Environmental And Organic Carbon Burial Changes Recorded By The Sediments Of Qilu Lake Since The Holocene

Since the mid-19th century,the global climate system has undergone profound changes,which has aroused widespread concern about climate change.The central Yunnan region,located in the transitional region between the Indian Ocean monsoon and the Asian summer monsoon,is sensitive to climate and environment changes and is an ideal region for paleoclimate and environment reconstruction.Based on the analysis of environmental proxy indicators such as geochemical indicators and grain size parameters of Qilu Lake Sediments in Central Yunnan,combined with "depth age" model,wavelet analysis and end element model,the Qilu Lake Basin is reconstructed to be about 11 Ka cal BP.The main driving mechanism of climate events,environment and carbon burial evolution recorded in Qilu Lake since the Holocene is discussed.The main results are as follows:(1)The environmental evolution history of Qilu Lake since Holocene was reconstructed according to the variation characteristics of environmental proxies in the sediments of Qilu Lake.The first stage was 445-281cm(11000-6000 cal.a BP): this stage presented a climate signal from dry and cold to warm and wet,which was in line with the transition state from the end of YD event to the early Holocene,during which the burial rate of organic carbon was relatively low.Second stage 279~51cm(6000~500cal.a BP): This period corresponds to the transition from Holocene Megatherma to late Holocene.Before 5000 cal.a BP,the vegetation coverage,air temperature and precipitation all reached the highest values since Holocene.After 5000 cal.a BP,the environmental proxies indicated that the primary productivity decreased,and the watershed transport capacity and chemical weathering level decreased.The climate changed to warm and dry,and the burial rate of organic carbon increased significantly in this stage.The third stage: 51~0cm(500 cal.a BP~ present),at this stage,human activities are more intense,and the reasons for the fluctuation of environmental proxy index are more complex,which may be caused by human regulation of hydrological balance,and the burial rate of organic carbon is also greatly increased.Four extreme climatic events have been identified since the Holocene,which are 8.2 ka event,5.2 ka event,western Zhou Cold Period(about 2.3 ka BP)and Little Ice Age of Ming and Qing Dynasties(about 0.4 ka BP).There are about 5 layers of snail shells in the core borehole,which correspond well with the climatic abrupt change events recorded by environmental proxy index.The causes of these abrupt change events are related to solar activity,atmospheric circulation and topographic factors.(2)By Qilu lake substitute index analysis and comparison,this paper discusses the Qilu lake since Holocene carbon buried drive mechanism,the first stage,the climate changes to the warm wet trend gradually,terrigenous and endogenous climate warm wet reason is growing at the same time,this period of organic carbon in lake sediments is internal and external source mixed input,exogenous input is given priority to,The burial rate of organic carbon in this stage was mainly affected by climatic conditions.In the second stage,the climate reached the optimum stage,during which the endogenous TN and TOC showed a negative correlation with OCBR,while the exogenous TN and TOC showed a positive correlation with OCBR,but the correlation between exogenous TN and TOC and OCBR was still high.The source of organic matter in this period was mainly exogenous,and the burial rate of organic carbon in this period was relatively high.In this stage,although human activities,but the impact on organic carbon burial rate is relatively small,the main impact on organic carbon burial is climate conditions.Third stage human activities is relatively strong,the use of a large number of phosphate fertilizer as well as the excessive use of lakes increase lake nutrient elements,combined with the agricultural activity changes of vegetation,cutting down vegetation increased soil erosion,this period of organic carbon source is relatively complex,organic carbon during this period buried under the influence of phosphorus and soil erosion,the rate of organic carbon buried a boost during this period,In this stage,the main influence on organic carbon burial is human activities.(3)The periodic change characteristics of the environmental proxy indicators in Qilu Lake since Holocene show that the proxy indicators in Qilu Lake in Holocene mainly have 2000 a,1000a,500 a and 250 a cycles.The main oscillation period of Qilu Lake in Holocene is 1000 a and 500 a,and runs through the whole Holocene,at 6000 cal a BP~ is obvious so far;The 250 a and 125 a cycles were obvious in the middle and late Holocene.(4)through the Qilu lake regional climate change characteristic and the solar radiation and the related research results,analyzes the driving mechanism,the results are as follows: after the YD event to the middle Holocene,the sun is strong,high solar radiation and temperature rise,will drive the intertropical convergence zone to strengthen and move to the north,and enhance the southwest monsoon in Asia;In the middle Holocene,the solar activity and solar radiation decreased,and the water vapor transport over The Yunnan Plateau moved southward slowly.Although the Indian summer monsoon weakened,the high SST and high sea level may have generated abundant water vapor and sent it to the Yunnan Plateau through the Southwest Asian monsoon,making this stage the most suitable period of the Holocene.Late Holocene solar activity continues to weaken,solar radiation declining ITCZ continued south,also continues to weaken,Indian summer monsoon rainfall,southwest of human activities on the Asian monsoon region over the past 2000 years,but the periodical change of climate environment in southwest Asia is mainly affected by low latitudes in the northern hemisphere the sun radiation and interaction control of the event,Anthropogenic activities can also exacerbate climate fluctuations.