Study On Lithogenic Flux Indicated By Dissolved Thorium Isotopes In Qinghai Lake Area And Its Environmental Significance

The Qinghai Lake basin,located in the northeastern part of the Qinghai-Tibet Plateau,has unique geographical and climatic characteristics.Qinghai Lake has unique advantages for studying the characteristics of budget and expenditure.This study evaluates the lithogenic flux input contribution of the lake based on the dissolved thorium isotope（Th）information in the water body,and explores its response potential to environmental changes,so as to provide more basic research data for the subsequent assessment of whether the U-series imbalance in lake sediments can provide the paleoclimate history of the basin.First,the concentrations of dissolved ²³²Th and ²³⁰Th were determined from 59 water samples collected from Qinghai Lake and its exogenous supply rivers;secondly,the scavenging residence time of Th in lake water was calculated using the ²³⁰Th concentration;Then,the flux of dissolved ²³²Th was calculated by combining the measured value of dissolved ²³²Th concentration and the scavenging residence time;Finally,using the ²³²Th content in the lithogenic clastic material and the solubility fraction of Th in the preliminary particulate matter data in the Qaidam area of Qinghai,the input flux of the lithogenic material in the lake water was calculated.The experimental and calculation results show that:（1）At the horizontal spatial scale on the surface of Qinghai Lake:there are significant differences in the distribution of dissolved thorium isotopes.The dissolved ²³²Th concentration,dissolved ²³²Th flux and lithogenic material flux all show the characteristics of high in the northwest and low in the southeast,especially at the entrances of the Buha and Shaliu Rivers.Nearby,the input of different lithogenic clastic material sources was confirmed.When using the Th solubility of 1%of the particles,the obtained fluxes of lithogenic in the surface lake water ranged from 0.03 to 25.25 g/m²/yr,which is consistent with the fluxes of settled particles in previous studies.In addition,the scavenging residence time of ²³⁰Th in the southeast lake basin is higher than that in the northwest lake basin,so the particle removal rate in the northwest lake basin is faster than that in the southeast lake basin,that is,the sedimentation rate in the southeast lake area is slower,and the resolution of the sedimentary record cores in the southeast lake area can be judged about 7 years.（2）On the spatial scale of the Qinghai Lake depth profile:the dissolved ²³²Th concentration increased slowly with depth,and the bottom ²³²Th concentration was higher in all three profiles.These trends suggest that additional ²³²Th entering the water column at the bottom of the lake may be a contribution from lake bottom sediments.The dissolved²³²Th flux and lithogenic flux increased with depth,and the average dissolved ²³²Th flux and lithogenic flux in the northwest of the lake basin were significantly higher than those in the southeast of the lake basin,which was related to the input of exogenous feeding rivers and the effect of hydraulic sorting.And from the ²³⁰Th scavenging residence time information of each station,it can be concluded that there are sedimentary record resolutions of about 7 years,4 years and 0.7 years in the southeast,northwest and southwest of Qinghai Lake,respectively.（3）Combining the thorium isotope information of the supply river and Qinghai Lake,it is concluded that:When a large number of exogenous feeding rivers were mixed into the northwest lake basin,the content of ²³²Th in the lake water and the corresponding lithogenic fluxes were mainly affected by the type and content of particles in the Buha and Shaliu rivers.In contrast,in the southern basin area with limited recharge rivers,the ²³²Th content of lake water far from the estuary was mainly affected by atmospheric dust.In addition,according to the ²³⁰Th normalization method(combining ²³²Th andτ_Th),the Buha and Shaliu Rivers located in the northwestern lake basin contributed about 90%of the lithogenic clastic flux to the lake.The lithogenic flux in the southeastern part of the lake is mainly controlled by the dust flux,with a value of about 0.109 g/m²/yr,while the higher lithogenic flux at the lake bottom may be due to the accumulation of sinking particulate matter and the resuspension of bottom sediments in September.This study confirms the utility of long-lived thorium isotopes in quantifying lithogenic input based on dissolved thorium content in lake water and provides some sedimentary resolution information for the Qinghai Lake sedimentary record.