Research paperOrganic compounds preserved in a 2.9 million year old stalagmite from the Nullarbor Plain, Australia
Research Highlights
► Organic matter preserved in a 2.9 Ma stalagmite can be effectively recovered. ► TMAH thermochemolysis indicates the presence of lignin associated products. ► Comparison with modern stalagmites indicates the signal can reflect overlying vegetation.
Introduction
The Nullarbor Plain in South Australia (Fig. 1) is one of the largest karst regions in the world at ~ 240,000 km2, and contains a number of remarkable speleothems that have long been of interest to cave scientists in terms of both their great age and unusual dark colouration (Caldwell et al., 1982). The region is also of considerable importance to understanding past climatic changes as it has a strong sensitivity to Southern Ocean climate dynamics, and can be shown to have experienced a very different climate state in the past, with satellite-derived digital terrain data showing clear evidence of ancient river channels and other landforms indicative of higher rainfall. Fossil evidence also indicates that even when the climate was semi-arid, a more diverse vegetation regime was present (Prideaux et al., 2007). The development of stalagmite proxies from this area is of obvious value as they will provide a major southern hemisphere terrestrial archive to complement the regional and global records recovered from marine and ice cores.
However, despite the importance of these stalagmites, they remain largely unstudied due to the lack of a reliable dating methodology, with the vast majority of samples being beyond the reach of the U–Th dating techniques used in other stalagmite research (i.e. older than 500 ka). This issue has recently been resolved by the successful development of a U–Pb chronology (Woodhead et al., 2006), meaning we can begin to explore this potentially extremely valuable palaeoenvironmental archive.
The use of organic matter in stalagmites as a palaeoenvironmental proxy is a growing field (for a review, see Blyth et al., 2008), with recent research indicating a connection between lipid biomarker content and both climate (Xie et al., 2003) and overlying vegetation regime (Blyth et al., 2007). Nullarbor stalagmites should be ideal for organic matter analyses as previous research has indicated that their dark colour may be due to unusually high levels of organics (Caldwell et al., 1982). A preliminary experiment with an undated sample showed a lipid biomarker signal comparable in concentration and constituent compounds to that from a modern African cave (Blyth, 2007, Blyth et al., 2008).
This work therefore qualitatively analyses organic compounds recovered via a sequential organic extraction from a Nullarbor stalagmite with the aim of establishing the feasibility of organic analyses in these samples, and their potential for use in palaeoenvironmental reconstruction.
Section snippets
Site and sample
The Nullarbor Plain is a flat-lying area where shallow marine limestones were exposed 15 million years ago due to a combination of uplift and global lowering of sea level. The current climate is arid to semi-arid, although it is clear from geomorphological evidence that this has not always been the case (Caldwell et al., 1982).
The stalagmite analysed (1411-MO6) was collected in 2005 from ‘1411’ cave (lat 31.85; long 127.57), and has been dated via U–Pb techniques to 2.9 ± 0.1 Ma (disequilibrium
Lipid results
Although a preliminary undated sample analysed via the same extraction method in 2006 showed a well preserved lipid signal (Blyth, 2007, Blyth et al., 2008), comparatively few lipid biomarkers were recovered from the current samples, with MO6b, MO6c, and MO6d containing only trace amounts of low molecular weight fatty acids below the contamination threshold associated with this method (0.3 μg for C16n-alkanoic acid; Blyth et al., 2006). MO6a does appear to contain a genuine lipid signal with a
Conclusion
Organic matter is extractable from the Nullarbor Plain stalagmites, and can be analysed at a molecular level. The results indicate that lipid analysis is not always feasible in these samples, although at this stage we cannot say whether this is due to a degradation of the signal in older samples, the lack of an original signal in some, but not all, samples, or the binding of the lipids into the bulk organic matter in some way that renders them unamenable to simple extraction. More promising is
Acknowledgements
Dr. Lisa Harris of the Mass Spectrometry Facility, Department of Chemistry, UCL, undertook preliminary analyses of the pink colouration in M06a. Funding for this work was provided by the McDonald Institute for Archaeological Research at the University of Cambridge, The Open University, and a Leverhulme Early Career Fellowship to AJB.
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