A REVIEW OF THE OXYGEN ISOTOPE COMPOSITION OF LACUSTRINE DIATOM SILICA FOR PALAEOCLIMATE RECONSTRUCTION

Abstract

Diatom silica (frustules) are a form of biogenic opal and contain oxygen isotopes that are being increasingly used in palaeoclimate studies. Frustules are comprised of an inner tetrahedrally bonded silica skeleton (Si-O-Si) with an outer, hydrous layer. The hydrous layer is freely exchangeable and must be removed prior to oxygen isotope measurement using oxidising reagents and/or high temperatures. Analysis of the oxygen isotope composition of diatom silica requires samples that are almost pure diatomite since extraction 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. Recent studies of lacustrine diatoms have shown that even a small proportion of contaminant can have a significant influence on the oxygen isotope value. All lake sediments require their own specific procedure and every sample must be scrutinised by microscopy to check for the level of contamination prior to analysis. Where sediment cannot be purified sufficiently, a semi-quantitative assessment of the diatom content can facilitate mass balance techniques. When robust preparation methods are used, diatom silica offers an important palaeoclimate proxy, providing an additional complementary, rather than an alternative, host of oxygen isotopes in carbonates. In lacustrine isotope studies it is often assumed, but rarely proven, that diatom silica form in isotope equilibrium and hence empirically derived palaeotemperature equations, usually made in vitro, can be used. Most studies lack the detailed investigation of contemporaneous materials from natural waters of the region under study to test these equations. Further investigations need to be conducted in order to understand the systematic relationship between temperature, water isotope composition and diatom isotope composition to enable quantitative interpretation of the sediment record. Experiments to test for vital effects and inter-specific differences have so far revealed little variation. It is useful to know when and where the diatom silica is formed in the modern environment, so details of diatom ecology are important in the interpretation of isotope data. Here we show that valuable palaeoclimate data can be gained from the oxygen isotope composition of diatom silica (δ18Odiatom), especially since diatoms are abundant in many lakes sensitive to climate variation where other hosts (e.g., carbonates) are absent. To date the most successful studies have been conducted in areas where the δ18Odiatom registers changes in the oxygen isotope composition of the lake water (rather than temperature) which is then related to other aspects of climate. In these studies, the range of values obtained is often greater than the error introduced by contaminating materials. © 2005 NERC. Published by Elsevier B.V. All rights reserved.