Search

The sedimentary rhythm is well developed in the environment which is controlled by strong seasonal climate changing condition. This rhythm is important for sediment core dating purpose. In this study, we examine muddy sediments on the inner continental shelf of the East China Sea that are influenced under seasonal change of the East Asian monsoon. We use Ensemble Empirical Mode Decomposition (EEMD) method to process sediment grain size and X-radiograph negatives grayscale value data in three sediment cores from this area and use the results to analyze the patterns of sedimentary variability and the causes of cyclic variability of the sedimentary process. The variability in the grain size and grayscale values of the core sediments has three components: the high-frequency variability, low-frequency variability and a secular trend. Among these components, the high-frequency component reflects variability on annual time scale can be well recognized, we suggest that the annual variability is derived from seasonal variation in the sediment flux from the Yangtze River to the sea, as well as the East Asian monsoon's impact on the sedimentary dynamics of the East China Sea's inner continental shelf. However, the other variability is hard to identify which cycles there are. This variability may reflect decadal winter monsoon oscillation and human activity of dam construction in the Yangtze River. Each annual variability presents a varve in the sediment sequence, which can be used to determine the sediment deposition rate and accurately date the core sediment. We found that the average deposition rates of the three cores from north to south on the inner shelf were 3.1 cm/yr, 1.0 cm/yr, 0.9 cm/yr, indicating that the Yangtze-derived sediment decreased with the increasing distance from the Yangtze estuary to the shelf. The deposition rate of each core varies over time, and the maximum deposition rate can be 3–5 times greater than the minimum rate. The sediment ages obtained from varve chronology and radiometric dating are fairly consistent on a centennial scale, differing only by 2–5 years. Thus, varve counting is a useful supplementary approach to radiometric age dating for constructing chronologies for core sediment in shelf seas influenced by monsoons. [ABSTRACT FROM AUTHOR]

Abstract Previous studies have considered that the wedge-shaped muddy body on the inner shelf of the East China Sea was formed by the deposition of resuspended fine-grained sediments from the Changjiang River estuary and its surrounding areas, which was transported southward by the Zhejiang-Fujian coastal currents. However, the occasional coarse-grained sedimentary layers in the muddy body have long been ignored. This study focuses on two sediment cores through the muddy wedge of the inner shelf. Grain sizes of the sediments with 0.25 cm interval and elemental compositions with 0.1 cm interval of the sedimentary cores were analyzed systematically, and the high-resolution sedimentary records of the cores were reconstructed by combining 210Pb and 137Cs isotopic dating. The event deposits resulting from intense typhoons were discerned by evidences of grain size anomalies and anomalous characteristics of elements. The possible mechanisms of the event deposits are discussed as well. The typhoon event layers (ELs) are a few mm to 8 cm in thicknesses, being in conformity with the underlying layers, having the sequences of fining upward in grain size. The ratio of Zr/Rb and content of Sr in the ELs increase sharply, are in agreement with the changes of sediments. The Core C0702 recorded 15 ELs, and Core DH6-1 recorded three in the past 150 years, the ELs were compared with the modern hurricanes. The cumulative thicknesses of the ELs account for 17.5% (C0702) and 7.6% (DH6-1) of the core total lengths. A concept model was developed to illustrate the sedimentary process of the formation of EL: coastal sediment was eroded and resuspended during intense typhoon period, transported by outward current in plume form and deposited on the inner shelf. The results suggest that summer typhoon plays an important role in the formation of the muddy wedge on the inner shelf of the East China Sea. Highlights • Event deposit of intense typhoon in the inner shelf of the East China Sea are identified and characterized. • The event layers have special sequence of fining upward in grain size, and high values of Zr/Rb and Sr. • Intense typhoon events recorded in cores are in good agreement with instrumental records of typhoons since 1949. • The event layers about 10% of total, suggesting typhoons play an important role on the formation of the muddy deposits. [ABSTRACT FROM AUTHOR]

