Milankovitch-paced South Asian monsoons during Marine Isotope Stage 5

Abstract

The orbital-scale variations and associated driving mechanisms of the South Asian summer monsoon (SASM) and South Asian winter monsoon (SAWM) remain poorly understood. In this study, we present the first record of a 4.3-m loess-paleosol sequence from the southern Tibetan Plateau, which provides insight into the changes of these two monsoons over most of the last interglacial-glacial cycle. The age framework of this sequence was established using twenty-one samples with absolute luminescence techniques. The SASM was represented by frequency-dependent susceptibility in percentage and redness, while the SAWM was reflected by the recently proposed sorting coefficient of grain size. The results suggest an anti-phase relationship between the SASM and SAWM. During marine isotope stage (MIS) 5, variations of the SASM and SAWM showed a strong correlation with the Northern Hemisphere summer (July) insolation (NHSI) at 65°N, in terms of both phase and amplitude. Stronger SASM (SAWM) corresponded to higher (lower) July NHSI at 65°N, supporting the Milankovitch theory. The variation amplitude of the SASM record in this study significantly differs from the that of stalagmite δ18O from the nearby Tianmen Cave, which presented no distinct difference between MIS 5e, MIS 5c, and MIS 5a. During MIS 4 and MIS 3, the larger global ice volume and lower atmospheric CO2 could have enhanced the SAWM but weakened the SASM, which might have restrained the response amplitude of the South Asian monsoons to the July NHSI at 65°N.