南方红土的剖面风化特征

对地处九江的南方红土所作的观察和元素地球化学分析表明 ,南方红土在剖面上存在风化强度的系统变化特征。剖面下部对应网纹红土的层位风化强度大大高于剖面上部对应褐黄色土的层位 ,其中氧化铁游离度从下部 90 %降至上部 4 5 %左右 ,相当于从砖红壤环境转变为红壤环境。这种风化强度的系统变化在北方黄土地层中没有相似表现 ,但可能与黄土记录的中更新世晚期以来粉尘通量逐步加大、干旱化程度逐步加深有着某种关联 ,暗示中更新世晚期以来可能存在较为普遍的气候恶化趋势。

THE WEATHERING SEQUENCE OF THE RED EARTH OVER SOUTHERN CHINA

RedearthinSouthernChinahasprovidedavaluablerecordforthepaleoclimatic changesinlowlatitudes.However,up_to_date ,thestudiesonthepaleoclimaticaspectsofthe redeartharestillscarce .Herewepresentapreliminaryresultofclimate_inducedgeochemical variationsofaredearthprofileinJiujiangCity ,JiangxiProvince .Thisredearthsectionis about 1 5mthick ,andconsistsoftwopedogenicunits.Theupperpartisalayerofyellowish siltyclay ,whilethelowerpartalayerofredclaywithyellowandwriteworm_shapedveins. Theresultrevealsthatapronounceddepletionofbases (Na ,K ,Ca)occurredinthelowerpart ofthesection .InthelowerparttheratioofFed/Fetisabout 90 % ,correspondingtothatof presentlaterite .However,intheupperpart,thisratiodecreasedtoabout 45% ,indicatinga significantweakeningintheintensityofweathering .TheCIAalsodecreasedby 1 0 % (from 85%to 75% )fromthebottomtothetopofthesection .Asignifieantlossinthe concentrationofsomemoremobileelements,forexample,Sr,Ba,Rbetc.,isobservedinthe lowerpartofthesection .Otherproxiesalsoshowaweakenedweatheringup_section .This weakeningtrendinweatheringintensityisnotobservedintheloesssequeneesofnorthern China ,butmaybereleventtothedryingprocessreflectedbythedustfluxrecordoftheloess deposits.Theweatheringsequencealsoindicatesthattheredearthisnotonlytheproduction ofeluvialweathering ,butisaccompaniedwithanacceleratedaccumulationdustmaterialsas well.Thesemayimplythatadeteriorationofclimatehasundergoneoverbothnorthand southernChinasincelatePleistocene .