzed for isotopes (δ13Corg and δ15N), radioisotopes (210Pb and 137Cs), bulk organic carbon and nitrogen, sediment grain size, major elements, trace metals and chemical properties (pH and Eh), in order to reveal the fate of the OC buried in the studied shelf over the last 70 years. The results showed that the input of terrestrial organic matter (TOM) from the Yangtze River into the ECS declined by 15% between 1980 and 2005, in concomitance with decreasing terrestrial-derived Ca and Zr. These changes were mainly due to alterations in the sediment load and cultivated land, induced by river damming and soil conservation practices. The amount of marine organic matter (MOM) was strongly and positively correlated with chlorophyll a concentrations and the loads of dissolved inorganic nitrogen and phosphate. The discharge of riverine nutrients from artificial fertilizers seems to be one of the key factors influencing marine productivity. OC burial fluxes varied between 33.8 and 54.7 g C m−2 yr−1 and were primarily related to changes in the mass accumulation rates (MARs) and OC%. Notably, due to the construction of the Three Gorges Dam (TGD) in 2003, the input of eroded material from the downstream and subaqueous delta of the Yangtze River increased, profoundly altering the local hydrodynamics and sedimentary redox levels. These changes may, in turn, accelerate the degradation of sedimentary OC, preventing carbon deposition and preservation.

Environmental factors shaping the archaeal community structure and ether lipid distribution in a subtropic river and estuary, China

Archaea are widespread and abundant in aquatic and terrestrial habitats and play fundamental roles in global biogeochemical cycles. Archaeal lipids, such as isoprenoid glycerol diakyl glycerol tetraethers (iGDGTs), are important biomarkers tracing changes in archaeal community structure and biogeochemical processes in nature. However, the linkage between the archaeal populations and the GDGT distribution in the natural environment is poorly examined, which hindered the application and interpretation of GDGT-based climate or environmental proxies. We addressed this question by investigating changes in archaeal lipid composition and community structure in the context of environmental variables along the subtropical Jiulong River Watershed (JRW) and Jiulong River Estuary (JRE) in southern China. The results showed that both the archaeal cells and the polar GDGTs (P-GDGTs) in the JRW and JRE were mostly autochthonous rather than exogenous input from surrounding soils. We further found that only five (Methanobacteriales, Ca. Bathyarchaeota, Marine Benthic Groups A (MBGA), Marine Benthic Groups B (MBGB), and Marine Benthic Groups D (MBGD)) out of sixteen lineages showed significant impacts on the composition of P-GDGTs, suggesting the significant contribution of those archaea to the changes of P-GDGT compositions. Salinity and total phosphorus (TP) showed significant impact on the distribution of both genetic and P-GDGTs compositions of archaea; whereas, sand and silt contents only had significant impact on the P-GDGTs. MBGD archaea, which occur widely in marine sediments, showed positive correlations with P-TEX86 in the JRW and JRE, suggesting that uncultivated MBGD might also contribute to the variations in TEX86 signals in marine sediments. This study provided insight into the sources of P-GDGTs and the factors controlling their distributions in river-dominated continental margins, which has relevance to applications of GDGT-based proxies in paleoclimate studies.

Impact of river channel shifts on tetraether lipids in the Rhône prodelta (NW Mediterranean): Implication for the BIT index as an indicator of palaeoflood events

We tested the applicability of the BIT (branched and isoprenoid tetraether) index as a proxy for palaeoflood events in the river-dominated continental margin of the Gulf of Lions (NW Mediterranean). We compared the concentrations of branched glycerol dialkyl glycerol tetraethers (br GDGTs) and crenarchaeol in suspended particulate matter (SPM) collected downstream in the Rhône River, as well as in surface sediments and a ca. 8m piston core from the Rhône prodelta. The core covered the last 400yr, with four distinct intervals recording the river influence under natural and man-induced shifts in four main channels of the river mouth (Bras de Fer, Grand Rhône, Pégoulier and Roustan). The results indicate that there are mixed sources of br GDGTs and crenarchaeol in the prodelta, complicating application of the BIT index as an indicator of continental organic carbon input and thus as a palaeoflood proxy. However, the sedimentary BIT record for the period when continental material was delivered by the river more directly to the core site (Roustan phase; 1892 to the present) mimics the historical paleoflood record. This shows the potential of the BIT index as a palaeoflood proxy, provided that the delivery route of the continental material by rivers to the core sites remains constant over time. The study also highlights the idea that shifts in river channels should be taken into account for the use of the BIT index as a palaeoflood proxy.

Variability of tetraether lipids in Yellow River-dominated continental margin during the past eight decades: Implications for organic matter sources and river channel shifts

Glycerol dialkyl glycerol tetraethers (GDGTs) and bulk organic geochemical parameters were examined for a short core from the Bohai Sea, a Yellow River-dominated continental margin. A three end member mixing model using branched/isoprenoid tetraethers (BIT) index, δ13C and C/N shows that the average fractions of soil, marine and plant organic matter (OM) during the period of 1933–2011 are 67.7% (38–92%), 26.1% (0–58%) and 6.2% (0–23%), respectively. Abrupt changes of sedimentary OM compositions around 1953, 1976 and 1996 are synchronous with the Yellow River mouth relocations. The BIT index values (0.33–0.80) present a stronger correlation with crenarchaeol abundance (R2=0.88) than branched GDGTs abundance (R2=0.27), suggesting that variations of marine Thaumarchaeota abundance rather than soil OM inputs is the first order factor controlling the BIT index values, although this proxy has been widely used for soil OM. The comparison between the BIT index, nutrient status and historical Yellow River sediment load indicates that the high sensitivity of the BIT index to the Yellow River channel shifts cannot be explained by a nutrient stimulation mechanism, but instead is likely caused by the restriction of Thaumarchaeota growth in highly turbid water due to the enormous sediment inputs from Yellow River. Our study demonstrates that local conditions should be considered when applying the BIT index as an environmental proxy.

Sources and distributions of tetraether lipids in surface sediments across a large river-dominated continental margin

Glycerol dialkyl glycerol tetraether (GDGT)-based proxies are increasingly used in modern carbon cycling and palaeoenvironmental investigations. It is therefore crucial to examine the robustness (sources, transport and degradation) of all GDGT-based proxies in continental margins, where sedimentation rates and extent of carbon cycling are high. We have analyzed the distributions of GDGTs in surface sediments from the Lower Yangtze River and East China Sea (ECS) shelf. The results revealed multiple sources and complex shelf processes that govern the distributions. The isoprenoid GDGT-inferred sea surface temperatures (SSTs) are robust and reflect the satellite-derived annual mean SSTs on the shallow ECS shelf, confirming an origin from surface water column-dwelling crenarchaeota. The input from methanogen-sourced, isoprenoid GDGTs is significant in the river surface sediments but they are almost absent from the ECS shelf. Branched GDGTs are also abundant in the river sediments, but ca. 95% are degraded in the Yangtze estuary, a much greater extent than observed for other terrigenous organic matter (OM) proxies. There is also evidence for production of branched GDGTs in the oxic ECS shelf water column and the anoxic sediments/waters of the Lower Yangtze River. As a result, branched GDGT-based proxies in the lower river and ECS surface sediments do not reflect the catchment environmental conditions. The effective degradation in the estuary and widespread aquatic contributions of branched GDGTs improves our understanding of how to use branched GDGT-based proxies in marginal seas.