黄河口夏季颗粒有机碳和沉积有机碳的来源组成及影响因素

Estuary is the key site of carbon cycling at the land-sea interface, thus clarifying the source, composition and spatial distribution of organic carbon is important for a deeper understanding of its fate in the marginal seas. In this study, surface particulate matter and sediments from the Yellow River estuary(YRE) were collected in August, 2022(after water-sediment regulation scheme), and we analyzed total organic carbon parameters(POC%, TOC%, C/N ratio and δ13C), mineral characterization parameters and environmental parameters in order to explore the source composition, distribution characteristics and influencing factors of organic carbon in the YRE in summer. The results show that the concentration of particulate organic carbon(POC) ranged from 0.33 to 36.95 mg/L, with high values nearshore and low values offshore because of riverine inputs, flocculation sedimentation and resuspension. The mean value of POC% is(1.25±1.40)%, and POC% and δ13CPOC are lower in the low-middle salinity area, suggesting the organic carbon is mainly derived from the terrestrial input of the Yellow River. POC% and δ13CPOC increased in the high salinity area, because the influence of river runoff weakened and phytoplankton blooms would lead to an increase of marine OC contribution, thus, the distribution of POC was mainly controlled by OC sources. The mean value of sedimentary organic carbon(TOC) content is(0.33±0.21)%, which is close to POC% in the low-middle salinity area. A large amount of particulate matter input from the Yellow River during the period of water-sediment regulation scheme and summer is deposited in the estuary region, which minimized the difference of the properties between sedimentary OC and POC. The spatial distribution of TOC in the estuary is controlled by the particle size effect. TOC% is negatively correlated with mean grain size(MGS), with decreasing grain size and increasing TOC% along the increase of salinity, indicating that coarser particles with low TOC% are deposited in the nearshore and the particles with high TOC% are transported to the offshore. The three-end member model by using C/N ratio and δ13C is applied to quantify the contribution of different OC sources of organic carbon and its distribution. The results show that soil and marine OC were dominant in POC, with contribution of soil OC decreasing from 51% to 19% offshore, and contribution of marine OC increasing from 24% to 68%. The contribution of soil OC accounted for about 40% in sedimentary OC. This study will provide support for further research on the transfer and preservation of OC from different sources in the YRE, which is of great significance for understanding the OC cycles in estuaries.