Sediment Organic Carbon Content Research Articles

Blue carbon dynamics across a salt marsh-seagrass ecotone in a cool-temperate estuary

Abstract Background Seagrass and salt marsh ecosystems are recognised for their role in climate change mitigation and adaptation given their carbon storage potential. However, factors driving variability in blue carbon ecosystems are understudied, yet are important to account for. Aims Examine the variability of sediment organic carbon (SOC) and its drivers (seagrass morphometrics and sediment nutrients) at different spatial scales; > 1 km, ~ 150 m and ~ 10 m across the salt marsh-seagrass ecotone. Methods We collected the top 5 cm of sediment in the Olifants River Estuary, a cool-temperate system in South Africa. Using a line transect approach, we sampled across the salt marsh-seagrass ecotone (~ 10 m) in triplicate transects (~ 50 m apart) at three sampling sites (1–3 km) and analysed for SOC and elemental nutrient (nitrogen and phosphorus) content. Seagrass morphometrics (shoot density, leaf length and number per shoot) were measured. Results There was significant (P < 0.05) spatial heterogeneity in SOC stocks between sites (1–3 km) and between salt marshes and seagrass, but low variability at ~150 m. We detected a significant decrease in SOC from salt marsh towards the seagrass edge, with seagrass SOC remaining uniform. Nitrogen content was positively correlated with SOC in seagrass and salt marshes (P < 0.05), but seagrass morphometrics were not significant drivers of SOC. Conclusions The dynamics of blue carbon differ between salt marshes and seagrass, with spatial heterogeneity of SOC at scales > 1 km, suggesting that future BC assessments need to account for spatial heterogeneity to improve the accuracy of carbon removal estimates.

Assessing controls on sedimentation rates and sediment organic carbon accretion in beaver ponds

Beaver dams trap sediment, promote channel-floodplain connectivity, modify biogeochemical cycling and organic carbon (OC) storage, and influence geomorphic form. Beaver-related sediment accumulation has been investigated at longer timescales (e.g., > 1000 years) and shorter timescales (< 10 years), but we lack information on sedimentation and sediment-associated OC accretion rates over multiple decades in relatively persistent beaver ponds (10–100 years old). We coupled field surveys of 45 beaver ponds with historical aerial imagery and radiometric dating with 7Be, 210Pb, and 14C to calculate sedimentation rates, mean sediment depth, and sediment OC content at two study sites in the southern Rocky Mountains, USA. Sedimentation rates in beaver ponds (median = 5.7 cm yr−1, mean = 11.6 cm yr−1) decreased with pond age. Incised, single threaded reaches had greater variability in mean sediment depth compared to less incised reaches. In less incised reaches, mean sediment depth and beaver dam height increased with pond age, indicating more stable dams and depositional environments. Sediment OC content within beaver ponds (median = 0.8 %, mean = 1.7 %) increased with finer sediment grain size distributions. Sediment OC accretion rates in ponds ranged between 0.13 and 23 Mg C ha −1 per year. We used Monte Carlo simulations to estimate it would take ∼100 years or more of uninhibited beaver activity for deposition to laterally reconnect adjacent terraces in the incised study reaches, a common objective within many stream restoration projects. Our findings show that beaver ponds in complex, multi-threaded reaches better retain fine sediment over longer timescales, highlighting the need to incorporate geomorphic context when considering whether beaver can help restore incised river channels and floodplain connectivity, retain fine sediment, and store OC on the landscape.

Molecular Composition Evolution of Dissolved Organic Matter With Water Depth in Prydz Bay of East Antarctic: Carbon Export Implications

