Climatic and non-climatic effects on the delta O-18 and delta C-13 compositions of Lake Awassa, Ethiopia, during the last 6.5 ka

Abstract

A comparison of a 6450 C-14 yr PO and delta(13)C record of authigenic calcite from Lake Awassa, Ethiopia, with other proxy climate records in the area suggests that the lake records long-term regional climate changes. Co-varying and increasing delta(18)O and delta(13)C values from similar to4800 BP suggest an aridification of climate after the early Holocene insolation maximum. After 4000 BP, humid conditions return until after similar to2800 BP when delta(18)O increases again, reflecting more and conditions recorded elsewhere in Ethiopia. In addition to these long-term changes, there are abrupt decreases in both delta(18)O(calcitic) and delta(13)C(calcitite) immediately after tephra layers. The likeliest explanation for these abrupt decreases in isotopes is the effect of tephra on the lake's catchment vegetation. delta(18)O, delta(13)C and lake-level measurements from Lake Awassa since the 1970s suggest that the lake is currently isotopically sensitive to short-term (annual-decadal) climate change. However, during this period, the catchment has undergone progressive deforestation that may have caused an increase in runoff. Caution is therefore required when reconstructing palaeoclimates as a contemporary lake may not always be a good analogue for lake hydrology in the past. (C) 2002 Elsevier Science Ltd. All rights reserved.