Organic Carbon In Surface Sediments Research Articles

Organic carbon in surface sediments of Chaunskaya Bay (East Siberian Sea): results of pyrolytic analysis using the Rock-Eval method

Relevance. Dictated by the need to assess functioning of the biogeochemical regime of the Arctic region by studying geochemical properties of organic matter of bottom sediments on the example of the Chaunskaya Bay (East-Siberian Sea). Aim. To study the spatial variability of geochemical parameters of organic matter of bottom sediments of the Chaunskaya Bay using the Rock-Eval method, as well as to identify a possible relationship between the parameter TOC and the pelite fraction. Objects. Samples of bottom sediments of the Chaunskaya Bay (East Siberian Sea). Sampling took place in stages from three horizons (upper 0–2 cm, intermediate 2–5 cm, lower 5–10 cm) during a comprehensive scientific expedition to the R/V "Academician Oparin" in September–October 2020. Methods. Granulometric composition of bottom sediments was determined using the Analysette 22 NanoTec particle analyzer (Fritsch, Germany). The analysis of hydrocarbon compounds of organic matter was performed using pyrolytic analysis on the device (Rock Eval 6 Turbo of Vinci Technologies, France). Results. The results of pyrolytic analysis considered by the authors have shown that such factors as the primary productivity of the waters of the studied water area and the processes of erosion of the coastal zone play a decisive role in the formation of the composition of the TOC in bottom sediments of the Chaunskaya Bay. We also do not exclude the contribution of river runoff to the composition of the TOC in bottom sediments; however, we consider it small due to the insignificant inflow of river sediments into the waters of the studied area. The pyrolytic data obtained by us indicate that both the marine component (primary productivity) and the terrigenous component (erosion of the coastal complex) are present in the composition of the TOC in bottom sediments of the studied area.

Correlation between polycyclic aromatic hydrocarbons and total organic carbon in surface sediments of the Bohai Sea and the North Yellow Sea.

The correlation between polycyclic aromatic hydrocarbons (PAHs) and organic carbon is an important indicator of the marine environment. In this paper, twenty-six surface sediment samples were collected from the Bohai Sea and the North Yellow Sea as the study area, and the contents of sixteen PAHs ranged from 71.34 ng·g-1 to 240.78 ng·g-1 with an average of 143.26 ng·g-1 by gas chromatography. The source analysis indicated the source of PAHs in the surface sediments of the study area was more complex, and the percentage of combustion sources was higher than that of petroleum sources, which might cause less negative ecological risks. The average content of total organic carbon (TOC) was 5.25%, as determined by automatic potentiometric titration, and the content of TOC was higher in the Bohai Sea than in the North Yellow Sea. TOC was significantly correlated with HWM-PAHs but was not correlated with LWM-PAHs, which proved organic matter was more effective in controlling high-molecular-weight PAHs than low-molecular-weight PAHs in the surface sediments of the Bohai Sea and the North Yellow Sea.

Lack of strong seasonality in macrobenthic communities from the northern Barents Sea shelf and Nansen Basin

The Barents Sea has been coined ‘the Arctic hotspot’ of climate change due to the rapidity with which environmental changes are taking place. This transitional domain from Atlantic to Arctic waters is home to highly productive benthic communities. This system strongly fluctuates on a seasonal basis in its sympagic-pelagic-benthic coupling interactions, with potential effects on benthic standing stocks and production. Recent discoveries have questioned the marked seasonality for several high Arctic seafloor communities in coastal waters of Svalbard. Still, the seasonal variability of benthic process in the extensive Barents Sea open shelf remains poorly understood. Therefore, we studied the seasonality of macrofauna communities along a transect in the northwestern Barents Sea comprising two hydrographic domains (Arctic vs. Atlantic Water, across the Polar Front) and three geomorphological settings (shelf, continental slope and abyssal plain). Overall, we did not find strong signs of seasonal variation in taxonomic community structure and functional diversity. However, we found some weak signs of seasonality when examining each station separately, especially at a station close to the Polar Front, with high seasonal fluctuations in abiotic drivers indicating a stronger pelagic-benthic coupling. The lack of seasonality found both at the shelf stations south and north of the Polar Front could be related to organic matter stored in the sediments, reflected in constant levels of total organic carbon in surface sediment across time for all stations. We did observe, as expected, highly spatially structured environmental regimes and macrofauna communities associated to them from shelf to slope and basin locations. Understanding the underlying spatio-temporal mechanisms by which soft-bottom benthic communities are structured along environmental gradients is necessary to predict future impacts of climate change in this area. Our results indicate that short-term climate driven changes in the phenology of pelagic ecosystem components might not be directly reflected in the Arctic benthic system, as seafloor processes seem to be partially decoupled from those in the overlying water.