AbstractThis study analyzes the molecular composition of dissolved organic matter (DOM) in Prydz Bay by Fourier Transform Ion Cyclotron Resonance mass spectrometry to probe the carbon sequestration capacity in the continental shelf system. Concentrations of particulate organic carbon (POC), particulate nitrogen and dissolved organic carbon (DOC) with water depth show that POC could be mainly decomposed into DOC and/or microbially degraded. Highly labile DOC is further degraded and remineralized by microorganisms within the upper 200 m, as evidenced by a downward enrichment of 13CPOC and increases in the average molecular weight, oxygen atom number (O) and double bond equivalents of DOM molecules, indicating that biodegradation is the main driver for particulate organic matter and DOM evolution with water depth. Semi‐quantitative calculation demonstrates that ∼83% of POC was transformed to DOC as well as dissolved inorganic carbon (DIC), and ∼30% of DOC further to DIC via microbial degradation within the upper 200 m in summer, resulting in a relatively low total organic carbon content in sediments of Prydz Bay. The newly transformed DIC and residue DOC can be preserved in the deep layer due to the formation of well stratified and stable water body in summer of Prydz Bay, ultimately entering the regional circulation system instead of being released back into the atmosphere. This could be one of the most important processes determining the atmosphere CO2 uptake in the continental shelf system of Southern Ocean.

The overlooked carbon cache: Unveiling organic carbon storage in small floodplain lake sediments under humid continental climate changes

Small floodplain lakes (SFLs), characterized by their diminutive size (<10,000 m2), have been consistently overlooked in assessments of organic carbon (C) stock and the impacts of climate changes. This study aims to ascertain both the quantity and quality of carbon stored within the sediments of SFLs. Furthermore, we endeavour to evaluate the influence of ongoing climate change on the floodplain lake structure within the continental humid climate zone on the example of the Vistula River valley and estimate the CO2 emission from dry floodplain lakes. The sedimentary organic carbon content (TOC) ranged from 2.3–64 g kg−1, averaging at 3.4 g kg−1. Consequently, the sediments in the analysed SFLs of the river section potentially harbour up to 5,785 Mg TOC. Notably, the abundance of easily soluble carbon fractions (humic and fulvic acids) was relatively low (∼15 %), implying a stable immobilization of organic C compounds and minimal impact of sediments on water quality. Satellite imagery analysis for the years 2017–2020 revealed a 31 % reduction in the total lake surface area within a 100-km-long segment of the Vistula River. The decreasing surface area of floodplain lakes and rising mean air temperatures increase CO2 emissions and have an additional impact on climate warming. Our research underscores that investigating lake C storage concerning climate change demands a nuanced approach. While higher temperatures stimulate primary production, the reduction in lake surface area and the escalating role of small lakes, such as SFLs, in C storage should be paramount considerations.

Seagrass decline weakens sediment organic carbon stability

Seagrass meadows are globally recognized as critical natural carbon sinks, commonly known as ‘blue carbon’. However, seagrass decline attributed to escalating human activities and climate change, significantly influences their carbon sequestration capacity. A key aspect in comprehending the impact of seagrass decline on carbon sequestration is understanding how degradation affects the stored blue carbon, primarily consisting of sediment organic carbon (SOC). While it is widely acknowledged that seagrass decline affects the input of organic carbon, little is known about its impact on SOC pool stability. To address this knowledge, we examined variations in total SOC and recalcitrant SOC (RSOC) at a depth of 15 cm in nine seagrass meadows located on the coast of Southern China. Our findings revealed that the ratio of RSOC to SOC (RSOC/SOC) ranged from 27 % to 91 % in the seagrass meadows, and the RSOC/SOC increased slightly with depth. Comparing different seagrass species, we observed that SOC and RSOC stocks were 1.94 and 3.19-fold higher under Halophila beccarii and Halophila ovalis meadows compared to Thalassia hemprichii and Enhalus acoroides meadows. Redundancy and correlation analyses indicated that SOC and RSOC content and stock, as well as the RSOC/SOC ratio, decreased with declining seagrass shoot density, biomass, and coverage. This implies that the loss of seagrass, caused by human activities and climate change, results in a reduction in carbon sequestration stability. Further, the RSOC decreased by 15 %, 29 %, and 40 % under unvegetated areas compared to adjacent Halophila spp., T. hemprichii and E. acoroides meadows, respectively. Given the anticipated acceleration of seagrass decline due to climate change and increasing coastal development, our study provides timely information for developing coastal carbon protection strategies. These strategies should focus on preserving seagrass and restoring damaged seagrass meadows, to maximize their carbon sequestration capacity.

