Provenance Of The Old Red Sand In Coastal Zones Of Southeast China And Its Implications For Climate And Land-sea Interaction During The Late Pleistocene

A large number of coastal dunes have been deposited in various regions of the Earth during the late Pleistocene.The formation of the ancient coastal dunes has been thought to be closely related to the evolution of paleoclimates and paleogeographical environments.The Old Red Sand is a kind of brown red,red or orange yellow semi-consolidated medium-fine sand sediment widely distributed in the southeast coast of China.They are located in the leading edge of the vegetation cover area with a height of 0-40 m above current sea level.Provenance and depositional process of the Old Red Sand in the southeast coastal zones of China are important to better understanding of paleoclimate,sea level change and land-sea interaction in East Asia since the late pleistocene.However,there are still many problems about the source-sink process of the Old Red Sand in southeast China.In this study,sediment samples were collected from Qingfeng,Pingtan and Keren,Jinjiang profiles in Fujian,Banmei,Huilai and Dongmenxia,Xuwen profiles in Guangdong.The four profiles of the Old Red Sand were mainly deposited during the last glacial period(70-11.5kaBP),while the profile of the Old Red Sand in the Qingfeng involves last interglacial and last glacial periods.The framework grains,the composition of transparent heavy minerals,the composition of clay minerals,the U-Pb age and the roundness of detrital zircons were systematically analyzed.Results show that quartz and feldspar were the main component in the samples from the four profiles,but the content of quartz and feldspar in the Qingfeng profile samples in the last interglacial period was significantly lower than that of the last glacial period.In terms of the composition of transparent heavy minerals,zircon+tourmaline+apatite+monazite are the main heavy minerals in four profiles,reflecting the source of magmatic rocks.In addition,the Old Red Sand sediments deposited during the glacial period have higher ZTR index(zircon+tourmaline+rutile as a percentage of the transparent heavy mineral)than that of interglacial period.In terms of structural characteristics,the roundness of samples deposited in the glacial period is relatively high,while the roundness of Qingfeng profile sediment deposited in the interglacial period was also lower than that in glacial period.Detrital zircon U-Pb ages of three Old Red Sand samples indicate mainly 80-160 Ma(Yanshanian period),200-300 Ma(indo-china period),400-500 Ma(caledonian period),and subordinate 700-1000 Ma(jinning period),1800-2000 Ma(paleoproterozoic period)and 2400-2600 Ma(neo-archean period)ages.These age distributions are similar to that of coastal river sediments in the southeast China,it is speculated that these detrital zircons first came from the sediment supply of coastal rivers in the southeast before entering multiple cycles.We suggest that the sediment source-sink processes and transport forcings were different during the last glacial and interglacial periods.During the interglacial period,sea level rose and the continental shelves flooded,the runoff of rivers in the southeast coast increased,resulting in the sediment carried by rivers also increased greatly.The Old Red Sand sources deposited in this period mainly came directly from the sediments of rivers and the supply of a small part of beach sand in the adjacent region.While in glacial period,the global sea level dropped and the continental shelves were exposed,runoff in southeastern coastal rivers was reduced with the East Asian winter monsoon strengthened,the sediment carried by rivers was significantly reduced.The Old Red Sand deposited in this period were mainly supplied from exposed beach sand(continental shelf sand)with multiple cycles,and the beach sand with multiple cycles should also come from the southeast coastal rivers.This study shows that the Old Red Sand is a good sedimentary record of climate change since the late Pleistocene,which is of great significance to the in-depth understanding of the geographical environment changes in the southeast China during this period.