Drivers of weathering-related magnetic concentration changes in lacustrine sediments of the Tibetan Plateau – cases in dry and humid climate

Abstract:

Magnetic susceptibility (MS) variations in lacustrine cores from the westerlies-dominated arid Qaidam basin (NE Tibetan Plateau) and the monsoon- dominated humid Heqing basin (SE Tibetan Plateau) have turned out to record changes in moisture fluxes throughout the Pleistocene. In both cases humidity-related weathering of magnetite to maghemite and hematite by low-temperature oxidation in the catchment is recognized as a main driver. In Qaidam basin fluctuations of low and high MS values show orbital cyclicities and reflect relatively higher and lower humidity, respectively, as revealed by calibration to pollen results. The range and possible changes of the catchment area, remains unclear. A tectonic control of the proxy record becomes evident by comparing results in an overlapping age interval (~2.7-1.7 Ma) of two cores in ~20 km distance. The records in the sediments of the Qaidam basin may be dominated by paleo-environmental changes near the lakeshore and controlled by regional water recycling. In Heqing basin the extent of the catchment is far clearer and processes in the catchment and their pathway into lake sediments are better understood. These processes comprise chemical weathering of limestone rocks, strong magnetic enhancement in soils by both enrichment of coarser-grained magnetite inherited from the bedrock and bacterial production of ultrafine magnetite. Relative changes of wind and surface water transport in more and less humid periods led to systematic variations of MS values and carbonate contents on a clear eccentricity (100-kyr) scale. Amplitudes of the cycles can be interpreted with the relative influence of the Indian summer monsoon in this area.