Simultaneous methane mitigation and nitrogen removal by denitrifying anaerobic methane oxidation in lake sediments

Methane (CH4) is a potent greenhouse gas, with lake ecosystems significantly contributing to its global emissions. Denitrifying anaerobic methane oxidation (DAMO) process, mediated by NC10 bacteria and ANME-2d archaea, links global carbon and nitrogen cycles. However, their potential roles in mitigating methane emissions and removing nitrogen from lake ecosystems remain unclear. This study explored the spatial variations in activities of nitrite- and nitrate-DAMO and their functional microbes in Changdanghu Lake sediments (Jiangsu Province, China). The results showed that although the average abundance of ANME-2d archaea (5.0 × 106 copies g−1) was significantly higher than that of NC10 bacteria (2.1 × 106 copies g−1), the average potential rates of nitrite-DAMO (4.59 nmol 13CO2 g−1 d−1) and nitrate-DAMO (5.01 nmol 13CO2 g−1 d−1) showed no significant difference across all sampling sites. It is estimated that nitrite- and nitrate-DAMO consumed approximately 6.46 and 7.05 mg CH4 m−2 d−1, respectively, which accordingly achieved 15.07–24.95 mg m−2 d−1 nitrogen removal from the studied lake sediments. Statistical analyses found that nitrite- and nitrate-DAMO activities were both significantly related to sediment nitrate contents and ANME-2d archaeal abundance. In addition, NC10 bacterial and ANME-2d archaeal community compositions showed significant correlations with sediment organic carbon content and water depth. Overall, this study underscores the dual roles of nitrite- and nitrate-DAMO processes in CH4 mitigation and nitrogen elimination and their key environmental impact factors (sediment organic carbon and inorganic nitrogen contents, and water depth) in shallow lake, enhancing the understanding of carbon and nitrogen cycles in freshwater aquatic ecosystems.

Unraveling Environmental Forces Shaping Surface Sediment Geochemical “Isodrapes” in the East Asian Marginal Seas

AbstractAs major sites of carbon burial and remineralization, continental margins are key components of the global carbon cycle. However, heterogeneous sources of organic matter (OM) and depositional environments lead to complex spatial patterns in sedimentary organic carbon (OC) content and composition. To better constrain the processes that control OM cycling, we focus on the East Asian marginal seas as a model system, where we compiled extensive data on the OC content, bulk isotopic composition (δ13C and Δ14C), total nitrogen, and mineral surface area of surficial sediments from previous studies and new measurements. We developed a spatial machine learning modeling framework to predict the spatial distribution of these parameters and identify regions where sediments with similar geochemical signatures drape the seafloor (i.e., “isodrapes”). We demonstrate that both provenance (44%–77%) and hydrodynamic processes (22%–53%) govern the fate of OM in this margin. Hydrodynamic processes can either promote the degradation of OM in mobile mud‐belts or preserve it in stable mud‐deposits. The distinct isotopic composition of OC sources from marine productivity and individual rivers regulates the age and reactivity of OM deposited on the sea‐floor. The East Asian marginal seas can be separated into three main isodrapes: hydrodynamically energetic shelves with coarser‐grained sediment depleted in OC, OM‐enriched mud deposits, and a deep basin with fine‐grained sediments and aged OC affected by long oxygen exposure times and petrogenic input from rivers. This study confirms that both hydrodynamic processes and provenance should be accounted for to understand the fate of OC in continental margins.

Restoration of Aegiceras corniculatum Mangroves May Not Increase the Sediment Carbon, Nitrogen, and Phosphorus Stocks in Southeastern China

