The distribution and seasonal variations of sedimentary organic matter in the East China Sea shelf

Highlights: • TOC, TN, OP, δ¹³C and δ¹⁵N in ECS surface sediments were studied in spring and fall. • Concentrations of TOC, TN and OP in autumn decreased compared with those in spring. • Increased δ¹⁵N values in fall indicated OM fractionation during degradation process. • Based on a model using δ¹³C, 0–34.6% of the sedimentary OM was of terrestrial origin. • The decomposition of OM provided relatively less phosphorus for the overlying water. - Abstract: We sampled the surface sediments of the East China Sea shelf (ECSS) in spring and autumn, 2014, and analyzed the biogenic element concentrations and stable carbon (δ¹³C) and nitrogen (δ¹⁵N) isotopic compositions to study the distribution and seasonal variations of sedimentary organic matter (OM). The average concentrations of TOC, TN and OP in autumn decreased obviously compared with those in spring. The increase of δ¹⁵N values in autumn indicated the priority utilization of ¹⁴N by bacterial decomposition activity. The values of δ¹³C were used to trace organic matter sources. The estimated percentages for terrestrial OM were in the range of 0–34.6%. They generally decreased seaward in nearshore areas, indicating the decrease of terrigenous influence. There was an obvious tongue-shaped region with relatively low percentages of terrestrial OM (<12%) in the northern part of the ECSS, which may be a reflection of the intrusion pathway of the outer seawater.