Investigating speleothem magnetism as a proxy for dust mobilization and rainfall

Abstract

Commonly used speleothem-based paleoclimate proxies such as δ18O, δ13C, and trace element ratios are capable of high temporal resolution, but their interpretations are often ambiguous due to the conflated effects of multiple forcings. This complexity motivates the development of targeted proxies that can track specific local hydrological conditions. To this end, the concentration of ferromagnetic Fe-oxide particles in speleothem laminae has been suggested as a potential tracer for local precipitation. Here we use the quantum diamond microscope (QDM) to obtain a sub-annual resolution time series of ferromagnetic content in a Brazilian speleothem from a well-ventilated cave environment covering the period between 1913 and 2016 CE. This high resolution allows us to quantify the correlation between speleothem magnetic properties and an instrumental precipitation record for the first time. We find that ferromagnetic content in the central column displays a modest but significant negative correlation with rainfall (R2=0.35;p=0.0027), while magnetism in the speleothem flanks shows a weaker, statistically insignificant relationship. Rock magnetic analyses of the speleothem further reveal ultrafine, pedogenic grains to be the dominant ferromagnetic carriers. Combining QDM and electron microprobe data, we show that these pedogenic grains were likely developed in surface soils and delivered into the cave as part of airborne, 10–200 μm silicate-carbonate soil agglomerates. Our results show that speleothem magnetism holds strong potential as a targeted proxy for paleorainfall and demonstrate a method for identifying the mechanism of magnetic enhancement, which sets the necessary foundation for any paleoclimatic interpretations.