Carbon Source Analysis And Its Carbon Cycle Significance Of Poxin Underground River System

Since the Industrial Revolution,the global atmospheric greenhouse gas concentration has risen sharply,which leads to global warming,resulting in a series of environmental problems.Global warming has attracted the attention of scientists over the world.The reason for global warming is that human activities lead to the increase of atmospheric greenhouse gases.Therefore,the reduction of atmospheric CO₂ and the storage of carbon sequestration technology have become one of the environmental problems that scientists need to solve urgently.However,when scientists research the carbon cycle,they find that the global carbon sources and sinks are not balanced in revenue and expenditure,and a considerable portion of carbon sinks are lost.This part of carbon sinks is called missing sink.Carbon sinks produced by neglecting geological processes may be an important part of carbon omission.Current studies have shown that 99.5%of the global carbon pool of carbonate rocks is actively participating in the carbon cycle.Taking the earth system science as the guiding ideology and considering the water-gas-solid interaction model,it is proved that karstification can produce net carbon sinks.Therefore,it is of great significance to study the mechanism of karst carbon cycle for global carbon cycle.In this study,the karst underground river system in southern China was taken as the research object.During March,June,October and December,2018,the main recharge tributaries of underground river pipelines,underground river skylights and springs were monitored and sampled.The pH value,water temperature,electrical conductivity,dissolved oxygen and other indicators of water body were tested in situ.Water samples were collected for analysis of cations,dissolved organic carbon?DIC?and its isotopes,dissolved organic carbon?DOC?and its isotopes,particulate organic carbon?POC?and its isotopes,in order to obtain hydrogeochemical and carbon cycle information of Poxin underground river system and understand typical karst underground.The conceptual model of carbon cycle in the karst underground river system is established based on the characteristics of river carbon cycle,which provides some basic theories for the establishment of carbon cycle model and mathematical model of karst underground river system.Our results show that the runoff in the study area is very sensitive to the monsoon climate change,and this difference is obvious in different seasons.The water samples of surface river water,spring water and underground river skylight were belonged to weak alkaline,and their hydrochemical characteristics are obviously different.The hydrochemical types are HCO₃-Ca species.DIC,POC and their isotopes varied sensitive to the real-time runoff.Besides flow control,DOC and its isotopes are also affected by other factors,and have obvious seasonal variations.DOC is the main organic carbon in the water samples in the study area,and POC content is less.Hydrochemical ions in the study area are mainly controlled by atmospheric precipitation,silicate weathering and carbonate dissolution.Carbonate dissolution is the main source of water solutes in the basin,accounting for more than 75%.DIC mainly comes from the dissolution of carbonate and the contribution of CO₂ in soil.The contribution of H₂CO₃ dissolving carbonate rock to HCO₃^-is the highest in this basin,followed by exogenous acid dissolving carbonate rock.The proportion of HCO₃^-produced by silicate weathering is relatively low.Under the influence of“fertilization effect”,there is a positive correlation between DOC and DIC.Due to the influence of hydrological conditions and metabolism of aquatic plants,the correlation between?¹³C_DOC and?¹³C_POC is obvious in spring and summer,but almost not in autumn and winter.Owing to the biological activity difference in Poxin basin,the DOC ratio of autochthonous and exogenous sources is different among different sampling sites due to their location,hydrological,topographic and geological conditions.The ratio of POC to POC is affected by the activities of aquatic plants,discharge and vegetation cover in the basin.Total organic carbon?TOC?is about 9%of HCO₃^-output.It also proves that the organic chemical processes in karst environment could not be ignored,and only inorganic processes are considered to underestimate the carbon output rate and carbon consumption in Karst environment.At the same time,it is also proved that there is still a considerable part of organic carbon sink in the Poxin underground river system,rather than a simple process of inorganic carbon consumption.According to the results of this study and previous studies,we established a carbon cycle conceptual model in the karst surface zone of the Poxin underground river basin,which reveals the mechanism of carbon cycle in the basin.We believe that in the karst underground river system,the endogenous part of organic carbon is also an important carbon sink.The model also provides some scientific basis for the theory of“carbon sink in carbonate weathering”...