Sr And Li Isotopes Trace Chemical Weathering During Key Periods Of The Cenozoic

Over the past 66 million years,the Earth’s climate has undergone a long-term cooling trend,punctuated by many climate transition periods.The study of climate change during the Cenozoic era has been a research focus,with significant implications for understanding the evolution of current and future Earth climates.Continental chemical weathering is a crucial part of the surficial geochemical process.Continental weathering can consume carbon dioxide,lower the concentration of greenhouse gases in the atmosphere and cause climate change.Therefore,investigating the connection between weathering processes and climate change is of great significance for understanding the triggering and recovery mechanisms of Cenozoic climate events.This study focuses on examining the relationship between continental chemical weathering changes and climate changes during crucial periods of the Cenozoic era.The first part of this research entailed analyses of strontium（Sr）isotopes from coral reef samples from the South China Sea’s NK-1 drill core,which provided a continuous record of seawater strontium isotopes from about 20 million to 3 million years ago.These strontium isotope changes reflect the intensity of global continental weathering during the Miocene climate change period.By combining the Sr isotope data with previous oxygen（O）and carbon（C）isotope records from deep-sea sediments from the Miocene,chemical weathering can be linked to climate change.The ⁸⁷Sr/⁸⁶Sr gradually increased from 0.70819 to 0.70908（+0.00089）from the early Miocene to the early Pliocene,indicating an increasing chemical weathering during the Miocene,while the climate gradually cooled as recorded by δ¹⁸O.This suggests that the long-term tectonic uplift during the Cenozoic era has caused an increase in continental weathering flux,which consumes greenhouse gases and promotes global cooling.During the Miocene Climate Optimum,the ⁸⁷Sr/⁸⁶Sr variation was relatively small（±0.00002）,while δ¹³C shows a significant positive shift.This result suggests that the carbon cycle disturbances,caused by volcanic degassing,led to a small warming trend in climate,and the burial of organic carbon became the primary carbon sink,instead of chemical weathering.This carbon cycle pattern may have continued until the end of the Middle Miocene Climate Transition.The short-term climate changes reflected by δ¹⁸O and the simultaneous ⁸⁷Sr/⁸⁶Sr variations suggest that glacial events may have a feedback effect on the continental weathering process.The second part of this study analyzed major and trace element,mineralogical,Sr isotopic,and lithium（Li）isotopic compositions from detrital sedimentary rock samples from the Lühe Basin,located on the southeastern margin of the Tibetan Plateau,to obtain geochemical indicators of weathering products from approximately 35 million to 26 million years ago.These samples may record changes in the continental weathering intensity in a tectonically active area in the Tibetan Plateau during the Eocene-Oligocene transition.The results show that the chemical index of alteration,Li abundance,and Sr isotopic compositions all rose up to a maximum during the EoceneOligocene transition,indicating an intensification of chemical weathering.However,the Li isotopic compositions remained relatively stable（between-7.7‰ to-9.0‰）during this transition period compared to the whole section（between-14.6‰to-5.0‰）.It suggests an incongruent weathering pattern with a moderate ratio of chemical weathering rate to physical erosion rate.Large amounts of clay minerals formed in the weathering regime with abundant fresh mineral supply and sufficient time for waterrock interaction.The enhanced chemical weathering during the Eocene-Oligocene transition indicates that the intensification of continental weathering controlled the long-term cooling of the Cenozoic era.The incongruent weathering pattern in the Lühe Basin during the Eocene-Oligocene transition may be related to changes in the erosion rate and hydrology in the Tibetan Plateau,as the global climate shifted from warmhouse to coolhouse.