Sediment Total Organic Carbon Research Articles

Offshore Wind Farm Footprint on Organic and Mineral Particle Flux to the Bottom

Offshore wind farms (OWFs) are an important source of renewable energy accounting for 2.3% of the European Union's electricity demand. Yet their impact on the environment needs to be assessed. Here, we couple a hydrodynamic (including tides and waves) and sediment transport model with a description of the organic carbon and mineral particle dynamics in the water column and sediments. The model is applied to the Belgian Coastal Zone (BCZ) where OWFs currently occupy 7% of its surface area which is estimated to double in the next 5 years. The impact of OWFs on the environment is represented through the filtration of the water column and fecal pellets production by the blue mussel, the dominant fouling organism. Our model simulations show that the impact of biodeposition on the mud particle sedimentation and on sediment composition is small compared to the fluxes associated with tidal deposition and resuspension and the lateral inputs. In contrast, the total organic carbon (TOC) flux to the sediment is significantly altered inside the OWF perimeters and TOC deposition is increased up to 50% in an area 5 km around the monopiles. Further away, the TOC flux to the bottom decreases with a notable effect up to 30 km away. The major changes are found along the direction of the main residual current and tidal ellipse's major axis. In addition, sub-mesoscale gyres act as retention areas with increased carbon deposition. A future OWF in the BCZ will be located close to gravel beds in a Natura 2000 area, considered as vulnerable habitats and biodiversity hotspots. The different scenarios for this OWF, varying in turbine number and positioning, are compared in terms of impact on the carbon and mineral particle deposition flux in the BCZ and, particularly, to these gravel beds. The scenarios show that the number of turbines has only a slight impact on the TOC deposition flux, unlike their positioning that significantly alters the TOC flux to the gravel beds. The TOC deposition flux exceeds 50%, when the turbines are placed next to the gravel beds; while a limited increase is simulated, when the turbines are located the farthest possible from them.

Coupled action of rock weathering and aquatic photosynthesis: Influence of the biological carbon pump effect on the sources and deposition of organic matter in Ngoring Lake, Qinghai–Tibet Plateau, China

A critical problem in the study of global climate change is how to identify “missing” carbon sinks. The carbon sink resulting from the weathering of carbonate and silicate minerals may make a major contribution, and it is often coupled with aquatic photosynthesis (the biological carbon pump effect - ‘BCP’). However, the lack of relevant field studies hinders our understanding of rock weathering and the carbon cycle. To address this issue, we analyzed the pH, [HCO3−], carbon isotopic composition of dissolved inorganic carbon (δ13CDIC), total organic carbon (TOC), total nitrogen (TN), TOC/TN, and the stable carbon isotopic composition of TOC (δ13Corg) and CaCO3 (δ13Ccarb) in the modern surface sediments of Ngoring Lake, in the northeastern Qinghai-Tibet Plateau (QTP), China. The results show that the efficiency of aquatic photosynthesis may be an important factor affecting the δ13CDIC in Ngoring Lake. δ13Corg in the surface sediments is positively correlated with the δ13Ccarb of inorganic carbon produced within the lake, especially in shallow water (<10 m). In addition, the increase in the TOC content of the surface sediment samples is accompanied by an increase in the CaCO3 content. These observations suggest that the BCP effect is the major mechanism driving the observed synchronous increases in TOC and CaCO3 in the sediments, and the range of observed carbon isotope values. Furthermore, organic carbon in the surface sediments is characterized by a relatively low C/N ratio and δ13Corg, suggesting it is mainly autochthonous, and is transformed within the aquatic ecosystem from rock weathering-derived DIC via the BCP effect. Therefore, the BCP effect plays an important role in the stabilization of the rock weathering carbon sink. Our results emphasize the importance of the carbon sink of coupled rock weathering and aquatic photosynthesis in the evolution of the carbon cycle in lakes in the QTP.