Iron (Fe), sulfur (S), and molybdenum (Mo) geochemistry in marine sediments impacted by hydrothermal plumes and/or cold seeps is complex and has not been systematically documented. Here we characterize Fe, S, and Mo diagenesis in sediments between the Minami‐Ensei Knoll hydrothermal field and a cold‐seep site of the middle Okinawa Trough. Results show that distances away from the hydrothermal field and the steep trough slope may significantly affect the transport of hydrothermal Fe. The transformation of hydrothermal reactive Fe to poorly reactive or unreactive Fe‐bearing phyllosilicates decreased the relative fractions of highly reactive Fe (FeHR) in total Fe (FeHR/FeT). Despite this, the standing stocks of Fe oxides in the methane‐free sediments have not been dampened, indicating no net impacts of hydrothermal Fe inputs on the size of Fe oxides. In the methane‐free sediments, low ratios of total reduced inorganic sulfide (TRIS) to total organic carbon (TOC) (TRIS/TOC), highly 34S‐depleted pyrite, and low Mo contents suggest that organoclastic sulfate reduction is at low rates and plays a limited role in carbon cycle. In the cold‐seep sediments, however, intense sulfate reduction coupled to anaerobic methane oxidation significantly elevate TRIS/TOC ratios, Mo enrichment, and isotope compositions of Mo and pyrite‐S. This pathway is expected to be important in carbon cycle in the basin due to the wide occurrence of cold seeps. Our results highlight the important controls of the local extreme depositional/diagenetic conditions on sedimentary S and Mo records, with implications for the reconstruction of paleoredox states of the past earth's surface. Plain Language Summary: Submarine methane‐rich cold seeps and hydrothermal venting exert important influences on the geochemistry of iron, sulfur, and molybdenum. However, the influences of the two extreme environments on the three interlinked elements in seafloor have not been well documented. Here, we used solid‐phase chemistry of four sediment cores located between a hydrothermal field and a cold‐seep site in the middle Okinawa Trough to document the impacts of the two extreme environments on the geochemistry of iron, sulfur, and molybdenum. Results show that hydrothermal Fe signature in the sediments is largely controlled by the distances away from the hydrothermal field and the steep slope of the trough; the hydrothermal Fe inputs have no net impacts on sedimentary iron oxide contents due to simultaneous inactivation of reactive Fe. In the methane‐free sediments, sulfate reduction (SR) is coupled mainly to oxidation of sedimentary organic carbon. In the cold‐seep site, however, SR coupled to anaerobic methane oxidation is the main pathway of carbon cycle, leading to significant enrichments of molybdenum and isotopically heavy pyrite. The differences highlight the important control of local extreme depositional/diagenetic conditions on geochemical records of iron, sulfur, and molybdenum, with implications for the reconstruction of paleoredox states in the past. Key Points: Distances away from the vent, steep basin slope and Fe inactivation control the signature of hydrothermal Fe in the basin sedimentsLow lability of organics limits sulfate reduction (SR) in CH4‐free sediments, while CH4 largely facilitates SR in cold‐seep sedimentsLocal extreme conditions exert key controls on S‐Fe‐Mo diagenesis in the basin [ABSTRACT FROM AUTHOR]

• The diatom assemblages are closely correlated with the environmental variables. • Three assemblages are distinguished with different hydrological characteristics. • Important for palaeoceanographic studies in the northern and middle Okinawa Trough. Determining the relationship between diatoms in surface sediments and the local sedimentary environment is important for paleoenvironmental reconstruction, including in marine settings. In this study, we used canonical correspondence analysis of 41 surface sediment samples from the northern and middle Okinawa Trough to analyze the relationship between diatom species and environmental variables. The principal results are as follows. (i) The diatom assemblages are closely correlated with the environmental variables, specifically, July sea surface salinity (SSS-Jul), February sea surface temperature (SST-Feb), and June sea surface temperature (SST-Jun). (ii) Three diatom assemblages are distinguished, representing environments with different hydrological characteristics. The high-temperature diatom assemblage may indicate a warm-water mass (the Kuroshio Current), and the polyhaline diatom assemblage can be used as an indicator of a high salinity water mass. The mixed diatom assemblage is associated with the warm-water masses of the Kuroshio Current, and shelf water masses with a low salinity and low temperature. (iii) The high-temperature and mixed diatom assemblages occur on the boundary of the continental shelf and Okinawa Trough, and are temperature sensitive; the high-temperature assemblage is located further upstream of the Kuroshio Current, indicating higher temperatures. Changes in the distribution of the polyhaline diatom assemblage within sediment cores can be regarded as an indicator of changes in water masses, and of possible salinity variations, in this area. These three assemblages are potentially important for future palaeoceanographic studies. [ABSTRACT FROM AUTHOR]

