Sedimentary Evolution In The Eastern Arabian Sea And Its Response To The Indian Summer Monsoon Since 1.2 Ma

Combined with?¹⁸O and AMS ¹⁴C chronology,a multiple-record of siliciclastic mass accumulation rate,grain-size,clay mineralogy and elemental geochemistry from the upper 141.5 m of the International Ocean Discovery Program Site U1456 have been investigated to trace the provenance of detrital sediments,and to establish proxies of Indian summer monsoon intensity,thus to constrain the evolution of the Indian summer monsoon and its forcing mechanisms by comparison with other monsoon records of previous studies.The major conclusions we have drawn are summarized as followed:The clay mineral assemblages at Site U1456 mainly consist of smctite?with an average value of 59%?and illite?with an average value of 33%?,with minor chlorite?with an average value of 5%?and kaolinite?with an average value of 3%?.The provenance analysis indicates that smectite predominatly originated from the chemical weathering of basalts in the Deccan Traps,while illite,chlorite,and kaolinite were primarily derived from the Indus River,which are the products of physical erosion from the Western Himalya;the contribution from the gneissic rocks in the southern India is insignificant.At millennial timescales,smectite/?illite+chlorite?and K/Al ratios reflect the relative contribution of the Deccan Traps to the Indus River and the intensity of chemical weathering in drainage basin of the Indus River,respectively.Both ratios show coherent variation patterns to the pervious Indian summer monsoon records,which suggests that evolution of the Indian summer monsoon should probably be responsible for the provenance changes at Site U1456 as well as weathering/erosion intensity in the source areas during the late Quanternary.In addition,the sea-level variations also palyed an important role in controlling the sedimentary records at Site U1456 in the late Quaternary.At orbital timescales,variations in paleoclimate indicators of smectite/?illite+chlorite?and?_K ^Alratios at the study site are coincident with the evolution of Indian summer monsoon intensity revealed by previous studies,characterized by general glacial-intergalcial cycles.During the period of strong summer monsoon?e.g.,interglacial periods?,the enhanced chemical weathering in the source areas resulted in higher input of smectite from the Deccan Traps.During the period of weakened summer monsoon?e.g.,glacial periods?,a weakening in chemical weathering is observed in the source areas.In particular,more siliciclastic sediments?illite and chlorite?were derived from the Western Hiimalya due to intensified physical erosion.Therefore,the clay mineralogy and elemental geochemistry composition at Site U1456 recorded the singals of the Indian summer monsoon variations,thus can be used as reliable proxies to indicate the evolution of summer monsoon.According to the selected monsoon proxies,the Indian summer monsoon evolution can be divided into two intervals since 1.2 Ma:?i?a period of high-frequency variability of Indian summer monsoon during 1.2-0.9 Ma;and?ii?a period characterized by low-frequency in the Indian summer monsoon since 0.9 Ma.A strengthened Indian summer monsoon was observed during the Mid-Pleistocene Transition?1.2-0.9 Ma?,Marine Isotope Stage?MIS?13 event and the Mid-Brunhes Event?^MIS 11?.Spectral analysis results further revealed that,except for the obvious precession cycle?23 kyr or 19 kyr?,a transiton from 29 kyr or 41 kyr?obliquity?periodicities to strong 100 kyr?eccentricity?periodicity was recorded in the monsoon proxies.Such results suggest that the evolution of Indian summer monsoon at orbital timescales is controlled by both high-and low-latitude processes.