Revisiting the marine reservoir age in Baja California continental margin sediments using 14C and 210Pb dating

Knowledge of the Marine Reservoir Effect (MRE) correction is fundamental in palaeoceanographic research to establish an accurate age-depth model for marine sedimentary records. However, during the last decades different MRE corrections have been applied in inconsistent ways for the same locality and same sediment cores, at Soledad Basin, Baja California, Mexico, creating confusion about the proper correction value of the marine reservoir effect (ΔR) to be applied. In contrast with the empirical approach previously used for assessing the ΔR value in Soledad Basin, in this study we applied an analytical approach based on the concurrent application of AMS- 14 C and 210 Pb xs dating techniques made on sedimentary total organic carbon and foraminifera to determine new regional ΔR values from newly collected sediment cores from this site. Our results from Soledad Basin show a ΔR of 206 ± 32 years for foraminifera and 706 ± 42 years for organic carbon. Modeled ages using these results, and compared with those previously applied for the basin, highlight the relevance of the correct use of the local reservoir age as it can generate an offset of approximately 150 years if the other published ΔR were used. These differences can shift core chronologies by several decades and thus yield significant errors in palaeoceanographic reconstructions, which will be important to remedy in future work. • We propose a new ΔR value for the Soledad Basin region. • Paired AMS-14C/210Pbxs dating technique was used to estimate ΔR value. • ΔR values are 706 ± 42 yr and 206 ± 32 yr for OM and foraminifera, respectively. • Our results offer a solution to existing discrepancies between ΔR used regionally. • Chronology obtained with our ΔR correlates most robustly with the timing of PDO.

Amino acid and chlorin based degradation indicators in freshwater systems

Lakes cover a global area that is about 35 times smaller than the oceans, but carbon burial in lakes and oceans are on the same order of magnitude. Hence, understanding the processes for such high organic carbon burial in lacustrine systems is essential. We applied proxies typically used for marine environments including amino acid (AA) content and their nitrogen stable isotope composition to the water columns and sediments of three lakes that differ in their trophic states and depositions rates of sedimentary terrestrial organic matter. Additionally, we used carbon isotope fingerprinting of AAs to characterise their sources and fates. We show that this set of proxies tracks sources and degradation processes in eutrophic lakes with high sedimentary total organic carbon and nitrogen content. Those lakes also have a high total hydrolysable amino acid (THAA) content as well as higher pigment concentrations. While the Chlorin degradation Index (CI) showed increasing values with depth, the patterns were less consistent for the Degradation Index (DI). In general, all parameters failed to describe degradation in the most oligotrophic lake due to the very low organic carbon and nitrogen content in the sediment. We show that AAs are mostly of autochthonous origin and that AA contribute 5–45% to the organic material in plankton, POM, and sediment. Proxies based on AA for bacterial reworking (ΣV) or trophic level (TL) showed increasing values in the water column but relatively stable values in the sediments. Furthermore, we show that methanotrophic bacteria led to increased values for the bacterial reworking proxy (ΣV) and extraordinarily low δ13C AA values (−30 to −60‰).

Organic biomarkers and Meiofauna diversity reflect distinct carbon sources to sediments transecting the Mackenzie continental shelf

During the Arctic spring freshet, the Mackenzie River delivers large amounts of dissolved and particulate terrestrial organic carbon to the Canadian Beaufort Sea shelf. This pulse of material and nutrients fuels phytoplankton blooms in late spring and summer and together with under ice primary production contributes to the organic carbon reservoir of the coastal shelf sediments. As a component of the Marine Arctic Ecosystem Study (MARES), samples transecting the Canadian Mackenzie Trough were collected in fall before ice cover to examine the balance of marine and terrestrial carbon in sediments and its relationship with sediment meiobenthic diversity and abundance. Organic biomarker analysis of lipid proxies together with total hydrolyzable amino acids (THAA) defined organic matter sources and potential lability. Across the shelf transect, consistent values observed in sedimentary total organic carbon (1.09%–1.62%) belied a shift in the balance of terrestrial and marine carbon sources. Predominately terrestrial inputs at the shallow site near the delta outflow appeared to constrain meiofaunal populations while the deepest location off shelf reflected lower labile marine organic carbon reaching the sediments that limited benthic production. Surface sediments at the mid-shelf, however, showed a near 3-fold increase (to 1.16 mg C OC-1) in contributions by diatoms and dinoflagellates markers plus a peak in total amino acids (9.53 mg g OC-1) which corresponded with higher meiofauna abundance. The combination of detailed organic analysis together with meiofaunal community measures suggest that mid-shelf sediments receive higher contributions of labile marine carbon which drive increased meiofauna diversity and abundance despite the large background of terrestrial organic material exiting the Mackenzie River outflow.