Abstract Widely distributed cold seeps are crucial sources of carbon to the seawater, whereas anaerobic oxidation of methane (AOM) and precipitation of authigenic carbonate might change the compositions of the methane-rich fluids and thus reduce the outputs of dissolved carbon to seawater. In this work, we analyze the pore water compositions of four gravity cores with high methane concentration abnormalities in pore water or in overlying seawater in the western slope of the Mid-Okinawa Trough. For the northern research area, active weak methane seepage through the seafloor is identified in the vicinity of mud volcanoes (sites C01 and C10), and strong emissions of gas and fluids may occur in the central mud volcanoes. In a submarine canyon at the south, C23 site exhibits rarely methane seepage, while high rate transportation of methane from deep sediments and associated AOM are identified at site C25 where small vertical faults and gas-bearing layers are developed at the surrounding. AOM at site C25 is indicated by (1) quasi-linearly decrease of sulfate concentrations with depths, (2) intense increases of dissolved inorganic carbon (DIC) concentrations and (3) significantly negative δ13C values (vary from −7.2 to −26.3‰) of DIC. Organoclastic degradation in the shallow sediments is not obviously recognized in the study area because of the low concentrations of NH 4 +, as well as negligible sulfate consumption at three of the four sites. Our data reveal that the cold systems are one of the DIC sources (the fluxes are ca. 11.2 mmol/m2/yr) to the seawater in the Okinawa Trough, which can be tracked by high depletion of 13C (δ13C DIC ranges from −7.2 to −26.3‰ VPDB). Highlights • Pore water geochemistry was firstly linked to methane seepage in the Okinawa Trough. • Concentrations of DIC and SO 4 2−, as well as δ13C DIC value reveal anaerobic oxidation of methane. • Seep-wide methane and DIC budgets were constrained, revealing cold systems are one of the DIC sources to the seawater. [ABSTRACT FROM AUTHOR]

To understand the spatio-temporal distribution and sources of Pb in the sediments of the Yangtze River Estuary and its adjacent areas, 25 surface sediments and 1 sediment core were collected from the study areas. The concentrations of Al and Pb of these sediments exhibit a decreasing trend from the nearshore towards the offshore, with higher concentrations in the coastal areas of the East China Sea (ECS) and southwest of Jeju Island. According to the stable isotopic ratios of Pb, in combination with the elemental ratios and clay mineral data, it is inferred that sedimentary Pb in the surface sediments of the coastal areas of the ECS may come primarily from the Yangtze River, while the Pb southwest of Jeju Island is probably derived from both the Yangtze and Yellow Rivers. The particulate Pb derived from the Yangtze River was possibly dispersed along two paths: the path southward along the coastline of the ECS and the path eastward associated with the Changjiang Diluted Water (CDW), which crosses the shelf of the ECS towards the area southeast of Jeju Island. Although the Yangtze River Basin witnessed rapid economic development during the period from the late 1970s to the middle 1990s, the influence of human activity on Pb concentration remained weak in the Yangtze River Estuary. Since the early 2000s, however, sedimentary Pb has been significantly increasing in the coastal mud areas of the ECS due to the increasing influence of human activity, such as the increase in atmospheric emission of anthropogenic Pb in China, construction of the Three Gorges Dam (TGD), and the construction of smaller dams in the upper reaches of the Yangtze River. Coal combustion and the smelting of non-ferrous metals are possible anthropogenic sources for the sedimentary Pb in the Yangtze River Estuary. [ABSTRACT FROM AUTHOR]

