A comparison of the palaeoclimate signals from diatom oxygen isotope ratios and carbonate oxygen isotope ratios from a low latitude crater lake

The analysis of oxygen isotope ratios (delta O-18) from authigenic lake carbonates has become a well-established palaeoclimate technique. Less common is the use of delta O-18 in biogenic silica (e.g. diatoms, sponge spicules, phytoliths), but the technique is being increasingly utilised for non-carbonate lakes. This paper aims to compare delta O-18(diatom) with delta O-18(carbonate) in a low latitude, closed basin lake. Due to the influence of pH, rarely are both carbonate and diatoms preserved in lake sediments in concentrations high enough to measure oxygen isotopes. The delta O-18 composition of both carbonate and silica should reflect the isotope composition and temperature of the lake water at the time of precipitation and hence theoretically they should be delta O-18(diatom) and delta O-18(carbonate) currently exists and that found differences between their equivalent. Only one comparative study of delta O-18 compositions, probably because they precipitated in different seasons. Unless there is evidence to suggest that silica and carbonate precipitate at the same time, their delta O-18 records are likely to be different but complementary. In this study, we show that delta O-18(diatom) and delta O-18(calcite) from Lake Tilo, Ethiopia show some similar broad climate trends. However, the delta O-18(diatom) curve is generally more variable and does not record two regional and events, picked up by the delta O-18(calcite) data. Several possible reasons for this are discussed: the precision of the delta O-18(diatom) extraction technique, contamination of the diatom samples from tephra (volcanic glass), vital effects in the diatom samples, differences in their respective equilibrium isotope fractionation rates and the possibility that the dominant diatom, Aulacoseira granulata, and calcite precipitated in different seasons during this time. Currently, the problems associated with the cleaning and extraction of delta O-18(diatom) suggests that details found in delta O-18(calcite) records may be lost in delta O-18 records where contaminants such as tephra and clay minerals are difficult to remove. (c) 2005 Elsevier B.V All rights reserved.