Phytoplankton Productivity And Community Structure Variations And Controlling Mechanisms In The Yellow Sea During The Holocene

The Yellow Sea (YS) is an important marginal sea in the northwestern Pacific andits environment and ecology have changed significantly during the Holocene. Thechanges were driven by the interactions of the Yellow Sea Warm Current (YSWC),the East Asian Monsoon (EAM) and the Kuroshio Current (KC). In order to study theHolocene environmental and ecosystem variations in the YS, high-resolution recordswhich could provide materials for paleoclimatic and palaeoceanographic research arenecessary. This thesis carries out the multi-biomarkers in three sediment cores (YS01,ZY1, ZY2) drilled from the same latitude section in the South Yellow Sea (SYS)central mud area to reconstruct the past sea surface temperature (SST), phytoplanktonproductivity and community structure changes. The biomarker method is based on theassumptions that sedimentary biomarker contents reflect phytoplankton productivityand biomarker ratios reflect community structure.During the early Holocene (10.5-7ka), the phytoplankton primary productivityand community structure changes in the SYS are mainly controlled by the sea-levelrising and display lower paleoproductivity and haptophyte contribution. The rapidsea-level rise event MWP-1C occurred in9.6ka was recorded in core YS01withincreased primary productivity and decreased terrestrial organic matter contribution.Reconstructed phytoplankton community structure variations may also be affected bythe East Asian Summer Monsoon (EASM) which could lead to the precipitationchanges. More runoff transported to the YS induced by stronger rainfall precipitationwould decrease the salinity, which would result in less haptophyte contribution. TheYS was in a low salinity estuarine environment stage during the early Holocene andhaptophytes contribute less compared with the Mid-late Holocene. During the Mid-late Holocene (7-0ka), after the intrusion of the YSWC at about6.8ka, the SYS environment has been under the influence of the YSWC, andproductivity-community structure has been affected by the YSWC strength-pathwayvariations. Phytoplankton productivity is enhanced and contributes more comparedwith terrestrial organic matters after the formation of the YS circulation system. Thecontent values of the marine biomarkers in the core ZY1and ZY2are similar, andthey also reveal similar temporal trends. From6ka to3ka, the biomarker contents inthe two cores were relatively low with small oscillations, followed by a distinctincrease at about3ka which indicated productivity increased with stronger East AsianWinter Monsoon (EAWM). The circulation system in the YS are strengthened byincreased EAWM. In addition, a stronger EAWM could increase nutrient levels ineuphotic layer and increase dust input, all enhancing productivity. For core YS01which locates to the west of the other two cores, the marine biomarker contents ishigher and lack of an increasing trend. We presume that the productivity in thislocation could also be affected by the YS continental shelf front upwelling besides theEAWM. In areas influenced by the YSWC, an increase in haptophyte contributionwould be caused by stronger YSWC influence with the EASM precipitationmodulating. The abrupt increase of productivity values and haptophyte contributionduring the late Holocene (3ka) may be related to the occurrence of stronger and morefrequent ENSO, reflecting the onset and enhanced magnitude of modern ENSOsystem.Some differences of the biomarker records between core ZY1and ZY2suggestspatial variations in responses to YSWC and KC forcing. When the KC wasintensified during the periods6-4.2ka and1.7-0ka, the YSWC extended eastward,exerting more influence on core ZY1. On the other hand, when the KC weakenedduring4.2-1.7ka, the YSWC extended westward, exerting more influence on the coreZY2.The SST results in core YS01show that three periods of the regional climatechanges can be identified during the Holocene. During the early Holocene (9.6-7ka),it shifted from cold to warm with large amplitude under the control of Northern Hemisphere solar irradiance; the middle Holocene (7-2.8ka), the higher SST lasteduntil6ka and then decreased with oscillation; the late Holocene (2.8-0ka), theclimate was replaced by a warm stage with small fluctuations. The SST changes arecontrolled by the YSWC and in good response to the KC variations.There are also eight cold events recorded in core YS01, which occurred at0.3-0.5ka,1.2-1.6ka,2.7-3.1ka,4.2-4.5ka,5.5-5.9ka,6.9-7.2ka,8.1-8.5ka and9.2-9.5kathrough the last9.6ka. The SST displays periodicity of~532yr during the last9.6kaand biomarker proxy A/B ratio displays periodicity of~1560yr after the YSWCintrusion. The cold events and the millennial scale climate cycle appear to correlatewell with records in the North Atlantic, the KC and other different regions of theworld, suggesting global climatic tele-connections. The SST records in core YS01indicate that the YS climate is affected by global and regional climate change.