The Depositional Sequence In Laxmi Basin(IODP 355 U1456A,East Arabian Sea)Unravels South Asian Monsoon Changes Since The Late Pliocene

The South Asian monsoon is one of the most active components of the Earth's climate system,which controls climate change in the South Asia region and parts of Southeast Asia,and affects the livelihood and development of nearly thirty billion people.The Indus fan in the Arabian Sea is the second largest submarine fan in the world.Sediment of Indus fan is mainly from the weathered and exhumed surface in Himalayan and Indian suture in South Asia.The deposition sequence in Indus fan is an ideal achieve for studying South Asian monsoon evolution during late Cenozoic era.In this thesis,the submarine sediments of the core U1456 section A(419 m in thick)from IODP 355 are investigated.The chronologic framework is based on the identification of a series of calcareous nannofossil and planktonic foraminifer events and paleomagnetic reversals.We measured the grain size and hematite contents and discussed their paleoclimate implications.As a result,we established time series of the monsoon intensity evolution over?3.4 Ma.This thesis draws some conclusions as following:1)On the basis of analysis of sedimentary characteristics and depositional environment,the depositions were mainly turbidite sediments,with the suspended sediments transport by ocean current.The existed biostratigraphic datum and paleomagnetic timing controls were used for liner interpolation and extrapolation to established the time scale for this depositional sequence.The age of the depositional sequence is from 0.1-3.4 Ma and there exists a hiatus between 1.8-2.4 Ma.2)The study of grain size characteristic and sedimentary process shows that the grain-size distribution is mainly controlled by strength of South Asian monsoon.Then the sea level may affect it a little bit.The monsoon dominated precipitation,intensity of weathering,transport of surface sediments,these factors largely dominated grain-size distribution in Indus fan.The strengthened South Asian monsoon precipitation brought more rainfall to landmass that transported coarser particles and more sediments to deposition in Arabian Sea.3)Using the diffuse reflectance spectroscopy and CBD technique,we establish a standard equation for hematite content estimation.Then we determine the content of hematite which changes between 0-0.58%in the sediment core.In general,the dry and hot environment is conducive to the formation of hematite,the temperature is the main controlling factor for the formation and conversion rate of hematite.And the change of hematite from 3.4 Ma has been well consistent with the redness of the same core and the sea surface temperature curve from West Arabian Sea.Therefore,the hematite content in this study can be used as a good indicator of temperature change.4)On the tectonic time scale,we reconstruct the monsoon climate change since-3.4 Ma.From 3.4?2.4 Ma,the South Asian monsoon is stronger,with more precipitation and high temperature,especially the time between 3.4?2.7 Ma,which have the increased South Asian monsoon precipitation and highest temperature.Then,from 1.8?1.2 Ma,the South Asian monsoon is still strong,the temperature decreased than the previous stage(3.4?2.4 Ma).From 1.2?0.1 Ma,the South Asian monsoon weakened,precipitation reduced,temperature dropped,climate became dry and cold.5)As aforementioned conclusions,the sediment accumulation rate was decreased and the grain size became finer after 1.2 Ma,associated with a weaker South Asian monsoon intensity in this time.At 1.2 Ma,the cycle of grain-size variation changed from?41 ka to?80 ka dominated at?1.2 M.These observations may support South Asian monsoon evolution was responsible to global temperature and ice volume change at the mid-Pleistocene Climatic Transition(MPT),in that the South Asian monsoon intensity shifted to a weak phase compared with the previous time.Our investigation use grain size of the deep-sea sediment as a novel proxy indicator of South Asian monsoon strength,we find the South Asian monsoon strength has a direct response to the MPT;this finding probably manifest temperature/ice volume is the dominated driver of South Asian monsoon evolution at tectonic and orbital time scales in Quaternary.