Anthropogenic impact on the organic carbon sources, transport and distribution in a subtropical semi-enclosed bay

Suspended particulate organic carbon (POC) and sedimentary total organic carbon (TOC) in coastal areas play critical roles in the global carbon cycle, yet sources and dynamics of coastal POC and TOC have been affected by various anthropogenic activities such as aquaculture, sewage discharge, dam construction and land reclamation. To better understand the anthropogenic impacts on coastal organic carbon, this study was carried out in a representative semi-enclosed bay, Dongshan Bay, Southeast China. Through analyses of stable isotopic compositions of both POC (δ13CPOC and δ15NPN) and TOC (δ13CTOC and δ15NTN), the ratio of total organic carbon vs. total nitrogen (C/N), grain size, Chl-a concentrations and hydrological parameters, our study led to the following main findings: 1) During flood season, the distribution of δ13CPOC, δ13CTOC, δ15NPN and δ15NTN values within the bay did not follow the conventional land-sea transition pattern. This distribution pattern indicated more terrestrial organic matter input seaward, which contrasts with the conventional organic matter distribution along the estuarine gradient. 2) Using the organic δ13C, δ15N and C/N signatures of different endmembers, we found that the sources of organic matter deposited in the bay were strongly related to anthropogenic activities, including municipal wastewater discharge, aquaculture, land reclamation and sluice-dyke construction. Furthermore, 3) by applying the Grain Size Trend Analysis Model and the previously-estimated residual current directions, we suggested that human activities have not only altered the sources of organic matter to the semi-enclosed bays, but also significantly modified their transportation and deposition patterns, and might influence the ultimate fate of organic matter into and out of Dongshan Bay. The conclusions of this study should be applicable to similar coastal bays around the world.

Direct‐Push Color Logging Images Spatial Heterogeneity of Organic Carbon in Floodplain Sediments

AbstractIn soils and sediments, large amounts of total organic carbon (TOC) mark reducing conditions. As dark sediment colors are good predictors for high‐TOC zones, they indicate hot spots of biogeochemical turnover and microbial activity. Traditionally, obtaining the sediment color or TOC at depth requires costly core sampling, resulting in poor horizontal resolution and related uncertainty caused by interpolation. We suggest using a direct‐push tool for optical screening of the sediment color to acquire multiple high‐resolution vertical color profiles and demonstrate its applicability to a biogeochemical transition zone in floodplain sediments, dominated by tufa. We use Gaussian mixture models for a cluster analysis of 35 color logs in the International Commission on Illumination (CIE) L*a*b* color space to identify three colorfacies that differ in lithology and TOC content: a dark colorfacies that agrees well with peat layers, a gray colorfacies associated with clay, and a creamy‐brown facies made of autochthonous carbonate precipitates. We test different approaches either to infer the TOC content from color metrics, namely, the lightness and chroma, across all facies, or to identify TOC ranges for each colorfacies. Given the high variability in TOC due to organic carbon specks in the tufa, the latter approach appears more realistic. In our application we map the 3‐D distribution of organic matter in a floodplain in distinct facies over 20,000 m2 down to 12 m depth. While we relate the sediment color only to the TOC content, direct‐push color logging may also be used for in situ mapping of other biogeochemically relevant properties, such as the ferric‐iron content or sedimentary structure.

Coastal urbanization alters carbon cycling in Tokyo Bay

The carbon budget of Tokyo Bay, a highly urbanized coastal basin, was estimated using a box model that incorporated inorganic and organic carbon data over an annual cycle (2011–2012). The surface water represented net autotrophic system in which the annual net community production (NCP) was 19 × 1010 gC year−1. The annual loading of dissolved inorganic carbon and total organic carbon (TOC) from freshwater inputs was 11.2 × 1010 and 4.9 × 1010 gC year−1, respectively. The annual TOC sedimentation rate was 3.1 × 1010 gC year−1, similar to the annual air–sea CO2 uptake (5.0 × 1010 gC year−1). Although the NCP and TOC loading from freshwater inputs were respectively 3.0 and 2.7 times lower than those in the 1970s, the TOC sedimentation rate was similar. Therefore, a relatively high carbon efflux from Tokyo Bay likely occurred in the 1970s, including CO2 efflux to the atmosphere and/or export of labile organic carbon to the open ocean. The changes in carbon flow between the 1970s and 2011–2012 resulted from improved water quality due to increased sewage treatment facilities and improved sewage treatment efficiency in the catchment, which decreased the amount of labile organic carbon flowing into the bay.