Changes in surface sediment carbon compositions in response to tropical seagrass meadow restoration

Seagrass meadows are declining at a global scale, threatening their capacity as blue carbon sinks. Restoration of seagrasses (via seagrass seeds or plant transplantation) may recover their carbon sequestration capacity. Previous studies have predominantly focused on sediment organic carbon (SOC), while variations in sediment carbon compositions remain poorly understood, limiting our comprehension of the influence of seagrass restoration on sediment carbon stability. Here, we researched the differences in surface (0–3 cm) sediment carbon compositions in response to tropical seagrass transplantation among species (Thalassia hemprichii and Enhalus acoroides); specifically, differences in labile, recalcitrant and refractory SOC, as well as sediment inorganic carbon (SIC) compositions variations under transplanted T. hemprichii and E. acoroides communities. It was found that seagrass transplantation enhanced suspended particle organic matter, and epiphyte and macroalgae input to surface sediment, which recovered the surface SOC concentration and stock rapidly to natural levels (increased ∼1.6-fold) within two years following transplantation. The elevated contribution of epiphyte and macroalgae significantly increased the surface labile sediment organic matter (SOM), but not the recalcitrant and refractory SOM composition after short-term transplantation. Meanwhile, surface SIC was significantly elevated, which might be mainly ascribed to allochthonous carbonate particle trapped under transplanted area with implications for carbon sequestration. The higher canopy and longer leaf seagrass species, E. acoroides, had elevated SOC, SIC and was more labile composition, compared to T. hemprichii transplant. Overall, this research suggests that tropical seagrass transplantation can increase the surface SOC, SIC concentration by increasing the labile organic matter and allochthonous carbonate particle input, respectively, with varying significantly among seagrass species.

Effects of herbivore on seagrass, epiphyte and sediment carbon sequestration in tropical seagrass bed

Herbivores strongly affect the ecological structure and functioning in seagrass bed ecosystems, but may exhibit density-dependent effects on primary producers and carbon sequestration. This study examined the effects of herbivorous snail (Cerithidea rhizophorarum) density on snail intraspecific competition and diet, dominant seagrass (Thalassia hemprichii) and epiphyte growth metrics, and sediment organic carbon (SOC). The growth rates of the herbivorous snail under low density (421 ind m−2) and mid density (842 ind m−2) were almost two times of those at extremely high density (1684 ind m−2), indicating strong intraspecific competition at high density. Herbivorous snails markedly reduced the epiphyte biomass on seagrass leaves. Additionally, the seagrass contribution to herbivorous snail as food source under high density was about 1.5 times of that under low density, while the epiphyte contribution under low density was 3 times of that under high density. A moderate density of herbivorous snails enhanced leaf length, carbon, nitrogen, total phenol and flavonoid contents of seagrasses, as well as surface SOC content and activities of polyphenol oxidase and β-glucosidase. However, high density of herbivorous snails decreased leaf glucose, fructose, detritus carbon, and total phenols contents of seagrasses, as well as surface SOC content and activities of polyphenol oxidase and β-glucosidase. Therefore, the effects of herbivorous snail on seagrass, epiphyte and SOC were density-dependent, and moderate density of herbivorous snail could be beneficial for seagrasses to increase productivity. This provided theoretical guidance for enhancing carbon sink in seagrass bed and its better conservation.

Distribution, sources and influencing factors of organic carbon in the surface sediments of the coastal tidal flats in Jiangsu Province

Coastal tidal flats are situated in the interaction zone between the ocean and land and are vulnerable to natural changes, human activities, and global changes; these areas serve as an important mixing zone and burial area for carbon and nitrogen storage. Coastal tidal flats contribute significantly to the global carbon cycle due to their high biological productivity, high sedimentation rate, and low decomposition rate. However, there is a lack of research on the sources and influencing factors of organic carbon in surface sediments in the coastal tidal flat area of Jiangsu Province. In this study, fourteen surface sediment samples were collected from the Jiangsu coastal tidal flats, and the distribution of organic carbon was analyzed. The sources and influencing factors of sediment organic carbon were also investigated by analyzing the contents, ratios and stable isotopes of carbon and nitrogen in the sediments. The results indicated that the total organic carbon (TOC) and total nitrogen (TN) contents in the surface sediments of Jiangsu coastal tidal flats ranged from 0.09% to 0.82% and from 0.01% to 0.1%, with mean values of 0.36% and 0.04%, respectively, and that there was a significant positive correlation between TOC and TN. Moreover, the high-value areas were located mostly along the borders or in sections covered in vegetation, whereas the mudflat areas without vegetation had lower values. Considering the status of the Jiangsu coastal zone, the abandoned Yellow River estuary, Yangtze River inlet, marine benthic microalgae, C3 plants and C4 plants were selected as end members of organic carbon concentrations. The contributions of different sources were quantified using a Bayesian mixture model (MixSIAR). Among them, the abandoned Yellow River estuary and Yangtze River inlet contributed 28.1% and 19.3%, respectively, followed by marine benthic microalgae with a contribution of 26%, whereas C3 and C4 plants made limited contributions of 9% and 8.8%, respectively. Additionally, the sediment particle size, hydrodynamics, and tidal flat vegetation such as Spartina alterniflora were the key factors influencing the organic carbon distribution. In general, this study contributes to a better understanding of the biogeochemical processes and sources of organic carbon in coastal tidal flats. It also provides a solid foundation for the creation of carbon sink measures in nearshore tidal flats.

