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

Abstract

The analysis of oxygen isotope ratios ( δ 18O) from authigenic lake carbonates has become a well-established palaeoclimate technique. Less common is the use of δ 18O 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 δ 18O diatom with δ 18O 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 δ 18O 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 equivalent. Only one comparative study of δ 18O diatom and δ 18O carbonate currently exists and that found differences between their δ 18O compositions, probably because they precipitated in different seasons. Unless there is evidence to suggest that silica and carbonate precipitate at the same time, their δ 18O records are likely to be different but complementary. In this study, we show that δ 18O diatom and δ 18O calcite from Lake Tilo, Ethiopia show some similar broad climate trends. However, the δ 18O diatom curve is generally more variable and does not record two regional arid events, picked up by the δ 18O calcite data. Several possible reasons for this are discussed: the precision of the δ 18O 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 δ 18O diatom suggests that details found in δ 18O calcite records may be lost in δ 18O diatom records where contaminants such as tephra and clay minerals are difficult to remove.