Lack of methane hotspot in the upstream dam: Case study in a tributary of the Three Gorges Reservoir, China

River damming has seen a growing trend in demand worldwide and the impounded reaches are considered hotspots of greenhouse gas emissions. However, it remains unclear how the spatial distribution of C-gas in sediments and methane (CH4) emissions of dammed tributary changes under different operation periods of the Three Gorges reservoir (TGR). We measured CH4 and carbon dioxide (CO2) concentrations in sediment and CH4 emissions from a dammed river of the TGR, and evaluated the effect of damming on the spatial variability of carbon in the sediment and on CH4 flux. It was found that damming led to a distinct spatial pattern of total organic carbon (TOC) in the sediment, which resulted in higher CH4 and CO2 in upstream sediment compared to the downstream. During the TGR impounding period, the upstream CH4 diffusive flux (0.253–0.427 mg m−2 h−1) across the water-air interface was higher than in the downstream (0.093 mg m−2 h−1), which was consistent with the spatial variation of CH4 in the sediments. However, the CH4 emission was predominantly by ebullition and the flux in the downstream (169.173 mg m−2 h−1) was significantly higher than upstream (12.23–123.05 mg m−2 h−1) in the discharging period. This can be attributed to a sharp increase in TOC in downstream sediment due to riparian zone soil erosion on both banks, which was caused by periodic large fluctuation in the water level, and a shallow water depth in the downstream. This study adds to our understanding of effects of the TGR's operation on CH4 emissions from a dammed tributary and suggests that the water level fluctuation of tributaries which has direct influence on ebullition and methane oxidation caused by manipulation of the TGR should not be overlooked.

Effect of water-sediment regulation operation on sediment grain size and nutrient content in the lower Yellow River

The Yellow River (YR) is famous for its high sediment concentration. The Xiaolangdi dam (XLD), the most downstream dam on the mainstream of the YR, plays an integral role in siltation mitigation, flood defence, agriculture, and hydropower generation. To alleviate siltation of the Xiaolangdi Reservoir (XLDR) and riverbed uplift of the lower reaches of the YR and strengthen flood control safety downstream, the XLDR began to implement water-sediment regulation in 2002. Water-sediment regulation (WSR) has significantly changed the hydrological processes, erosion and deposition, and sediment transport in the downstream channel, and the downstream water environmental conditions have also changed. In this study, 10 sections in the Lower Yellow River (LYR, from the XLDR to the Lijin section) and 4 sections in the soft floodplain area were sampled in the field to analyse their surface sediment grain size composition and nutrient content. At the same time, data on the average annual median particle sizes (D50) of channel sediments and suspended particles in 7 typical sections in the LYR from 2004 to 2015 were statistically analysed. The results show that the channel sediment grain size in the LYR decreased along the river course, and the D50 ranged from 0.20 mm in the HYK section to 0.08 mm in the LJ section. Moreover the D50 of the suspended sediment was less than 0.05 mm. The D50 of the channel and suspended sediment were both ranked as follows: non-flood season > flood season > WSR period. After WSR, fine silt particles with diameters of 0.01–0.05 mm were deposited on the soft floodplain. The channel sediment size was mainly coarse particles with diameters of 0.05–0.20 mm due to channel erosion. The suspension of fine silt particles due to flood scouring led to the total organic carbon (TOC) and total nitrogen (TN) absorbed in the particles to also be resuspended in the water. As a result, the TOC and TN contents in the channel sediments decreased by approximately 40% after WSR.

