Disentangling The Sources And Distribution Of Sedimentary Organic Matters In The Yellow River Estuarine Wetlands Using Multi-tracer Approach

Coastal wetlands are located at the boundary between ocean and land.The biogeochemical process in the coastal wetland is complicated and volatile due to the frequently exchange of materials between the adjacent ecosystems. Being an important part of global carbon cycle, the biogeochemical process of sedimentary organic matters in estuary wetlands is an important proposition in Wetland Research and has been focused on for the recent decades. The source of sedimentary organic matters is essential for understanding their distribution, migration, degradation and transformation. It will help us insight the knowlogement of the role which estuary wetlands in translating the terrestrial organic matter to open sea.As a typical coastal wetlands, the Yellow River Estuarine Wetland (YREW) is accumulated by massive amount of mud and sand which arise from the Yellow River, is the most integrated, broadest and youngest wetland ecological system in warm temperate zone of China, and is one of the best areas for studying the formation, evolution and development of new coastal wetlands in the world. Surface sediments, six sediment cores were collected from different areas of the YREW in different seasons. Grain size, Total organic carbon (TOC), Total nitrogen (TN) and stable carbon (δ¹³C) compositions, as well as biomakers (n-alkanes and GDGTs) were measured. C/N ratio, Carbon Preference Index (CPI), four Branched and Isoprenoid ether Index (BIT) which been used to indicate the source of sedimentary organic matters were calculated. Two and three end-member mixing models which were established based onδ¹³C and BIT respectively, were used to distinguish the contribution of different source to sedimentary organic matters. The influences of environment conditions on the spatial and temporal distribution of indexes were discussed based on the analysis of various parameters, as well as on the combination of hydrological and environmental difference between sample time. The vegetation differences among sample sites were also taken into consideration.Experimental data indicated that silt was the predominant composition of sediments, and the average percentage was 75%. Spatial and temporal variations of the TOC, TN, C/N ratios andδ¹³C of sediments were significantly different. C/N ratios andδ¹³C indicated that the predominate source of sedimentary organic matters was terrestrial in area A (located at the area of core recovery in the YREW) and B (below the pontoon which is the last pontoon before The Yellow River enters the sea), but in area C (near the countryhero) was marine. From near river bank sampling point to the far river bank direction, the influence of terrestrial inputs reduced gradually, but the influence of sea source inputs elevated gradually. The content and distribution of organic matters were significantly different between the month April and June. C/N ratio showed a strong terrestrial inputs in June, but not consistent withδ¹³C. Quantification of terrestrial OC in surface sediments was calculated by two end-member mixing models based onδ¹³C, the results showed that terrestrial OC accounts for 52% in area A, and in area B and C accounts for 44% and 34%, respectively.The predominant composition of n-alkanes was high molecular weight and strong odd-even carbon preference. Similar to bulk chemical parameters, the n-alkane proxies had different area and season variation tendency. The n-alkane proxies in April were higher than in June, 2009, demonstrated that had higher terrestrial inputs in June, which consistented withδ¹³C. From near river bank sampling point to the far river bank direction, the influence of terrestrial inputs reduced gradually, but the influence of sea source inputs elevated gradually.GDGTs analysis showed that there were amount of Crenarchaeol (produced by marine planktonic Crenarchaeota) and Branch-GDGTs (derived from presumably anaerobic bacteria) in surface sediments of YREW. The BIT value was higher in April than in June 2009, especially in area A, indicated the proportion of soil organic matter was high in area A, intermediate in area B, and low in area C. Application of three end-member mixing model including C/N ratios,δ¹³C and BIT, suggested that the proportion of soil organic matter was higher in area A and B, their accounts for 89% and 68%, respectively, while in area C the proportion of marine organic matter was higher which accounts for 39%. The contribution of terrestrial sources calculated from three end-member mixing model was greater than that from two end-member mixing model, indicating that the contribution of soil organic matter may been ignored by two end-member mixing model.