Climate-Driven Provenance Variation And Drainage Evolution Of Small Mountainous Rivers In Taiwan Since The Last Ice Age

Small mountainous rivers,with the drainage basins less than 104 km2 and the elevation of headwater higher than 1000 m,play a crucial role in the global land-ocean source-sink system.The mountainous rivers,which cover less than 10%of drainage area of the earth,collectively account for?45%of detrital sediments,?30%of the dissolved solids and abundant particle organic carbon to the global ocean.Taiwan is located at the boundary of the Eurasian and Philippine Plates and is in a subtropical monsoon area.Therefore,this region is strongly influenced by active tectonic activity,abundant rainfall and frequency typhoon events.As a result,Taiwanese rivers contribute number of sediments to adjacent oceans,such as Taiwan Strait,Okinawa Trough,East China Sea,the northern shelf and slope of South China Sea and thus Taiwan is regarded as an ideal site to study modern geological process such as rapid,short-term and large-flux sediment transport events.It is well known that climate and landscape of Taiwan varied significantly since last ice age.In this case,sediment transport process and sediment composition in the Taiwanese river systems might greatly varied and influenced sedimentary records in adjacent sinks.However,the evolution of drainage basin of Taiwanese rivers since last ice age,which is important to better understand the sediment source to sink processes in East Asia continental margins,remain understudied.This study targets a sediment core with a length of 98 m,which was drilled at the estuary of the Choshui River.The methods include petrography,heavy mineral analysis,clay mineral analysis,major and trace elemental geochemistry and detrital zircon U-Pb geochronology.We present an integrated provenance analysis of sediments deposited in the estuary since 60 ka to unravel the evolution of river catchment of the Choshui River.The clay minerals,heavy minerals and detrital zircon U-Pb ages indicate that since the last ice age,the provenance of estuarine sediments of Choshui River has changed.The lower content of smectite,higher illite crystallinity index,lower ZTR index and detrital zircon U-Pb age all indicate the Coastal Plain and Western Foothills were main source area of Choshui River in last ice age.Since the ending of last ice age,the relative contents of smectite raised,the illite crystallinity index declined,ZTR index increased and the detrital zircon U-Pb age sees more similar to upstream,indicating that the Central Range and Hsueshan Range contributed more sediments to the estuary.Combined with the climatic evolution history,we propose that the provenance variations were resulted from river drainage changes.Specifically,the river was shorter and with less runoff because of glaciers,less rain and weak East Asia monsoon conditions during the last ice age.By contrast,due to the declined mountainous glaciers,powerful monsoon,abundant rainfall and more frequency typhoons,the source of the Choshui River extended headward to the present position in the deglacial age.Our findings provide essential information for sediment source to sink study on East Asia continental margins and are also beneficial to researchers who are interested in millennial-scale evolution of mountainous rivers in other regions of the world.