Climate variability and its magnetic response recorded in a lacustrine sequence in Heqing basin at the SE Tibetan Plateau since 900 ka

Abstract

The lacustrine deposits in Heqing basin provide an excellent archive for long-term highresolution palaeoclimate studies in the monsoon-dominated southeastern Tibetan Plateau region. In this study, we investigate the climatic significance of magnetic parameters for analysing the variability of the past climate. For this, we performed comprehensive time-series and statistical analysis on previously published proxy data from a 168-m-long drill core (Core-HQ) that spans ~900-30 ka, comprising mainly magnetic parameters and carbonate content (CC). Moreover, we investigated magnetic properties of modern soil in the catchment that predominantly formed on limestone bedrock. Key findings are: (1) modern soils and sediments of Core-HQ both contain a mixture of magnetite (Mt), maghemite (Mgh) and hematite (Ht), but magnetic concentration of the soils is one order higher; (2) a superparamagnetic (SP) fraction of Mt/Mgh dominates in the soils whereas in Core-HQ the SP contribution is generally very low; (3) a larger grain-size fraction of Mt/Mgh and Ht is also present in the soils. We explain variations of magnetic concentration and CC in Core-HQ by an increased wind transport of soil and a decreased surface water transport of carbonate and soil in less humid periods. Low-temperature oxidation of magnetite in the catchment is as another crucial process that reflects weathering conditions and is likely sensitive to humidity; the degree of LTO can be semi-quantified by the magnetic parameters ARM/SIRM and S-ratio. Combining CC, ARM/SIRM and S-ratio values, we derive a weathering intensity (WI). The WI index variation along Core-HQ shows strong fluctuations on a 100-kyr eccentricity scale in the lower part, especially during ~630-380 ka, followed by a long period (~320-80 ka) of persistently weaker weathering (drier?) conditions with low variability, and a rapid return of much stronger weathering (wetter?) conditions at ~80 ka. We suggest that a reduced influence of the Indian summermonsoon accounts for less moisture supply to the region and lower climatic variability.