Elemental And Isotopic Characteristics Of Sedimentary Organic Matter From The Inner Shelf Of The East China Sea

Quantification of terrestrial versus marine organic matter in sediments accumulated in the river-dominated continental margins for the last 50-200 years is crucial for understanding the role of climate and the impact of human on the modern carbon burial.In this study,three box sediment cores,namely DH1-1,DH3-3 and DH5-1,collected from the inner shelf of East China Sea?ECS?were analyzed for the contents of organic carbon?OC?and total nitrogen?TN?and their stable isotopic compositions??3C,?15N?,along with grain size and calcium carbonate?CaCO3?content to delineate the different sources of organic matter and estimate their modern burial rates.The results show that the sediments accumulated close to the Changjiang?Yangtze River?Estuary?CJE?contain low OC and TN contents with high mean grain size?e.g.,core DH1-1?,while the sediments accumulated relatively far away from the CJE contain high OC and TN contents with low mean grain size.This phenomenon implies the transportation of fine-grained particles?enriched in organic matter?to a longer distance from the CJE,indicating the effect of Zhejiang-Fujian Coastal Current on the hydrodynamics sorting along the Min-Zhe Mud Belt.Narrow molar C/N ratios?5.6-7.7?and ?13C values?-22.4 to-21.2‰?in these cores with insignificant down-core variation reflect the predominant contribution of marine-sourced organic matter with a slight incorporation of terrestrial organic matter.An obvious 3‰decrease of ?15N in core DH1-1 from the bottom to the top suggests the utilization of chemical fertilizer due to agriculture in the Changjiang Basin,considering the increasing of dissolved inorganic nitrogen?DIN?export in Changjiang over the past decades.In order to distinguish the relative contribution of marine-and terrestrial-sourced OC,two end-member ?13C mixing model is employed and the results indicate the predominant burial of marine OC?DH1-1:70%;DH3-3:77%and DH5-1:65%?in the study area.Furthermore,the burial rates of terrestrial and marine OC in the entire shelf of the ECS are estimated at 0.8-3.0 Mt C yr^-1 and 1.8-5.9 Mt C yr^-1 with average burial efficiencies of around 9.1%and 3.9%,respectively.