| The global climate has undergone many harmful changes in recent years,including the global warming,melting glaciers,rising sea levels and other extreme weather events,which has caused many serious negative effects to our human society and the earth ecosystem in which we live.Thus,the environment issues have been highly focused on in the academic community.Pliocene(5.3~2.6Ma)is a continuous warming period,the global mean temperature,the atmospheric CO2 concentrations,the mainland construction and ocean depth of which is most similar to modern times,meanwhile,the paleo-monsoon variation of Chinese Loess Plateau(CLP)in Pliocene is one of the one of the main components of the Paleocene climate.Therefore,reconstructing the features,processes and driving mechanisms of Pliocene paleo-monsoon evolution is considered to be crucial for modelling and predicting global warming in the near future.In the past studies of red-clay,scholars mainly explain the changes of East Asian Summer Monsoon through magnetic susceptibility,particle size,pollen,carbonate content,iron element and so on.However,the results obtained by former people may be very different and controversial due different proxies have different levels of sensitive to climate,especially temperature and precipitation,or the fact that one proxy is affected by temperature and precipitation at the same time.Here we first investigated the glycerol dialkyl glycerol tetraethers(GDGTs)in Shilou red-clay sequence in east CLP,using the detection of GDGTs compounds in red clay by high performance liquid chromatography-mass spectrometry(HPLC-MS)to discuss the composition and distribution characteristics of different GDGTs.The GDGTs climate proxies formula proposed by previous scholars for other geological carriers are applied to Shilou red-clay sequence,we combined the magnetic chronology data from previous scholars to discuss the availability of different formulas,and reconstruct the paleo-climate evolution in Shilou area during Pliocene.Finally,we discuss the possible driving mechanism of drought and moisture variation comprehensively.The results are as follows:(1)Nearly all GDGTs compounds were detected in the red clay section of Shilou,in which the content bacteria brGDGTs was 1.84 times higher than that archaea iso GDGTs.The main iso GDGTs were cren and GDGT-0,followed by GDGT-1.GDGT-2 GDGT-3 and cren' were much lower.Bacteria brGDGTs includes ?a??b??c??a??b??c??a??b and 6-methyl isomers ?a'??b'??c' and ?a'??b' compounds,?c??c' were not detected in all samples.brGDGTs were center on ?a??b??a??a'??b??b'??a and ?a',while other brGDGTs were lower.(2)The concentrations of different GDGTs in the Shilou red clay profile changes very strong.Whereas,the variation of composition and distribution of GDGTs compounds is mainly due to the changing of GDGTs microbial community controlled by atmospheric temperature,soil p H and water content et cetera,reflecting the extremely strong sensitivity of microorganisms that synthesize GDGTs compounds to environmental changes,supporting the availability of using GDGTs compounds in red clay to reconstruct paleo-climatic and environmental changes.(3)The changing trends of the concentrations of iso GDGTs and brGDGTs were vary similar,and there were obvious stages during the Pliocene: 5.2~3.7 Ma,the total concentrations of two GDGTs varied greatly and contains multiple peaks;3.7~3Ma,the total concentrations were stable and in low value;3~2.6Ma,the total concentrations were extremely high.(4)Of all the previous temperature substitution formulas based on bacterial brGDGTs,only MATmrs formula can be used in Shilou red clay section.However,the change of Pliocene paleotemperature reconstructed by this proxy index is opposite to the global cooling during the Pliocene,thus can not be used to reflect the change of ancient temperature in Shilou area.However,the temperatures obtained by using brGDGTs relative palaeotemperature proxies formulas are very consistent with the change of magnetic susceptibilities in the loesspalaeosol and red clay sequences in the Loess Plateau of China.Therefore,brGDGTs relevant palaeotemperature proxies may indicate other environmental factors,such as paleomagnetism changes.(5)According to the Pliocene paleoclimate changes in the Shilou red clay section reconstructed from Ri/b and BIT proxies,Pliocene arid environment changing can be broadly characterized as the following four stages: the first stage is from 5.2 to 4.7 Ma,the variation frequency of arid and humid environment in the study area is very large;The 2nd stage is from 4.7 to 3.7Ma,the degree of drought increased,and there have been a number of extreme drought events;The 3rd stage is from 3.7 to 2.9Ma,the humidity increased and remain stable;The final stage is from 2.9 to 2.6Ma,during which stage,aridity appeared again in the study area.By comparing our result with monsoon variation reconstructed by others,including East Asian monsoon changing from magnetic susceptibility data,we found they were in good consistency.(6)The results of the reconstruction of Ri/b and BIT indexes were compared with the magnetic susceptibility,particle size,magnetic susceptibility,pollen,snail fossils,Rb/Sr and Zr/Rb data,n-alkane data and East Asian monsoon indicators from other red clay profiles near the study area in Chinese Loess Plateau,and the good agreement between them was found.Meanwhile,the magnetic susceptibility and particle size index can not indicate the extreme drought events indicated by the Ri/b index,indicating the accuracy of the Ri/b index in reconstructing drought events.(7)The paleo-hydrology variation during Pliocene in Shilou area we reconstructed can be matched perfectly with tectonic activity events including the rapid uplift of Tibetan Plateau and the closure of Panama Seaway,and is also coincident with the increase of global ice sheet,the change of equatorial Pacific temperature difference caused by the ENSO(El Ni?oSouthern Oscillation)phenomenon.The extreme drought events during 4.7~3.7 Ma were mainly controlled by the ENSO events.The sudden change of humidity after 3.7 Ma is mainly affected by the rapid uplift of the Tibetan Plateau.After 4.7 Ma,the increasing of humidity was also affected by the closure of the Panama Seaway and the increasing of global ice sheet. |
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