Sediment Organic Carbon Research Articles

Thermal stability of sedimentary organic carbon in a large river dominated marginal sea

The thermal stability of organic carbon (OC) in marine sediments is one of the critical factors that influences its burial efficiency in marine environments. However, the distribution patterns and influencing factors of the thermal stability of OC in marginal seas remain poorly understood. In this study, we conducted the thermal gravimetric analysis (TGA) of OC in surface sediments of the Changjiang Estuary (CE) and its adjacent East China Sea (ECS) shelf. Both labile and refractory organic matter (OML and OMR) contents derived from the TGA were higher in the CE and Zhe-Min Coast (ZMC) mobile muds relative to those in the sandy areas. The average Carbon Reactivity Index (CRI) is 69.3 ± 4.2 %, ranging from 62.8 % to 85.1 %. Most of the stations in the CE and ZMC mobile muds were characterized by relatively low CRI values, while only some stations in the inner estuary and outer shelf had higher CRI values. As a result, the CRI values correlated reversely with the OC contents and positively with the median grain size, especially in sandy sediments. Despite being sandy sediments, there were significant differences in the thermal stability of OC among the three different sandy sediment areas, with the highest CRI value in the inner estuary, lower CRI values in the outer estuary and outer shelf sediments, possibly related to the sources and composition of OC in different regions as shown by the negative correlation between CRI and δ13C for sandy sediments. Compared with certain European marginal seas, the sedimentary OC (SOC) in the CE and ECS shelf exhibits greater thermal stability, which is probably linked to the reduced preservation efficiency of OC caused by the extensive sediment dynamics in this area. This study supports the notion that organo-mineral interactions and the sources are two major factors controlling the reactivity of OC.

ENSO modulates soil organic carbon retention and deposition in the East China Sea

Accurately quantifying the contribution of terrestrial soil organic carbon (SOC) in marine environment is still a challenge. Moreover, reassessing the effect of climate change on SOC variability is necessary, with the increasing variability of El Niño under greenhouse warming. Here, we present new insights from bacterial 3-hydroxy fatty acids (3-OH-FAs) by systematically analyzing the distributions of 3-OH-FA in soils, lake/river sediments and marine sediments, and propose a novel proxy (RIA, the ratio of odd carbon number iso to anteiso 3-OH-FAs) to quantificationally evaluate the contribution of OC from different components. The reliability of the RIA proxy is tested by using a three end-member model and Monte Carlo simulations along a transect from the Yangtze River estuary to the ECS shelf. Then the RIA proxy is applied to a core in the East China Sea covering the past 700 years, and the results reveal that El Niño (La Niña) induced more (less) precipitation and increased (decreased) SOC deposition. This study provides a novel approach for quantifying terrigenous OC inputs and burial in marginal seas and shows the impact of El Niño-Southern Oscillation (ENSO) modes on SOC loss and burial in marginal seas under global warming.

Seaweed burial mitigated the release of organic carbon and nutrients by regulating microbial activity

Seaweed debris is susceptible to being buried in sediments due to natural environmental changes and human activities. So far, the effect of buried seaweeds on the environment and its decomposition mechanism remains unclear. This study simulated the decomposition of seaweed Gracilariopsis lemaneiformis for 180 days with different burial depths (0 cm and 10 cm) and burial weights (10 g and 20 g). Our findings revealed that compared with Gracilariopsis decomposition on the sediment surface, the seaweed buried in sediment slowed down the release of N, P, and dissolved organic carbon (DOC) by enhancing the activity of diverse anaerobic microbes (i.e. Draconibacterium, Desulfuromusa, Sediminispirochaeta), which were associated with organic matter decomposition. The enhanced burial quantity of Gracilariopsis resulted in a 3.28 % increase in sediment OC and enriched the humification degree of DOC in seawater. These results highlight the role of seaweed burial in enhancing OC sequestration in marine environments.

