Changes Of Organic Carbon Sinks From Closed Basins Since The Last Glacial Maximum And Quantitative Evaluation Of Human Impacts

The Fifth Assessment Report of IPCC indicates that greenhouse-gas emissions resulted from human activities contribute significantly to global warming since the middle of twentieth century.Therefore,global carbon cycle has received considerable attention from the international community.Closed basins occupying about one-fifth of the world's land area can substantially affect global carbon budgets.Those regions have relatively isolated and integrated carbon cycle,which makes them ideal candidates to explore the impact of climate change on regional carbon sinks.However,previous studies have not considered closed basins as an unique geographic unit,making the mechanism of carbon balance and the actual magnitude of organic carbon sinks in closed basins still unclear.Moreover,closed basins spatially concurring with arid/semi-arid climates have been intensively affected by human activity,but studies regarding quantitative evaluation of human impacts on closed basins have received little attention.This paper presented a series of regional and global studies on the significance of terminal lake carbon sink,the influence of carbon reservoir,the estimation of organic carbon sink since the LGM,the impact of human activities on organic carbon sink of the closed basins.Finally,we reconstructed organic carbon source of terminal lake,evaluated human impacts on terminal lake carbon sink,as well as proposed some strategies for closed basins carbon sequestration.Our results contribute to the understanding of long-term carbon storage mechanism in arid and closed basins,and also provide the basis for the sustainable restoration of arid degraded ecosystems and future mitigation policies towards global climate change.The main results are as follows:?1?Through organic geochemical proxies analysis of 230 topsoil samples and 12terminal lake sediments,we investigated organic carbon source of terminal lake in the closed Qilian Mountains basins.Then,we using organic geochemical proxies from 43terminal lakes around the world validated the regional results on the global scale.The results indicated that organic carbon in terminal lakes of closed basins containing abundant terrestrial input from the catchments was an effective indicator for implying carbon change of the whole basins.?2?The ¹⁴C/OSL dating results obtained from twelve lake records of the closed Qilian Mountains basins were used to explore the reservoir effect of those region,and the results showed that the reservoir effect was relatively weak in the closed Qilian Mountains basins.Global chronologies analysis was in accordance with the results of closed Qilian Mountains basins,indicating that the reservoir effect would not have changed the results of millennial-scale carbon evaluations of closed basins.?3?Based on 12 organic carbon records of the closed Qilian Mountains basins,the results suggested that 0.90 Pg C organic carbon was stored in terminal lakes of the closed Qilian Mountains basins since the LGM,with carbon accumulation rate of10.74 g C m^-22 yr^-11 in the modern times,6.31 g C m^-22 yr^-11 in the mid-Holocene and 1.76g C m^-22 yr^-11 in the LGM period.In order to accurately evaluate the carbon budget of closed basins,we defined a weight coefficient according to the geophysical-chemical characteristics of terminal lake water to represent proportion of organic carbon content in carbonate,sulfate and chlorine basins.The millennial-scale organic carbon storage in terminal lake records of 39 closed basins around the world were then reassigned in the proportion according to their lake types?carbonate:sulfate:chlorine=0.021:0.221:0.757?.The results showed that the global carbon storage of the terminal lakes was 80.56 Pg C since the LGM,and carbon accumulation rate in modern times was 17.54 g C m^-22 yr^-1,higher than 6.36 g C m^-22 yr^-11 in the mid-Holocene and 2.25 g C m^-22 yr^-11 in the LGM.It is obvious that carbon storage of terminal lakes in closed basins indicated an increasing trend since the LGM,with small carbon sinks during the LGM periods,relatively high carbon accumulation in the mid-Holocene,and abnormally high carbon deposition in the modern time.?4?Based on the studies regarding carbon source and storage estimation of terminal lake carbon sink from closed regions,we found that organic carbon sink in terminal lakes of closed basins was certainly not as significant as other carbon sinks,since its magnitude was much smaller than the peatland,soil and vegetation.However,it was still an important component of the terrestrial carbon budget,because the sink could effectively indicated the change of organic carbon storage of closed basins.On this basis,we further evaluated the influence of human activities on carbon sinks of global closed basins.This paper proposed that intense human interference around closed basins began with last 3000 yr.The rate of human impacts to organic carbon was estimated to be 22.79%,and human-induced soil organic carbon emissions in the past three thousand yrs amounted to 207 Pg C.