Origin and Isotopic Composition of Aragonite Laminae in an Ethiopian Crater Lake

Abstract

Millimetre-scale white aragonite laminations alternating with dark diatom-rich organic layers are present in the uppermost sediments of the crater lakes Hora and Babogaya, at Debre Zeit, Ethiopia. The sediment accumulation rate calculated from lamina counts matches that estimated from a 210 Pb chronology, indicating that the laminations were deposited annually. The oxygen-isotope composition of the surface white layer is equivalent to that of surface water, which shows that white-layer aragonite is formed in isotopic equilibrium with the lake water, and suggests that isotopic analyses of these layers may provide valid paleoclimatic information. Because the aragonite is probably precipitated during dry-season mixing, aragonite δ 18 O values for individual laminae reflect the composition of the entire lake, integrated over its water-residence time of about 10 years. The sedimentary record of oxygen-isotope variations should therefore be interpreted as a proxy-climate record with decadal rather than annual resolution. Comparisons between δ 18 O values for the laminae and climate data for equivalent years show no clear relationships, so calibration of the sedimentary record requires a more detailed understanding of the climatic controls on the isotopic composition of these groundwater-fed lakes. An isotopic mass-balance model of the lake's response to rainfall variation shows (1) that the oxygen isotope composition of the lake waters varies by about 1 ‰, which is comparable to the range of δ 18 O values determined from the individual laminae; and (2) that modelled lake level is a reasonable match to observed levels, confirming that climate changes can interpreted from the oxygen-isotope record. 2