Distinct patterns of sedimentary phosphorus fractionation and mobilization in the seafloor of the Black Sea, Marmara Sea and Mediterranean Sea

Phosphorus (P) is a key element to all life that is used for structural and functional component of all organisms. The cycling of sedimentary P may differ depending on the redox-conditions of the overlying waters affecting the dynamics, and distribution of P-fractions and the elements that are highly coupled to P cycle. Though biogeochemistry of water column in the three interconnected marine basins of Black, Marmara and Mediterranean Seas have been studied extensively, few studies were carried out to understand sedimentary P dynamics in these regions. In this study, therefore, the biogeochemical cycling of sedimentary P and related variables such as porewater nutrients, sedimentary organic carbon, nitrogen and reactive iron were studied in selected sites at the three interconnected marine basins: Black Sea, Marmara and Northeastern (NE) Mediterranean Sea. The pool of “potentially mobile P” was also determined for the studied sites. The study results showed that porewater and sediment biogeochemistry displayed great variability in the studied sites with the maximum concentrations of porewater phosphate, ammonium, reactive silicate, surface sediment organic carbon, nitrogen, phosphorus and total phosphorus (TP) measured in the hypoxic Marmara Sea and suboxic/anoxic Black Sea. The decline in the TP concentrations of all sediment core samples indicated P-mobilization to the overlying water. The pool of “potentially mobile P” varied between 0.023 and 0.148 mol/m2 in the studied sites with the maximum values recorded in suboxic and anoxic/sulfidic parts of the Black Sea. This study predicts that the deoxygenation and eutrophication would further lead to the preferential release of P in these three interconnected marine basins, hence changing the remineralization, N/P molar ratios and eventually transform the deep-water nutrient stocks with implications for internal N/P control on marine ecosystems.

Composition, distribution, and source of organic carbon in surface sediments of Erhai Lake, China

Lake sediment is an important organic carbon (OC) sink. Nevertheless, few studies have been conducted on sediment organic carbon (SOC) in lakes, and the effects of environmental variables on SOC pools remain poorly understood. We combined physicochemical and spectroscopic analyses to investigate the composition, distribution, and source of OC in surface sediments of Erhai Lake, southwest China, and explored the relationships between environmental variables and its SOC pool. The SOC pool consists of relatively high proportions of labile organic carbon fractions, mainly from algal production, which are rapidly decomposed and exhibit high turnover rates. The relative content of humus carbon ranges from 13.5 % to 20.5 %, with fulvic acid carbon predominating (average 52.95 %), indicating weak humification and a relatively active humus carbon pool. The dissolved organic matter in water column and sediments of Erhai Lake is largely influenced by endogenous production, with a great contribution from phytoplankton. Surface sediments contained more protein-like components than overlying waters (80.0 % vs. 63.0 %), attributed mainly to abundant algal deposition and intense bacterial metabolism. Among environmental variables, sediment chlorophyll a showed the strongest relationship with the SOC pool, and was associated with rapid decomposition and promotion of the humification process, which supported the conclusion that algae had an important influence on the SOC pool. The SOC pool in the southern region of the lake is mainly contributed by algae, other microorganisms, and sewage, exhibiting a greater potential to release organic matters into the water column. The center and northern SOC pools show relatively stable characteristics and stronger OC sink capacity, mainly because of the input of terrestrial refractory organic matters from runoff. Our data shed light on the OC storage mechanisms in the surface sediments of Erhai Lake and provide theoretical bases for enhancing the OC sink of sediments in the lake.

