East Asian Monsoon Variations During the Late Miocene-Late Pliocene Inferred from Palygorskite Record in Chinese Red Clay Deposits, Northwestern China

摘要

The concentration and morphological characteristics of palygorskite through the late Miocene to late Pliocene were investigated in four Red Clay sequences on the Chinese Loess Plateau (CLP) using scan electronic microscope (SEM). Two significant environmental and paleoclimatic shifts recorded by palygorskite microtextures, which reflect changes in the East Asian monsoon, occurred at similar to 3.6, similar to 3.2 Ma. Prior to 3.6 Ma, palygorskite with a 'woven' appearance is a ubiquitous component in the Red Clay deposits, revealing an extremely warm-wet climate due to the strong East Asian summer monsoon. SEM investigations point to a marked decrease in the accumulation of palygorskite with a 'woven' appearance and an increase in the accumulation of a palygorskite-smectite aggregate consisting of platy smectite transforming to palygorskite during the interval of 3.6-3.2 Ma, indicating a climate change from warm-wet to warm-dry. In terms of rainfall and temperature, precipitation decreased and temperature dropped during similar to 3.6-3.2 Ma compared to the stage before 3.6 Ma. This decrease was followed by an increase in the accumulation of eolian palygorskite and a sharp decrease in palygorskite with extremely minor 'woven' appearance and platy-silky aggregate during similar to 3.2-2.6 Ma. During late Pliocene, a minor to trace amount of eolian palygorskite occurs in the Red Clay deposits. Comparison of our record with published paleoclimatic records from continental and oceanic sediments indicate that there is a good match between palygorskite morphological types distribution, drying in the interior of Asia and global cooling, which indicates that global cooling may be the most likely driver of stepwise drying in interior Asia. Palygorskite distribution in the Red Clay deposits also suggests a stronger East Asian summer monsoon during the late Miocene when compared to the early Pliocene. And, palygorskite may be regarded as a sensitive mineral responding to the strengthening of the East Asian winter monsoon and global cooling.