A Study On Sources Of Different Grain-Sized Fractions Of Organic Matter In The Surficial Sediments From The Northern South China Sea

Continental shelves and coastal areas are important systems in the production and storage of organic carbon (OC). The sediments in these areas account for ⁹0% of OC burial in the ocean. Identifying the transportation and transformation of these OC will help us understand the carbon cycle clearly. As a start of transportation and transformation, the source of OC is very important, because it is always a crucial factor controlling the molecular composition and biogeochemical characteristics of OC. Generally, coastal sedimentary OM receives inputs from multiple sources, including terrestrial C3 vascular plant debris, soil-derived OM, estuary plankton and marine plankton. Therefore, these sedimentary OM is the mixing of OM from different sources. They are, on the other hand, not homogeneous in terms of particle size and chemical characteristics. There may be obvious difference in source among variant grain-sized sedimentary OM. However, there are few researches in this field, especially in the South China Sea (SCS).In order to identify distinctly the sources of sedimentary OM in the SCS further, two sediment samples from the north SCS, close to Pearl estuary, were separated into nine fractions (>250,250～160,160～63,63～50,50～40,40～32,32～20,20～10μm,<10μm) using the wet-sieving method. Elemental contents (OC% and TN%) and stable carbon isotopic composition (δ¹³C) were analyzed and then employed to trace the sources of OM. At last, a three-end-member mixing model was applied to assess quantitatively contribution of different sources of OM in grain-seized fractions.The result indicates that the main origins of sedimentary OM in this area are C3 plant debris, soil-derived OM and marine plankton. At A10 site, C3 plant debris is the significant contributor to >10μm fraction, while OM from marine plankton is primary only in <10μm fraction. At A6 site located in continental shelf, on the contrary, the signature of ocean plankton is very obvious. Oceanic OM accounts for 67.63% in the finest particles (<10μm). OM from C3 plant debris, whereas, is minor in the whole sample except >63μm fraction (²5%). As a whole, C3 plant debris contribution is...