A review of the oxygen isotope composition of lacustrine diatom silica for palaeoclimate reconstruction
Introduction
Stratigraphic changes in the oxygen isotope composition of authigenic minerals in lakes are routinely used to reconstruct palaeoclimate (see reviews in Buchardt and Fritz, 1980, Siegenthaler and Eicher, 1986, Ito, 2001, Schwalb, 2003, Leng and Marshall, 2004). Carbonates (e.g., marl, ostracods, molluscs) are the most commonly used material and the technique is now well established, but in non-alkaline, dilute, and/or open lakes, carbonates may be rare or absent. Such lakes are common at high-altitudes and are ideal for investigating climate change using oxygen isotopes because the isotope composition of the lake water is often similar to the composition of meteoric water (on either an annual or seasonal basis). These lakes nearly always contain diatom silica, which can be used as an alternative to carbonates in isotope studies. In simple systems, the oxygen isotope composition of diatom silica (δ18Odiatom) is controlled by water temperature and by the isotope composition of the lake water from which the diatom frustule is formed (Fig. 1). If diatom silica is precipitated in isotope equilibrium with the lake water, then mineral–water fractionation equations (often referred to as palaeotemperature equations after Craig, 1965) can be used to estimate variations in past temperatures providing there is no change in the oxygen isotope composition of the lake water. The interpretation of oxygen isotope compositions, in practice however, is complicated because the oxygen isotope composition of the water can be affected by changes in hydrology and climate. Knowledge of the factors that may have influenced the isotope composition of the lake water (δ18Olakewater on Fig. 1) is vital to the interpretation of the δ18Odiatom signal. Perhaps the most critical control on a lake's isotope hydrology is the extent to which the lake is open (has an outflow and a short residence time) or closed (no effective outflow, and a long lake water residence time). The oxygen isotope composition of water in hydrologically open lakes usually reflects the isotope composition of precipitation, both rain and snowfall, received by the lake (δ18Oprecipitation or δp). Whereas closed lakes tend to have δ18O that respond predominantly to the balance between precipitation (P) and evaporation (E). Although closed lake basins can be important in terms of area, the majority of lakes are open to some extent and their relative sensitivity to changes in P and E will depend on their water residence time. If a lake has a low P/E, then the isotope signal will record P/E changes since the amount of change due to temperature fluctuation will be small in comparison to the potential change due to changes in the P/E balance (see Table 1 in Leng and Marshall, 2004).
Research on diatom-based oxygen isotopes was developed by oceanographers (Labeyrie, 1974, Labeyrie and Juillet, 1982, Labeyrie et al., 1984) and has produced many notable records, especially from the opal-rich sediments of the Southern Ocean (Shemesh et al., 1993, Shemesh et al., 1995). Sustained work on lacustrine diatoms is a more recent development, in part because of even more challenging methodological considerations (Binz, 1987), and also because the application of stable isotope methods to lakes has developed more slowly than in the oceans. This review of the rapid expansion of the literature in this field during the last decade seeks to highlight the factors that influence the measured isotope composition of lacustrine diatom silica and the assumptions that need to be made in using isotope variation to indicate past changes in climate. The most important considerations are the need to analyse a pure diatom sample, to measure the oxygen isotope ratio incorporated at the time of formation, and the need to calibrate the isotope records using studies of the isotope systematics of the modern lake system and the link to climate. We describe a number of studies from late Quaternary lake sediments to illustrate how the oxygen isotope composition of diatom silica has been successfully used to demonstrate climate change.
Section snippets
Diatoms in lakes
Diatoms are photosynthetic algae (2–200 μm) that form a shell, or frustule, composed of opalline or biogenic silica (SiO2·nH2O) (Round et al., 1990). The precise method of formation is still not fully understood but most models propose the formation of a matrix of organic fibrils and microtubules onto which the silica is deposited. More recently, Sumper and Kröger (2004) have suggested that silica formation takes place in intracellular compartments termed silica deposition vesicles (SDVs). The
Purification techniques
Nearly pure samples of diatom frustules from naturally occurring diatomite deposits, or artificially concentrated sediment samples, are required for oxygen isotope analysis, since the method generally used (i.e., fluorination techniques following Clayton and Mayeda, 1963) will liberate oxygen from all the components in the sediment; for example, silt, clay, tephra, carbonates and organic matter (Juillet-Leclerc, 1986). Natural diatomites are relatively rare, although careful choice of sites can
Diatom oxygen systematics
Changes in the oxygen isotope composition of diatom silica can be used to infer changes in either temperature or the oxygen isotope composition of lake water, although both are affected by climate dynamics and lake hydrology.
Vital effects
Disequilibrium effects (Fig. 1) (commonly known as ‘vital’ effects in the case of biogenic carbonates) include a variety of rate effects and micro-environment induced changes that cause the mineral to have an isotope composition that is different from that predicted purely by thermodynamics (cf., Holmes and Chivas, 2002 show a table of ostracod species effects). In one of the earliest studies of lacustrine δ18Odiatom, Binz (1987) found little difference in fractionation between three cultured
Comparison between carbonate and diatom silica records
Combining isotope analysis of different authigenic minerals offers the possibility of obtaining seasonally specific information. For example, where carbonates and diatom silica co-occur they may provide different, yet complementary, oxygen isotope signatures weighted by different seasonal biological productivities. This has been undertaken in the marine environment, and a recent example from the NW Pacific showed that combining δ18O from foraminifera and diatoms provided inter seasonal ocean
Summary
Analysis of the oxygen isotope composition of diatom silica requires samples that are almost pure diatomite since fluorination and other analytical techniques will liberate oxygen from all the components in the sediment. There is a generally acceptable protocol involving chemistry, sieving and settling techniques and more recently laminar flow separation. However, all lakes require a dedicated procedure and every sample must be scrutinised with microscopy. Where sediments cannot be purified
Acknowledgements
We wish to thank all our friends and colleagues who have helped with the challenges of using diatom oxygen isotopes in palaeoclimate research. In particular work with (in alphabetical order) Rick Battarbee, Sarah Davies, Roger Flower, Françoise Gasse, Viv Jones, Angela Lamb, Anson Mackay, Sarah Metcalfe, David Morley, Neil Roberts, Ninis Rosqvist, Nadia Solovieva, Alayne Street-Perrott, George Swann, Jo Thorpe and Jonathan Tyler. Peter Greenwood built the original fluorination line at NIGL and
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