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


Gleixner,  Gerd
Molecular Biogeochemistry Group, Dr. G. Gleixner, Department Biogeochemical Processes, Prof. S. E. Trumbore, Max Planck Institute for Biogeochemistry, Max Planck Society;

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Hu, S., Goddu, S. R., Herb, C., Appel, E., Gleixner, G., Wang, S., et al. (2015). Climate variability and its magnetic response recorded in a lacustrine sequence in Heqing basin at the SE Tibetan Plateau since 900 ka. Geophysical journal international, 201(1), 444-458. doi:10.1093/gji/ggv033.

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.