Organic Carbon Burial And Climate Change In The Early-Middle Holocene Of Xingyun Lake In Central Yunnan

As global warming and carbon emissions become more prominent,research on the carbon cycle has attracted considerable attention.The early-mid Holocene is a typical warming period in the history geology,which is similar to the present global warming.The systematic study of the carbon sources and sinks in the early-mid Holocene will help to understand the current changes of global carbon cycle.Lake is an important part of the global carbon cycle,and affects the migration,transformation and burial of organic carbon in aquatic and terrestrial ecosystems.Therefore,it will provide the scientific support for the study of global carbon cycle and its influencing factors by understanding and reconstructing lake organic carbon burial in the early-mid Holocene.Based on the analysis of the geochemical and physical proxies and organic carbon accumulation rate in the Xingyun Lake sediment,which is located in the monsoon region of southwest China,the carbon burial characteristics and influencing factors in the early-mid Holocene of Xingyun Lake have been discussed combined with correlation analysis and wavelet analysis.The main conclusions are as follows:1.The organic carbon accumulation rate(OCAR)varies from 14 g m^-2 a^-1 to 69 g m^-2 a^-1with an average of 42 g m^-2 a^-1 between 8900 cal.a BP and 2000 cal.a BP.Through a binary model and the quantitative analysis,it is shown that the sources of organic carbon of Xingyun Lake was mainly derived from exogenous organic carbon during the 8900-4700 cal.a BP,OCAR in this period is kept in high range with an average of 48 g m^-2 a^-1,and the fluctuation trend is consistent with that of the total organic carbon(TOC)content.From 4700 cal.a BP to 2000 cal.a BP,the results indicate a decrease OCAR in the lake catchment,the organic carbon in lake sediments is also dominated by exogenous input in 4700 to 3800 cal.a BP,while organic carbon in Xingyun Lake was mainly imported from internal sources since 3800 cal.a BP,the value of OCAR has decreased significantly.And there is a significantly positive correlation between OCAR and chemical proxies(e.g.TOC,TN,C/N,C/P),redness(a*)and yellowness(b*).2.Based on the data normalization processing of climate proxies(e.g.,TOC,C/N,chroma and grain-size)from Xingyun Lake,the results of the power spectrum analysis and the Morlet wavelet transform were obtained.It is shown that 1000-a quasi-periodic signal were obvious in the early-mid Holocene.Through the analysis of quasi-periodic signal and the charactertics of environmental proxies,the variation of lake catchment environment could be divided into two stages.Zone 1(564?405cm)corresponds to early Holocene and the optimum period.From 8900 to 4700 cal a BP,the higher values of TOC and TN content and the C/N ratio are probably induced by enhancement of biological productions,and the organic matter was derived from terrestrial plants.The strong surface runoff and hydrodynamic of lakes catchment indicate the warm-humid climate,and it causes the low Ca CO₃content and high?lf,and the sediment of Xingyun lake is mainly composed of silt.Zone 2(405?277cm)corresponds to the period of mid-to late-Holocene.From4700 cal.a BP to 2000 cal.a BP,the gradually decreasing of TOC and TN content,as well as the low C/N values,suggest the decreasing precipitation and arid climate.the fine grain-size,high brightness(L*),and decreased a*,b*and?lf indicate there was the weak hydrodynamic force and surface runoff of the catchment,and the decreased biological productivity and precipitation in the lake catchment occurred since 3800 cal.a BP.The climate change into warm and dry.3.Moreover,the research on the influence factors of climate change and OCAR of Xingyun Lake reveal that the Indian summer monsoon(ISM)intensified with an increase by low-latitude solar insolation(LSI)during the early-mid Holocene,and the OCAR was mainly controlled by climate change.During 8900-4700 cal.a BP,ISM intensified brought the warm-humid climate,the increased biological productivity and the OC input of the lake catchment,OCAR is larger,except for the period of 7400?6600 cal.a BP.Since 4700 cal.a BP,ISM has weakened and the OCAR has reduced with the decrease of temperature and humidity in the lake basin,especially since 3800cal.a BP,the source of organic carbon of lake sediments changed from exogenous input into endogenous input,OCAR decreased significantly.