Chemical Weathering Rate And Karst Carbon Sink Of Typical Dolomite Catchment In Subtropical Area

The greenhouse gas emission dominated by CO₂ by human activities is the main reason leading to the global warming,which threatens the survival of humankind.Climate change is not only a simple scientific problem,but an important international political issue.The chemical weathering of carbonate rocks consumes atmosphere CO₂ and form karst carbon sink.Therefore,the chemical weathering rate and its karst carbon sink effect from carbonate rocks weathering have been attracting the attention of researchers at home and abroad.It is found by the researches in the past that the carbon sink from the chemical weathering of carbonate rocks is an important part of terrestrial carbon sink.However,the studies in the past mainly focused on karst areas dominated by limestone,and insufficient attention has been paid to the chemical weathering rate and its karst carbon sink from subtropical dolomite catchments.Theoretically,the dissolution rate of dolomite is lower than that of limestone,however,the solubility of dolomite is stronger than that of limestone in natural conditions.In China,the distribution area of dolomite or strata with dolomite account for a big percentage of that of carbonate rocks,and the distribution of dolomite is as popular as limestone in the world;therefore,more attention should be paid to the chemical weathering rate and its karst carbon sink effect of dolomite catchments.The Shanmuhe Catchment and its sub-catchment Huangzhouhe Catchment inside the Shibing Dolomite Karst World Natural Heritage Property are typical representatives of subtropical dolomite catchments.Shanmuhe Catchment covers 242.00 km2,192.34 km2 of which are dolomite,accounting for 80%,and Huangzhouhe Catchment covers 49.19km2,45.27 km2 of which are Cambrian pure dolomite or relatively pure dolomite,accounting for 92% of the total area,which make the catchment a reference site in China or even the whole world to study subtropical dolomite karst.In order to study the hydrochemistry features of subtropical dolomite karst area,estimate the chemical weathering rate and its karst carbon sink flux,enrich the karst carbon sink study types in China and improve the accuracy on estimating the karst carbon sink potential,Shanmuhe and Huangzhouhe catchments were chosen as the study area,petrologic features,hydrogeochemical features and their spatiotemporal dynamic variations of both the surface and underground waters of the Shanmuhe Catchment were studied,hydrogeochemical characteristics of subtropical dolomite karst area were revealed.After that,the Huangzhouhe Catchment were selected for further study,to monitor the hydrochemistry and real-time monitoring on the discharge in the outlet of the catchment,and finally,the chemical weathering rate and karst carbon sink flux of the typical subtropical dolomite catchment were calculated using solute load method.Conclusions were drawn as follow.?1?The Shanmuhe Catchment especially Huangzhouhe Catchment inside Shibing Karst World Natural Heritage Property are typical representatives of subtropical dolomite catchments with outcropping dolomite in Qingxudong formation,Shilengshui formation of Cambrian and Loushanguan formation of Cambrian-Ordovician.The rock in Qingxudong formation in the study area is composed of both dolomite and limestone:the content of CaO is between 16.56%⁴7.20%,most are between 28.04%³2.05%;that of MgO is between 0.92%²1.71%,most are between 18.65%¹9.80%;Ca/Mg ratio is between 0.97³6.65,most are between 0.97¹.42.The rock in Shilengshui formation in the area is dominated by dolomite,accompanied by small part of calcite dolomite: the content of CaO is between 22.76%⁵3.39%,most are 27.36%³2.95%;that of MgO is between 1.25%²1.62%,most are 16.26%²1.62%;Ca/Mg ratio is between 0.99³0.51,most are 0.99¹.64.The rock in Loushanguan formation in the area is composed of pure dolomite: the content of CaO is between 25.82%³6.75%,most are 25.82%³0.90%;that of MgO is between 15.62%²1.70%,most are19.44%²1.70%;Ca/Mg ratio is between 0.97¹.68,most are 0.97¹.09.Based on the structures,the dolomite which is the dominant rock types in Shanmuhe and Huangzhouhe catchments are divided into grain dolomite and crystalline dolomite,andthe limestone into intraclast limestone and crystalline limestone etc.?2?It was found by mercury injection and permeability tests that,the mercury injection volume and the porosity of the dolomite in the study area are bigger than those of limestone,indicating that the water storage capacity in the pore system in the dolomite is better than that in the limestone;the area of the pore system of the dolomite is bigger than that of the limestone,indicating that the contact area between rock and mercury of the dolomite is bigger than that of the limestone,namely the area for water-rock interaction is bigger in dolomite;although both the medium and average pore diameters of the dolomite are smaller than those of limestone,the threshhold pressure of the dolomite is smaller than that of limestone,which means the porosity system of the dolomite is more homogeneous than that of the limestone,and the overall permeability of the dolomite is better,which can also be shown through the smaller tortuosity in the dolomite.All these features demonstrate that the lithology features of the dolomite is good for karst