Seasonal and annual variations of sediment trapping and particulate organic carbon burial in Yellow River reservoirs

The Yellow River is distinguished by the highest sediment load in mainland China and significant siltation in artificial reservoirs along its main channel, reducing water availability, sediment trapping, and carbon burial in the hydrological project. Since 2002, the Water Sediment Regulation Scheme (WSRS) has been progressively implemented as a hydraulic management strategy to mitigate reservoir sedimentation in the middle-lower basin reservoirs. However, this substantial release of sediment and water has also affected river morphology, carbon burial, and sediment trapping.This research assesses the evolution of particulate organic carbon burial and sediment trapping in eight Yellow River reservoirs from 2002 to 2018, covering the upper and middle-lower basins and two reservoirs in the Yiluo River sub-basin. We calculated the annual and seasonal variations using hydrological data from 11 stations along the Yellow River. A hydrological framework was developed to calculate sediment trapping, and a Monte Carlo analysis was performed to estimate particulate organic carbon burial across all the evaluated reservoirs.We found that sediment trapping and carbon burial in upper basin reservoirs are most influenced by shifts in the precipitation regime, particularly by natural events such as severe droughts or heavy rainfall. A strong correlation was observed between annual precipitation variations and sediment load. In middle-lower basin reservoirs, the major artificial sediment regulation is strongly linked to a significant reduction in sediment trapping and particulate organic carbon burial. This impact is especially notable in the Yiluo River Sub-basin reservoirs, which have experienced a >90 % reduction compared to levels before the WSRS implementation.Finally, we highlight the consequences of climate change and artificial water management strategies in reservoirs, demonstrating how both affect their capacity for sediment trapping and impact their role as significant carbon sinks.

Terrestrial input and biological processes drive varying mineral/organic matrix-related mercury sequestration and deposition in the East Siberian Arctic Shelf

Thawing Arctic permafrost is releasing massive amounts of terrestrial mercury (Hg) into the Arctic shelf, which is expected to form a Hg sink in sediments. Here we analyzed the coupling roles of terrestrial input and biological processes in varying mineral/organic matrix-related Hg sequestration and deposition on the East Siberian Arctic shelf (ESAS), using Pearson and partial correlation analysis. Results show that the occurrence of Hg in the Laptev Sea under the Lena River-dominated input exhibits strong correlations with the organic carbon (OC) content and sediment specific surface area. This indicates that the prolonged riverine loading processes facilitate Hg sequestration by organic/mineral matrices and these land-derived mineral-OC-Hg complexes remain stable during cross-shelf transportation. By contrast, the comparative samples with coastal erosion-dominated Hg input in the western East Siberian Sea shows limited Hg complexation, primarily due to the absence of regulation by riverine processes and/or the high sedimentation rates. The correlations between Hg and OC source proxies (δ13C and lignin) indicate that these matrix-free Hg could be more readily sequestrated by the marine OC, which facilitates its deposition via the biological pump. While unexpectedly low Hg abundance in the Chukchi Sea may indicate that the effective biological scavenging of Hg sequestration could be constrained by insufficient terrestrial Hg input. Our findings of the matrix-related Hg sequestration by mineral and/or OC association in this study may shed light on the biogeochemical cycle of Hg in the Arctic aquatic regime, which could reduce the methylmercury formation in water column, and the methylation within sediments needs further exploration.

