Determination of the optimized late Pleistocene chronology of a lacustrine sedimentary core from the Heqing Basin by geomagnetic paleointensity and its paleoclimate significance

Abstract

Long and continuous lacustrine sedimentary sequences from SW China faithfully recorded the evolution of the Indian summer monsoon (ISM). However, the limited dating range (<50 ka) of 14C dating has restricted high-resolution research on the last glacial cycle.We presented a relative paleointensity (RPI) record from a lacustrine sedimentary core from the Heqing Basin to optimize its AMS 14C chronology. Detailed rock magnetic analyses indicated that SD and vortex state magnetite (and/or maghemite) was the dominant magnetic mineral of lacustrine sediments. Moderate variations in concentration-dependent (SIRM, ARM) and grain-size-dependent (ARM/SIRM) parameters satisfied the criteria for the RPI studies. Consequently, we determined the RPI by NRM20-40 mT/SIRM20-40 mT, where NRM20-40 mT denotes the NRM after demagnetization between 20 and 40 mT. Using 6 AMS 14C dates younger than 40 ka BP as constraints, 7 age control points were determined by correlating the lows in the RPI record with those of the target paleointensity stacks PISO-1500. We established a reliable chronological framework of the upper 11.1 m of the HQ drill core by binomial fitting based on the 7 RPI age control points and the climatic transition termination II. The consistent variations in our RPI curve and other global target paleointensity curves indicated that the optimized chronology was reliable.In our optimized chronology, the visible precession cycle in ARM/SIRM indicated that the ISM was a direct response to solar insolation during the last glacial cycle. The strong ISM suggested by the high content of Mesic conifers and tropical and subtropical trees during MIS 3 shifted to MIS 5a. Thus, the “MIS 3 Mega-paleolake” hypothesis in China, which was mainly deduced by the 14C age model, was challenged.