Combining sterols with stable carbon isotope as indicators for assessing the organic matter sources and primary productivity evolution in the coastal areas of the East China Sea

Abstract

To identify the sedimentary organic matter (OM) sources and reconstruct primary productivity evolution process in the coastal areas of the East China Sea (ECS), carbon, nitrogen and carbon stable isotope (δ13C) combined with sterols were studied in the surface and core sediments. The quantitative calculations of δ13C model and characteristic sterols suggested that the OM is derived from mixed marine and terrestrial inputs. Sinking marine phytoplankton was the dominant marine OM (MOM) source, with the average MOM proportion calculated by two end-member model of δ13C and characteristic sterol analysis of 73% and 62%, respectively. Terrestrial organic matter (TOM) displayed a decreasing trend seaward, attributed to the Changjiang River input as the dominant terrestrial source pathway. The decreased proportion of TOM with age in the core sediment was mainly controlled by the fluctuating changes of freshwater and sediment influxes of the Changjiang River. The sum content of epi-brassicasterol and dinosterol increased with age in recent 30 years, suggesting the increasing phytoplankton productivity. Enhanced anthropogenic nutrient inputs from the Changjiang River as well as the incremental nutrient supplement from the deep waters of the coastal upwelling area aggravated the phytoplankton blooms. Besides, the increasing ratio of epi-brassicasterol to dinosterol contents (B/D ratio) with age revealed the increasing diatom productivity relative to dinoflagellate productivity. This variation was mostly attributed to the natural forcing of the increased nutrient supplement from intensified coastal upwelling in the Zhejiang coastal mud area.