Cenozoic Carbon Dioxide: The 66 Ma Solution

The trend in partial pressure of atmospheric CO2, P(CO2), across the 66 MYr of the Cenozoic requires elucidation and explanation. The Null Hypothesis sets sea surface temperature (SST) as the baseline driver for Cenozoic P(CO2). The crystallization and cooling of flood basalt magmas is proposed to have heated the ocean, producing the Paleocene–Eocene Thermal Maximum (PETM). Heat of fusion and heat capacity were used to calculate flood basalt magmatic Joule heating of the ocean. Each 1 million km3 of oceanic flood basaltic magma liberates ~5.4 × 1024 J, able to heat the global ocean by ~0.97 °C. Henry’s Law for CO2 plus seawater (HS) was calculated using δ18O proxy-estimated Cenozoic SSTs. HS closely parallels Cenozoic SST and predicts the gas solute partition across the sea surface. The fractional change of Henry’s Law constants, Hn−HiHn−H0 is proportional to ΔP(CO2)i, and Hn−HiHn−H0×∆P(CO2)+P(CO2)min, where ΔP(CO2) = P(CO2)max − P(CO2)min, closely reconstructs the proxy estimate of Cenozoic P(CO2) and is most consistent with a 35 °C PETM ocean. Disparities are assigned to carbonate drawdown and organic carbon sedimentation. The Null Hypothesis recovers the glacial/interglacial P(CO2) over the VOSTOK 420 ka ice core record, including the rise to the Holocene. The success of the Null Hypothesis implies that P(CO2) has been a molecular spectator of the Cenozoic climate. A generalizing conclusion is that the notion of atmospheric CO2 as the predominant driver of Cenozoic global surface temperature should be set aside.

Assessment of coral reef connectivity in improved organic carbon storage of seagrass ecosystems in Palk Bay, India

The increase in climate-related extreme events and ecosystem degradation demands consistent and sustainable climate mitigation efforts. Seagrass playing a key role in nature-based carbon sequestration mitigation strategy. Here, we investigated the role of coral reef connectivity in blue carbon dynamics with seagrass meadows with coral reef connectivity (SC areas) and without coral reef connectivity (SG areas) in Palk Bay, India. The high sediment organic carbon was recorded in SC areas (90.26 ± 25.68 Mg org.C/ha) and lower in SG areas (66.96 ± 12.6 Mg org.C/ha). The maximum above-ground biomass (AGB) was recorded in Syringodium isoetifolium (35.43 ± 8.50) in SC areas and the minimum in Halophila ovalis (7.59 ± 0.90) in SG areas, with a similar trend observed in below-ground biomass (BGB). Our findings highlight the importance of coral reefs in enhancing the blue carbon potential of seagrass ecosystems and underscore the need for integrated conservation and restoration strategies for coral reefs and seagrasses.

Carbon stocks in mangrove ecosystems of Sri Lanka: Average contributions and determinants of sequestration potential

Mangroves are crucial in carbon sequestration despite covering only a small percentage of the Earth's surface. Latitudinal gradient primarily determines the distribution of climatic zones of distinct sunlight, temperature, and precipitation patterns, which influence the community structure of the mangrove ecosystem. The tropical climate of Sri Lanka contributes to the island's lush mangrove forests. The present study estimated that the average island's carbon sequestration capacity of Sri Lankan mangrove ecosystems is 524.25 t/ha representing a substantial volume of carbon storage that contributes to offset greenhouse effect due to increasing atmospheric CO2. Our results substantiated that rainfall positively influences total carbon sequestration capacity of mangrove ecosystems.It was also revealed that a positive relationship exists between vegetation structural complexity and sediment organic carbon, highlighting the influence of vegetation structure, that is primarily dependent on climatic conditions, on production of organic matter and sediment carbon sequestration. Globally, the carbon sink function of mangrove ecosystems is reported to be highest in the tropical areas and it declines towards sub-tropical higher latitudes while those in the southern hemisphere perform better carbon sinks than those in the northern hemisphere.The vertical distribution of total organic matter content in mangrove sediments was revealed to be in a descending order, manifesting the weak tidal removal of surface organic matter under the microtidal conditions in Sri Lankan marine waters, thus qualifying mangrove ecosystems in microtidal coasts as effective carbon sinks.

