Molecular organic matter in speleothems and its potential as an environmental proxy

Abstract

Organic matter preserved in speleothems has considerable potential to record changes in the surrounding environment, particularly in the overlying vegetation. Here, we review three types of organic matter analysis relevant to speleothems: organic fluorescence, lipid biomarker analysis, and amino acid racemisation. Organic matter luminescence provides a useful non-destructive and rapid method for assessing dissolved organic matter quantity and quality, while biomarker analysis (amino acids and lipids) has the potential to provide a more detailed signal related to specific parts of the surrounding ecosystem such as the dominant vegetation regime and bacterial activity. Amino acid analysis has yet to prove demonstrably useful in stalagmites, due to the inability to characterise the sources of proteinaceous matter. However, the small but increasing body of work on lipid biomarker analysis in stalagmites has shown that a wide variety of recognisable biomarkers are preserved over long periods of time (>100 ka), can be recovered at temporal resolutions of <10 yr, and show meaningful changes through time. This approach is therefore of considerable potential value to Quaternary science.

Introduction

Speleothems (chemically precipitated cave deposits) occupy a valuable niche as proxies for palaeoenvironmental and climatic change. Caves provide sheltered environments that are more climatically stable than the Earth's surface (Wigley and Brown, 1976), whilst speleothems are generally chemically closed systems that alter little after lithification (Wigley and Brown, 1976). Therefore, the environmental records contained within speleothems are subject to far less geochemical and taphonomic disturbance than those in surface sedimentary deposits. Furthermore, a single speleothem can contain a multitude of proxies, including oxygen and carbon isotopes, trace elements and organic material, while the incremental nature of growth provides the opportunity for high-resolution dating. This means that, given sufficient material, a wide range of environmental and climatic parameters can be interpreted, including vegetation change, organic matter fluxes, variations in amount and seasonality of surface precipitation and temperature, and groundwater storage.

The vast majority of speleothem studies focus on inorganic proxies, in particular oxygen isotopes, which can be tied to other major climate records such as ice and ocean sediment cores (Fairchild et al., 2006). Until recently these studies have been generally unsuccessful at reconstructing climate, due to the complex forcing mechanisms affecting oxygen isotopes (McDermott, 2004). However, careful modern process studies mean that workers are now starting to provide quantified isotopic palaeoenvironmental data (e.g. Dorale et al., 1998; McDermott et al., 2001; Wang et al., 2001; Dykoski et al., 2005; Baker et al., 2007).

By and large, until the 1990s, organic material, although recognised as a significant minor component of speleothems (James et al., 1994), was mainly of interest as a potential contributor to the carbon isotope record, and as a source of speleothem colour (e.g. Caldwell et al., 1982). However, more recently there has been an increased focus on the actual organic matter itself. This has been brought about by the recognition of organic fluorescence in stalagmites (Baker et al., 1993; Shopov et al., 1994), and the recovery of usable pollen records (Bastin, 1978; Brook et al., 1990; Burney et al., 1994; Brook and Nickmann, 1996; McGarry and Caseldine, 2004). More recently, the field has been advanced by investigations into organic biomarkers, a proxy that is widely used in sediment and soil studies (e.g. González-Vila et al., 2003; Hanisch et al., 2003; Xie et al., 2003a; Zhang et al., 2004; Kim et al., 2007), and has been demonstrated to have potential in stalagmite work (Rousseau et al., 1995; Xie et al., 2003b, Xie et al., 2005; Blyth et al., 2006, Blyth et al., 2007).

This paper reviews how organic matter in speleothems is currently used as an environmental proxy, and how improved analytical techniques can advance this further. It focuses on the organic matter trapped in the calcite during precipitation. ‘Whole’ contributions, such as pollen, are not considered here as they have been recently reviewed elsewhere (McGarry and Caseldine, 2004).

Organic matter in speleothems is important in environmental research because it can provide information on a number of different environmental variables including climate and ecosystem change. The latter is especially important, as organic matter in stalagmites, and particularly biomarkers, have the potential to record detailed changes in the vegetational and microbial ecosystems. Currently, records of such changes in the terrestrial environment are obtained from studies of soil and sediment cores and micro-fossil records (e.g. Ficken et al., 1998; Huang et al., 1999; Brincat et al., 2000). However, speleothems provide comparative environmental stability, with samples preserved for the Quaternary or longer (Richards and Dorale, 2003), and can be dated with relative ease, potentially back to several million years (Woodhead et al., 2006). Given this and the number of speleothem proxies available for multi-disciplinary work, molecular palaeoenvironmental information derived from stalagmites and flowstones is very valuable.

We therefore believe that a review is timely to gather together the small but increasing body of work on organic matter in speleothems, and identify how each proxy can contribute to palaeoclimatic studies. In presenting an overview of the subject, we can also identify where future work is needed to advance the field, and where additional novel approaches might be taken.