Source And Deposition Process Of Inorganic Carbon From Paleolake In The Closed Basins

According to the IPCC Fifth Assessment Report,anthropogenic greenhouse gas emissions(e.g.,carbon dioxide),are most likely the main cause of modern global warming.Understanding and identifying the processes and mechanisms of the global carbon cycle is important to achieve a goal of limiting global warming to 1.5°C by the end of the century.Global closed basins occupying almost one-fifth of the world's land area spatially concur with arid and semiarid areas.Its carbon cycle is an important part of the global terrestrial carbon cycle.In recent years,negative CO₂fluxes have been found in these regions.Additionally,it is also known that carbonate enrichments exist in lake sediments of inland river basins.Whether there is a link between them is unclear.In this paper,three paleolakes and their basins of the Hexi Corridor,Zhuyeze,Yanchi,and Huahai are the research objects.The traditional paleoenvironmental research methods are combined with the study of inorganic carbon sinks.And the surface soil,groundwater,river water,and lake water from basins of Zhuyeze,Yanchi,Huahai in the Hexi Corridor are collected.Combined with our team's research of paleolake sediments in the study area,we conduct a comprehensive analysis for indicators related to inorganic carbon.The goals of this paper are as follows:establishing databases of regional inorganic carbon,revealing the basin-wide distribution of inorganic carbon,clarifying the source and deposition process of inorganic carbon from paleolake sediments in the Hexi Corridor during the Holocene,and exploring the relationship between inorganic carbon sink and environmental change.This study can provide scientific support for the assessment of carbon sink and environmental change in closed basins.The main conclusions are as follows:(1)The average inorganic carbon content of 0-20 cm topsoil in the Hexi Corridor is 11.2 g·kg^-1.Affected by differences in the internal climate of the basin,the inorganic carbon content of the topsoil tends to decrease gradually as the altitude decreases.The dissolved inorganic carbon(DIC)content of the closed basin in the Hexi Corridor ranges from 14.53 to 2463 mg/l,and the DIC content of surface water and shallow groundwater increases from upstream to downstream.This feature may be related to the strong evaporation in the basin.The Holocene average inorganic carbon content in Inoze,Yanchi and Huahai is 5.21%,0.70%and 2.88%respectively.The Holocene total inorganic carbon content in Zhuyeze,Yanchi and Huahai is 0.318 Pg,0.003 Pg and 0.160 Pg respectively,and the deposition rate is41.15 g C m^-2 yr^-1?2.40 g C m^-2 yr^-1and 29.92 g C m^-2 yr^-1 respectively.During the Holocene,the deposition rate of inorganic carbon in paleolake of the closed basins in the Hexi Corridor was much higher than that of organic carbon.(2)The groundwater of the closed basin in the Hexi Corridor is mainly affected by the atmospheric precipitation,melted ice and snow in the Qilian Mountains.The shallow groundwater in the middle and lower reaches is also supplied by river leakage and agricultural irrigation.There are mutual transformations between groundwater and surface water in the basin.The DIC age of groundwater in the closed basins of the Hexi Corridor generally gradually increases from upstream to downstream.The shallow groundwater DIC from river leakage and irrigation infiltration is relatively young,and its DIC age is on the centennial or millennial scale.The age of deep groundwater and some shallow groundwater downstream are close to the Last Glacial Maximum and the Middle Holocene,and its replenishment source and renewal rate are closely related to paleoclimatic changes.The contribution rate of soil CO₂ in the unsaturated zone to the shallow groundwater DIC in the study area is as high as about70%,and the input carbon source of groundwater may be soil mainly.The results show that the CO₂ absorbed by the saline-alkali soil enters the groundwater in the Hexi Corridor,at the same time,it is slowly transported downstream along the direction of the groundwater flow until deposited in the terminal area of the basin.(3)The grain size of surface soil in the upper reaches of the closed basin in the Hexi Corridor is mainly affected by the hydrodynamics of the river.In the lower reaches,under the influence of modern climate aridity and human activities,most of the lower reaches of the basin dry up.The sedimentation of the river no longer works and is adjacent to the desert,which is mainly manifested by near-source wind deposition.The rivers and wind in the study area jointly affect the materials in the watershed,thereby affecting the transportation methods and depositional environment of sediments.The climate of the river basin in the Hexi Corridor is relatively dry,and the content of detrital minerals is much higher than that of other mineral types.The content of carbonate minerals gradually decreases from upstream to downstream in the basin.Affected by temperature and precipitation,the weathering intensity of the river basin in the Hexi Corridor increases from west to east,while the difference in chemical weathering and erosion between the upper and lower reaches of the basin is relatively small.The paleolake sediments in the Hexi Corridor are mainly eolian sediments,shallow lacustrine facies and lacustrine facies sediments.Inorganic carbon is mainly deposited in the still water environment with a high lake level and weak hydrodynamic force,and the accumulation rate of inorganic carbon in the eolian deposition is low.(4)In the Early and Mid Holocene,precipitation increased in the river basins in the Hexi Corridor,and caused the lake water level to rise and the inorganic carbon content increased.Since the Late Holocene,climatic drought caused the lake to shrink and the inorganic carbon content gradually decreased.The inorganic carbon deposition of ancient lakes in the Hexi Corridor is mainly related to the changes in lake water level under the combined influence of the East Asian summer monsoon and the westerly wind.