A 200-kyr Magnesium Isotope Record Of Continental Weathering On Northwestern Australian Margin And Its Significance Of Paleoclimatology

In recent years,with rapid development of geochemical analysis technology,huge potential and many applications for non-traditional stable isotopes have been developed in various fields of geoscience.Magnesium isotope proxies have been widely used to investigate earth surface processes,especially their quite advantageous in tracking silicate weathering.This study presents new multi-proxy records of grain size,Mg isotope and major element compositions of down-core sediments from Site U1483,which was drilled on the northwestern Australian margin during the International Ocean Discovery Program（IODP）Expedition 363.These records are used to reconstruct the terrestrial sediment supply to the southwestern Timor Sea as well as the continental weathering and erosion history in the source region over the last200 kyr,and further to explore the implication ofδ²⁶Mg of fine-grained siliciclastic marine sediments on tracing silicate weathering.The results reveal that terrestrial siliciclastic fraction of sediments at IODP Site U1483 are mainly composed of clay silts andδ²⁶Mg of the silts oscillates in amplitudes of larger than0.25‰.The Chemical index of alteration（CIA）and two physical erosion parameters including mass accumulation rates and Ti/Ca ratios are reported.Over the last 200 kyr,theδ²⁶Mg index shows a shape of curve in a great consistence with that of the CIA.It suggests that Mg isotope compositions are indicative of the weathering intensity in the source region,and the influence of source lithology,grain size sorting,and early diagenesis are negligible.The weathering and erosion records from Site U1483 almost synchronously co-vary with the published records of Australian summer monsoonal precipitation and the sea surface temperature from the same site,but had low correlation with the sea level record from the equatorial Pacific,indicating that the weathering and erosion of the source region on orbital timescales were mainly controlled by the Australian paleo-monsoon system,pointing to the“climate-driving weathering”pattern.Comparison of our records with orbital parameters,low latitude monsoon and high latitude temperature records suggests that the Intertropical Convergence Zone（ITCZ）tends to having been in the‘expansion’state during the last two ice age terminations（T-I and T-II）.The comparison also shows that interhemispheric precipitation,temperature and climate-driving continental weathering changed nearly in phase with one another during the terminations,indicating that the low-latitude monsoon system is not only closely related to the position of ITCZ and but also possibly affected by high-latitude climates through the teleconnection process.In addition,the low values of monsoon strength,weathering and erosion during Marine Isotope Stage（MIS）5/4 transition are possibly linked to the millennial-scale global cooling event caused by Toba eruption.Therefore,besides the orbital change,high-latitude climates and millennial-scale events may also largely influence the Australian monsoon system,and subsequently changes the evolution of climate-driving continental weathering.This study shows that theδ²⁶Mg index may provide a new isotopic perspective on the regional continental weathering in the context of the past climate evolution.