Subaqueous speleothems from the Flinders Ranges as palaeoclimate archives for the arid zone

Abstract

There is a distinct paucity of palaeoclimate data across the Southern Hemisphere, particularly in the subtropics. In Australia, the subtropics are largely dominated by arid environments, which do not lend themselves to the preservation of palaeoclimate archives. Erosion and reworking of sediments and low-resolution archives results in discontinuous records that usually cannot be dated to a high degree of accuracy and precision. Alternatively, speleothems can offer high resolution multi-proxy records of past climate that can be tethered to accurate and precise chronologies. Mairs Cave, in the central Flinders Ranges, lies on Australia’s southern arid margin and is filled with subaqueous speleothems, a unique and largely unexplored archive. The subaqueous speleothems in Mairs Cave represent a rare and valuable opportunity to reconstruct the past climate of Australia’s southern arid margin.

In this study, uranium-thorium disequilibrium dating, stable-isotope analysis and trace-element analysis were performed on two subaqueous speleothems from Mairs Cave. The results indicate that subaqueous growth began ~89 ka BP, with two periods of continuous growth around the middle of the Last Glacial Period, a burst of growth immediately after the Australian Last Glacial Maximum, and a final burst of growth in the mid-Holocene. The timing of these intervals suggests a strong association between rainfall at the cave site and summer insolation intensity in the Southern Hemisphere, indicating that the growth of the subaqueous speleothems was sustained by episodes of tropical moisture governed by orbital forcing. An inverse relationship is observed between growth rate of the subaqueous speleothems and temperatures in the North Atlantic (interpreted from the NGRIP oxygen isotope record). This relationship is attributed to variations in the strength of the Atlantic Meridional Overturning Circulation, and demonstrates the influence of sub-orbital climate changes superimposed over orbital forcing.

Magnesium concentration in the speleothems demonstrates a strong association with both insolation and NGRIP records, and is interpreted as a potential indicator of palaeotemperature. This interpretation is in opposition to classical speleothem studies of stalagmites, but in agreement with a recent study of subaqueous speleothems. Contrary to expectations, the oxygen isotope record exhibits a positive relationship with insolation. This is tentatively attributed to the ‘temperature effect’, but further research is required to test this hypothesis. Aside from this unresolved question that has arisen, the growth rate and magnesium records are testament to the potential of the subaqueous speleothems from Mairs Cave as palaeoclimate archives for Australia’s southern arid margin.