Sources And Sinks Of Sedimentary Organic Matter In The River-dominated Continental Shelves

The river-dominated continental shelves could serve as a key role for the study of land-ocean interaction in the coastal zone (LOICZ) in the context of the global change. Worlds 10 largest rivers transport-40% of fresh water and particulate matter into its associated marginal shelves, and over 80% of the global carbon burial occurs in these systems (Berner,1982; Hedge and Keil,1995). The sedimentary organic matter in the shelf settings should be an important process in the global cycle of carbon and land-ocean interaction.With large inputs of particulate materials and anthropogenic pollutants via rivers (e.g. Yellow River-the second largest sediment-load river in the world) and atmospheric deposition, the Bohai and Yellow Sea located in East China are the typical enclosed-semienclosed shelf seas with active land-Ocean interaction. The relative enclosed setting, large river inputs and typical shelf hydrodynamics forced the sink characteristics in this region. In this work, the soures and sinks of sedimentary organic matter (SOM) as well as the organic pollutant materials were examined in order to better characterize the importance of organic carbon burial in these shelf regions around the world, and also to evaluate the human influence in these shelf regions.Based on the analyses of particulate organic carbon (POC) and particulate nitrogen (PN) in the Bohai Bay and its adjacent sea, the results show that the spatial distribution patterns of POC and PN are similar in the study area. The concentrations of POC and PN in the coastal regions are higher than those in the outer shelf, and the concentrations of POC and PN are higher at the bottom water layers than those at the surface water layers. Biological effect, thermocline and concentration of TSM are the main factors affecting the spatial distribution of POC. The POC/PN ratio to evaluate the sources of POC is influenced by the presence of particulate inorganic nitrogen (PIN). Removing the content of PIN in the samples, the POC/PON ratio suggested that the terrestrial input originating from the nearby rivers is also an important source of POC, in addition to the major contribution from the marine primary production. This method could be applied to identify the sources of POC in other coastal areas of China.According to the analysis of total organic carbon (TOC), total nitrogen (TN), stable carbon isotopic composition and aliphatic hydrocarbons in the surface and cores sediment samples from the Bohai Sea, the results indicate that TOC showed a direct relationship with sediment grain size with the finest sediment having the highest TOC, suggesting that the hydrodynamics in Bohai was the main driving force in the accumulation of SOM.δ13C, the corrected TOC/ON ratios and TAR of n-alkanes indicated a mixed marine/terrestrial source for the SOM. The ubiquitous presence of UCM, composition patterns of hopanes and steranes, and PCA results indicated that the petroleum contamination in Bohai. The Huanghe River Estuary (HRE) and its adjacent area is the main sink for the Huanghe river-derived OC. This draws a similarity with other large river-dominated ocean margins (RiOMar) in the world, suggesting that the preservation of terrestrial OC in these large river-dominated coastal margins should be an important process in the global cycle of carbon. For the core samples, the C/N ratio of the bulk organic matter and the composition of the n-alkanes suggested that the sedimentary organic matter (OM) was of mixed marine and terrestrial sources. The presence of unresolved complex mixtures (UCM) in the surface layers and the patterns of biomarkers (hopanes and steranes) indicated more petroleum-derived inputs in recent years due to oil exploration and heavy traffic in Bohai. The coarser sediment grain size and decreasing C/N ratios since the mid of 1970s could be attributed to the relocation of the Yellow River course into the sea from the central Bohai to the south Bohai to decrease the influence of Yellow River-derived sediments and associated OM on the central Bohai.Based on the molecular composition feature of n-alkane in surface sediments, the dispersal of terrestrial organic matter (TOM) in the YS was identified. The results indicated that the distribution of n-alkanes content was influenced by the hydrodynamics, with the highest content found in the fine-grained mud area. The concentration of n-alkanes in the northern YS together with the correlation with TOC was higher than that of the southern YS, which could be related with the different sediment grain size and contribution of TOM in various regions. The concentrations and relative abundance of TOM in shelf mud areas increased distinctly, suggesting the central mud areas in the middle shelf of YS could be the main sink for the TOM. The PCA analysis also revealed that the riverine TOM from Huanghe could transport along the shelf under the special hydrodynamics, and then deposit and accumulate in the regions far distance from the original entrance. This source to sink process draws similarity with TOM in other shelf margins in the world.The fine-grained mud areas induced by the specific hydrodynamic conditions on the shelves of the Bohai and Yellow Seas are important sink for the organic carbon (OC). The weighted average accumulation rates of OC and alkanes in Bohai Sea were calculated as approximately 28 tC/km2/yr and 17.2 kg/km2/yr, respectively, which is higher than the mean values of the whole areas of the Bohai and Yellow Sea. The total OC budget for the entire Bohai could be estimated as 2.1 Mt/yr. This average OC accumulation rate in Bohai Sea draws similarity with other river-dominated ocean margins (RiOMar) in the world, suggesting the regional OC burial in the Bohai Sea could serve as an important role in the carbon sink by comparing with other RiOMar and coastal systems with high primary production preservation. The large riverine inputs of the terrestrial organic matter; relatively enclosed marine environments and special hydrodynamic conditions could serve as the key factor in maintaining the high carbon sink in this region, which are essential for the understanding of the role of large rivers and associated margins in the global cycles of carbon.Fifty-five surface sediment samples covering virtually the entire Bohai Sea (Bohai) were analyzed for organochlorine pesticides (OCPs). The results showed that DDTs and HCHs could be the most two important pesticides contaminants in Bohai. (DDE+DDD)/DDT ratios indicated that the degradation of the parent DDT occurred significantly. High concentrations of DDTs were observed in the coastal areas especially the isolated sites neighboring the harbor or port regions, suggesting the recent DDT inputs from wastewater derived from agriculture activity (e.g. dicofol) and chemical plant as well as the usage of antifouling paint. The contribution of the previous and current input of pesticides in the coastal areas was distinguished by means of principal component analysi. And the distribution of these relatively "old" accumulated pesticides could also be influenced by the distribution of organic matter due to the post-depositional sorption in addition to the input sources. DDTs and chlordanes are the two-main species of pesticides with more ecotoxicological concern in Bohai.The vertical distribution patterns of total PAHs in the two cores in Bohai was mainly dominated by the abundance of the low-molecular weight component, which influenced by the petroleum-related contributions. The pyrogenic PAHs in Bohai two cores were mainly from the incomplete combustion of coal or biomass burning. In addition, in the context of the large river dominated conditions, the biogenic contribution from higher plants for the low molecular PAH compounds could be inevitable. The vertical record of DDTs could reflected the official ban of the production for these chemicals in 1980s and the large-scale land use transformation and citifying process in the following 1990s. The 16 PAHs variation in the sediment core in the central mud of southern Yellow Sea follows closely with the stage features of historical economic development in China. The historical profile of PAH distributions in this region is different from those in the United States and Europe, owning to the different evolution course of energy structures between these western and oriental countries.