Regional Climate Change And Human Activities Since The Little Ice Age Recorded By Lacustrine Sediments In Huangmao Lake, Jiangxi

The global warming has accelerated significantly,it made the instability of the climate increased Since the Little Ice Age.There are many lakes in the middle and lower reaches of the Yangtze River,which contains abundant sedimentary records.It is an ideal place to rebuild the paleoclimate environment and records of the human activities.This paper took Huangmao Lake —a small lake in the middle and lower reaches of the Yangtze River as research object.The sediment cole HMT-1 from Huamgmao Lake was testing indicators of grain-size,element geochemistry organic carbon isotope,,TOC,pollen and others,based on accurate sediment chronology of 210Pb?137Cs and SCP?carbon particle?.By comparing the proxies to document and meteorological data,this paper systematically described the regional climate change recorded by lacustrine sediments over the past 240 years?particularly modern global warming?,flood events and the records of human activities.And the study draw the following conclusions:?1?The change characteristics of grain-size identify the periods between the depth which between70-37cm?1800-1900 AD?and the top which deposited at 20 cm?1970-2010AD?as the regional climate environment with unstable characteristics,and flood events occurred frequently at the periods.?2?According to the characteristics of the change of Rb/Sr??¹³C,we can divide the profile into four phases: 1769-1800 AD,1800-1900 AD,1900-1970 AD,1970-2010 AD.a.Phase 1?80-70 cm,1769-1800AD?: Rb/Sr increases rapidly,the ratio rises from 1.2 to 1.8,?¹³C shows a trend of increasing volatility,it indicates that at this period,the chemical weathering gradually weakened and the climate conditions deteriorated gradually.b.Phase 2?70-37 cm,1800-1900AD?: The values of Rb/Sr and ?¹³C are the highest values of the whole core,it was the cold phase which is not conducive to chemical weathering,and terrestrial plant growing.In the 1920 s,the floods of Huangmaotan catchment occurred frequently?such as 1823 AD,1831 AD,1839 AD,1869-1870 AD?,the climate of the region is characterized by instability.c.Phase 3?37-20 cm,1900-1970AD?: The values of Rb/Sr??¹³C reduced rapidly,and then they remained at a lower value,with the less volatile,the result reflected the rapid rising of the temperature after the Little Ice Age.d.Phase 4?20-3cm,1970-2010AD?: Rb/Sr is at a lower value,and ?¹³C is the lowest value of the whole core.the floods of Huangmao Lake occured frequently?such as 1983 AD,1988 AD,1998-1999 AD?.That indicates the region was in the condition of warm and wet,but the regional climate was in unstable condition.?3?The comparison of Rb/Sr and ?¹³C in the sediment cole HMT-1 Huangmao Lake with meteorological data of Nanchang since 1950 AD shows that,Rb/Sr has better response to regional rainfall in the warm and humid period recently,and ?¹³C has better response to regional temperature.?4?The grain size parameters?mean particle size,?coarse+sand?/clay?,elements characteristics?Zr/Rb,Zr/Fe,Ti/Rb ratio?,meteorological rainfall data,the index of drought and flood and document flood records has better corresponding relationship.The result indicates Zr/Rb,Zr/Fe and Ti/Rb ratios and?coarse silt +sand?/clay ratio,mean grain size are sensitive to flood-event deposits,especially Zr/Rb ratios.The flood deposition indexes also further confirm that 1820s-1870 s,1970s has been characterized by unstable climate and frequent flood events in the Huangmao Lake.?5?The pollen analysis showed that: the reduction of arbor pollen obviously in 1930 s,which related to outbreak of the Anti Japanese War;In the last 19950 s,the Pinus pollen and tree pollen decreased significantly,which on account of the steelmaking and deforestation;the broad-leaved trees of Salix pollen increased significantly in 1960 s,which related to the planted protection forests of willows;the increasing of tree pollen has related in 70 s and 90 s due to the afforestation policy;Cruciferae pollen increased observably and became the dominant species in the herb since 1970 s,which related to fine varieties introduced and the large-scale cultivation of oilseed rape.