Terrestrial and marine biomarker estimates of organic matter sources and distributions in surface sediments from the East China Sea shelf

Abstract

Revealing of the sources and distributions of sedimentary organic matter in the East China Sea (ECS) is important for understanding its carbon cycle, which has significant temporal and spatial variability due to the influences of recent climate changes and anthropogenic activities. In this study, we report the contents of both terrestrial and marine biomarkers including ∑C 27+C 29+C 31 n-alkanes (38.6–580 ng/g), C 37 alkenones (5.6–124.6 ng/g), brassicasterol (98–913 ng/g) and dinosterol (125–1521 ng/g) from the surface sediments in the Changjiang River Estuary (CRE) and shelf areas of the ECS. Several indices based on biomarker contents and ratios are calculated to assess the spatial distributions of both terrestrial and marine organic matter in the ECS surface sediments, and these results are compared with organic matter distribution patterns revealed by the δ 13C (−20.1‰ to −22.7‰) and C/N ratio (5–7.5) of total organic matter. The contents of terrestrial biomarkers in the ECS surface sediments decrease seaward, controlled mostly by Changjiang River (CR) inputs and surface currents; while higher contents of the two marine biomarkers (brassicasterol and dinosterol) occur in upwelling areas outside the CRE and in the Zhejiang–Fujian coastal zone, controlled mostly by marine productivity. Four proxies, f Terr( δ 13C) (the fraction of terrestrial organic matter in TOC estimated by TOC δ 13C), odd-alkanes (∑C 27+C 29+C 31 n-alkanes), 1/ P mar-aq ((C 23+C 25+C 29+C 31)/(C 23+C 25) n-alkanes) and TMBR (terrestrial and marine biomarker ratio) (C 27+C 29+C 31 n-alkanes)/((C 27+C 29+C 31) n-alkanes+(brassicasterol+dinosterol+alkenones)), reveal a consistent pattern showing the relative contribution of terrestrial organic matter (TOM) is higher in the CRE and along the Zhejiang–Fujian coastline, controlled mostly by CR inputs and currents, but the TOM contribution decreases seaward, as the influences of the CR discharge decrease.