Organic carbon cycling in the sediments of Prydz Bay, Eastern Antarctica: Implications for a high carbon sequestration potential

Understanding the dynamics of sedimentary organic carbon (SOC) in the productive continental marginal sea surrounding Antarctica is crucial for elucidating the effect of this sea on the global carbon cycle. We analyzed 31 surface sediment samples and eight sediment cores collected from Prydz Bay (PB) and the adjacent basin area. The element and stable isotope compositions, grain size compositions, and biogenic silica and lithogenic minerals of these samples were used to evaluate the spatial variations in the sources, transport mechanisms, and preservation patterns of SOC, with a particular focus on the efficiency of the biological carbon pump (BCP). Our findings reveal that the SOC originated from mixed marine/terrestrial sources. The δ13C values were higher in the Prydz Bay Gyre (PBG) region than in the open sea area. Biogenic matter-rich debris, associated with fine-grained particles (silt and clay), was concentrated in the PBG, while abiotic ice-rafted debris and coarse-grained particles were preferentially deposited in the bank and ice shelf front regions. Lithogenic matter predominated in the basin sediments. The annual accumulation rate of SOC in PB ranged from 1.6 to 6.2 g·m−2·yr−1 (mean 4.2 ± 1.9 g·m−2·yr−1), and the rates were higher in the PBG than in the ice shelf front region. Estimates based on our tentative box model suggest that the efficiency of the BCP, which refers to the proportion of surface-produced organic carbon successfully transferred to deep waters, is approximately 5.7 % in PB, surpassing the global average (∼0.8 %) and the efficiencies reported for other polar environments. Furthermore, our calculations indicate that the SOC preservation efficiency (the ratio of preserved to initially deposited organic carbon in sediments) in PB is approximately 79 % ± 20 %, underscoring the significant carbon sequestration potential within PB. The results of this study have important implications for the effects of sediment dynamics on the carbon cycle in the sea surrounding Antarctica.

Organic carbon oxidation and the associated pyrite formation in deep sediments at the Nankai subduction zone

The preservation of organic carbon (OC) in marine sediments is crucial for Earth's climate by sequestering CO2. While OC is mostly remineralized before it reaches the seafloor, a notable fraction is buried in deep-sea sediments, particularly at subduction zones known for their enhanced sedimentation rates that promote OC burial. The mechanisms behind OC recycling and decomposition within subduction zone sediments are governed by biologically mediated redox reactions involving a range of electron acceptors, which facilitate the sequential oxidation of OC. However, direct evidence regarding such effects on OC in deep sediments remains limited. In this study, we examine the contents of pyrite and trace metals of molybdenum (Mo), vanadium (V), and cesium (Cs) in organic-rich deep sediments from the Nankai subduction zone. We find that the pyrite, sulfate, diagenetic illite, and OC contents covary in Unit V, highlighting the important role of organoclastic sulfate reduction and biotic smectite-to-illite reaction at 60–70 °C in OC oxidation and pyrite formation. The profiles of Mo, V, and Cs also align with these processes in the same interval. Our findings contribute to an in-depth understanding of OC dynamics in subduction zones, emphasizing the complex interactions within deep sediments that may influence the global carbon cycle.

Short-term sedimentary evidence for increasing diatoms in Arctic fjords in a warming world

Arctic fjords are hotspots of marine carbon burial, with diatoms playing an essential role in the biological carbon pump. Under the background of global warming, the proportion of diatoms in total phytoplankton communities has been declining in many high-latitude fjords due to increased turbidity and oligotrophication resulting from glacier melting. However, due to the habitat heterogeneity among Svalbard fjords, diatom responses to glacier melting are also expected to be complex, which will further lead to changes in the biological carbon pumping and carbon sequestration. To address the complexity, three short sediment cores were collected from three contrasting fjords in Svalbard (Krossfjorden, Kongsfjorden, Gronfjorden), recording the history of fjord changes in recent decades during significant glacier melting. The amino acid molecular indicators in cores K4 and KF1 suggested similar organic matter degradation states between these two sites. In contrast to the turbid Kongsfjorden and Gronfjorden, preserved fucoxanthin in Krossfjorden indicated a continuous increase in diatoms since the mid-1980s, corresponding to a 59 % increase in biological carbon pumping, as quantified by the δ13C of sedimentary organic carbon. The increasing biological carbon pumping in Krossfjorden is further attributed to its hard rock types in the glacier basin, compared to Kongsfjorden and Gronfjorden, which are instead covered by soft rocks, as confirmed by a one-dimensional model. Given the distribution of rock types among basins in Svalbard, we extrapolate our findings and propose that approximately one-fifth of Svalbard's fjords, especially those with hard rock basins and persistent marine-terminated glaciers, still have the potential for an increase in diatom fractions and efficient biological carbon pumping. Our findings reveal the complexity of fjord phytoplankton responses and biological carbon pumping to increasing glacier melting, and underscore the necessity of modifying Arctic marine carbon feedback to climate change based on results from fjords underlain by hard rocks.