Anaerobic oxidation of methane with denitrification in sediments of a subtropical estuary: Rates, controlling factors and environmental implications

Anaerobic oxidation of methane with denitrification (DAMO), as an important microbial process regulating methane emission, has been widely reported in freshwater ecosystems. However, the DAMO process and associated biogeochemical controls in estuaries remain poorly understood. Here, we used 13C- and 15N-labelling experiments to quantify the potential rates of DAMO and determined the crucial factors controlling the DAMO rates in the sediment of Yangtze Estuary. Potential rates of DAMO varied greatly across the estuary, ranging from 0.07 to 0.28 nmol CO2 g−1 d−1. Salinity negatively affected the DAMO and also showed an indirectly negative influence on DAMO process by high salinity inhibition on NO3− availability and denitrification. Nitrate concentrations were significantly correlated with the DAMO rates. Denitrification rates showed positive correlation with DAMO rates, implying that nitrate reduction drives the DAMO process. Sediment total organic carbon and NH4+ had important effects on DAMO rates. These results together indicate that DAMO process can occur and the DAMO rates were mainly controlled by sediment NO3− and denitrification in estuary. We further conclude that increasing NO3− load can drive the DAMO process with more important implications on methane sink in estuarine ecosystems.

Mid-Holocene environmental change and human response at the Neolithic Wuguishan site in the Ningbo coastal lowland of East China

Coastal wetlands provided a favorable settling site for Neolithic people because of their highly exploitable biomass, but were vulnerable to marine hazards such as coastal flooding. The Chinese Hemudu culture persisted for ~2000 years (7200–5300 cal. year BP) in the Ningbo coastal lowland of East China. This study explores the Hemudu people’s survival strategy using sedimentological and chronological records, and organic and acetic-acid-leachable alkaline-earth (Ca, Sr, and Ba) chemistry on a well-dated profile from the coastal Wuguishan site in the Ningbo Plain. Analyses of alkaline-earth elements in surficial sediments collected from present-day alluvial plain, tidal river, and saltmarsh/tidal flat environments in the Ningbo Plain were also undertaken to explain sedimentary environmental changes and their linkage to Hemudu activity at the Wuguishan site. Results indicate high sediment acetic-acid-leachable Ca and Sr contents with high Sr/Ba ratios, and high sediment total inorganic carbon contents at the site during 6300–6000 cal. year BP, which coincided with a marine incursion at the nearby Neolithic Yushan site. However, the increasing sediment total organic carbon contents and decreasing δ13C values suggest that the Wuguishan site evolved from an upper tidal flat to a saltmarsh environment, attracting settlement by the prehistoric Hemudu people after ~6200 cal. year BP. Sr and Ca contents and Sr/Ba ratios decreased after ~6000 cal. year BP, indicating that the site developed into a low-salinity marsh in the supratidal environment after rapid accumulation caused by a storm event at ~6020 cal. year BP. Furthermore, the high Sr and Ba contents in the layers of Hemudu Culture Period III indicate the Hemudu people’s consumption of seafood and their adaption strategy for living in the vulnerable coastal wetland.

Glycerol dialkyl glycerol tetraethers in surface sediments from three Pacific trenches: Distribution, source and environmental implications

Glycerol dialkyl glycerol tetraethers (GDGTs) have been widely used to elucidate sources of sediment total organic carbon (TOC), past temperature and presence of methanogenesis in diverse environments. However, their applicability to hadal trenches with their unique deposition dynamics remains unknown. Here, we analyzed GDGTs and their stable isotope values and content of TOC in surface sediments from the Kermadec Trench region (KT; 6080–10010 m), New Britain Trench region (NBT; 1553–8931 m), and Atacama Trench region (AT; 2560–8085 m). These regions are at very different distances from terrestrial sources and have varying net primary productivity (NPP) in the waters above them. The GDGT concentration was highly variable (54.5–2416 μg g−1 TOC) within and between trench regions and was not directly related to local NPP or apparent terrestrial inputs. This finding is presumably due to complex deposition dynamics within the trench interior. Isoprenoidal GDGTs (isoGDGTs; 75.4–99.1%) were dominant over branched GDGTs (brGDGTs, 0.91–24.6%) in all samples, leading to low levels of Branched versus Isoprenoidal Tetraether (BIT) index (0.01–0.27). Thus, sediment TOC is mainly derived from marine sources. However, compared to adjacent non-hadal sites, trench axis sites have a higher BIT index, lower acyclic hexa-/pentamethylated brGDGT and lower δ13C, supporting relative enrichment of terrestrial organic carbon at the trench axis. The application of TetraEther indeX of tetraethers consisting of 86 carbon atoms (TEX86) resulted in sea surface temperature (SST) estimates of 18.9–23.7 °C in the KT, 28.6–30.2 °C in the NBT, and 17.9–20.4 °C in the AT. The close agreement between TEX86-SST with observed in situ SST suggests that isoGDGTs are not selectively degraded during the transport towards the hadal realm, and that TEX86 from hadal settings robustly records an integrated regional SST signal.

