Inner Shelf Of East China Sea Research Articles

Decadal variations of total organic carbon production in the inner-shelf of the South China Sea and East China Sea

Abstract Organic carbon content is one of the major proxies of aquatic primary production and implication of environmental changes. However, there is a scarcity of information regarding the decadal variation of organic carbon production in inner-shelf of South China Sea (SCS) and East China Sea (ECS). To bridge this gap of information two sediment cores were collected from the inner shelf of SCS (Leizhou Peninsula) and ECS. Then, Total Organic Carbon (TOC), Total Inorganic Carbon (TIC) and Total Nitrogen (TN) content were examined. The TOC content in the Leizhou Peninsula averaged 0.56% and varied from 0.35% to 0.81%. At decadal time scale, the TOC content of Leizhou Peninsula erratically increased in the last century, and it distinctly peaked after 1990’s. This is related to increased primary production due to the increased input of anthropogenic nutrients and rainfall level. The TOC content of the inner shelf of ECS averaged 0.5% and varied narrowly from 0.46% to 0.53%. The TOC:TN ratio of ECS averaged 5.65 and varied from 4.42 to 7.85, indicating there is high degree of organic matter degradations. The TN:TP ratios were below 10 for the periods before late-1990’s, indicating that there had been a potential nitrogen limitation in the inner shelf of ECS. Generally, this study showed that the TOC and TIC of ECS and SCS inner-shelf had undergone substantial changes in the last century.

Using multi-radiotracer techniques to better understand sedimentary dynamics of reworked muds in the Changjiang River estuary and inner shelf of East China Sea

Reworked muds, including mobile muds, resuspended sediments, and bio-mixed muds are ephemeral layers in the benthic boundary of large-river delta-front estuaries (LDEs). In this study, multiple radiotracers (210Pb, 137Cs, 234Th, 7Be, 235U, 228Th, 228Ra, 226Ra and 40K) were combined to evaluate the sedimentary dynamics of the mud belt on the inner shelf of the East China Sea (ECS) adjacent to the Changjiang River Estuary. We first categorized eight types of typical 210Pb profiles in marine sediments, and specified which ones could be used as possible evidence of reworked muds. The types of 210Pb profiles observed in previous studies and this study were then used to examine the depth distribution of reworked mud layers of the benthic boundary of ECS. We found that reworked mud depth in the along-shore mud belt of the inner shelf of ECS was deeper than those in the outer shelf and slope area, reaching up to 150cm deep in the Changjiang River LDE, and up to 50cm deep in the Zhejiang–Fujian coast mud area. Sediment core tracer profiles also showed that the reworked mud layer could be further divided into several sub-layers with distinct redox conditions and microbe (sulfate-reducing bacterial) abundance. SRB abundance intensively fluctuated with changing redox environment, and larger abundance values were usually observed in anoxic conditions. Based on 7Be and 234Th, the top layer (ca. 2–4cm thick) was the most modern and active zone, showing evidence of reworking on seasonal timescales. The deeper sections of the reworked mud layer were generally modified over longer timescale (years) which were examined using the more long-lived tracers, e.g., 137Cs, 228Thex, and 228Ra/226Ra ratio. We found for the first time in this region that age of the reworked layers was usually much younger than underlying accumulating (burial) layers. We posit that this multi-radionuclide approach is critical for evaluating the complex biogeochemical sediment record on delta-front estuaries of large rivers subject to changing climate conditions and human impacts over the last several centuries.

Summer carbonate chemistry dynamics in the Southern Yellow Sea and the East China Sea: Regional variations and controls

Surface partial pressure of CO2 (pCO2) and pertinent parameters (i.e., pH, total alkalinity, dissolved oxygen, chlorophyll a) were investigated in the southern Yellow Sea (SYS) and the East China Sea (ECS) basing on two surveys conducted in June and August of 2013. The results suggested carbonate chemistry dynamics and related controlling factors were provided with significant temporal and spatial variations in different subregions of these two continental shelf seas. The western of SYS (SYSW) was CO2-undersaturated both in June and August, with the average FCO2 −1.88mmolm−2d−1 and −3.72mmolm−2d−1, respectively. The phytoplankton initiated CO2-absorption and the suspended sediment induced CO2-emission jointly controlled the air–sea CO2 exchange there. The center of SYS (SYSC) also behaved as an obvious CO2 sink (−1.57mmolm−2d−1and −3.99mmolm−2d−1 in June and August, respectively), probably due to elevated TA/DIC ratio and the subsequent effects of spring bloom. As for the Yangtze River estuary (YRE), it changed from an obvious CO2 sink (−1.28mmolm−2d−1) in June into a very weak CO2 source (0.04mmolm−2d−1) in August. This change was probably associated with the rising of seawater temperature and monthly variation of Yangtze River discharge. The inner shelf of ECS (ECSS) experienced obvious air–sea CO2 flux changes during from June (−8.88mmolm−2d−1) to August (−0.36mmolm−2d−1) as well. Biological DIC consumption in the upper layer and DIC regenerated from respiration in the subsurface jointly controlled this pCO2 variation. As a whole, the SYS and ECS acted as an obvious CO2 sink during summer and could absorb atmospheric CO2 with the average air–sea flux (FCO2) −2.68mmolm−2d−1.The summary of air–sea CO2 flux in the ECS and SYS during recent two decades indicated the ECS served as quite a stable CO2 sink, whereas the SYS experienced obvious change. Discharge of Yangtze River and anthropogenic nutrients loading could profoundly affect the variations of pCO2 and FCO2 in future.