Mangrove ecosystems play important roles in mitigating climate change but are suffering from an area decrease. The sediment in a mangrove ecosystem is a crucial component in estuarine carbon and nutrient cycles. However, it is unclear whether the restoration of mangroves enhances their sediment carbon sequestration and nutrient storage. To investigate the restoration effects of native Aegiceras corniculatum mangrove forests, the contents and density of sediment organic carbon (SOC), total nitrogen (TN), and total phosphorus (TP) were quantified in an unvegetated mudflat (MF), an area with mature A. corniculatum (AC), and two restored A. corniculatum mangroves planted in 2002 (02AC) and 2008 (08AC) in Quanzhou, southeastern China. Compared with the MF, mangrove restoration increased the SOC and TN contents in the sediment, primarily at the 0–40 cm depth for SOC and 0–20 cm for TN, while the TP content was not impacted by restoration. The sediment under AC showed a higher SOC content only at the 0–10 cm and 20–40 cm depths compared to restored 02AC and 08AC. The restoration of A. corniculatum mangroves did not significantly increase the stock of sediment SOC, TN, and TP relative to the MF, which might be attributed to the decrease in bulk density and the relatively short restoration period. Thus, further continuous monitoring is needed to verify whether the sediment carbon pool can be promoted on a longer-term scale by the restoration of A. corniculatum.

Functional composition outweighs taxonomic and functional diversity in maintaining ecosystem properties and processes of mangrove forests

AbstractBiodiversity loss can have significant consequences for human well‐being, as it can affect multiple ecosystem properties and processes (MEPP) that drive ecosystem services. However, a comprehensive understanding of the link between environmental factors, biodiversity, and MEPP remains elusive, especially in mangrove ecosystems that millions of people along tropical coastlines worldwide depend upon. Here, we collated a comprehensive dataset on forest inventory, plant traits, and environmental factors across 93 plots in the Sundarbans Reserved Forests, Bangladesh. The functional composition (FC) of leaf area showed a stronger positive association with MEPP, being determined by total biomass and productivity of the mangroves, sediment organic carbon, and ammonium, phosphorus, and potassium contents of the sediment, than species richness (SR) or functional diversity (FD). Further, FC mediated a strong negative association of sediment salinity, and a positive association of SR, with MEPP. The similar but opposite total associations of SR and sediment salinity with MEPP suggest that species‐rich mangroves could offset the negative impacts of rising salinity on MEPP. When focusing on a single aspect of MEPP, both FD and FC mattered, with the FD of leaf area showing a strong association with mangrove productivity and sediment potassium content, while the FC of leaf litter nitrogen showed the strongest associations with sediment ammonium and phosphorus contents. Therefore, to sustain mangrove ecosystems as a reliable nature‐based solution for climate change mitigation, conservation and (re‐)establishment projects should prioritize regionally dominant species with high leaf area and nitrogen content, plus functionally different species to support the ecosystem processes and services provided by mangroves.

Sediment Carbon Stock in Natural and Transplanted Mangroves in Bahrain, Arabian Gulf

Mangroves in the Arabian Gulf provide several biological, ecological, and environmental services. They are also considered among the largest carbon sinks. However, mangroves along the coastlines of the Arabian Gulf have degraded in recent decades, mainly due to urbanization and coastal development. Therefore, restoration and afforestation programs have been initiated to enhance the services and functions of mangrove ecosystems and as part of national targets to mitigate climate change. Increasing carbon sinks by quadrupling the current areas covered by mangroves through afforestation programs by 2035 is one of the strategies to mitigate climate change in Bahrain. The aim of the present study was to estimate the organic carbon stocks in the sediments of natural and transplanted mangroves in Bahrain. Within the protected areas of Tubli and Arad Bays, sediment samples were taken down to a depth of 70 cm from natural and transplanted mangroves as well as a bare mudflat. The findings of the present study indicated that the total sediment organic carbon concentrations at three sampling sites of natural and transplanted mangroves and the mudflat were 200.54 ± 24.52, 112.36 ± 55.51, and 81.56 ± 8.92 Mg C/ha, respectively. The natural mangroves in Tubli Bay differed considerably from those in Arad Bay (p ≤ 0.001), based on the concentrations of organic carbon in sediments. However, there was a noticeable similarity seen in the organic carbon of the mangroves in Arad Bay that were transplanted 25 years prior and the natural mangroves in Tubli Bay, indicating the importance of a long-term mangrove afforestation strategy to mitigate climate change in the Arabian Gulf.