Impact of intensive land use on heavy metal concentrations and ecological risks in an urbanized river network of Shanghai

• Concentrations and spatial distribution of HMs in the river network were influenced by intensive land use. • AVS and TOC in river sediments influenced the migration and transformation of Cd, Hg and Zn. • Hg and Cd were the main contributors of RI, HMs in the central city area showed the highest potential ecological risk. • HMs in the river network primarily originated from industrial activities, traffic emissions and coal-burning emissions. Heavy metal pollution characteristics in the urbanized river network are closely related to urban land use. The existing studies mainly focused on the pollution characteristics of heavy metals in single rivers, but fewer studies reported the impact of land use on the concentrations and ecological risks of heavy metals in surface water and sediments of the urban river network. This study combined land use types with heavy metal pollution in rivers, and explored the specific impact of intensive land use on the characteristics of heavy metal pollution in rivers, which provided scientific basis for the study of heavy metal pollution in plain river networks. The concentrations, spatial variations and potential ecological risk levels of cadmium (Cd), chromium (Cr), copper (Cu), mercury (Hg), lead (Pb) and zinc (Zn) in surface water and sediments from five different intensive land use areas of Shanghai river network (industrial areas, agricultural areas, central city, suburban towns and Chongming Island) were analyzed. Results showed the concentrations of Cd, Cr, Cu, Pb and Zn in surface water of the Shanghai river network were relatively low, but Hg pollution existed in some rivers. Influenced by intensive land use, heavy metal concentrations in the central city were generally higher than those in other areas. Heavy metals in sediments had considerable ecological risk, in which Hg and Cd were the main contributors, and showed the highest potential ecological risk in central city. Acid volatile sulfides (AVS) and total organic carbon (TOC) in river sediments affected the migration and transformation of Cd, Hg and Zn. Industrial activities, traffic emissions and coal-burning emissions were the main sources of heavy metals. Intensive land use caused by anthropogenic activities led to the weakening of hydrodynamic conditions and the discharge of a large number of pollutants into the river network. In the central city with intensive anthropogenic activities, heavy metals in the river network showed higher levels than those in other areas. This study found that intensive land use and high-intensity anthropogenic activities led to the high pollution level of heavy metals in the plain urbanized river networks. Quantitative analyses of the effects of intensive land use on heavy metal pollution in urbanized river networks were suggested to conduct further to protect the urban river water quality.

Influences of tropical monsoon climatology on the delivery and dispersal of organic carbon over the Upper Gulf of Thailand

Organic carbon cycling in SE Asia is data-sparse and poorly studied, although high sediment yield and organic carbon intensity occur in the tropical region. In the present study, we evaluated the role of tropical monsoon pertaining to fluvial discharge, sediment load, coastal current and water stratification on seasonal organic carbon dynamics during four sampling campaigns in the Upper Gulf of Thailand (UGoT). This study demonstrates that particulate organic carbon (POC) is closely correlated with the river influx of suspended sediment, which is generally regulated by the local rainfall. Higher POC is found near the large estuarine section (Chao Phraya River, CHAO) during southwest monsoon period and the small estuarine section (Mae Klong River, MK) during November 2013 when tropical cyclones impacted. POC in the estuarine sections is more significantly influenced by the seasonal shift compared with the coastal sections. Land-derived organic matter prevails in the small estuarine and coastal sections, while marine-derived organic matter dominates in the estuarine sections impacted by CHAO and MK. Anthropogenic inputs and dam regulation also influences the river export of organic matter during the high precipitation periods. Total organic carbon (TOC) however displays less significant seasonal monsoon variations than POC. Further, TOC tends to accumulate in the sub-silt fraction of sediments, which mainly occurs in the small estuarine and eastern coastal sections and is more obviously influenced by marine-derived factors. TOC in sediment of the CHAO and MK influenced sections however displays more seasonal variations with prevailing river input evidenced by coarser sediment and higher C/N ratios. Moreover, the almost year round water stratification across the region acts as the barrier in retaining organic carbon in the estuaries and their vicinities from dispersal into the lower portion of Gulf of Thailand. High sedimentation rate (~1.1 cm·yr−1) further facilitates the organic carbon burial in the study area. The delivery, dispersal and burial of organic carbon are closely associated with the climate-controlled precipitation, and thus the tropical monsoon climatology under the global warming in particular is an important factor influencing the organic carbon in the UGoT.