Exploring Changes in Fishery Emissions and Organic Carbon Impacts Associated With a Recovering Stock

International objectives for sustainable development and biodiversity conservation require restoring fish populations to healthy levels and reducing fishing impacts on marine ecosystems. At the same time, governments, retailers, and consumers are increasingly motivated to reduce the carbon footprint of food. These concerns are reflected in measures of the EU Common Fisheries Policy (CFP) and the CFP Reform Regulation, which highlighted a need to move from traditional single-stock management toward an ecosystem approach to fisheries management (EAF). Using publicly available landings and effort data combined with estimates of adult population biomass, we develop methods to explore the potential for lowering emissions intensity and impacts on organic carbon stocks through ending overfishing and rebuilding stocks. We use the recent recovery of European hake (Merluccius merluccius) stocks in the Northeast Atlantic as a case study. With a focus on the hake fisheries of France, Spain, and the United Kingdom, we compare 2008 and 2016 fishing years. We make an initial estimate of the influence of changing stock status on greenhouse gas emissions during the fishery phase from fuel use and investigate the potential disturbance of organic carbon in the ecosystem, specifically via identification of bottom trawling overlap with organic-rich muddy sediments, and directly on storage in hake biomass. Our findings indicate that recovery of the hake stock was associated with reductions in overall emissions intensity from fuel and proportional impact on hake populations, however, total emissions from both fuel and landings increased, as did likely disturbance of sedimentary organic carbon in surface sediments due to benthic trawling. Ultimately, the aims of this analysis are to further explore the climate impacts of fisheries and overfishing, and to inform development of EAF in the EU.

The sources characteristics of stable isotope organic carbon and nitrogen in suspended particles and surface sediments in Lake Erhai and their water quality implications

为探究湖泊水体悬浮颗粒物和沉积物有机碳、氮来源及水质指示意义，分析了2013-2014年洱海悬浮颗粒物和表层沉积物有机碳同位素（δ¹³C）、氮同位素（δ¹⁵N）和C/N比值时空变化特征及与水质的关系.结果表明：①洱海悬浮颗粒物δ¹³C、C/N、δ¹⁵N在旱、雨季差异显著（P<0.05），旱季变化范围分别为-31.75‰~-18.21‰（均值-25.34‰±4.14‰）、9.1~16.9（均值13.3±2.7）、4.9‰~7.4‰（均值6.4‰±1.3‰），雨季变化范围分别为-14.7‰~-23.8‰（均值-20.2‰±3.3‰）、4.6~8.9（均值7.1±1.6）、7.4‰~10.8‰（均值9.3‰±1.8‰）.悬浮颗粒物有机碳来源在旱季以陆源C3植物为主（46.0%±6.9%），转变为雨季以浮游植物为主（43.3%±6.1%）；氮来源在旱季以陆源植物为主（40.7‰±6.5%），转变为雨季以湖内水生植物和浮游植物为主（39.9%±6.6%）.表层沉积物δ¹³C（-24.0‰~-14.6‰（均值为-18.7‰±4.7‰））和C/N（9.1~15.5（均值为12.1±3.3））均无显著季节差异（P>0.05），δ¹⁵N在旱、雨季差异显著（P<0.05），变化范围分别为1.9‰~4.9‰（均值为3.6‰±1.5‰）和0.7‰~7.8‰（均值为4.2‰±1.8‰）.表层沉积物有机碳来源在旱、雨季均以陆源C4植物为主（48.2%±19.1%），氮来源旱季以陆源植物为主（44.3%±10.1%），转变为雨季以化肥为主（30.3%±6.8%）.两者有机碳与氮来源差异揭示水生和浮游植物来源的有机碳与氮易降解，对水质影响较大，而陆源C4植物和土壤来源的有机碳和化肥来源氮易沉积，对水质影响相对较小.②随机森林回归分析表明，悬浮颗粒物有机质（POM）来源差异（δ¹³C、C/N）、氧化还原电位和水温（WT）是影响水体多营养循环指标的重要因子，重要度为4.0%~6.9%；POM、C/N和WT是影响叶绿素a的重要因子，重要度为9.3%~10.7%，说明POM来源特征结合水环境因子显著影响水质，而表层沉积物有机质来源相对稳定，对水质无显著影响.为防控洱海水质持续下降，除了加强外源控制，雨季还应重点采取控藻措施.;In order to explore the sources of organic carbon and nitrogen in suspended particles (SP) and sediments, and their implication for water quality, the spatiotemporal variation characteristics of stable isotopic carbon (δ¹³C), nitrogen (δ¹⁵N) and C/N in SP and surface sediments of Lake Erhai were determined in 2013-2014, and their effects on water quality were analyzed. The results showed that, ① The δ¹³C, C/N and δ¹⁵N of SP were significantly different between dry and wet seasons (P<0.05), and varied in the range of -31.75‰ to -18.21‰(the mean was -25.34‰±4.14‰), 9.1 to 16.9(13.3±2.7) and 4.9‰ to 7.4‰(6.4‰±1.3‰) in the dry season, and -23.8‰ to -14.7‰(-20.2‰±3.3‰),4.6 to 8.9(7.1±1.6) and 7.4‰ to 10.8‰(9.3‰±1.8‰) in the wet season, respectively. The end-member mixing model showed, the sources of organic carbon in SP transformed from the terrestrial C3 plants dominated (46.0%±6.9%) in the dry season to phytoplankton dominated (43.3%±6.1%) in the wet season; the nitrogen sources in SP transformed from the terrestrial plants dominated (40.7%±6.5%) in the dry season to aquatic plants and phytoplankton dominated (39.9%±6.6%) in the wet season. There were no significant differences in δ¹³C and C/N in surface sediments between dry and wet seasons (P>0.05), and the δ¹³C and C/N varied in the range of -24.0‰ to -14.6‰(-18.7‰±4.7‰)) and 9.1 to 15.5(12.1±3.3), respectively. The δ¹⁵N of surface sediments was significantly different between dry and wet seasons(P<0.05), and varied in the range of 1.9‰ to 4.9‰(3.6‰±1.5‰) and 0.7‰ to 7.8‰(4.2‰±1.8‰), respectively. The organic carbon in surface sediments in dry and wet seasons mainly originated from the terrestrial C4 plants, which contributed 48.2%±19.1% of the total organic carbon, and the nitrogen sources in surface sediments transformed from the terrestrial plants dominated (44.3%±10.1%) in the dry season to fertilizers dominated (30.3%±6.8%) in the wet season. The different sources of organic carbon and nitrogen between SP and surface sediments revealed that the organic carbon and nitrogen from phytoplankton were easily degraded, leading to the deterioration of water quality; while the organic carbon from terrestrial C4 plants and soil erosion and nitrogen from fertilizer are easy to deposit, which had little effect on water quality. ② Correlation analysis and random forest regression analysis showed that the source difference of particulate organic matter(POM) (δ¹³C and δ¹⁵N), oxidation-reduction potential and water temperature (WT) in SP were key indicators in the multi-nutrient cycling index for lake water with the importance for 4.0% to 6.9%, while POM, C/N and WT in SP were key indicators in chlorophyll-a for lake water with the importance for 9.3% to 10.7%, which indicated that the source characteristics of POM combining with environmental factors had a significant effect on water quality. However, the source characteristics of organic carbon and nitrogen in surface sediments have no significant effect on water quality. For the protection of Lake Erhai, in addition to further strengthening the control of external loads, the phytoplankton biomass should be mainly controlled in the wet season to avoid further deterioration of water quality.