Spatial variation of organic carbon storage and aggregate sizes in the sediment of the Zhangjiang mangrove ecosystem

Sediment organic carbon (SOC) in mangrove ecosystem, known as blue carbon, is a crucial component in estuarine carbon cycles. To get a deep insight into the mechanism of SOC accumulation, the sediment physicochemical properties, SOC density and aggregate sizes were investigated among mangrove (Kandelia obovata), saltmarsh (Spartina alterniflora) and mudflats at different elevations (upland mudflat, midland mudflat, lowland mudflat). The results showed that the higher SOC content was observed both in the 0–60 cm of mangrove and in the 0–40 cm of S. alterniflora compared with mudflats (P < 0.05). The SOC density in the top meter ranged from 99 to 202.7 Mg·ha−1 and had vertical difference at all sites. The highest SOC density were found in mangrove, while no difference was observed between S. alterniflora and mudflats. The mean weight diameter of aggregates was higher in the top 60 cm sediment of mangrove while S. alterniflora had no change indicating the capacity of aggregate formation. RDA analysis showed SOC storage could be more explained by the SOC content than bulk density, all sites in the top 40 cm could be well separated. The MWD had the 8.6 % individual effects on SOC content, reflecting the aggregated protection. The mangrove root system favored aggregates formation and physical protection, which facilitate the SOC accumulation. These findings revealed that mangroves restoration could effectively enhance SOC storage, and the mudflat is also significant SOC pool owning to the large bulk density and area. The invasive S. alterniflora did not enhance the SOC storage compared with mudflats and should be controlled in the future work.

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.

Plutonium in sediments of the Eastern Guangdong coast-its sources and their contribution

The 239+240Pu activities and 240Pu/239Pu atom ratios of surface sediments from the Eastern Guangdong coast (EGDC) were determined by sector field ICP-MS in order to examine the sources of plutonium (Pu) and quantify their contributions. The 239+240Pu activities in the EGDC ranged from 0.113 to 0.451 Bq kg−1, with an average of 0.225 ± 0.090 Bq kg−1 (n = 17). Consistently high 240Pu/239Pu atom ratios, ranging from 0.218 to 0.274 (average = 0.254 ± 0.014, n = 17), indicate a non-global fallout Pu source in the EGDC. The horizontal distribution of the 240Pu/239Pu atom ratios in the EGDC sediment suggests the non-global fallout Pu is sourced from close-in fallout from the Pacific Proving Grounds (PPG). Using a simple two end-member mixing model, we calculated the relative proportions of Pu from the PPG and global fallout in the EGDC to be 57 ± 9 % and 43 ± 9 %, respectively. Moreover, from the well-defined relationship between 239+240Pu activity and total organic carbon content in sediments and a two end-member mixing model using δ13C, we further calculated the Terr-global fallout (riverine input) and Mar-global fallout (direct atmospheric deposition) to be 11 ± 2 % and 32 ± 6 %, respectively. Finally, from the activity levels and atom ratios of Pu isotopes in the EGDC, we established a baseline for future use in environmental risk assessment related to nuclear power plant operations.

Effects of Fe(II) and organic carbon on nitrate reduction in surficial sediments of a large shallow freshwater lake

Nitrate-reducing ferrous [Fe(II)]-oxidizing (NRFO) has been reported from lake sediments as a natural reduction pathway. However, the effects of the contents of Fe(II) and sediment organic carbon (SOC) on the NRFO process still remain unclear. In this study, the influences of Fe(II) and organic carbon on nitrate reduction were analyzed quantitatively at two typical seasonal temperatures (25 °C representing summers and 5 °C for winters) by conducting a series of batch incubation experiments, using surficial sediments at the western zone of Lake Taihu (Eastern China). Results showed that Fe(II) greatly promoted NO3‾-N reduction by denitrification (DNF) and dissimilatory nitrate reduction to ammonium (DNRA) processes at high-temperature (25 °C, representing summer season). As Fe (II) increased (e.g., Fe(II)/NO3‾ = 4), the promotion effect on NO3‾-N reduction was weakened, but on the other side, the DNRA process was enhanced. In comparison, the NO3‾-N reduction rate obviously decreased at low-temperature (5 °C, representing the winter season). NRFO in sediments mainly belongs to biological rather than abiotic processes. A relatively high SOC content apparently increased the rate of NO3‾-N reduction (r = 0.023–0.053 mM/d), particularly on the heterotrophic NRFO. It is interesting that the Fe(II) consistently remained active in the nitrate reduction processes no matter whether SOC was sufficient in the sediment, particularly at high-temperature. Overall, the combining effects of both Fe(II) and SOC in surficial sediments made a great contribution towards NO3‾-N reduction and N removal in a lake system. These results provide a better understanding and estimation of N transformation in sediments of the aquatic ecosystem under different environmental conditions.