process,good for the seepage of groundwater and the process of water-rock interaction,and the dolomite presents entire chemical dissolution features,leading to the complete solution and solubility equilibrium of dolomite.?3?Controlled by the geology and lithology,the water in the small non-karst area in northern Shanmuhe Catchment is HCO₃-Ca type and that in the dolomite area is HCO₃-Ca·Mg type,dominated by Ca²⁺,Mg²⁺,and HCO₃-.Compared with the impure dolomite catchment,the concentration of HCO₃-and Mg²⁺/Ca²⁺ of pure dolomite catchment is higher: for surface water,the average concentration of HCO₃-is 4.3mmol/L,while that in impure dolomite area is 3.7 mmol/L;for the underground water,the average concentration of HCO₃-is 5.5 mmol/L,while that in impure dolomite area is5.2 mmol/L.Although the dissolution rate of dolomite is slower than that of limestone,more HCO₃-could be dissolved out because of its higher solubility and enough water-rock interaction time.The formation and variation of both the surface and underground water in the study area are controlled by the weathering of carbonate rocks,and human activities have little impact on the hydrochemistry.The dissolved inorganic carbon of the underground water in the study area mainly comes from the CO₂ from the oxidation of organic matter decomposition and the dissolution of the carbonate rocks,which contribute to the DIC in 50.9% and 49.1% respectively.?4?The saturation index including both SIc and SId of the allogenic water from the northern part of the Shanmuhe Catchment are below zero,which means the allogenicwater has strong dissolution ability on the carbonate rocks.When the water enters into the dolomite karst area,the concentrations of Ca²⁺,Mg²⁺ and HCO₃-increased evidently,and the saturation index including both SIc and SId are over zero,reaching oversaturated state.It is indicated that although the dissolution rate of dolomite is slower,the solubility equilibrium are attained in natural conditions.The reason may be that,because the dolomite karst process and its aquosity is more homogeneous,it leads to diffuse aquifer,bigger contacting area between rock and water,longer time for water-rock-atmosphere interaction and more sufficient chemical reaction in dolomite catchment scale.In the end,both the surface and underground water in dolomite catchment can reach solubility equilibrium.?5?The dissolved inorganic carbon,Ca²⁺ and Mg²⁺ in subtropical typical dolomite catchments exhibit chemostatic behavior.In 2015,the coefficient of variations of the monthly average rainfall and discharge in Huangzhouhe dolomite Catchment are 0.7 to1.133,which are much higher than those of the concentrations of Ca²⁺,Mg²⁺ and HCO₃-?0.1 to 0.2?,which indicates that the subtropical dolomite catchment shows chemostatic behavior in monthly scale: their stream concentrations of weathering products such as Ca²⁺,Mg²⁺ and HCO₃-typically vary slightly while the discharge varies by several orders of magnitude.This behavior ensures the accurancy of karst carbon sink flux calculation by low-frequency sample collection on HCO₃-and high-frequency monitoring on the discharge in typical dolomite catchments.?6?The mass concentration of Ca²⁺ and Mg²⁺ in Huangzhouhe Catchment were used to calculate the chemical weathering flux and chemical weathering rate.It is found that the chemical weathering flux and chemical weathering rate of Huangzhouhe Catchment in the hydrologic year from March 2015 to March 2016 are 119.65 t·km-2·a-1 and 46mm/ka respectively,which are high.?7?Solute load method was used to estimate the karst carbon sink flux in the typical dolomite catchment.It is found that the karst carbon sink flux of Huangzhouhe Catchment in the hydrologic year from March 2015 to March 2016 is 57.58tCO₂·km-2·a-1,which is evidently very high and tremendous.It may be because of the high solubility of dolomite and the plentiful time in catchment scale for the water-rock-atmosphere interaction to reach solubility equilibrium.Therefore,the karst carbon sink in subtropical dolomite catchments cannot be ignored.In a word,the research on chemical weathering rate and karst carbon sink flux insubtropical typical dolomite catchments is deficient.It can be said that the chemical weathering rate and karst carbon sink flux in subtropical typical relatively pure dolomite area were monitored and calculated for the first time in this thesis.It is found that high solubility and low dissolution rate are the two main factors affecting the chemical weathering intensity of dolomite.Theoretically,the dissolution rate of dolomite is lower than that of limestone,however,in natural conditions of the catchment,the water-rock-atmosphere interaction can hold enough time to make the CaMg?CO3?2-CO₂-H2 O system reach solubility equilibrium because of the special lithological structure and hydrogeological structure of the dolomite,as a result,the dissolution intensity is determined by the intensity of water circle and solubility.Therefore,the higher solubility of dolomite leads to the higher karst carbon sink,which means the chemical weathering of dolomite may play a significant role in global carbon cycle.The conclusions of this thesis get important break and innovation in the study of karst carbon sink in subtropical typical dolomite catchments,which provides new evidences for karst carbon sink calculation for China or even the whole world.