Characteristics Of Sediments In The Equatorial Western Indian Ocean Since MIS 7 And Implications For Sediment Sources And Paleoenvironment

The western Indian Ocean is surrounded by the Indian Peninsula,Africa and the southern Asia,and the seafloor is spread with an inverted"Y"shaped mid-ocean ridge.Under the influence of monsoon,circulation and deep water mass,the mixed sediments composed of biological,terrigenous,autogenic,volcanic and hydrothermal materials have been formed in the deep sea of western Indian Ocean.The study of sediment characteristics and distributions can not only understand the sources of sediment,but also can provide scientific basis for the reconstruction of palaeosedimentology and climate environment.In this paper,clay minerals,major and rare earth elements were analyzed in the non-carbonate surface sediments of the equatorial western Indian Ocean.The sediment sources and the influencing factors of sedimentary characteristics were discussed.The CJ02-01 core sediment was selected to analyse the changes of sediment inputs and redox environment,investigate the effects of water mass,weathering and climate change on sediments on the glacial-interglacial scale since the MIS 7 through physical parameters,clay minerals,element ratios and Sr-Nd isotope indexes.The characteristics of clay minerals and rare earth elements indicate that the non-carbonate sediments in the equatorial western Indian Ocean were mainly mixed from North Africa-Arabian Peninsula,Indus river and Deccan Trap.The sediments in zoneⅠwere mainly clayey calcareous ooze,and the average content of palygorskite was20%.The contents of Fe and Mn in the southeastern and central of zoneⅠwere higher than that of continental weathering materials,indicating that there was a slight hydrothermal input.The sediments of zone II,being influenced by the Antarctic bottom water,were mainly calcareous clay,and the content of palygorskite was 24%,which received more aeolian dust than that in zoneⅠ.Significant positiveδCe anomalies were observed in all sediments,indicating that the study area was in an oxidizing environment.Due to the blocking of the CIR,the Antarctic bottom water mainly flows through the zoneⅡ,which is located on the western side of the ridge with a depth of more than 3800 m.The sedimentary water in zoneⅡis more oxidized than that in zoneⅠ,and Ce and Mn in the sediments are more enriched.Through AMS¹⁴C dating of planktic foraminifera,oxygen isotope and paleomagnetic method,the dating frame of CJ02-01 was established.The bottom boundary age is about 212 ka.On the glacial-interglacial scale,the weathering indexes and detrital input indexes of sediments in the equatorial western Indian Ocean have exhibited an obvious characteristic of cycle.The lower kaolinite/（illite+chlorite）and the higher K/Al in MIS 2,4 and 6 indicated that physical erosion was dominant in continental weathering debris sources and the chemical weathering was weak during the glacial period.Ti/Ca was higher in MIS 2,4 and 6,and the maximum value appeared in MIS 6（0.0038）,while it decreased to 0.0018 in MIS 5.The magnetic susceptibility reached the maximum value(7 SI×10^-5)in MIS 2 and 4,which increased by 9 SI×10^-5 compared with MIS 1 and 3.The median particle size decreased significantly in MIS 5 and MIS 1.All these indexes indicated that the input of continental weathering materials increased and the proportion of biogenic materials decreases in the study area during the glacial period.The abundance of palygorskite was lower in MIS 1,3 and 5,but increased（by13%）in MIS 2,4,and 6,indicating that the input of the North Africa-Arabian Peninsula aeolian dust increased in the glacial period.εNd and（Gd/Yb）_N of aeolian dust and Deccan Traps sediments were different.However,the meanεNd and（Gd/Yb）_N values of glacial and interglacial sediments in the study area are-9.09 and-9.03,and（Gd/Yb）_N values are 0.8991 and 0.8991.The changes of values were small.It indicated that during the glacial period,the input of Deccan Trap sediments increased at the same time as the input of aeolian dust increased.The detrital input indexes（magnetic susceptibility,median particle size）in the study area were compared with the South Asian summer monsoon intensity and global sea level curve.When the intensity of the South Asian summer monsoon decreased in MIS 2,4 and 6,the detrital input increased.The trend of global sea level curve was generally consistent with the detrital input index,and the continental weathering materials increased when the sea level dropped.The source of the proportion of deep water mass in different periods is the main factor affecting the redox environment at the glacial-interglacial scale in the study area.In MIS 2,3b,4 and 6,a cooler climate corresponds to a higher Mn/Al ratio,indicating that the increase of oxygen-rich AABW input and the decrease of oxygen-poor NADW input during the glacial period.