Weak and intermittent anoxia during the mid-Tournaisian (Mississippian) anoxic event in the Montagne Noire, France

AbstractThe mid-Tournaisian black radiolarian cherts of the Lydiennes Formation are exposed in deep-shelf successions of the Puech de la Suque and Col des Tribes sections of the Mont Peyroux Nappe area in the Montagne Noire, southern France. This interval represents the mid-Tournaisian anoxic event that is also termed the Lower Alum Shale Event. This event is associated with a global marine transgression that was characterized by increased productivity and drastic facies changes from pelagic carbonate sedimentation to the widespread deposition of black organic-rich siliceous shales and radiolarites in many parts of the world. In the present study, high-resolution inorganic geochemistry and framboidal pyrite analyses were employed to decipher changes in depositional conditions during the mid-Tournaisian anoxic event in the Montagne Noire. The results show that the total organic carbon contents of sediments associated with the Lower Alum Shale Event vary from 0.09 to 1.9 wt %. These low to moderate total organic carbon contents, high U/Th, low Corg/P and intermediate V/Cr ratios, enrichment in redox-sensitive trace elements, such as U, Mo and V, as well as varying sizes of pyrite framboids, indicate periodic dysoxic to anoxic bottom-water conditions during deposition of the studied sediments. Anomalous Hg spikes (&gt;500 ppb) are also reported in the mid-Tournaisian deep-water marine succession of the Montagne Noire in the present study, which confirm a possible influence of increased regional volcanic activity during this environmental turnover.

The Geochemical and Environmental Characteristics of Trace Metals in Coastal Sediment Discharge off the Mailiao Industrial Zone of Central Western Taiwan

The geochemical fractions of trace metals in the coastal sediments of the central western Taiwan were examined, employing the Tessier sequential extraction method, and the metals contamination status of the analyzed sediments were also evaluated in the present study. Based on the metal fraction present in sediments, trace metals can be divided into three groups: (1) Al, Cr, Fe, Ni and Zn; (2) Cu and Pb and (3) Mn. In group (1) metal, the metals’ total concentrations were chiefly dominated by the residual fraction, exceeding 80% of the total concentration pool. In group (2) metal, the metals’ total concentrations were dominated by three labile fractions, carbonate, Fe-Mn oxides and organic, accounting for nearly 65% of the total pool, and the residual fraction contributed 35% of the total concentrations. Over 90% of Mn total concentrations were shared by three fractions, carbonate, Fe-Mn oxides and the residual fraction. The statistic results indicated that the total organic carbon contents in sediments played a more important role in influencing the metals contents in sediments. The contamination assessment results suggested that the Mailiao coastal sediments were minorly contaminated by trace metals. Lead should be paid more concerns because Pb total concentrations at some sediments exceeded the value (46.7 mg/kg) of effect range low, USA, and Pb was mainly present in the labile fraction.

Bioaccumulation of per- and polyfluoroalkyl substances by freshwater benthic macroinvertebrates: Impact of species and sediment organic carbon content

