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Speleothem oxygen isotopic records from the Last Glacial Maximum

P. Treble

Speleothems (cave stalagmites and flowstones) have the potential to provide the terrestrial equivalent of the detailed multi-proxy palaeoclimate records derived from deep-sea sediment cores and corals. Speleothems are usually continuous, well-dateable by U-series and preserve stable isotopes and trace elements which may serve as first order climate proxies for temperature and rainfall variations.

Figure1 Site locations of speleothem flowstone c records; Margaret River
(southwest Western Australia), Mole Creek (Tasmania) and Mt Arthur( South Island New Zealand). Average Synoptic features for Southern Hemisphereare shown.

 

Oxygen isotopes are the key for palaeoclimate reconstruction from speleothems. The isotopic composition in speleothem calcite (delta18Oc) is dependent on the isotopic composition of its meteoric rainfall source (delta18Ow) and also the cave temperature. Changes of global ice volume also can affect the net composition. Thus, speleothems are potentially a record of both temperature and rainfall variations, which may be expressed in a simple equation as follows:

This study compares oxygen isotope records from cave flowstones from southwest Western Australia and Tasmania with records previously published from South Island New Zealand. These sites (Figure 5) were chosen in order to compare the speleothem records from sites that are climatically similar today and also known to have experienced similar temperature depressions at the Last Glacial Maximum. Furthermore, these sites are sensitive to the north-south migration of the rain-bearing low pressure systems and fronts of the southern hemisphere westerlies.

 

Comparing the results of the speleothem c records reveals an unexpected anomaly: those from Tasmania move positively with post-glacial warming, while those of southwest Western Australia and New Zealand move negatively. The difference suggests that either these sites were climatically less similar at the LGM than previously thought, or that an additional process overshadows the rainfall isotopic shift, but is not controlled by temperature, nor does it affect the speleothem sites uniformly. It is clear that records of rainfall isotopes independent of cave temperature are required to resolve this. It is proposed to reconstruct the LGM rainfall isotopes by direct isotopic analysis of the trapped water (fluid inclusions) in the speleothem calcite, in the expectation that this technique will resolve the divergence between New Zealand and Tasmanian speleothem records, and may also resolve a longer-standing question of Australian Quaternary science concerning the behaviour of the southern hemisphere westerly wind and frontal systems during the Glacial Maximum.