Abstract

Speleothems are a key archive for past terrestrial climate information due to their potential for long, continuous, high-resolution, precisely-dated proxy records. The concentration and grain-size distribution of allogenic magnetic minerals incorporated into speleothems can be used to reconstruct past hydrological regimes. We use principal component analysis of first-order reversal curves to characterise the magnetic minerals incorporated into a 33 kyr flowstone record from Aotearoa New Zealand. Three end members (EMs) represent (1) a component with a broad spectrum of grain sizes, similar to that found in the overlying soil (EM1); (2) a coarse multi-domain component, with grains ranging up to several hundred microns (EM2); and (3) a fine, single-domain to vortex component (EM3). We interpret EM1 and especially EM2 as proxies for cave stream flooding and EM3 as a proxy for soil erodibility and transport through infiltration or possibly aeolian processes. We find increased soil erodibility during the period 30-20 kyr BP, corresponding to the extended Last Glacial Maximum (eLGM). Flooding frequency is high during the periods 27-25 kyr BP and 9-0 kyr BP. eLGM flooding episodes as recorded in the flowstone may be enhanced by the deposition of the Kawakawa/Oruanui tephra, providing a rich source of magnetic material. This study shows the potential for coarse, multi-domain magnetic material incorporated in speleothems as a source of palaeoenvironmental information, especially in regions characterised by magnetite-rich soils and frequent, high-energy hydrologic events.

view journal