Increased petrogenic and biospheric organic carbon burial in sub‐Antarctic fjord sediments in response to recent glacier retreat

AbstractFjords are recognized as hotspots of organic carbon (OC) burial in the coastal ocean. In fjords with glaciated catchments, glacier discharge carries large amounts of suspended matter. This sedimentary load includes OC from bedrock and terrigenous sources (modern vegetation, peat, soil deposits), which is either buried in the fjord or remineralized during export, acting as a potential source of CO2 to the atmosphere. In sub‐Antarctic South Georgia, fjord‐terminating glaciers have been retreating during the past decades, likely as a response to changing climate conditions. We determine sources of OC in surface sediments of Cumberland Bay, South Georgia, using lipid biomarkers and the bulk 14C isotopic composition, and quantify OC burial at present and for the time period of documented glacier retreat (between 1958 and 2017). Petrogenic OC is the dominant type of OC in proximity to the present‐day calving fronts (60.4 ± 1.4% to 73.8 ± 2.6%) and decreases to 14.0 ± 2.7% outside the fjord, indicating that petrogenic OC is effectively buried in the fjord. Beside of marine OC, terrigenous OC comprises 2.7 ± 0.5% to 7.9 ± 5.9% and is mostly derived from modern plants and Holocene peat and soil deposits that are eroded along the flanks of the fjord, rather than released by the retreating fjord glaciers. We estimate that the retreat of tidewater glaciers between 1958 and 2017 led to an increase in petrogenic carbon accumulation of 22% in Cumberland West Bay and 6.5% in Cumberland East Bay, suggesting that successive glacier retreat does not only release petrogenic OC into the fjord, but also increases the capacity of OC burial.

The Carbon-Sulfur Link in the Remineralization of Organic Carbon in Surface Sediments

Here we present the carbon isotopic composition of dissolved inorganic carbon (DIC) and the sulfur isotopic composition of sulfate, along with changes in sulfate concentrations, of the pore fluid collected from a series of sediment cores located along a depth transect on the Iberian Margin. We use these data to explore the coupling of microbial sulfate reduction (MSR) to organic carbon oxidation in the uppermost (up to nine meters) sediment. We argue that the combined use of the carbon and sulfur isotopic composition, of DIC and sulfate respectively, in sedimentary pore fluids, viewed through a δ13CDIC vs. δ34SSO4 cross plot, reveals significant insight into the nature of carbon-sulfur coupling in marine sedimentary pore fluids on continental margins. Our data show systemic changes in the carbon and sulfur isotopic composition of DIC and sulfate (respectively) where, at all sites, the carbon isotopic composition of the DIC decreases before the sulfur isotopic composition of sulfate increases. We compare our results to global data and show that this behavior persists over a range of sediment types, locations and water depths. We use a reactive-transport model to show how changes in the amount of DIC in seawater, the carbon isotopic composition of organic matter, the amount of organic carbon oxidation by early diagenetic reactions, and the presence and source of methane influence the carbon and sulfur isotopic composition of sedimentary pore fluids and the shape of the δ13CDIC vs. δ34SSO4 cross plot. The δ13C of the DIC released during sulfate reduction and sulfate-driven anaerobic oxidation of methane is a major control on the minimum δ13CDIC value in the δ13CDIC vs. δ34SSO4 cross plot, with the δ13C of the organic carbon being important during both MSR and combined sulfate reduction, sulfate-driven AOM and methanogenesis.