Monitoring of Nutrient Status at Ratargul Swamp Forest in Sylhet, Bangladesh

Nutrient concentration is an important parameter to recognize critical processes of an ecosystem and environmental quality. This paper is based on a study carried out to evaluate the nutrient status in sediments, water and plant leaves from Ratargul swamp forest of Bangladesh during the period from September 2016 to August 2017. The samples were collected from twelve sampling stations of four different canals during dry (September 2016 to April 2017) and wet (May to August, 2017) seasons, respectively. The nutrient contents such as total nitrogen (TN), available phosphorus (AP) and total organic carbon (TOC) were analysed in the laboratory of the Bangladesh Institute of Nuclear Agriculture (BINA), Mymensingh, Bangladesh. The TN, AP and TOC in sediments were ranged from 2.22 to 5.23, 0.032 to 0.035 and 2.6 to 4.52%, respectively, whereas TN and AP in water were found 5.34 to 22.54% and 0.002 to 0.366%, respectively. Result showed that the TOC in plant leaves was higher in wet season than in dry season, where the TN and AP were higher in dry season than in wet season due to high temperature which increases the rate of photosynthetic activities in the dry season. In both Korochand Hijal plants, the TOC and AP were higher in green leaves than brown leaves, while TN was higher in brown leaves than green ones. Moreover, a statistically significant positive correlation was found among the samples. The study concluded that the nutrient status in sediments, water and leaves were perfectly natural during both the dry and wet season.

Long-lived radionuclides in marine sediments from the Beibu Gulf, South China Sea: Spatial distribution, controlling factors, and proxy for transport pathway

The source and transport process of sediments are of great concern in marine sedimentology. In this study, naturally occurring radionuclides (238U, 226Ra, 228Ra, and 40K) in surface sediments and sediment cores were comprehensively analysed using high-purity germanium (HPGe) γ spectrometry in order to explore radionuclide fingerprints as novel proxies for tracking the source and transport pathway of sediments in the Beibu Gulf. We found that the radioactivity level of sediments in the Beibu Gulf was lower than that in other Chinese seas, following a comprehensive compilation and comparison of radioactivity in the region, indicating the Beibu Gulf is a high-quality marine environment. An interesting “butterfly pattern” was exhibited in the horizontal distribution of radionuclides in surface sediments from the Beibu Gulf, mainly attributable to the controlling factors of the non-linear negative regulation of sediment grain size and positive role of total organic carbon in sediment. Furthermore, the vertical distribution of radionuclides in sediment cores was affected by the redox state and a physical/biological disturbance, which were demonstrated by the simultaneous analysis of redox-sensitive element Mn and 210Pb, respectively. Additionally, the 226Ra238U endmember model and 232Th238U endmember model were proposed for quantifying local input (85%) and trans-region input (15%) of marine sediments in the Beibu Gulf in the context of the Guangdong-Qiongzhou Strait-Guangxi framework. We inferred that the Beibu Gulf was at risk of contamination by artificial radionuclides derived from nearby Guangdong Province, which has the highest number of nuclear power plants in China, through a trans-region input by ocean currents. Our study not only provided spatiotemporal distributions of radionuclides in the Beibu Gulf and their controlling factors (sediment grain size, total organic carbon, redox state, physical/biological disturbance, and ocean currents) but also indicated the radionuclide fingerprints (226Ra/238U and 232Th/238U) as potential geochemical proxies for quantifying the distinct sources of radionuclides and other associated chemicals (e.g., heavy metals, organic pollutants) in sediments.