The increasing influence of oyster farming on sedimentary organic matter in a semi-closed subtropical bay

Oyster farming activities play a pivotal role in the biogeochemical cycles of coastal marine ecosystems, particularly in terms of sedimentary carbon cycling. To gain deep insights into the influence of expanding oyster culture on the sedimentary carbon cycle, surface sediments were collected from the Maowei Sea, which is the largest oyster farming bay in south China, based on six filed surveys between July 2010 and December 2022. The sediment samples were analyzed for total organic carbon (TOC), total nitrogen (TN), stable carbon and nitrogen isotopes (δ13C and δ15N) to evaluate the inter-annual variations in the source contribution to sedimentary organic matter (SOM). The results revealed that the average contents of sedimentary TOC and TN were 0.67 ± 0.41 % and 0.06 ± 0.03 %, respectively. Fluctuations in the C/N molar ratios ranged from 5.8 to 23.6, with an average of 12.6 ± 2.9, indicating a significant terrestrial input contribution to SOM in the study area. Furthermore, the integration of stable isotope analysis and Bayesian mixing model demonstrated a gradual increase in the mean proportion of shellfish biodeposition to SOM, from 12.0 ± 5.6 % in July 2010 to 21.1 ± 7.3 % in December 2022, consistent with the progressive expansion of oyster aquaculture along this coastal area, thereby emphasizing the substantial influence of oyster farming on SOM composition. With the anticipated expansion of oyster farming scale and production in the future, shellfish biodeposition is expected to assume a more important role in shaping SOM dynamics and sedimentary organic carbon cycling in coastal waters. Overall, this study provided an important perspective for better assessing the impact of expanding mariculture scale on coastal biogeochemical cycles, thereby making valuable contributions to future policy formulation concerning mariculture and ecological conservation.

Cyanobacteria decay alters CH4 and CO2 produced hotspots along vertical sediment profiles in eutrophic lakes

Cyanobacteria-derived organic carbon has been reported to intensify greenhouse gas emissions from lacustrine sediments. However, the specific processes of CH4 and CO2 production and release from sediments into the atmosphere remain unclear, especially in eutrophic lakes. To investigate the influence of severe cyanobacteria accumulation on the production and migration of sedimentary CH4 and CO2, this study examined the different trophic level lakes along the middle and lower reaches of the Yangtze River. The results demonstrated that eutrophication amplified CH4 and CO2 emissions, notably in Lake Taihu, where fluxes peaked at 929.9 and 7222.5 μmol/m2·h, mirroring dissolved gas levels in overlying waters. Increased sedimentary organic carbon raised dissolved CH4 and CO2 concentrations in pore-water, with isotopic tracking showing cyanobacteria-derived carbon specifically elevated CH4 and CO2 in surface sediment pore-water more than in deeper layers. Cyanobacteria-derived carbon deposition on surface sediment boosted organic carbon and moisture levels, fostering an anaerobic microenvironment conducive to enhanced biogenic CH4 and CO2 production in surface sediments. In the microcosm systems with the most severe cyanobacteria accumulation, average CH4 and CO2 concentrations in surface sediments reached 6.9 and 2.3 mol/L, respectively, surpassing the 4.7 and 1.4 mol/L observed in bottom sediments, indicating upward migration of CH4 and CO2 hotspots from deeper to surface layers. These findings enhance our understanding of the mechanisms underlying lake sediment carbon emissions induced by eutrophication and provide a more accurate assessment of lake carbon emissions.