Per- and polyfluoroalkyl substances (PFAS) in aquatic environments have caused global concern due to their persistence, toxicity, and potential bioaccumulation of some compounds. As an important compartment of the aquatic ecosystem, sediment properties impact PFAS partitioning between aqueous and solid phases, but little is known about the influence of sediment organic carbon content on PFAS bioaccumulation in benthic organisms. In this study, three freshwater benthic macroinvertebrates – worms (Lumbriculus variegatus), mussels (Elliptio complanata) and snails (Physella acuta) – were exposed for 28 days to PFAS spiked synthetic sediment equilibrated with a synthetic surface water. Using microcosms, sediment organic carbon content – 2%, 5% and 8% – was manipulated to assess its impact on PFAS bioaccumulation. Worms were found to have substantially greater PFAS bioaccumulation compared to mussels and snails. The bioaccumulation factors (BAFs) and biota sediment accumulation factors (BSAFs) in worms were both one to two magnitudes higher than in mussels and snails, likely due to different habitat-specific uptake pathways and elimination capacities among species. In these experiments, increasing sediment organic carbon content decreased the bioaccumulation of PFAS to benthic macroinvertebrates. In worms, sediment organic carbon content was hypothesized to impact PFAS bioaccumulation by affecting PFAS partitioning and sediment ingestion rate. Notably, the BSAF values of 8:2 fluorotelomer sulfonic acid (FTS) were the largest among 14 PFAS for all species, suggesting that the benthic macroinvertebrates probably have different metabolic mechanisms for fluorotelomer sulfonic acids compared to fish evaluated in published literature. Understanding the impact of species and sediment organic carbon on PFAS bioaccumulation is key to developing environmental quality guidelines and evaluating potential ecological risks to higher trophic level species.

Relationship between organic carbon and sediment properties in mangrove forests of three Bushehr coastal areas

One of the main ecosystem services of mangroves is enhancing carbon sequestration, most of which is done by sediments. Therefore, recognizing sediment properties is useful to evaluate the factors affecting carbon uptake in mangrove sediments. To identify the relationship between sediment organic carbon (SOC) content and sediment properties, sampling from three mangrove forests in Bushehr (Iran) coastal regions (Asalouyeh, Basatin, and Malegonzeh) was performed at 6 different stations, and the SOC of the samples were measured by Walkley and Black method. The particle diameters were performed by the sieving and hydrometer method and the results were analyzed (Gradistat. v8). The results showed that the highest percent of particles were silt, ranging from 93.6 to 96.6%. Several sediment grain size properties were correlated with SOC. Two factors were analyzed in principal component analyses, which were responsible for 75.8% of the samples attrinbutes. Factor 1 was organic carbon (SOC), skewness, sand, mean size, sediment density, clay, and silt summation (clay & silt). Factor 2 included clay, sorting, and kurtosis. Organic carbon was positively correlated to sorting, clay & silt percent, and grain size (ɸ). Meanwhile, the SOC was negatively correlated to sediment density, C, sand percent, and skewness. Generally, by decreasing the size of the sediment particles, the amount of SOC increased. Areas with fine-grained sediments appear to be able to absorb more organic carbon.

Distribution and Genesis of Organic Carbon Storage on the Northern Shelf of the South China Sea.

The sediments distributed in the marginal seas of the continental shelf are important burial materials for global organic carbon (OC). There have been many estimates of the global continental shelf OC reserves, but due to the limited acquisition of measured data, the estimated results have great uncertainty. The vast continental shelf in the northern part of the South China Sea (SCS) provides a good place for the storage of OC. Based on a large amount of sediment OC data obtained from the northern coast of the SCS, the OC storage in the surface sediment (0~10 cm) in the study area (approximately 8.63 × 104 km2) was accurately calculated as 51 Tg. The study area covers different regions, such as estuaries, open seas, strait areas and upwelling development areas, and the OC content of each area is quite different. According to provenance analysis, the source of OC in sediments is mainly from the input of Pearl River runoff. The OC content is significantly higher and less affected by sediment particle size in the Pearl River Estuary and the surrounding areas; meanwhile, the OC content gradually decreases with the distance from the Pearl River Estuary. Far from the western Pearl River Estuary, the sediment OC content is mainly controlled by the particle size of the sediments and is significantly correlated with silt and clay content. The deposition rate is also an important factor affecting the burial of OC, for the high deposition rates correspond to the high levels of OC in the nearshore estuarine areas, as well as the low deposition rate region having low OC content in the sediments even though it has a high productivity of OC, such in as the upwelling sea area on the eastern side of Hainan.