Effects of marine pollution, climate, and tidal range on biomass and sediment organic carbon in Chinese mangrove forests

Mangroves are among the most important carbon reservoirs for atmospheric CO2. Nonetheless, how the biomass and sediment organic carbon (SOC) of mangrove forests are affected by marine pollution, in addition to climate and tidal range, on a regional scale is unknown. This study aimed to assess how spatial patterns of biomass and SOC could be influenced by the climate, tidal range, and marine pollution in China’s mangrove forests. The result showed that aboveground biomass (AGB), surface SOC content (SOCC), and SOC density (SOCD) showed clear latitudinal change patterns on a regional scale and decreased in a non-linear manner with increasing latitude. Correlation analysis and principal component analysis (PCA) indicated that latitudinal variation in AGB, SOCC, and SOCD was mainly regulated by temperature-related indicators. However, AGB, SOCC, and SOCD were not found to be correlated to annual precipitation. Besides, the tidal range was found to be negatively correlated to biomass and SOC content/stock. Tidal range is one important abiotic predictor used to understand the latitudinal patterns of mangrove biomass and SOC dynamics. Correlation analysis and PCA revealed that mangrove biomass and SOC content/stock were negatively affected by heavy metal pollution, and they were disproportionately negatively correlated to marine heavy metal concentrations in the coastlines neighboring mangrove forests. Environmentally-sound technology should be adapted to prevent, control, and remediate heavy metal pollution to increase carbon storage in China’s mangrove forests. Mangrove biomass and SOC content/stock were also found to be negatively related to the suspended solid concentration in the coastlines neighboring mangrove forests. In contrast, vegetation biomass was found to be positively correlated to ammonia nitrogen content. However, biomass was found to be not correlated to nitrate-nitrogen, nitrites-nitrogen, inorganic forms of nitrogen, and non-ionic ammonia. Additional studies are needed to confirm the mechanisms by which different forms of nitrogen enrichment affect biomass storage.

Sediment carbon storage increases in tropical, oligotrophic, high mountain lakes

• El Sol and La Luna are valuable sentinels of global change by displaying detectable signs of anthropic disturbances from 1950 onwards. • The higher mass accumulation rates and organic carbon burial rates from 1950 onwards are result of human activities and atmospheric dust deposition. • Sharing the same climatic conditions, climate change unlikely caused the recent higher organic carbon burial observed in both lakes. High mountain lakes are valuable sentinels of global change because they are sensitive to environmental stress and integrate changes in the atmosphere and their catchment areas. This study tested the hypothesis that local and regional anthropogenic stressors have affected productivity, sedimentation, and organic carbon burial in two tropical high mountain lakes in central Mexico. We studied changes in the water column (Secchi disc depth, total suspended solids (TSS), and chlorophyll- a concentrations) and surface sediments (chlorophyll- a and organic carbon concentrations) in El Sol Lake and La Luna Lake during the period 2000–2018, and organic carbon burial rates in sediment cores (∼1884–2014) dated with lead-210 ( 210 Pb). We observed increasing water turbidity, TSS, and organic carbon in surface sediments in El Sol Lake. Different responses of the two lakes were caused by lower pH in La Luna Lake and a threefold residence time of TSS in El Sol Lake compared to La Luna Lake, mostly attributable to their different surface/volume ratios. Organic carbon burial rates were slightly higher at La Luna Lake until 2000, when they became higher at El Sol Lake due to increasing sediment accumulation and organic carbon concentrations. In both lakes, results show significantly higher organic carbon burial rates since the 1950s, likely resulting from the deposition of human-induced wind-blown particles derived from activities at the volcano slopes and long-distance transport from highly urbanized areas. Anthropogenic impacts rather than climate change, therefore, caused the recent higher organic carbon burial rates observed in both lakes. Methods and findings from this study provide a valuable basis for comparing changes in other high mountain lakes worldwide.

