Environmental magnetism of late Holocene stalagmites from semi-arid karst in southern Australia

Abstract

Whilst significant progress in speleothem environmental magnetism has been made in recent years, our knowledge of how magnetic proxies respond to Earth-system processes in arid/semi-arid environments is limited. In this study, we investigate calcite stalagmites that form under evaporative conditions near cave entrances in areas that receive low precipitation (∼250 mm/year). Such stalagmites have yet to be explored in environmental magnetism, and are typically avoided in paleoclimatology due to complications in their geochemistry. Proxies based on detrital inclusions, such as speleothem palynology, have, however, proven favorable in these conditions. Here we report the first combination of speleothem magnetism with published pollen and charcoal records from two stalagmites formed over the last ∼2400 years at Webbs Cave, located on the ∼230,000 km² Nullarbor Plain in southern Australia. Rock magnetic experiments were performed using anhysteretic and isothermal remnant magnetizations, hysteresis loops, and low-temperature remanence measurement to investigate the concentration, phases, and grain size of the magnetic mineral assemblage. Analysis of detrital particles and speleothem porosity was undertaken using x-ray microtomography. The magnetic mineralogy of both stalagmites is typical of stalagmites globally in being dominated by magnetite or maghemite with dominant grain size distributions in the superparamagnetic to single-domain range, which occurs alongside goethite and potentially hematite. Both the low and high coercivity fractions co-vary and are consistent with local soils, furthering the link between stalagmite magnetism and pedogenesis. However, rather than increasing with precipitation, we infer that magnetic particle concentrations can increase during drier climates, based on correlations to the stalagmite pollen records. This likely results from increased detritus in available drip waters, along with increased deposition of localized cave dust during (dry) periods of slow stalagmite growth. While the magnetic particles in near-entrance stalagmites may derive from multiple transport modes, their local soil provenience forms the main control on their magnetic properties.