Orbital forcing of Middle Miocene East Asian summer monsoon variability recorded by aeolian sediments on NE Tibetan Plateau

Abstract

An accurate chrono-stratigraphy of aeolian sediments is crucial for understanding East Asian Summer Monsoon (EASM) variability. However, there are few studies of EASM variability recorded in aeolian sediments on the northeastern margin of the Tibetan Plateau, especially high-resolution aeolian sedimentary sequences spanning the period of 15–10 Ma. We studied an aeolian red clay section (89.1 m) with interbedded fluvial sediments, in the Jianzha Basin, northeastern Tibetan Plateau, using the integration of magneto stratigraphy, cyclo-stratigraphy, and detrital zircon provenance analysis. Magneto stratigraphy revealed 12 normal and 11 reverse polarity zones that are well correlated with chrons C5n.2n to C5Bn.2n of the Geomagnetic Polarity Time Scale (GPTS); this constrains the age of the section to the Middle Miocene. Subsequently, we use the frequency-dependent magnetic susceptibility as an EASM proxy, combined with spectral analysis in the depth domain, to identify Earth orbital periodicities. Gaussian band-pass filtering enabled us to extract the 405-kyr eccentricity signal, which provided a high-resolution astronomical time scale for the interval of 15.231–10.439 Ma. Detrital zircon U-Pb provenance tracing and sediment accumulation rate analysis revealed a provenance shift between 14.08 Ma and 10.2 Ma, which we attribute to the rapid uplift of the West Qinling Mountains at ∼ 13–12 Ma. The relationship between the variation of monsoon proxy indicators in the section and the global marine oxygen isotope (δ¹⁸O) record indicates that EASM evolution during the Middle Miocene was primarily a response to global temperature changes. On an orbital time scale, the frequency-dependent magnetic susceptibility record shows a significant long-eccentricity (∼405 kyr) periodicity component, indicating that EASM variations during the Middle Miocene were forced mainly by eccentricity. We conclude that a combination of eccentricity-modulated low-latitude summer insolation and Antarctic Ice Sheet fluctuations drove the eccentricity-paced precipitation variability on the northeastern Tibetan Plateau during the Middle Miocene.