Beryllium And Potassium Isotope Geochemistry During Continental Weathering Process

To our humankind's knowledge,Earth is currently the only habitable planet.Earlier studies suggested that the rate of CO2 consumption by continental weathering is controlled by climate,and therefore it is a thermostat that regulates the Earth's climate and habitability.This seems to be consistent with the Be isotopic record of Cenozoic seawater.However,modern observations and other weathering proxies do not support this hypothesis.This contradiction is well-known as the"Be isotope weathering paradox"problem.One reason this cannot be solved is that the previous understanding of the Be cycle,especially the behavior of Be during the coastal process,is not accurate.Another reason is that all existed weathering proxies cannot be directly interpreted as variations in the continental silicate weathering process,and we need reliable silicate weathering proxies.In order to solve the'Be isotope weathering paradox'problem and to explore new silicate weathering proxies,on the one hand,this thesis tries to build a more accurate geochemical numerical model to estimate how the ¹⁰Be/⁹Be ratio of seawater respond to changes in continental weathering and erosion rates,and reinterpreted the records of the Late Cenozoic seawater ¹⁰Be/⁹Be.On the other hand,This study systematically investigated the K isotope composition of samples from the large river basins and small granite basins in China,and calculated modern K cycle fluxes and the sensitivity of seawater K isotope ratios to the continental weathering process,in order to test the reliability of K isotopes as a silicate weathering proxy.The main conclusions are listed below:1.This study built a new Be cycle model(Be-LI2017 model)by modeling the behavior of Be during continental weathering and coastal scavenging process.The Be-LI2017 model can reproduce the ¹⁰Be/⁹Be ratio of the modern ocean more precisely than the previous models,which indicate that the model can more accurately describe the Be cycle than the previous models.The calculation results of the Be-LI2017 model show that when the erosion rate is higher than 20t/km2/yr,the increase in Be weathering flux is offset by the enhanced estuarine scavenging,which makes the seawater ¹⁰Be/⁹Be insensitive to changes in continental denudation rate.When the erosion rate is lower than 20 t/km²/yr,as denudation increases,the increase in Be weathering flux cannot be offset by the enhanced estuarine scavenging due to the small physical erosion flux.The seawater ¹⁰Be/⁹Be ratio is negatively correlated with the continental denudation rate.2.The interpretation of ¹⁰Be/⁹Be records of the late Cenozoic seawater by the Be-LI2017model shows that due to the large uncertainty in seawater records,the uncertainty of the continental erosion and weathering rate estimated by the model is extremely huge,and its range is similar to the estimations by previous researchers using other weathering and denudation proxies.This indicates that all the proxies are consistent with the idea that the erosion and weathering rate of the felsic continent was increasing during the late Cenozoic.The'Be isotope weathering paradox'is thus solved.This confirms that the weathering of the felsic continent is the driver of climate change during orogenesis.Therefore,Earth's habitability cannot be maintained without other thermostats.In addition,as this thesis proves that the seawater ¹⁰Be/⁹Be is not sensitive to changes in the weathering and erosion process of the continent,now all the existing weathering proxies are ambiguous.Therefore,new reliable weathering proxies are urgently needed.3.The time-series?⁴¹K data of the Yangtze River sampled at Nanjing range from–0.40‰to–0.34‰excluding the samples in the early-flood season,which is very close to the annual average?⁴¹K(-0.41±0.07‰).This indicates that river waters sampled at any time,apart from the early part of the flood season,are representative of the annual riverine K isotopic composition.In small granite basins,the?⁴¹K of the coarse fraction of river sediment varies from–0.68‰to–0.48‰,which is very close to the?⁴¹K of bedrock.Compared to the coarse fraction,the?41K value of clay fraction is lower,ranging from–0.74‰to–0.58‰.Overall,the?⁴¹K value of river sediments is lower than the?⁴¹K(-0.28‰?-0.03‰)of river water in the granite watersheds.This shows that during the weathering process,heavier K preferentially enters the riverine dissolved loads,and the secondary solid weathering product preferentially incorporates lighter K.In order to estimate the K isotope fractionation factor during chemical weathering,three approaches are used.The K isotope fractionation factor estimated using paired water-clay samples and batch fractionation model is–0.55±0.29‰.The K isotope fractionation factor obtained using the batch fractionation model and the Rayleigh fractionation model are-0.91?-0.40‰and-0.79?-0.20‰,respectively.The results obtained by different calculation methods are very similar.4.The riverine dissolved?⁴¹K is negatively correlated with the silicate weathering intensity.Based on this,the average?⁴¹K of the global river runoff is estimated to be-0.22±0.04‰with the modern global average weathering intensity and the regression function between river dissolved?41K and weathering intensity.This provides important new information for the estimation of the fluxes associated with the modern geological K cycle.Combined with Monte Carlo calculations,the uncertainties of these fluxes are greatly reduced.The K flux released by the hydrothermal fluid and that taken by the diagenesis of marine sediment are precisely estimated to be 5-15.8×10¹²g/yr and 41-52×10¹²g/yr,respectively.At the same time,the K isotope fractionation factor of marine sedimentary diagenesis is estimated to be-0.6?-0.3‰.Based on these new K geochemistry data,this research further calculates how seawater?⁴¹K would respond to changes in continental weathering and other K cycle process.The results show that changes in continental weathering,especially changes in continental weathering driven by orogeny,can significantly change the K isotope value of seawater.Therefore,the K isotope ratios of paleo-seawater can be used as a new recorder for the ancient continental weathering process...