Establishment of Stalagmite 232Th as a Novel Proxy for Dust Flux and the Reconstruction of the Asian Monsoon System during the Late Pleistocene

Abstract

Aerosols are minute particles that are suspended in the Earth’s atmosphere. The variation in composition and size control the impact they have on global climate. This study focuses on mineral dust as a subspecies of aerosols and the reconstruction of its flux in Asia during the late Pleistocene. Stalagmites are upwards growing speleothems (the combined term for stalactites, stalagmites and other cave deposits) composed of calcite or aragonite. Their global distribution, semi-continuous growth and 234U/230Th datable laminae make them valuable climate archives. This study establishes a correlation between atmospheric aerosols and stalagmite 232Th concentrations to take advantage of the absolutely dated stalagmite aliquots and establish a proxy for paleo-dust flux. This proxy is applied to a reconstruction methodology for Asian dust flux during the late Pleistocene. East Asian dust flux and monsoon intensity are critical systems to understand when modelling the effects of future climate change
on the region.

194 Asian stalagmite 232Th records are interpolated and compiled to produce a median Asian 232Th curve. A high degree of correlation (r2 =0.32, p <0.00001, n =213) between this median 232Th curve and a global sea level curve (taken from Siddal et al., 2003) establishes a climatic control on the 232Th curve. This curve is evaluated against a Northern Hemisphere (65°) summer insolation model, a compiled East Asian stalagmite d18O record (taken from Cheng et al., 2016), NGRIP d18O (taken from North Greenland Ice-Core Project Members, 2004) and a Westerlies d18O record (taken from Cheng et al., 2016).

During a period of low insolation variability (40-20 ka), and subsequent constant monsoon intensity, the correlation between the stalagmite 232Th and d18O records decreases noticeably. During this period, stalagmite 232Th continues tracking regional dust fluxes, but no longer matches the EA stalagmite d18O record. This period coincides with the occurrence of North Atlantic Dansgaard-Oeschger (D-O) and Heinrich Events, which have been shown to affect Asian moisture transport and Indian Ocean surface temperatures. Due to the large amount of moisture the EAM sources from the Indian Ocean, these surface temperatures changes are recognizable in the EA stalagmite d18O record. Because overall monsoon intensity does not change, stalagmite 232Th does not record D-O events, while stalagmite d18O does. This creates the decoupling of the two records observed between 40 and 20 ka.

The application of 232Th as a proxy for Asian dust flux in this study demonstrates its validity as a climate proxy. The potential generation of an absolutely dated dust-flux record reaching back 500kyrs creates the opportunity to use this methodology in future studies of dust flux reconstruction for all regions of the globe.