Quantifying Marine Sedimentary Carbon: A New Spatial Analysis Approach Using Seafloor Acoustics, Imagery, and Ground-Truthing Data in Scotland

Marine sediments are important repositories of particulate organic matter, effectively burying organic carbon (OC) over geological timescales thus providing a climate regulation service. However, the spatial distribution of this marine sedimentary OC store is not well constrained. In this study we leverage a high resolution multibeam echosounder (MBES) survey taken at Loch Creran, a model fjordic site on the west coast of Scotland, to develop a new methodology for predicting the distribution of OC in surface sediments. Using an integrated approach, we combine use MBES survey, video imagery and ground-truthing data to produce a high-resolution (2 m x 2 m) map of surficial carbon and calculate a 10 cm stock. We find that the backscatter survey reliably uncovers a heterogeneous seabed and that OC correlates strongly with the MBES backscatter signal as a function of sediment composition. We estimate that there are approximately 12,346 ± 2,677 tonnes of OC held within the top 10 cm of mixed sediments across the MBES survey area (7.96 km2; 60% of the total area), upscaled to 20,577 ± 4,462 t of OC across Loch Creran (13.27 km2). Normalised by area, we find that mixed fine sediments with small fractions of sand and gravel are more effectivehold more carbon OC stores than homogenous fine sediments. This initial work proposes a novel methodological approach to using high resolution MBES surveys to improve the spatial mapping of sedimentary C carbon (C) and identification of C hotspots, enabling consideration of this resource in sedimentary carbon accounting, seabed management and climate mitigation strategies.

Enhanced but highly variable bioturbation around seamounts in the northwest Pacific

Seamounts are a unique ecosystem in marine environment, but the relevant understanding is limited. In this study, sedimentation and bioturbation around the Pako Guyot of Magellan, and the Lamont, Scripps, Arnold, and Pot Guyots of Marcus-Wake seamounts in the northwest Pacific were evaluated using 230Thex and 210Pbex as tracers. Our results showed that the linear sedimentation rate and the mass accumulation rate ranged from 0.12 to 2.50 mm/ka and from 0.06 to 1.14 kg/m2/ka with averages of 1.27 ± 0.80 mm/ka and 0.49 ± 0.30 kg/m2/ka respectively. The accumulation flux of organic carbon in surface sediments was estimated to be 0.10–4.52 g C/m2/ka. The bioturbation coefficients ranged from 1.01 to 27.1 cm2/a with an average of 10.8 ± 9.2 cm2/a, which is higher than those in abyssal sediments or predicted by traditional empirical equations. The enhanced bioturbation supports the view that seamounts are hotspots for pelagic and benthic organisms. The bioturbation intensity showed a great variability with the maximum around 40 km away from the edge of seamount summit. The bioturbation coefficient correlated positively with sedimentation rate and accumulation flux of organic carbon in surface sediments, indicating that the supply of organic matter is a main driving force for enhanced bioturbation around the seamounts. The increase in sedimentary organic matter promotes the activities of benthic organisms. More research is needed to gain a deep understanding of bioturbation in seamounts in the context of future climate change.

Interfacial oxygen nanobubbles reduce methylmercury production ability of sediments in eutrophic waters

Eutrophication can induce hypoxia/anoxia and rich organic matter at the sediment-water interface in surface waters. When eutrophic waters are impacted with mercury (Hg) pollution, methylmercury (MeHg) production ability (MPA) of surface sediment would increase and more MeHg might be produced. To tackle this risk, this study firstly collected samples of surface sediment and overlying water from a typical eutrophic lake—Taihu Lake. Then from a sediment-water simulation system, we demonstrated that eutrophic waters were able to methylate Hg spontaneously, and that sediment is the major Hg sink in the system. After the addition of HgCl2 solution (approximately 1 mg L−1 in the slurry), MeHg concentrations in the sediment increased by 11.7 times after 48 h. The subsequent column experiments proved that O2 nanobubbles could significantly decrease the MPA of surface sediment, by up to 48%. Furthermore, we found that O2 nanobubbles could remediate anoxia mainly by increasing dissolved oxygen (from 0 to 2.1 mg L−1), oxidation-reduction potentials (by 37% on average), and sulfate (by 31% on average) in the overlying water. In addition, O2 nanobubbles could also help decrease organic matter concentration, as was revealed by the decline of dissolved organic carbon in the overlying water (by up to 57%) and total organic carbon in surface sediment (by up to 37%). The remediation of anoxia and reduction of organic matter could contribute to the decrease of hgcA gene abundance (by up to 86%), and thus result in the reduction of MPA after the addition of O2 nanobubbles. This study revealed the risk of MeHg production in case Hg pollution occurs in eutrophic waters and proposed a feasible solution for MeHg remediation.