Species choice in mangrove reforestation may influence the quantity and quality of long-term carbon sequestration and storage

Despite carbon sequestration being an important service of mangrove ecosystems, many mangrove reforestation projects have little consideration of the carbon sequestration capacity of species to be planted. Species selection is mostly based on growth rate and convenience in planting. In this study, to compare the quantity and quality of carbon stored in soil, four habitats were selected in Haijiang River Estuary, southern China to assess the contribution by different mangrove species to sediment carbon pool. Two 12-year-old mangrove forests of the exotic Sonneratia apetala and native Kandelia obovata, respectively, and the adjacent sandflat and mudflat as unvegetated referencing sites had been studied. The total sedimentary organic carbon and active sedimentary organic content in sediment suggested that after 12 years of growth, (1) mangrove forests significantly increased the organic carbon content of sediment; (2) total organic carbon in the K. obovata forest was higher than that of the S. apetala forest; but (3) the carbon pool of the K. obovata forest was less stable than that of the S. apetala forest. These results corroborated with other studies that the sediment carbon pool of S. apetala forests reached a stable state after 13 years of growth, while that of K. obovata forests gradually stabilised upon long-term (>13 years) growth. Our study confirms that K. obovata is more conducive to capture carbon in long-term mangrove reforestation projects, demonstrating that the provision of this service may not be directly related to apparently relevant plant traits such as growth rate.

High Seasonal Variability in Sediment Carbon Stocks of Cold‐Temperate Seagrass Meadows

AbstractSeagrass meadows have a high ability to capture and store atmospheric CO2 in the plant biomass and underlying sediment and thereby function as efficient carbon sinks. The seagrass Zostera marina is a common species in the temperate Northern Hemisphere, a region with strong seasonal variations in climate. How seasonality affects carbon storage capacity in seagrass meadows is largely unknown, and therefore, in this study, we aimed to assess variations in sedimentary total organic carbon (TOC) content over a 1‐year cycle in seagrass meadows on the Swedish west coast. The TOC was measured in two Z. marina sites, one wave exposed and one sheltered, and at two depths (1.5 and 4 m) within each site, every second month from August 2015 to June 2016. We found a strong seasonal variation in carbon density, with a peak in early summer (June), and that the TOC was negatively correlated to the net community production of the meadows, presumably related to organic matter degradation. There was seasonal variation in TOC content at all sediment sections, indicating that the carbon content down to 30 cm is unstable on a seasonal scale and therefore likely not a long‐term carbon sink. The yearly mean carbon stocks were substantially higher in the sheltered meadow (3,965 and 3,465 g m−2) compared to the exposed one (2,712 and 1,054 g m−2) with similar seasonal variation. Due to the large intra‐annual variability in TOC content, seasonal variation should be considered in carbon stock assessments and management for cold‐temperate seagrass meadows.

Assessment of Sediment and Benthic Macrofauna Distribution at Dar es Salaam Harbour Channel

Sediment and macrobenthic fauna distribution at Dar es Salaam harbour channel were investigated to attest the influence of sediment grain size distribution and total organic carbon contents on benthic macrofauna dominance and diversity. Sampling campaign was conducted from January to April, 2019 in 25 sampling stations along the channel. Sediment grain sizes ranged between 0.7405 and 0.00273 mm with an average of 0.0804 mm classified as medium silt. Sediment distribution showed an onshore-offshore pattern consisting of fine dark clay-silt sediments in the southern part of the harbour close to Mwalimu Nyerere Bridge, medium (sandy) at the harbour and coarse sandy offshore. Five classes of macrofauna were identified with sedentary Polychaetes (Diopatra cuprea) found in tubes and free forms being dominant in most of the sediment types, however more abundant in clay-silt. High species diversity index values were encountered in sediments with medium total organic carbon (TOC) and sediment grain sizes (sand-silt). Apparently, low species similarity indices were observed in all sediment type indicating significant dissimilarities in species composition among sediment grain sizes. Thus, the distribution of benthic macrofauna species along Dar es Salaam harbour channel is strongly influenced by the sediment grain sizes and organic carbon contents in sediments.