ABSTRACT

Surficial sediment samples were collected from two transects on the shelf of the East China Sea: one across the shelf and the other along the coast, both starting near the Changjiang river mouth. They were analyzed for contents of total nitrogen (TN%) and total organic carbon (TOC%) and for their isotopic compositions (δ¹⁵N and δ¹³C_TOC). The TOC% and TN% values varied concomitantly in the ranges of 0.2 to 1.5% and 0.02 to 0.19%, respectively. The δ¹³C_TOC value ranged from −25.1 to −19.6‰. The lower δ¹³C_TOC values (<−21.5‰) all occurred in the inner shelf corresponding to higher TOC% (>0.5%). Away from the coastal zone, TOC% dropped sharply with corresponding increase in the δ¹³C_TOC value. The δ¹⁵N value, varying from 3.6 to 4.9‰, displayed a more complicated distribution pattern. Distinctively lower δ¹⁵N values (<4.1‰) occurred near the Changjiang river mouth and near the shelf edge. Isotopically light riverine nitrogenous materials and preferential uptake of isotopically light nitrogen in the rich supply of nutrients in the Changjiang plume may have contributed to the observed low values near the river mouth, while nitrogen fixers in the Kuroshio waters may have contributed to those near the shelf break.