Cenozoic Chemical Weathering Process And Related Controlling Factors In Jiuquan And Wuyu Basins On The Northern And Southern Tibetan Plateau

The Tibetan Plateau is the most typical and strong interaction region between the lithosphere,biosphere,hydrosphere and atmosphere.It is therefore a natural laboratory to trace the interaction between multi-circles and to promote theoretical innovations and breakthroughs in Earth science.Silicate chemical weathering involves the interaction of the atmosphere,hydrosphere,lithosphere,and biosphere.At tectonic scales it modulates atmospheric CO₂ levels and affects the global carbon cycle.In the late 1980s,based on theoretical analysis and numerical simulation,the famous theory of‘the uplift of the Tibetan Plateau-intensified weathering-global cooling'suggested that the strong uplift of the Tibetan Plateau in the Cenozoic facilitated the exposure of fresh rocks to the surface,thereby accelerated the silicate chemical weathering and therefore the rapid depletion of atmospheric CO₂.This process would ultimately induce the decrease of global temperatures.Since then,researchers have carried out a large number of related studies based on marine sediments and modern processes and obtained many important evidences and conclusions.These make the study of‘tectonic uplift-chemical weathering-climate change'a hot scientific issue on global change and geodynamics.However,debates on the hypothesis of‘Tibetan Plateau uplift-chemical weathering-global cooling'are still existed due to the lack of Cenozoic silicate chemical weathering records that retrieved directly from the Tibetan Plateau region.Therefore,obtaining more reliable Cenozoic silicate chemical weathering records in and around the Tibetan Plateau becomes the key to in-depth understanding of process that involves plateau uplift,silicate weathering and climate change.The Jiuquan Basin is located in the northern part of the Tibetan Plateau,where the westerlies,winter monsoons and East Asian monsoons meet in the arid region of inland Asia.The Wuyu Basin in the southern part of the Tibetan Plateau is located in the alpine region and is under the control of the South Asian(India)monsoon.They are typical regions where tectonics,climate and continental weathering interact,and are ideal regions for establishing high-resolution chemical weathering sequences.The sediments in the basin are the products of weathering and denudation of surrounding mountains,and record the tectonic activity,weathering and denudation,and the evolution of regional paleoenvironment.Clay minerals are usually served as sensitive proxy to reconstruct regional chemical weathering history,in that they are the direct product of silicate chemical weathering and have strong stability during sedimentary burial.In addition,variations and assemblage of minerals(such as Quartz/Feldspar)can reflect the strength of chemical weathering to some extent,and are thus used to explore the intensity of silicate chemical weathering.Therefore,we selected the Cenozoic sediments from the Jiuquan Basin in the northern margin of the Tibetan Plateau and the Wuyu Basin in the southern plateau.Based on mineralogical analysis of sediments and elemental geochemical analysis of clay fractions(<2?m)of the two basins,we obtained continuous chemical weathering intensity records in the Jiuquan Basin since 40 Ma and in the Wuyu Basin since 18 Ma.Combined with previous studies and a comparative study between the obtained chemical weathering records and regional tectonic events and global/regional climate changes,we discussed the associated controlling factors on chemical weathering intensity changes in the studies regions.The results promote the in-depth study of process that involves plateau uplift,silicate weathering and climate change.The main results are as follows:(1)We reconstructed the chemical weathering intensity record in the Jiuquan Basin since 40 Ma.The results indicate that the chemical weathering intensity of this region display a decreasing trend during 40 Ma to 26 Ma,which was mainly controlled by the global cooling.It is followed by a subsequent increase during 26 to 23 Ma and 17 to 14Ma,which might be related to tectonic activity of the Tibetan Plateau and the strengthening of the East Asian monsoon at these time intervals.The strong chemical weathering intensity during 17-14 Ma is also considered affected by a warm-humid global climate during the middle Miocene climate optimum(MMCO)interval.The significant decrease of the chemical weathering intensity since 14 Ma should be a response to the coeval global cooling and the intensified aridification in the inland Asia that related to the tectonic uplift of the Tibetan Plateau.(2)We reconstructed the chemical weathering intensity record in the Wuyu Basin,southern Tibetan Plateau since 18 Ma.The results indicate a strong chemical weathering intensity in the studied region during 18 Ma to 16 Ma.We explained this as a joint effect of a warm-humid global climate during the MMCO interval and the synchronous tectonic activity of the plateau uplift and the South Asian monsoon.The subsequent strong chemical weathering intensity during 7.2 Ma to 3 Ma was considered affected by the plateau uplift and the South Asian monsoon.This is followed by a weakened chemical weathering intensity since 3 Ma,which is presumed to be the result of the combined effect of the plateau tectonic uplift that drove the plateau into an alpine climate environment and coeval global cooling.(3)The comparative analysis of silicate chemical weathering intensity records retrieved from the Jiuquan and Wuyu Basins in the northern margin and southern part of the Tibetan Plateau indicates that chemical weathering intensity in these regions is controlled by both global climate and tectonic activity of the Tibetan Plateau,with climate as predominant controlling factor.The late Cenozoic global cooling and the coeval uplift of the Tibetan Plateau and the resulting aridification in the NE Tibetan Plateau could jointly explain the synchronously gradual weakening of the chemical weathering intensity in the Jiuquan Basin.While for the Wuyu Basin,uplift of the Tibetan Plateau would facilitate the intrusion of the South Asian monsoon into the basin,favoring an intensified weathering intensity within the basin.But when the uplifted plateau enters the cryosphere,in combination with the reinforced global cooling,the chemical weathering intensity in the Wuyu Basin would also be weakened.