Spatial Distribution and Isotopic Signature of Organic Carbon and Nitrogen in Surface Sediments from Bohai Sea

The Bohai Sea is a semi-enclosed shallow sea receiving increased loading of organic matters and nutrients as a result of anthropogenic activities. As a result, the Bohai Sea has suffered serious eutrophication and a bottom hypoxia zone has formed in recent years. A clear understanding of the formation mechanism of hypoxia zone and dynamic of eutrophication process demands better characterization of sources of carbon (C) and nitrogen (N) in marine sediments. We investigated the spatial distribution and stable isotopic signature of organic C and N in suspended particles and surface sediments from the Bohai Sea. Stable isotope values (δ¹³C and δ ¹⁵N) and elemental ratios (C/N) of organic material (OM) are used as qualitative tracers to elucidate the sources of organic matter in the marine environments. The total organic carbon (TOC), total nitrogen (TN) contents and the C/N ratios in marine sediment varied from 0.062-0.876%, 0.010-0.132% and 5.50-12.28, respectively. TOC and TN contents has a significant positive correlation (R²=0.914). In particular, the hypoxia zone of the Bohai Sea has a higher TOC and TN content, suggesting accumulation of OM through eutrophication processes. The TOC and TN contents showed strong correlation with particle size, which indicate that sediment transport process play an important role in the distribution of OM. The values of δ¹³C and δ¹⁵N of sedimentary organic matter in Bohai Sea ranged from -23.593‰ to -19.538‰ and +2.799‰ to +8.066‰, respectively. The spatial distribution of δ¹³C values suggested the dominance of marine source in most area of the Bohai Sea and increased contribution from terrestrial sources. Using a simple mixing model, the contribution of marine and terrestrial inputs were estimated based on δ¹³C values. The results suggested that most sediment samples collected (>90.7%) in Bohai Sea were dominated by marine derived carbon except nearshore, where shows a higher loading of OM from riverine and wastewater sources.