The Source And Preservation Of Organic Matters In Sediments Of Different Areas In The Northern South China Sea

The marginal seas,representing the interfaces between terrestrial and marine environments,play significant roles in the global carbon cycle.The sedimentary organic matter in the marginal seas is a mixture of terrigenous and marine organic matter,and the origin and preservation of the organic matter were influenced by the climate changes and human activities.In order to better understand the global carbon cycle,the source,transformation,and fate of organic matter in the marginal sea sediments were examined.By studying the bulk properties(grain size,specific surface area,POC/TN,?13C and clay minerals)and biomarkers(n-alkanes and fatty acids),the content,composition;,sources and preservation of sedimentary organic matter in the northern South China Sea were investigated.The studied areas included the east area of Hainan Island,the adjacent area near the Zhujiang estuary,Shenhu area,and DongSha area.In the northern South China Sea along Zhujiang(Pearl)River delta(ZJ)and waters to the east of Hainan Island(HN),the sedimentary organic matter is mainly of a marine origin.With increasing water depth,more degradation was observed for sedimentary organic matter in seas close to Hainan and deep slope of the South China Sea,which have been widely observed previously,however,this phenomenon was not observed in the ZJ samples.Such a difference could be resulted from selective adsorption by clay minerals.Core sediments of HN:The S10 station is in the upwelling region,so the average grain size is larger than that of S5 station and the mean OC%is lower than that of S5.Two reasons may explain these differences.First,in upwelling region the sediments were mixed evenly and the impact of clay minerals is not obvious,so the organic matter content of whole column is uniformity and the degradation degree is higher.In contrast,in the S5 station,the impact of clay minerals is obvious,so the clay minerals can adsorb and protect organic matter.Second,the higher proportion of coarse particles in the S10 station prvodie an unfavorable condition for the preservation of organic matter.In surface sediments,the grain size distribution also plays a role in the preservation of organic matter.Both core and surface sediments are mainly derived from marine sources.Therefore,it is necessary to considere minteral property when discussing burial and degradation of organic matter in continental slope of the north South China Sea.The burial and preservation of organic matter in surface sediments,shallow and deep cores are all influenced by clay minteral property.The sediments of Shenhu area has higher OC%and smaller mean grain size than that of Dongsha area,but both areas are dominated by marine organic matter and have simiar distribution patterns during glacial/interglacial cycles:less autochonthous contribution during the glacial stages and more marine organic matter inut during the interglacial stages.The ratio of semctite/(illite+ chlorite)is closely related to climatic conditions:lower values during glacial stages and higher during interglacial stages,consistent with previous research results,so we divided the geological age of CH03 and DS08-13 stations according to previous research results.The degradation degree of organic matter in Shehu area is higher than Dongsha area and the sedimentation rates of the Shehu area is lower than Dongsha area.Because in Dongsha area,chlorite%increases as the depth increases and chlorite has a strong adsorption capacity than illite,so sedimentary OC there was better protected by chlorite from degradation with increasing depth.Despite differences in the source and preservation of organic matter in two stduied areas,both of them are rich in organic matter.These results could be explained from the following aspects:first,the deep-water bottom curent could bring large amount of organic matter to Shehu area,providing substrates for the formation of gas hydrate;second,in Dongsha area the coarse particle is not better for the preserve of organic matter.However,in Shehu area,the sediments are composed of more fine particles,so is better for the preserve of organic matter.Im summary,the environment of the marginal seas is complex and as a hotspot of the global carbon cycle.In the Northern South China Sea,the source and preservation of the organic matter in the sediments had distinct patterns.This thesis considered mutltiple factors such as marine environment and clay minerals on the source and preservation of the organic matter in the sediments of north South China Sea in roder to better understand carbon cycle in this marginal sea.