| The amount of biogenic components such as total organic carbon(TOC),calcium carbonate(CaCO3)and opal in sediments can be used as proxy indicators of paleoproductivity to understand the effect of past climatic condition on productivity changes.Iron,as a nutrient,plays an important role in the growth of organisms,especially diatoms.In this study,three short sediment cores(34ab,11ab and 9b)collected from the Sunda shelf and slope in the southern South China Sea were to determine the contents of TOC,CaCO3,opal,total iron and reactive iron,and their mass accumulation rates were applied to evaluate the impact of paleoclimate change and human activities in the study area.Results indicated that the distribution of biological components is closely related to the anticyclonic eddy in the South China Sea.The flux of each biogenic component in the convergence center is higher due to the downwelling and lateral transport,while the productivity is higher due to the compensation flow(upwelling)at the edge of the anticyclonic eddy.The results show that the contents of TOC and opal are relatively low during the period from～4.0 ka(thousand years)to～2.0 ka,but their mass accumulation rates show an increaseing trend,indicating the presence of a dry and cold climate in the Sunda Shelf and slope during this period.From～2.0 ka to～0.5 ka,the content and mass accumulation rate of each biological component fluctuated slightly,suggesting that the climate was relatively stable in this period.From～0.5 ka to present,the contents of TOC and CaCO3 and their mass accumulation rates decreased,mainly due to the decrease of temperature.In addition,RFe/TFe ratio continues to increase from～2.0 ka to present,which response to chemical weathering,indicating that the impact of human activities is gradually increasing.In the shelf area,the shallow water depth and upwelling make the content of CaCO3 higher.In slope region,the lower productivity and strengthened dissolution make the content of CaCO3 lower.Therefore,the distribution of CaCO3 is mainly affected by water depth and productivity.Opal is affected by calcium carbonate dilution in the continental shelf,while higher content of opal in the continental slope is related to the increase of iron content.Therefore,the distribution of opal is not only controlled by CaCO3 dilution,but it also controlled by iron content.OC:Fe ratio is usually used to assess the associations between organic carbon and reactive iron.In the shelf area,low TOC is attributed by the dilution of CaCO3,and TOC is simply adsorbed with reactive iron(OC:Fe<1).On the other hand,in the slope area,the source of TOC is consistent with that of calcium carbonate,TOC precipitates or forms chelates(OC:Fe>1)with reactive iron. |
