Sources of organic carbon in surface sediments of Hangzhou Bay and Zhejiang coastal area: Implication from total organic carbon, lignin and algal pigments

In this study, surface sediment samples (n = 49) were collected from Hangzhou Bay and Zhejiang coastal area. We combine bulk parameters (total organic carbon, TOC and stable carbon isotope signature, δ¹³C) with both terrestrial (lignin) and marine (pigments) sources' biomarkers to elucidate the sources of organic carbon. The high contents of TOC were related to inputs from Ou River, Hangzhou Bay, Changjiang Diluted Water and Yong River. Stable carbon isotope signature (δ¹³C) and C/N ratio of the bulk organic matter indicated that the organic carbon in these samples was mainly derived from marine organic matters together with a little contribution from terrestrial sources. The eight lignin-phenols normalized to organic carbon (Λ₈) in this study were relatively low, which was attributed to the strong tidal forces in Hangzhou Bay and the shallow water depth in coastal areas. The S/V and C/V ratios of lignin-phenols demonstrated that lignin in our research area was mainly derived from angiosperms, with a small contribution from gymnosperms. The concentrations of nine pigments detected in this study were relatively low, and the concentration of fucoxanthin reduces with distance from the shoreline (in the southern area of 29°N), while the case was in the contrary for prasinoxanthin and zeaxanthin. Chlorophyll a was only detected in a few samples with the lowest average concentration. There was no obvious distribution characteristic of organic carbon, lignin and pigments from north to south of the entire study area, accompanied with some partial distribution features, which could be attributed to the complicated hydrodynamic conditions and diverse organic carbon sources.