Carbon isotope stratigraphy and biostratigraphy, and associated organic carbon deposition during the Late Cenomanian to Early Turonian in the Tarfaya Basin, Morocco

The Cenomanian-Turonian (C/T) boundary event, also known as Oceanic Anoxic Event 2 (OAE2), is characterised by significant organic carbon preservation and a positive carbon isotope excursion that is recorded globally in sedimentary strata. In this study of the Tarfaya Basin, Morocco, three outcrop sections were logged - at Lagguing East, Tazra, and En-Naila, dated from Late Cenomanian to Early Turonian. New dating, based on ammonites and planktic foraminifera, combined with high-resolution δ13C isotope stratigraphy, yields a more precise delineation of the C/T boundary in the studied section. A total of 99 samples were collected for combined sedimentological, mineralogical, and geochemical analyses to allow a better understand of the palaeoenvironments of deposition and interpret controlling factors on organic enrichment. Five lithofacies have been identified, all indicative of a relative deep marine setting. Variations in lithofacies can be correlated with relative sea level changes, which align with the global eustatic curve.The OAE2 interval comprises black mudstones that signal anoxic conditions, leading to significant organic matter (OM) preservation, even though they appear weathered in the studied section. Notably, organic matter-rich mudstones were evident during the post-OAE2 Early Turonian maximum marine transgression. Trace element enrichment in these organic-rick mudstones indicates that organic matter accumulation was accompanied with increased sea surface productivity and associated with oxygen-depleted bottom water conditions, which facilitated organic matter preservation and enrichment. Upwelling has also been previously described as a process to deliver nutrients to the shelf, enhancing productivity. Some units with depleted organic content are the result of dilution from continental derived clastics. Others may originally have had high TOC, but were influenced by weathering or diagenetic process, changing from the typical black mudstones to yellow in colour. The identified carbon isotopic values and organic carbon enrichment in the C/T strata of the Tarfaya Basin reflect signals of global perturbations in the carbon cycles.

Widespread crab burrows enhance greenhouse gas emissions from coastal blue carbon ecosystems

Fiddler crabs, as coastal ecosystem engineers, play a crucial role in enhancing biodiversity and accelerating the flow of material and energy. Here we show how widespread crab burrows modify the carbon sequestration capacity of different habitats across a large climatic gradient. The process of crab burrowing results in the reallocation of sediment organic carbon and humus. Crab burrows can increase more greenhouse gases emissions compared to the sediment matrix (CO2: by 17–30%; CH4: by 49–141%). Straightforward calculations indicate that these increased emissions could offset 35–134% of sediment carbon burial in these two ecosystems. This research highlights the complex interactions between crab burrows, habitat type, and climate which reveal a potential lower carbon sink function of blue carbon ecosystems than previously expected without considering crab burrows.

Hotspots of Dissolved Arsenic Generated from Buried Silt Layers along Fluctuating Rivers.

Previous studies along the banks of the tidal Meghna River of the Ganges-Brahmaputra-Meghna Delta demonstrated the active sequestration of dissolved arsenic (As) on newly formed iron oxide minerals (Fe(III)-oxides) within riverbank sands. The sand with high solid-phase As (>500 mg/kg) was located within the intertidal zone where robust mixing occurs with oxygen-rich river water. Here we present new evidence that upwelling groundwater through a buried silt layer generates the dissolved products of reductive dissolution of Fe(III)-oxides, including As, while mobilization of DOC by upwelling groundwater prevents their reconstitution in the intertidal zone by lowering the redox state. A three end-member conservative mixing model demonstrated mixing between riverbank groundwater above the silt layer, upwelling groundwater through the silt layer, and river water. An electrochemical mass balance model confirmed that Fe(III)-oxides were the primary electron acceptor driving the oxidation of DOC sourced from sediment organic carbon in the silt. Thus, the presence of an intercalating silt layer in the riverbanks of tidal rivers can represent a biogeochemical hotspot of As release while preventing its retention in the hyporheic zone.

