Paleoclimate And Paleoenvironment Evolutions In The Northwestern South China Sea Over The Past 36 Kyr BP And The Forcing Mechanisms

The East Asian monsoon?EAM?is the main factor that affects the climate change in most regions of East Asia.Owing to relatively high deposition rate and good carbonate preservation conditions,the northern South China Sea?SCS?slope sediments are considered to a record the EAM evolution accurately.The repeated rise and fall of sea level in geological history may lead to variations in sediment transport distance,the relative contribution of different provenances,and water currents.The northern SCS has a wide continental shelf,so sea level change in history have caused major changes in the geography pattern of land and sea.Therefore,the terrestrial weathering dominated by the EAM and sea-level change may be the two main factors that affect the northern SCS slope deposition.Most of the high-resolution paleoclimatic and environmental records revealed by previous generations were concentrated in the Pearl River Mouth Basin and Taixina n Basin in the northeastern SCS.There remains insufficient research on the Qiongdongnan Basin in the northwestern SCS,which limits our comprehensive understanding of the sedimentary environment and EAM evolution of the northern SCS.In this paper,sediment grain sizes,geochemical and environmental magnetic properties for the core PC338 retrieved from the northern continental slope of the Qiongdongnan Basin are comprehensively analyzed to?1?identify the main sediment provenance,and?2?discuss the influences of sea level changes and EAM on the sedimentary environment of the northern SCS slope.Accurate age was obtained with AMS¹⁴C measurements,and a layer of turbidite was identified through grain size analysis.The reliable age-depth model was obtained after removing the turbidite.The end-member model?EMM?was chosen to unmix grain sizes and extract useful end-members?EM?.The principal component analysis was used to analyze major elements and extract effective chemical elements to record surface weathering.Systematic magnetism was tested,and the response of magnetic properties to climate environment is discussed.Effective East Asian summer monsoon?EASM?indicators can be extracted to restore the climate environment in the area since the past 36 kyr.Finally,the mainly EAM forcing mechanism is discussed in combination with the achievements obtained from Southern China area.Based on this study,the main understanding is as follows:1.Comparison and analysis of the sediment rare earth elements?REEs?and the latest results of provenance identification suggest that the main provenances for the PC338sediments are the Red and Pearl Rivers.2.Due to the lower sea level,a layer of turbidite in the core was formed during the last glacial period?LGM?.Based on the palaeogeographic analysis,a probable triggering mechanism can be brought up.The sea level at that time was¹20 m lower than present,so the Red River estuary was face to the slope area.The hyperpycnal flow was formed in the estuarine sediments and gradually developed into turbidity flow after the long-distance migration.The turbidity scouring action resulted in better connectivity between the paleo-Red estuary and the sedimentary area,which promoted coarse-grained sediments,high sedimentation rate,increased Si O₂ content and magnetic mineral input after LGM.3.With the decrease of sea level,the distance from the estuary to the sedimentary area was shortened.This resulted in that the sediment and magnetic(?_ARM/?_lf)grain sizes were coarsened,a large number of magnetic minerals from the Red River with high hematite were imported,the content of the most coarse-grained grain component EM4?¹5-100?m?increased,the content of the finest-grained component EM1?⁰-2 m?decreased,and sediment deposition rate increased.4.EM2?²-10 m?obtained from EMM analysis may represents stable river input of clay and fine silt.Because it is mainly controlled by terrigenous weathering and rainfall under the action of EASM,it can be used as an index for EASM intensity.5.Principal component 1?PC1?contains K₂O,Al₂O₃,TFe₂O₃ and MgO,which can record surface chemical weathering steadily.6.When the sea level was lower than-90m,the magnetic minerals were more easily aggregated with the coarse-grained sediments from the Red River.However,when the sea level was higher than-90 m,the magnetic minerals may be mainly suspended in fine grain.Therefore,the high?_lf value over the highstand periods?³2-36 kyr BP and⁰-15 kyr BP?corresponds to the strong summer monsoon,while during the lowstand period?25-32 kyr BP?the?_lf response to EASM is completely opposite.7.EM2,PC1 and?_lf can effectively record the EASM intensity over the past³6 kyr records.Global events/stages,including the LGM,Heinrich stadials,B?lling-Aller?d and Younger Dryas,are well documented.The 8.2 ka and 10.3 ka cooling events may be imprinted in changes of grain sizes and elemental geochemical indices.8.The EASM variation trend is consistent with the 30°N insolation,indicating that the summer monsoon is mainly forced by the summer insolation of the northern hemisphere.The Holocene Optimum?HO?occurred in the early Holocene,which suggests that the enhanced insolation had a strong driving effect on the tropical convergence zone?ITCZ?in the early Holocene.In addition,the cold climate events have global consistency and may be related to the weakening of the Atlantic Ocean Turnover Circulation?AMOC?.