Abstract: Biogenic silica (BSi) is an important parameter for understanding biogeochemical processes and paleoceanographic records in the ocean, but this proxy still has many challenges when used to reconstruct changes in the paleoproductivity and evolution of the environment, one of which is the grain size effect. We analyzed the BSi distribution in different size fractions from 8 surface sediments collected in the East China Sea (ECS). We observed the particulate characteristics of diatoms in the water and assessed the grain size effect of BSi. The results suggest the following conclusions: (1) the BSi content of the surface sediments in the ECS is generally below 1%, and the BSi content of different size fractions varies significantly, with largest fraction <16μm, which is approximately 1.1–1.8 times that in the bulk sediments. (2) The variation in the BSi content in different size fractions is largely controlled by the species of diatoms and their cell sizes. In the East China Sea, nano-diatoms are the dominant species, with dominant cells of 2–14μm, resulting in a high BSi content in the fraction <16μm. (3) The hydro-dynamic condition affects the diatom cell size distribution and sediment character, and, thus, has a significant influence on the BSi content of different size fractions. Our research suggests that the grain size distribution of the bulk sediment should be considered when using BSi as a proxy for diatom primary production. [Copyright &y& Elsevier]

• Cold seeps were linked to fault scarps and dome-structures in the Okinawa Trough. • Numerical models were used to quantify CH 4 fluxes and biogeochemical processes. • Cold seeps are C sources to the seawater, and Seep-wide budgets were constrained. The methane-enriched fluids in cold seeps are likely to crystallize as gas hydrates and serve as crucial sources of carbon to seawater. In this research, we analyzed the pore-water composition in terms of CH 4 , dissolved inorganic carbon (DIC), Cl−, Br−, SO 4 2−, Na+, Mg2+, Ca2+, Sr2+, and NH 4 +, and the δ13C DIC , δ13C CH4 , and δD CH4 values of two gravity cores and six remotely operated vehicle (ROV) video-guided push cores retrieved from fault scarps and dome-like structures (DSs) on the western slope of the mid-Okinawa Trough. In addition, a reaction–transport model was applied to quantify the methane fluxes and related biogeochemical processes. Active seepage of biogenic (δ13C CH4 ~ –70‰ V-PDB) and thermogenic (δ13C CH4 = −40‰ to −56‰ V-PDB) methane was identified on fault scarps and dome structures, respectively. Methane seepage was controlled by the transport and dissolution of the ascending gas rather than by clay dehydration or gas hydrate dissociation-induced fluid advection. The high methane concentrations and shallow sulfate–methane transition zones (SMTZs; between 0.1 and 0.4 mbsf) at sites R3-C2, R4-C4, and R6-C1 suggest strong methane seepage at three of the four studied DS (the highest gas dissolution rates are 6450, 1475, and 515 mmol m−2 yr−1). Site GC08, located along a fault scarp, exhibits a moderate methane seepage; the SMTZ is located at ~2.5 mbsf, and the rate of anaerobic oxidation of methane (AOM) is 130 mmol m−2 yr−1. The methane migrating from depth is mainly consumed by AOM. However, ~12%–66% of the methane released from the two most intensive seep sites escapes to the water column. The precipitation of high–Mg calcite (at all sites) and aragonite (only at site R3-C2) has fixed 27%–50% (average = 39%) of the DIC. Therefore, the carbon outputs to the water column have been reduced. In the study area, the area-weighted seafloor CH 4 and DIC fluxes are ~30 and 20 mmol m−2 yr−1, respectively. Together, they correspond to ~7%–14% of the organic carbon burial rate, indicating that sediments could not be simply regarded as a stable carbon sink because they provide methane and a certain amount of DIC to the water column. Our findings contribute to the results of the ongoing efforts in understanding carbon cycling in submarine cold seep systems. [ABSTRACT FROM AUTHOR]