Millennial-scale East Asian Summer Monsoon variability recorded in grain size and provenance of mud belt sediments on the inner shelf of the East China Sea during mid-to late Holocene

The response of the monsoon climate on the inner shelf of the East China Sea (ECS) to abrupt climate changes events within the monsoonal Yangtze River drainage is contentious. Repositioning of the monsoon front is thought to have been responsible for the changes in hydroclimate over the Yangtze River drainage, which has exerted great impact on the sedimentation of the inner shelf of the ECS during the Holocene. Here, we present high-resolution analyses of grain size and provenance of the sediments from core MD06-3040 recovered from the inner shelf of the ECS during MD155-Marco Polo II-IMAGES XIV cruise. Significant variability occurred in the core MD06-3040 with notable decrease in East Asian Summer Monsoon precipitation (presumably drought events) at 6000 BP, 5300 BP, 4500 BP, 3700–3300 BP, 2200 BP, 1400–1300 BP, and 800–600 BP, which coincide with the δ18Ow record in the northern ECS and stalagmite Δδ18Osp records from southern China. Similar patterns of these temporal variations suggest that the Yangtze River discharge associated with EASM precipitation has been a dominant control on the sedimentation on the inner shelf of ECS.

Distribution of organochlorine pesticides in sediments from Yangtze River Estuary and the adjacent East China Sea: Implication of transport, sources and trends

Thirty-eight surface sediments and two sediment cores were collected from Yangtze River Estuary (YRE) and adjacent East China Sea (ECS) to study the distribution patterns, origins, potential transport and burden of organochlorine pesticides (OCPs) in the marginal sea. Residues of OCPs ranged from 0.190 to 5.17ngg−1 dry weight with a distinct “band type” pattern under the control of Yangtze River inputs and ECS circulation system. Differences in transmission media and redox conditions in situ respectively resulted in the divergent distributions of α/β-HCH and DDD/DDE. The compositional pattern analysis implied that OCPs in the inner shelf of ECS were derived from both “weathered” and fresh sources, whereas those in the outer shelf of ECS had undergone high metabolism. Concurrent with the land–sea migration, vertical profiles of sediment cores showed increasing trends or rebound since the 1990s, characteristic of two evident “jumps” of DDE+DDD/DDT and DDT/DDE ratios. Moreover, the primary distribution pattern founded for HCHs and the considerable mass inventories calculated (6.20 metric tones for OCPs) together suggested that the contaminated sediments in the studied area to be a potential source of OCPs to the global ocean.

Rare earth element geochemistry in the inner shelf of the East China Sea and its implication to sediment provenances

Gravity core EC2005, located in the mud wedge off the Zhejiang-Fujian coast in the inner shelf of the East China Sea (ECS), was analyzed for its lithology, grain size, rare earth elements (REE) and AMS 14C dating. Results revealed high-resolution paleoenvironmental evolution and multiple switches of sediment provenances in the inner shelf of ECS. The lithology, grain size and REE concentrations of sediments varied significantly down the core. Mean grain size ranged from 8 to 121 μm, and the values of ΣREE were 152.8–227.9 μg/g. The chondrite- and shale-normalized REE patterns indicated their similarity to the terrigenous sediments in the study area. Between 17.3–12.3 ka BP the sediments in the inner shelf of ECS were mainly originated from local drainage basins. The source then gradually switched to the Yangtze River between 12.3 and 9.8 ka BP. Since about 10–9.8 ka BP, the sediments in the study area were dominantly derived from the Yangtze River. The continuous rising of sea level and formation of coastal current along the inner shelf played a key role in the switches of sediment provenances.

Contrasting chemical and isotopic compositions of organic matter in Changjiang (Yangtze River) estuarine and East China Sea shelf sediments

To examine the source and preservation of organic matter in the shelf sediments of the East China Sea (ECS), we measured bulk C/N and isotopes, organic biomarkers (n-alkanes and fatty acids) and compound-specific (fatty acids) stable carbon isotope ratios in three sediment cores collected from two sites near the Changjiang Estuary and one in the ECS shelf. Contrasting chemical and isotopic compositions of organic matter were observed between the estuarine and shelf sediments. The concentrations of total n-alkanes and fatty acids in the shelf surface sediments (0–2 cm) were 5–10 times higher than those in estuarine surface sediments but they all decreased rapidly to comparable levels below the surface layer. The compositions of n-alkanes in the estuarine sediments were dominated by C26-C33 long-chain n-alkanes with a strong odd-to-even carbon number predominance. In contrast, the composition of n-alkanes in the shelf sediment was dominated by nC15 to nC22 compounds. Long-chain (>C20) fatty acids (terrestrial biomarkers) accounted for a significantly higher fraction in the estuarine sediments compared to that in the shelf sediment, while short-chain (<C20) saturated and unsaturated fatty acids were more abundant in the shelf surface sediments than in the estuarine sediments. Stable carbon isotopic ratios of individual fatty acids showed a general positive shift from estuarine to shelf sediments, consistent with the variations in bulk δ 13CTOCTOC. These contrasts between the estuarine and shelf sediments indicate that terrestrial organic matter was mainly deposited within the Changjiang Estuary and inner shelf of ECS. Post-depositional diagenetic processes in the surface sediments rapidly altered the chemical compositions and control the preservation of organic matter in the region.