Dual biomarker traceability and ecological risk assessment of centennial organic contamination in South Dongting Lake

Enhanced anthropogenic activity strength has altered the watershed particulate transport and material cycle resulting in organic pollutant deposition changes in Dongting Lake associated with unclear ecological risk. In the present study, dual biomarkers i.e. n-alkanes and polycyclic aromatic hydrocarbon (PAHs) were applied in the 210Pb-dated sediment cores for traceability of centennial organic pollutants in the lake mouth area. The partial least squares path model and risk quotients method were used to explore the controlling pathways and ecological risk. The results show a range of sedimentary organic carbon (C), nitrogen (N), and phosphorus (P) was at 1.76–185.66, 0.97–89.80, and 0.01–0.97 g m−2 yr−1 with total reserves of 51.68, 18.44, and 0.27 t ha−1, respectively, over the past 179 years. The presence of PAHs rapidly increased by 2.47 fold from 535.60 ng g−1, while PAHs and carcinogenic PAHs (ΣCPAHs) burial fluxes increased by about 6 and 5 folds, respectively. Accompanied by anthropogenic activities and climate change, the exotic sources gradually becoming predominant. The n-alkane diagnostic ratios indicated a shift of organic matter (OM) from autotrophic bacteria, algae, and phytoplankton-derived sources to macrophyte and terrestrial plants. The exotic origins rose to approximately 73.61 %, while endogenous sources decreased to 26.39 %. The direct effects of anthropogenic activities and their indirect negative impacts on climate and sedimentary structure are the key ways for sediment material loading. The nutrient accumulation in sediments coincides with the lake's eutrophication history over the past decades. The ΣCPAHs accounted for about 89.37 ± 17.14 % of the total TEQ, reflecting a strong ecological risk. The contribution of anthropogenic activities such as fuel usage, fertilizer application, hard pavement coverage, and OM loss from the ecosystem to the sources of organic pollutants and their component types may be a focus of attention in the middle reaches of the Yangtze River plain lake.

Both nutrients and macrophytes regulate organic carbon burial: Insights from high-resolution spatiotemporal records of a large shallow lake (Baiyangdian) in eastern China

Both ecological regime shifts and carbon cycling in lakes have been the subject of global debates in recent years. However, the direct linkage between them is poorly understood. Lake Baiyangdian, a representative large shallow lake with the coexistence of a macrophyte-dominated area (MDA) and an algae-dominated area (ADA) in eastern China, allowing better understanding of the relationship between regime shifts and organic carbon (OC) burial in lakes. On the basis of Bayesian isotopic mixing modelling of C/N ratios and δ13C values, the sediment OC is primarily of autochthonous origin. The mean OC burial rate (OCBR) was 39 g C m−2 yr−1 before eutrophication occurred in 1990 and increased approximately 2.7-fold to 106 g C m−2 yr−1 after eutrophication. Partial least squares path modelling revealed that this change can be largely attributed to enhanced primary productivity and rapid burial as a result of intensified human perturbation. In terms of spatial patterns, the OCBR was greater in the MDA than in the ADA, which may be related to the different burial and mineralization processes of debris from macrophytes and algae. It then deduced that a decrease in the OCBR and an increase in the mineralization rate might have occurred after a shift from a macrophyte-dominated state to an algae-dominated state. Our findings highlight that eutrophication generally increases OC burial by enhancing lake primary productivity. However, once nutrient levels reach a critical range, lake ecosystems may shift from a macrophyte-dominated state to an algae-dominated state, which can lead to a significant reduction in the carbon burial capacity of lakes. Therefore, more attention should be given to avoiding shifts in eutrophic lakes, as such shifts can alter carbon cycling.

