Paleoclimate And Palaeoenvironment Implication Of Molecule Organic Geochemistry Records From Dongsha Area?northern South China Sea Since 50Ka

The South China Sea?SCS?,a large marginal sea between the Asian continent and the Pacific Ocean and rimmed by a broad continental shelf,is dominated climatically by the East Asian monsoon.Moreover,sedimentation rates are generally high,sediments accumulating in the northern SCS therefore potentially provide an opportunity for relatively well-resolved reconstructions of past environmental changes.However,the current research is mainly concentrated in the last glacial stage.Absent information is obtained on the paleoenvironmental changes since the Marine Isotope Stage3?MIS3?in Dongsha area.It is significant for the comprehensive understanding of the ancient marine environment since the late Quaternary period.Therefore,it is necessary to carry out the research on the paleoclimate/environment reconstruction in the area since the MIS3.The total organic carbon?TOC?,the total nitrogen content?TN?,C/N ratio,the stable carbon isotope of bulk sediment organic matter(?¹³C_org),the n-alkanes and glycerol dialkyl glycerol tetraethers?GDGTs?in the DH-CL13 Core,collected from Dongsha area in the northern SCS,were analyzed to reveal the composition of sedimentary organic carbon and the paleoenvironmental changes since the MIS3.The values of TOC ranged from 0.26%to 0.73%.The TOC values were higher in the Holocene than in the glacial time.The TOC kept stable from the MIS3 to MIS2.However,the TOC increased and then decreased during the Holocene.The values of TN ranged from 0.03%to 0.11%.The values of C/N ratio ranged from 5.1 to 23.7.The?¹³C_org values were between-24.5‰and-21.5‰,indicating that the sedimentary organic matter was derived from both marine and terrigenous.The?¹³C_org values were relatively depleted during the MIS3 and MIS2 and changed into more positive in the Holocene,suggesting that the increased TOC in the Holocene was due to the enhancement of the marine organic carbon input.The increase of the marine primary productivity might be ascribed to the raise of the nutrient input associated with the strengthened Kuroshio Current due to the sea level rise.Based on TOC,TN,C/N ratio,the?¹³C_org,the n-alkanes and GDGTs of DH-CL13 core,we divided our study core into three stages:I stages?the MIS3?,from 50.0.to 25.0 ka B.P.;II stages?the MIS2?,from 25.0 to 12.0 ka B.P;III stages?the Holocene?,from 12.0 to1.2 ka B.P.The concentration of long-chain n-alkanes ranged from 0.09 to 1.345?g/g?dry weight?,with obvious odd carbon preferences,suggesting that they mainly derived from the terrigenous higher plants.Whearas the concentration of short-chain n-alkanes were0.015 to 1.464?g/g,with an even carbon preferences,indicating its marine bacteria origin.Carbon Preference Index?CPI?values for the core ranged from 1.4 to 5.7.The gradually decreasing CPI and increasing Pmar-aq trend in the DH-CL13 profile indicated that the contribution of higher plants to sedimentary organic matter decreased from the MIS3 to the Holocene.The nC₃₁/nC₂₇7 ratio ranged from 0.25 to 2.56,with the lowest values in the Holocene and peak values in the MIS3.ACL_25-33 values varied from 27.5to 29.9,and showed an overall decline from the MIS3.The molecular composition indicated that the n-alkanes were mainly derived from algae,microbes and terrestrial higher plants.The composition of n-alkanes suggested that the percentages of the herbaceous plants decreased in the North-east SCS from the MIS3 to the Holocene.Isoprenoid GDGTs are dominated in the DH-CL13 core,which comprising from49.0 to 97.0%of the total GDGTs.The branched/isoprenoid tetraether?BIT?index ranged from 0.04 to 0.58.The BIT index were higher in the glacial time than in the Holocene.The BIT index kept stable from the MIS3 to MIS2.However,the BIT index decreased during the Holocene.The contents of isoGDGTs slightly increased.Correlating with our data,the increase of the marine primary productivity might induce archaea bloom.From 50.0 to 39.2 ka B.P.and 39.2 to 1.2 ka B.P.,the Methane Index?MI?and the TOC kept stable.The values of MI range from 0.14 to 0.26.The low MI values may indicate the predominance of non-methanotrophic marine Crenarchaeota,which represent normal marine sedimentary condition.We have generated a record of TEX^H₈₆-derived sea surface temperature?SST?over the last 50.0 ka B.P.for DH-CL13 core from the northern SCS.The value of SST ranged from 22.0?to 29.5?,with the lowest values in the MIS3 and peak values in the Holocene.Form 39.2 to 30.2 ka B.P.,GDGT-1 and especially GDGT-2 concentrations were generally higher than the other stages.Moreover,the values of GDGT-2/Crenarchaeol ratio ranged from 0.08 to 9.95,with some peak values.Similarly,the values of MI ranged from 0.15 to 0.92,with some peak values.High MI values correspond to large production of GDGT-1,-2 and-3,which may indicate strong impact of anaerobic oxidation of methane?AOM?microbial communities.We applied the RI-OH index of hydroxy-GDGTs to reconstruct SST form 39.2 to 30.2 ka B.P.in northern SCS.The value of SST increased.Form 39.2 to 30.2 ka B.P.,high MI values and relatively high percentages of GDGT-1,-2 and-3 might indicate strong impact of anaerobic oxidation of methane?AOM?microbial communities.All the results showed that the core sediment recorded the millennial scale climate cycles such as Heinrich cold events,suggesting the climatic tele-connections between Dongsha area in the South China Sea and the high latitude.