Reactive Iron and Iron‐Bound Organic Carbon in Surface Sediments of the River‐Dominated Bohai Sea (China) Versus the Southern Yellow Sea

AbstractFactors influencing reactive Fe cycling and its protection of organic carbon (OC) in sediments are poorly understood. Here we comparatively study Fe speciation and Fe‐associated OC (Fe‐OC) in surface sediments of the Bohai Sea (BHS) and southern Yellow Sea (SYS), two seas with common sediment sources but different depositional regimes. Though significant sequestration of highly reactive Fe (FeHR) is expected in the estuarine system stream from the river‐dominated BHS, this pool is, however, slightly enriched in the BHS sediments relative to their source material. This reconfirms a previous speculation of sedimentary FeHR enrichment in semi‐protected settings. Relative to the BHS, the SYS sediments are depleted of FeHR, despite common sediment sources of these two areas. Estuarine pre‐enrichment and subsequent redistribution of FeHR in the BHS, and aging of Fe‐bearing authigenic clays during transport from the BHS to the SYS are potential mechanisms for the depletion. The fractions (fFe‐OC) of Fe‐OC in total OC in sediments of the two seas are at the lower end for soils and sediments, indicating Fe being a minor “rusty sink.” 13CFe‐OC fractionations indicate preferential sequestration of terrestrial OC by Fe oxides in the BHS, in contrast with preferential retention of marine OC in the SYS. Different fractionations of 13CFe‐OC in the two seas are a net result of selective adsorption of OC by Fe oxides and selective stabilization of OC during Fe reductive dissolution. Preferential sequestration of terrestrial OC may exert an important influence on distribution and compositions of OC buried in the river‐dominated system.

Quantifying Degradative Loss of Terrigenous Organic Carbon in Surface Sediments Across the Laptev and East Siberian Sea.

Ongoing permafrost thaw in the Arctic may remobilize large amounts of old organic matter. Upon transport to the Siberian shelf seas, this material may be degraded and released to the atmosphere, exported off‐shelf, or buried in the sediments. While our understanding of the fate of permafrost‐derived organic matter in shelf waters is improving, poor constraints remain regarding degradation in sediments. Here we use an extensive data set of organic carbon concentrations and isotopes (n = 109) to inventory terrigenous organic carbon (terrOC) in surficial sediments of the Laptev and East Siberian Seas (LS + ESS). Of these ~2.7 Tg terrOC about 55% appear resistant to degradation on a millennial timescale. A first‐order degradation rate constant of 1.5 kyr−1 is derived by combining a previously established relationship between water depth and cross‐shelf sediment‐terrOC transport time with mineral‐associated terrOC loadings. This yields a terrOC degradation flux of ~1.7 Gg/year from surficial sediments during cross‐shelf transport, which is orders of magnitude lower than earlier estimates for degradation fluxes of dissolved and particulate terrOC in the water column of the LS + ESS. The difference is mainly due to the low degradation rate constant of sedimentary terrOC, likely caused by a combination of factors: (i) the lower availability of oxygen in the sediments compared to fully oxygenated waters, (ii) the stabilizing role of terrOC‐mineral associations, and (iii) the higher proportion of material that is intrinsically recalcitrant due to its chemical/molecular structure in sediments. Sequestration of permafrost‐released terrOC in shelf sediments may thereby attenuate the otherwise expected permafrost carbon‐climate feedback.

Geochemistry of organic carbon in surface sediments of a summer hypoxic region in the coastal waters of northern Shandong Peninsula

The geochemistry of sedimentary organic matter (SOM) in coastal areas is complex due to its multiple sources and intricate hydrological features. In this study, the biogenic element concentrations and stable carbon (δ13C) and nitrogen (δ15N) isotopic compositions in the coastal surface sediments of northern Shandong Peninsula, along with some parameters related to water quality, were analyzed to investigate the temporal-spatial variations in SOM and the processes that control its distribution. The results revealed that marine autogenous organic matter is a major contributor to SOM, accounting for 75.4 ± 3.3%, 60.8 ± 6.6% and 67.4 ± 10.3% in August and November 2015 and March 2016, respectively. In summer, TOC and TN concentrations were significantly higher than those in autumn and spring. The relatively high abundances of SOM were found in the offshore areas in summer and spring, which was contrary to those in autumn. Riverine discharge, nutrients, primary productivity and dissolved oxygen (DO) dynamics could all influence the composition and contents of SOM in different seasons. In summer, high primary productivity and hypoxia condition led to high SOM values. In comparison, SOM contents were relatively low due to sufficient DO in bottom water in autumn and spring. Dissolved nutrients in seawater could affect the accumulation of autogenous organic matter by impacting upon primary production. In summer, nitrate in surface water had the most obvious effects on autogenous organic carbon (AOC) and may be the principal factor of limiting the growth of phytoplankton. In autumn, nitrate as well as dissolved silicate had more effects on AOC storage. However, phosphate had the most obvious influence on AOC storage in spring.