Spatiotemporal distribution patterns of iron, manganese, and arsenic within the river infiltration zone and the potential geochemical activity at key interfaces

The river infiltration zone is a mixed area of surface water and groundwater under the riverbed and along the riverbank, which is greatly affected by river infiltration. Currently, there is a lack of understanding regarding the migration and transformation processes of Fe, Mn, and As, along with their correlation with organic carbon in the river infiltration zone affected by riverbed scouring and siltation processes. In this study, based on techniques such as pore water sampling and the Tessier sequential extraction, the spatiotemporal variations in Fe, Mn, and As concentrations and possible geochemical processes at the riverbed sediment–water interface and along the river infiltration flow path were revealed. The results indicated that the potential geochemical activity of Fe, Mn, and As and the distribution of organic carbon in the riverbed sediment responded to riverbed scouring–siltation processes. Compared to the high water level period, the potential geochemical activity of Fe, Mn, As, and organic carbon in the riverbed sediment generally exhibited an upward trend during the low water level period, indicating more intense Fe and Mn biogeochemical reactions. During river infiltration, the labile organic carbon (LOC) was preferentially utilized, and with increasing depth, the sedimentary organic carbon (SOC) continuously transformed into LOC and dissolved organic carbon (DOC). Affected by riverbed scouring processes, the transition time from oxidizing to reducing conditions in the pore water increased, and the Fe, Mn, and As reduction zones moved deeper and farther from the riverbank, resulting in a significant expansion of the groundwater pollution area. The reductive dissolution of iron and manganese oxides/hydroxides and the oxidation of organic matter–bound Fe, Mn, and As were the main biogeochemical processes contributing to the release of Fe, Mn, and As in the river infiltration zone. These research results have crucial theoretical significance and practical application value for the sustainable utilization of groundwater resources and the remediation of groundwater pollution.

Sediment porewaters serve as a transient organic carbon pool at the land-ocean interface

The organic carbon (OC) cycle at the land-ocean interface is an important component of the global carbon budget, yet the processes that control the transfer, transformation, and burial of OC in these regions remain poorly understood. In this work, we examined sedimentary OC (SOC) in short core sediments, dissolved inorganic carbon (DIC), dissolved organic carbon (DOC), and chromophoric dissolved organic matter (CDOM), as well as other solutes in sediment porewaters of the Changjiang Estuary and adjacent East China Sea (ECS) shelf. The main goal of this work is to investigate the variation of the sources and composition of different forms of carbon in estuarine sediments associated with different sedimentary regimes, to further understand the role of sediment porewater in carbon sequestration at the land-ocean interface. Concentrations of Fe2+ and Mn2+ in porewaters of the muddy sediments are much higher than those in the sandy sediments, and SO42− decreases with depth in the deep sediment layer, indicating the degradation of SOC in mobile muds is mainly driven by suboxic and/or anoxic diagenetic processes (e.g., iron-manganese reduction). The accumulation of DIC in the muddy sediment is higher compared to the sandy sediment, indicating relatively complete SOC remineralization. The DOC in porewaters of the muddy areas is mainly composed of highly degraded and low molecular weight humic-like substances (C1), whereas in the sandy area, porewater DOC is mainly composed of less degraded and high molecular weight protein-like substances (C2 and C3). The average DOC stock (28.5 t/km2) in the upper 30 cm sediment porewaters is significantly higher than that of DIC (12.5 t/km2) in sandy area, but less in muddy areas (17.0 t/km2 of DOC vs. 25.4 t/km2 of DIC). The total DOC stock in sediment porewaters of the sandy area accounted for ∼61 % of DOC stock in water column of the ECS, indicating that the porewater is an important DOC pool in the ECS. However, this DOC pool is rather transient due to its high reactivity and mobility, especially in sandy area. Nevertheless, compared with other marine environments, the carbon stock of DOC (average of 43.8 t/km2) in porewaters of stable sedimentary environments is much higher than that of DIC (average of 21.7 t/km2). This work further supports the notion that sedimentary regime plays an important role in OC cycling at the land-ocean interface and highlights the significance of sediment porewaters as a vast carbon pool in marine ecosystems.