| As the major reservoir of terrestrial and marine OC, the continental marginal seas are the important regions of land and ocean interaction and play important role in the global material cycle. In this study,20 surface sediments,3 box-core sediments and 1 gravity-core sediments were collected from the Changjiang Estuary and adjacent East China Sea (ECS) inner continental shelf, and the organic carbon (OC) and total nitrogen (TN) contents, stable carbon (δ13C) and nitrogen (δ15N) isotopes, sediment compositions, specific surface area,210Pb activity, ligin and phytoplankton chlorins contents in these sediments were analyzed. The effects of hydrodynamic sorting processes on the distribution of different sources of OC and the differences in early diagenetic processes between different regions were studied, and the eco-environmental changes during the past 200 years were reconstructed. The major results of this study are as followed:(1) Sedimentary OC sources of the surface sediments of Changjiang Estuary and ECS inner shelf were determined using three end-member mixing model based on the Monte-Carlo (MC) simulation. The results showed that marine OC was the predominant OC source (64±11%), accounting for an increasing fraction (46%to 91%) along the coast and seaward, while soil-derived OC and C3 vascular plant detrital OC decreased from 36% to 7%(mean:27±8%) and from 25% to 2%(mean:9±4%). Large fragments of fresh lignin-rich C3 vascular plant detrital OC were deposited mainly near the river mouth, whereas fine-grained lignin-poor soil-derived OC was delivered south further along the coast, indicating the remarkable influence of hydrodynamic sorting processes on the selective transport of terrestrial OC.(2) Chlorophyll-a (Chl-a (mean:71±83 nmol g-1 OC), pheophytin-a (PHtin-a) (mean:295δ340 nmol g-1 OC), pheophorbide-a (PHide-a) (mean:185±151 nmol g-1 OC), pyropheophytin-a (pPHtin-a) (mean:156±69 nmol g-1 OC), sterol chlorin esters (SCEs) (mean:181±112 nmol g-1 OC) and carotenol chlorin esters (CCES) (mean: 87±39 nmol g-1 OC) were the major types of sedimentary pigments in the Changjiang Estuary and ECS inner shelf. Two regions with high sedimentary chloropigment (Chlorins) contents located in the Changjiang Estuary and Zhe-Min coast, respectively, which probably were due to the nutrient inputs from Changjiang diluted water and upwellings in the Zhe-Min coast. Higher Chl-a/Pheopigments (Pheo-a) ratios in the coastal regions (e.g. outer edge of maximum turbidity zone in the Changjiang Estuary, mouth of the Hangzhou Bay and upwelling region in the Zhe-Min Coast), probably were mainly due to the shorter retention time of particles in the water column and seasonal occurrences of hypoxia in bottom water in these regions.(3) Based on the comparison of annual variations of δ13C and Λ8 contents in the surface sediments of Changjiang Estuary and ECS inner shelf, different variation trends were observed. The Λ8 in the sediments of Changjiang Estuary and ECS inner shelf showed a decreasing trend, while the δ13C in OC of Changjiang Estuary surface sediments increased from-24%o in 2000 to -19.4‰ in 2006 and decreased to -23.3‰ in 2011. This different variation trend between δ13C and Λ8 probably indicated that the Three Gorges Dam intercepted part of terrestrial particles which leading to the decrease of Ag, however, the decreasing flow velocity and increasing transparency promoted the algal blooms in the Changjiang watershed and the increasing inputs of freshwater algae:derived OC to the estuarine region.(4) Based on the analysis of 210Pbexcess activity and biogeochemical parameters (e.g. elemental contents, stable isotopes, lignin and sedimentary contents) in core sediments collected from Changjiang mud area, Zhe-Min coastal mud area and non-mud area, remarkable differences in early diagenetic processes were observed between these regions. Changjiang Estuary mud area suffered the most severe physical disturbance, and had the lowest TOC/SSA ratio (0.44±0.08 mg OC m-2). The δ13C of lost OC (-25.1‰) in this region indicated the terrestrial OC contributed most to the lost OC. Zhe-Min coastal mud area had the highest OC loss rate (45.0±15%), and the δ13C of lost OC was around -21.9‰, indicating marine OC contributed most to the remineralization of sedimentary OC. Stronger diagenetic processes (e.g. dissimilatory reduction of Fe (Ⅲ)-and Mn (Ⅳ)-oxides and sulfate) were observed in the Zhe-Min coastal mud area comparing with the non-mud area, and resulted in the decreasing trend of TOC%,δ13C and Λ8 downward. The non-mud area had the most stable sedimentary environment and highest TOC/SSA ratio (0.55±0.12 mg OC m2), indicating the highest preservation rate of sedimentary OC.δ13C of lost OC (around-20.8‰) indicated the marine OC was the major contributor to the lost OC, and the weaker diagenetic processes had limited influence on the vertical distribution of sedimentary OC in this region.(5) Based on the analysis of elemental contents, stable isotopes, lignin and sedimentary chlorin contents in the gravity core located in the southern Zhe-Min coastal mud area, the relative contribution of terrestrial OC decreased from 41% to 28% but the degradation state of lignin phenols showed increasing trend (P/(S+V):from 0.12 to 0.22; 3,5-Bd/V:from 0.03 to 0.09) after 1970, which probably were mainly due to the trapping of riverine sediments and riverbank erosion caused by enhanced dam construction and excessive deforestation. The results also showed that Λ8 ranged from 0.35 mg 100 mg-1 OC to 6.92 mg 100 mg-1 OC, and lower values mainly corresponded to the worst flooding events in the Changjiang watershed (e.g. flooding event in 1998) and weaker East Asian Winter Monsoon (EAWM), while higher lignin contents were more correlated to the strengthening of EAWM, which indicated remarkable influences of hydrodynamic sorting processes and regional climate variability on the southward delivery of terrestrial OC. Sedimentary chlorin contents ranged from 663.4 nmol g-1 OC to 74.9 nmol g-1 OC, and decreased exponentially downward. Through the usage of sedimentary chlorin contents and degradation model, variation trend of phytoplankton biomass was observed, and the results showed that historical changes of phytoplankton biomass was decoupled from the fluxes of Changjiang riverine nutrients but corresponding well to the fluctuation of regional climate variability, with higher phytoplankton biomass during positive/warm phase of PDO and/or El Nino Years and lower algal biomass in the negative/cold phase of PDO and/or La Nina Years. The intrusion of phosphate-enriched water into the Zhe-Min coast dominated by the regional climate variability probably was the major reason for the increasing phytoplankton biomass or algal blooms happened hundred years ago. |
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