Paleoenvironmental Changes In The Southwestern Okhotsk Sea Since The Late Quaternary

The Okhotsk Sea is the second largest marginal sea in the northwestern Pacific Ocean,characterized by seasonal sea ice cover,high productivity and the source area of modern North Pacific Intermediate Water(NPIW),forming a complex environment with"sea-land-atmosphere-ice"interaction.As an important area for material and energy exchanges between low and high latitudes,the Okhotsk Sea plays an important role in the global ocean circulations and climate change.In this paper,an 8.8-m core(LV55-40-1)recovered from the southwestern Okhotsk Sea was used to establish the age model based on the AMS¹⁴C dating of planktonic foraminifera,Ba/Ti ratio,and productivity proxies(opal and chlorin)compared with the oxygen isotope records of LR04 and Greenland ice core,the basal age of the core was dated to?110 ka.The sedimentology,elemental geochemistry,and paleoproductivity analyses were used to have revealed the history of sea-ice variations and its mechanisms in the southwestern Okhotsk Sea at orbital-millennial scales since?110 kyr,to have discussed the changes in the source of terrigenous material in the sediments of the study area since the late Quaternary,and to have elucidated the variations in paleoproductivity and controlling factors in the study area.Sedimentology and ice-rafted debris(IRD)show that the Okhotsk Sea have been dominated by seasonal sea-ice since the past?110 kyr,and four sea ice expansions occurred in MIS 5b,MIS 4,mid-MIS 3 and early MIS 1,which were controlled mainly by decreased autumn insolation on an orbital timescale and show a strong 20-kyr cycle throughout the sequence.Superimposed on the orbital-scale changes,millennial-scale variations in sea ice were also observed,with 19 expansion events that coincided with Dansgaard-Oeschger stadials(DOS).Millennial scale sea ice variations were most likely controlled by both the Arctic oscillation(AO)and the East Asian summer monsoon(EASM).Terrigenous material is mainly controlled by transportation by sea ice.The source of terrigenous material in the study area can be divided into three phases:last glacial(MIS 5-MIS 2),the deglacial period(?19 ka)-middle Holocene(?8 ka),and since the middle Holocene(?8 ka).The northern shelf was the main source during the last glacial,which the sea ice formed along the northern shelf to capture terrigenous material and extended to the southern and southwestern Okhotsk Sea under the influence of northward and northeastward geostrophic winds.During the last deglacial-middle Holocene,the source of terrigenous material changed to a mixed input from the northern shelf,the Amur River and the Sakhalin Island,during which the runoff from the Amur River increased and was transported to the study area by sea ice and the Okhotsk Sea Intermediate Water(OSIW).In the Middle to Late Holocene,the Sakhalin Island and the Amur River were the main source,and under the influence of eastward and southeastward geostrophic winds,sea ice formed on the east coast of Sakhalin Island moved toward the study area.In addition,the sediments were mainly influenced by the input of volcanic material from the Kuril Islands-Kamchatka Peninsula at?31ka and?27 ka.Productivity proxies show productivity in the southwestern Okhotsk Sea was generally low during last glacial(MIS 5-MIS 2),mainly caused by the long duration of sea ice and the strong pycnocline produced by sea ice melting in summer.During the last deglacial,the silica productivity(diatom)remained low,while the calcium productivity(coccolithophore)increased in the B(?)lling-Aller(?)d and Preboreal,which probably controlled by the increased stratification of surface waters and weakened upwelling of North Pacific Deep Water(NPDW)resulting in reduced supply of silicates,to suppress the production of diatom and the bloom of coccolithophore.Since the middle Holocene,due to the disappearance of surface water stratification and the weakening of OSIW have made it easier for silicates in the deep water to enter surface,and silica productivity has gradually increased to modern levels.In addition,11 peaks of chlorin during MIS 5-MIS 2 corresponded to the warm Dansgaarde-Oeschger interstacials(DOI),which may be related to the slight bloom of coccolithophore caused by the increase of the Amur River runoff and the rapid retreat of sea ice during DOI.