High-resolution Chronostratigraphy And Palaeoceanographic Evolution Since Late Pleistocene,Offshore Southwest Australia

Offshore Southwest Australia is characterized by complex surface and subsurface oceanic currents.Among these,the Leeuwin Current（LC）holds significant importance as a poleward flowing,eastern boundary current that conveys warm and oligotrophic water of low salinity.The Leeuwin Current has been identified as an extension of the Indonesian Throughflow（ITF）,which forms a hydrographic connection between the Indian and Pacific Oceans.The Indonesian Throughflow carries warm,low saline and oligotrophic water from the Indo-Pacific Warm Pool（IPWP）to the Indian Ocean passing through the Indonesian archipelagos.The Leeuwin Current is crucial for maintaining a balance of heat transfer between the equatorial and polar regions by transferring heat from the equatorial region and providing warm water to polar regions.Consequently,the Leeuwin Current is a vital component of the southern hemisphere’s global thermohaline circulation.Moreover,beyond its global significance,the Leeuwin Current and Indonesian Throughflow also play an essential role in the rainfall system of the Australian continent and the Asian region.Therefore,the understanding of Leeuwin Current dynamic in the modern setting and from past records is crucial for future analogues.Previously,several studies documented that the behaviour of the Leeuwin Current is changing spatially and temporally during the different climatic cycles（glacial-interglacial）.In the last two decades,the spatial variation of the Leeuwin Current is focused although it has never been studied at mid-latitudes beyond the last glacial cycle.Most researchers assumed that the Leeuwin Current probably had not extended up to mid-latitudes during glacial periods.Furthermore,the previous studies did not comprehensively explore the combined impacts of the changes in Indonesian throughflow and its parallel influence on the strength of Leeuwin Current.Likewise,the Subtropical Front（STF）is another important component of the oceanographic system in offshore southwest Australia,which has a great influence on the Leeuwin Current dynamics.However,the changes in the Subtropical Front,Leeuwin Current and its counter current have never been researched at mid-latitudes beyond the last glacial-interglacial cycle.The planktonic foraminifera are the most important proxy for palaeoceanographic reconstruction in the Indian Ocean.However,the detailed ecological classification in offshore southwest Australia is unknown.Previously,the planktonic foraminifera have been vastly studied at low latitudes but at mid-latitudes remains poorly known.Therefore,a detailed ecological grouping at mid-latitudes is crucial for understanding the changes in different water masses.To know the possible mechanism for changes in the various oceanographic components such as temperature and productivity,the influence of astronomical forcing is important to understand.The influence of astronomical forcing on paleotemperature and paleoproductivity during MIS18-1 is unknown at mid-latitudes offshore southwest Australia.To overcome the above-mentioned issues,a high-resolution chronostratigraphic framework is established and then it is globally correlated.Then comprehensive insights have been provided into the paleoceanography during MIS 18-1 offshore southwest Australia.To achieve the under-discussion objectives,the core samples of International Ocean Discovery Program（IODP）Hole 369-U1516B were used,which has been drilled at 34°S offshore Southwest Australia,providing an excellent opportunity to investigate the oceanic circulation and associated climate changes at mid-latitude offshore southwest Australia.The main findings are as follows.1.Late Pleistocene chronostratigraphy and biostratigraphy of Mentelle Basin and its implications for global correlation are established.A new high-resolution chronostratigraphy has been constructed for IODP Hole 369-U1516B offshore southwestern Australia.Planktonic foraminiferaδ¹⁸O was employed to construct an astronomically tuned age model for Hole U1516B.Theδ¹⁸O record shows significant cyclicality’s of 100 ky eccentricity,41 ky obliquity and 23 ky precession.Biostratigraphic analysis was performed for Hole U1516B using planktonic foraminifera,nannofossils,radiolarian taxa and diatoms.Seven planktonic foraminifera events were recorded,including the PT1a and PT1b boundaries.Eight nannofossil events were recorded including the boundaries between CN14a,CN14b and CN15.The planktonic foraminifera datums marked in Hole U1516B are mostly synchronous with datums reported in the southern hemisphere,but diachronous with datums in the northern hemisphere.The nannofossil datums marked in Hole U1516B have a close affinity with globally reported datums,but small inconsistencies are probably due to strong ecological control.The diatom events are inconsistent and only recorded in short intervals during interglacials and several key radiolarians taxa are absent.2.The ecological groups of planktonic foraminifera have been applied for illustrating the changes of various water masses.The planktonic foraminifera species with more than 1%were applied for ecological classification by means of principal component analysis.Five components were obtained from the scree test.The first component represents the sea surface temperature,whereas the other components are uncertain.The biplot of component 1 and component 2 divided planktonic foraminiferal species into four ecological groups.The ecological Group-I is composed of planktonic foraminiferal species preferred to warm and oligotrophic surface waters.However,some species show deeper settings with warm water conditions.Group-II consists of surface-to-subsurface dwelling species that represent cool and nutrient-rich water.Group-III describes