黄土高原西部上新统风成红粘土的微形态特征及其古环境意义

Micromorphology of Pliocene aeolian red clay in western Chinese Loess Plateau and its paleoenvironmental implication

黄土高原中新统-上新统风成红粘土沉积序列是记录东亚季风演化和亚洲内陆干旱化历史的重要载体。红粘土沉积前可能存在相当程度的风化过程,准确区分物源区原生信号和沉积后次生成壤信号是红粘土古气候重建的基础。偏光显微镜下土壤微形态可直观反映沉积结构和成壤程度。本研究以黄土高原西部董湾剖面厚约73.7 m的中新统-上新统(7.1~3.52 Ma)红粘土及其中的钙积层为对象,在野外考察的基础上,对该剖面上部12 m厚度的红粘土进行详细的土壤微形态分析和扫描电镜观察,并与上覆第四纪黄土沉积对比。结果显示,大部分红粘土及碳酸钙淀积层中,光性定向粘土不显著,粘化程度低,次生碳酸盐及晶体沿根孔和裂隙分布,古土壤总体为中等或弱发育程度。部分红粘土层中存在粘粒胶膜或铁锰结核,指示土壤发育相对较强。碳酸钙的淋溶淀积形式和红粘土序列中普遍存在的铁锰胶膜,与红粘土的透水性差、裂隙发育、地形和地下水位波动等因素有关,与古气候之间的关系存在一定的不确定性。红粘土序列中水的参与度高于第四纪黄土,流水侵蚀和地下水位波动影响区域之间地层对比和红粘土理化性质,亟需综合多种研究手段更准确认识红粘土的古气候和古环境意义。

Aeolian accumulation in the Chinese Loess Plateau(CLP)including Quaternary loess and Miocene-Pliocene red clay has documented the evolution of Eastern Asian Monsoon,history of Asian interior drying and uplift of the Tibetan Plateau.Compared to the overlying loess sequences,the red clays in source areas might have undergone to an extent weathering and pedogenic process before they deposited in the CLP.It is thus essential to distinguish the secondary pedogenic signals after deposition from the original signals in sources in order for paleoclimatic reconstruction.Micromorphology features under(plane and crossed)polarized light microscopy are dedicated for characterizing sedimentary structure and pedogenesis.In the present study,we studied micromorphology of red clay sequence in Dongwan(34°59'31″N,105°47'12″E;elevation 1879 m)which is located in the northwest of Qin'an County in western CLP.The Dongwan section was paleomagnetically dated 7.1~3.52 Ma ranging from Late Miocene to Pliocene in thickness 73.7 m,and its aeolian origin was confirmed.The Dongwan section is characteristic of interbedded red clay and calcareous layers.The carbonate leaching and precipitation in the upper part of Dongwan section are diverse,including leaching carbonate,calcareous nodules,carbonate beds,carbonate in vertical cracks and lattice carbonate.The micromorphology features of red clays and carbonate of the topmost 12 m in thickness(Late Pliocene,about 3.52 Ma)were specifically investigated in comparison with overlying with combination of scanned electronic microscopy(SEM)and field observations.26 intact samples were used to make thin sections,71 bulk samples for carbonate content and 5 samples for SEM.Compared to the Quaternary paleosols,Dongwan red clay and carbonate layers are less clayey due to much less optically oriented clay.Secondary carbonate leaches and crystalizes as calcite in root channels and cracks,indicating weak and moderate development.Red clay samples with clay coating and Fe-Mn coating or nodules suggest strong development.The diverse carbonate leaching and universe Fe-Mn coating in such section are related to the water permeability,cracks,topography and variation of water table,which raises the uncertainties for paleoclimate interpretation.The role of water in red clay sequence is more evident than Quaternary loess,but not always suggesting strong pedogenesis.In contrast,fluvial erosion and variation of water table considerably affect the physical and chemical features and regional comparison between red clay sequences.The paleoclimatic and paleoenvironmental implications of red clay sequences in the CLP should be re-evaluated by multi proxies.