Resolving conflicting models of late Miocene East Asian summer monsoon intensity recorded in Red Clay deposits on the Chinese Loess Plateau

Abstract

Considerable debate exists regarding how the East Asian summer monsoon (EASM) varied over the late Miocene, limiting our ability to understand the long-term evolution and forcing of this system. Much understanding of EASM variation over the late Cenozoic is based on loess magnetic susceptibility (χlf) records from the Chinese Loess Plateau (CLP). During Pliocene-Quaternary times, χlf decreases from southeast to northwest across the CLP, consistent with the modern monsoonal climate pattern. However, the χlf records over the late Miocene show different patterns, with lower values on the central CLP sandwiched between higher values of the eastern and western CLP, a pattern inconsistent with predictions based on monsoon climate. Here we propose a hypothesis to explain this conundrum, and advance knowledge of late Miocene EASM variations. As with other parts of the CLP, high late Miocene χlf values on the western CLP were caused by increases in the ultrafine ferrimagnetic grain content. However, we argue that part of the ultrafine ferrimagnetic grain increase on the western CLP was due to detrital input derived from eroding surface soil cover on the uplifting bounding mountains of the northeastern Tibetan Plateau and western Qinling. Loess provenance data and independent evidence regarding uplift timing of the northeastern Tibetan Plateau support our hypothesis. We propose that the eastern CLP χlf more faithfully reflects EASM intensity; indeed, eastern CLP χlf data show relatively low values over the late Miocene, indicating a weak EASM, consistent with inferences from marine sediments. Therefore, our work reconciles conflicting evidence regarding late Miocene EASM intensity and deepens our understanding of the underlying monsoon forcing mechanisms. We conclude that global climate, instead of Tibetan uplift, is the major mechanism driving EASM precipitation evolution over the late Miocene.