Decoding climate cycles from the Red Clay sequence on the Chinese Loess Plateau

Abstract

The loess-Red Clay sequences on the Chinese Loess Plateau (CLP) are encoded with past variations of East Asian monsoons. In comparison with the loess sequence, the Red Clay sequence has lower dust accumulation rate, older depositional ages and was deposited in a warmer climate. These factors result in stronger degree of mineral oxidation, lower content of fine ferrimagnetic minerals and higher content of hematite. Therefore, magnetic parameters widely used in the loess sequence, such as magnetic susceptibility, may not be as effective in indicating past precipitation in the Red Clay sequence. It is notoriously well known that Red Clay lacks precessional cycles, the most obvious period dominating summer insolation. Here we explore the potential of “hard’’ isothermal remanent magnetization (HIRM), a parameter estimating hematite content, in recording past climatic changes in the Red Clay sequence. In contrast with the magnetic susceptibility record which only shows prominent 1 Myr and eccentricity cycles, the HIRM record shows all orbital and their harmonic cycles. Intriguingly, we also detected multiple millennial cycles from HIRM and found the amplitude variations of multiple millennial cycles are controlled by precession, obliquity, and their amplitude modulators. This work extends detecting of precession and multiple millennial cycles from the CLP deposits back to the late Miocene for the first time and revolutionize our thinking regarding how East Asian summer monsoons varied based on the CLP records for the late Miocene-Pliocene interval.