Sedimentary Records Of Climate Change In Huguangyan Maar Lake Over The Past Two Millennia

Huguangyan Maar Lake(HGY)is located in the tropical-subtropical Leiqiong Peninsula,and its climate shows a great seasonality and is influenced by the East Asian Monsoon system as represented by the East Asian summer monsoon(EASM),East Asian winter monsoon(EAWM)and Indian summer monsoon(ISM),and the Siberian High.In addition,the Leiqiong region is one of the areas most influenced by tropical cyclone activity originated in western North Pacific and South China Sea.Consequently,sediments in HGY can record information about changes in climate,lake water level,and human activity,and are therefore ideal sedimentary archive for the reconstruction of paleo-climate and paleoenvironment.Surface sediment samples(76 samples)and core HML2(309 samples for geochemical elements)were collected at 1 cm intervals from the HGY,and HML2 was further subsampled at high resolution at 0.3 cm intervals(1054 samples in total)for grain size analysis.The age model for core HML2 was established through 210 Pb and 137 Cs dating for the top part and comparison of magnetic susceptibility with the AMS14C-dated core SUS for the rest part.Geochemical analysis of bulk sediments and grain size distributions of terrigenous sediments were used to reconstruct the historical changes in precipitation,heavy rainfall,human activity and tropical cyclone activity in HGY's catchment over the past two millennia.The main results and conclusions are as follows:1)The PC3 from principal component analysis(PCA)is charaterized with high loadings of geochemically transported elements and covaries with the tranditional rainfall proxies including Sr/Rb,Ca O/Al2O3,Mg O/Al2O3 and Si O2/Al2O3 ratios,and thus was used to indicate the historical variations in precipitation and ISM.Their consistent variations revealed that plentiful rainfalls occurred during AD 85-195,336-475,670-723,1353-1663 and 1867-2009,causing intensified chemical weathering under tropical high temperatures.2)The PC1 represents detrital elements and fluctuates synchronously with sand content and mean grain size,and therefore can be a useful proxy for sever rainfalls in the catchment.The temporal variaions in PC1 combined with sand content,mean grain size and magnetic susceptibility revealed that heavy rainfall occurred at higher frequencies during AD 203-326,475-662,800-1230,1269-1377,1434-1475 and 1573-2009.3)Surface and core samples were separated along the PC2 axis characterized by high loadings of geochemical inert elements,suggesting that significant difference exists between modern sediments and core sediments in terms of composition and content of inert elements.Moreover,the temporal variation in PC2 fluctuates synchronously with the published black carbon record,reflected human activities in this area.Thus,PC2 can be used to indicate human activities influencing the HGY area and reveals sharply increased human activities since 1949.4)Three end members(EM1,EM2 and EM3)were obtained through an end-member modeling of the grain size distribution of the surface and core sediments.Through comparisons with the frequency curves of published end members and the geochemical PCs in this study,the coarse EM1 was interpreted as typhoon-induced changes in hydrodynamics and precipitations;the fine-grained clayey EM2 represents background depositions through air transportation;and the silty EM3 indicates monsoon-induced vegetation changes in the catchment,which may be related to the variations in ISM.5)Based on the temporal variation in the typhoon-induced EM1,tropical cyclones prevailed during AD 81-112,194-216,270-415,455-700,810-1225,1275-1340,1400-1445,1468-1508,1554-1600,1655-1872,1938-1959 and 2005-2013.A frequency analysis with detrended EM1 revealed cycles of 84 a,130 a,260 a and 753 a in the tropical cyclone activity over the last two millennia.Furthermore,comparisons with climate forcing showed that tropical cyclone activities may be caused by secular variations in SST,air temperature in HGY,ENSO,ITCZ and solar activity.Moreover,tropical cyclone activities seemed to be coupled with the EASM,but decoupled with both EAWM and ISM.