deep cool waters while Group-IV signifies thermocline.The variation in Group-I shows changes in the warm oligotrophic waters of the Leeuwin Current.The changes in Group-II indicate the changes in the cool and nutrient-rich waters of the Leeuwin Undercurrent（LUC）and West Australian Current（WAC）.The high relative abundance of Group-II indicates the domination of the Leeuwin Undercurrent and West Australian Current with weakening Leeuwin Current.Similarly,a high relative abundance of Group-III indicates ventilation of intermediate and deep water masses.The group-IV shows the changes in the strength of South Indian Central Water（SICW）and the thermocline water.3.The palaeoceanographic evolution during MIS 18-1 is illustrated that the Leeuwin Current reached mid-latitudes during glacial periods with probably absent during MIS 12.In order to analyse various oceanographic components in offshore southwest Australia,several proxies have been used,including benthic foraminifera（%）,planktonic foraminifera（counts in%）,stable isotope(δ¹⁸O andδ¹³C)records of planktonic foraminifera,a modern analogue technique based sea surface temperature,mixed layer thickness,seasonality,Mg/Ca-based sea surface temperature,sea water oxygen isotope(δ¹⁸O_sw),menardiiform plexus,warm and cool planktonic assemblages ratio,and benthic and planktonic foraminifera assemblages ratio.Various planktonic foraminiferal assemblages were also grouped into tropical,subtropical,subpolar,and polar species to better understand the variation of various oceanographic components.Furthermore,paleo-bathymetric maps were constructed based on various sea levels to understand the changes in the pathway and strength of the Indonesian Throughflow and Leeuwin Current.The proxies used for sea surface temperature and paleoproductivity reveal that the Leeuwin Current reached subtropical middle latitudes（34°S）during glacial periods except for MIS 12.During MIS 12,the sea surface temperature dropped by 4-5°C,high productivity,high mixed layer thickness and surface dweller extremely declined,which indicates that the Leeuwin Current probably did not reach middle latitudes（34°S）.During MIS 10,the Leeuwin Current could be also weakened but had enough strength to suppress the upwelling.Similarly,the Leeuwin Current could be weakened during MIS 13 and 9,even though MIS 13 and 9 are interglacial periods.The Leeuwin Current could be strong during interglacial periods,particularly during MIS 11 where the subsurface water also show warming.The detailed sea surface temperature reconstruction and bathymetric analysis reveal that the Leeuwin Current could be mainly controlled by the global sea level.The high sea level drop leads to the subaerial exposure of shallow shelves of the Indonesian archipelagos,which potentially restricts the Indonesian Throughflow and subsequently Leeuwin Current.The Leeuwin Undercurrent and West Australian Current were dominated during the glacial periods indicated by the decline in warm surface dwellers,decline in sea surface temperature and increase in the relative abundance of subsurface and deep-dwelling species.The planktonic foraminiferal assemblages show that the Subtropical Front and Subpolar Front migrated northward during the glacial periods with maximum migration during MIS 12 and 10.The northward migration of the Subtropical Front during MIS 12and 10 is also one of the reasons which reduced the strength of Leeuwin Current.The paleoclimate is also briefly discussed with the help of various proxies including seawater oxygen isotope(δ¹⁸O_sw)seasonality and precipitation proxies from central and northern Australia.These proxies suggest that the climate could be dry and cool during glacial periods,and wet and warm during interglacial periods.The higher sea surface temperatures of the Indo-Pacific Warm Pool,Indonesian Throughflow and Leeuwin Current provided moisture to the atmosphere during interglacial periods.4.The high influence of astronomical forcing on sea surface temperature and paleoproductivity was revealed.The spectral analyses and wavelet transformation reveal that the temperature and paleoproductivity proxies have significant cyclicities.The 100 ky eccentricity cycle is common and dominant in all proxies,which are obvious after the middle Pleistocene transition at the middle and low latitudes.However,most of the records also show the influence of high latitude forcing obliquity.The sea surface temperature record of Hole U1516B shows in-phase relation with orbital obliquity,whereas it shows anti-phase relation with local insolation at mid-latitude（34°S）.The high influence of high latitude forcing on the sea surface temperature in Leeuwin Current can be explained by the changes in the northern hemisphere ice sheet extension and subsequent changes in the global sea level.The global sea level potentially controls the warm water supply to the location of Hole U1516B.Another possible mechanism is the northward migration of subtropical front and westerlies,which potentially affect the sea surface temperature in the region.Conversely,the paleoproductivity indicator Globigerina bulloides shows anti-phase relation with orbital obliquity with exceptions.In addition,the sea surface temperature from both hemispheres has been compared,which indicates the higher influence of obliquity on the southern hemisphere than the northern hemisphere.The possible reason for the higher influence of obliquity on the southern hemisphere could be the low continental area and low vegetation.In addition,the comparative analysis of dust inflexes,Fe concentration,atmospheric carbon dioxide（CO₂）and atmospheric temperature（Δt）and the sea surface temperature of Hole U1516B show that the sea surface temperature record of the Leeuwin Current is unique and has no obvious relationship with the above-mentioned proxies in the southern hemisphere.