| Under the background of global change and for the sustainable development ofmankind, we need to master the principles of natural variations and understand themechanism of change to provide reference for future survival. The study of pastchanges is an important way to understand the pattern of natural variation. Onlysufficient understandings of the past is obtained can we effectively assess the impactof human activities on the modern environment and then be prepared to face thefuture challenges brought about by global changes. Continuous deposited marinesediments which bury rich information of past change is a good carrier for the studyof paleoclimate change. Under the functions of marine dynamic system and riverinput, large continuous deposited and organic-rich contained mud areas were formedin the Yellow Sea during the Holocene, which makes it a good place for past changestudy. In this paper, two gravity cores N04-A and A02-C were studied in the36°Nsection in the central mud area of the Yellow Sea, in which biomarker method wasapplied for reconstructing paleoenvironment records to explore hydrological andecosystem structure condition variations in36°N section during the Holocene,providing long timescale background information of natural changes for theassessment of the impact of human activities on the Yellow Sea. The mainconclusions are as follows:(1) TheU k’37-SST record of core N04-A showed a rapid significant increase at7.6cal ka B.P., perhaps indicating the intrusion of the YSWC and its influence on thissampling site.(2) TheU k’37-SST variation in the36°N section during the Holocene can bedivided into3stages: StageⅠ(8.5-7.2cal ka B.P.), SST fluctuated with an increasingtrend, mainly due to the high solar radiation in the northern hemisphere and the influence of the Yellow Sea Warm Current(YSWC); StageⅡ(7.2-2.7cal ka B.P.), SSToscillated stably relatively, reflecting the relative stability of hydrological conditionsat this time in the study area; Stage Ⅲ(2.7cal ka B.P.-present),U k’37-SST fluctuatedin the range of high value, mainly related to the strengthening of the YSWC at thisperiod.(3) In4.6-2.7cal ka B.P.,U k’37-SST record of the two cores both displayed acertain degree of response to the PME of Kuroshio, reflecting the important influenceof the YSWC on the hydrological under the influence of Kuroshio.(4) TheU k’37-SST records in cores N04-A and A02-C both had good responses tothe Holocene cold events in North Atlantic, indicating that SST in36°N section of thecentral Yellow Sea was under the combined effects of local hydrological conditionvariations and global climate changes.(5) The difference ofU k’37-SST records between the core N04-A and A02-C aremainly reflected in two aspects: In the respect of fluctuation amplitude, SST record ofA02-C is less obvious than the one of core N04-A; in the respect of the cut-off time ofthe significant change of SST, the SST record in core A02-C showed0.2-0.3ka aheadof time than the one of core N04-A. These differences are probably related to agedating error, different time scale corresponding of one sample and the geographicalposition difference of the two cores relative to the axis of the YSWC.(6) Marine phytoplankton primary productivity in36°N section during theHolocene could be divided into2stages. Before3.0cal ka B.P., marine primaryproductivity recorded by both cores oscillated in range of a relative low value; after3.0cal ka B.P., marine primary productivity recorded by both cores had an increasingtrend. The phenomena that marine ancient primary productivity greatly increased after3.0cal ka B.P. may be related to the high frequency of ENSO activities during the lateHolocene and a slight enhancement of the East Asian Winter Monsoon (EAWM) inthis period.(7) The variation of phytoplankton relative ratio in36°N section in the central Yellow Sea during the Holocene was mainly manifested on the changes ofcoccolithophores. However, the relative proportions of diatoms and dinoflagellateswere not particularly large. During8.5-7.2cal ka B.P., the relative contribution ofcoccolithophores in two sites both decreased, mainly related to the influence of strongEast Asian Summer Monsoon. During7.2-5.0cal ka B.P., the relative contribution ofcoccolithophores in the site of N04-A decreased while it had an increasing trend in thesite of core A02-C, mainly due to the wetward drift of the YSWC caused by thestrong ESWM. During5.0-2.7cal ka B.P., the relative contribution ofcoccolithophores in the site of N04-A increased while it had a decreasing trend in thesite of core A02-C, mainly related to the less westward drift of the YSWC caused bythe weak KS and EAWM. From2.7cal ka B.P. to present, the relative contribution ofcoccolithophores in two sites both had a significant increase, mainy because of thegreater effect of the YSWC caused by the increasing strength of the KS.(8) After7.2cal ka B.P., the changes of coccolithophore relative contribution intwo sites exhibited both similarities and differences, reflecting the variation of theinfluence extent on the study area by the YSWC under the impact of Kuroshio andEAWM, indicating the significant influence on the ecosystem structure of the studyarea by the YSWC and the sensitive response of phytoplankton structures to thehydrological condition changes in this region. |
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