A 9000-year oxygen and carbon isotope record of hydrological change in a small Ethiopian crater lake

The stable isotope composition of authigenic calcites in a core from Lake Tilo, a small crater lake in the Ethiopian Rift Valley, provides a subcentury scale record of lake response to climatic change over the last 8850 14C yrs (9850 cal. yrs). An unusually high range of δ18O and δ13C values (~15‰) is attributed, in part, to major changes in hydrothermal groundwater flux. Although hydrothermal groundwater influx to the lake was high during the early Holocene, its flow rate was relatively stable and thus climatically induced changes to the water budget can be inferred from variations in δ18O and δ13C ratios. A major decline in hydrothermal groundwater input from ~5500 14C yrs BP increased lakewater residence time and led to substantial increases in δ18O and δ13C, before the mid-Holocene transition to more arid conditions. During the last ~2700 14C years diagenetic processes have resulted in extremely varied δ18O and δ13C values. The Holocene isotope record from Lake Tilo challenges the widely held assumption that crater lakes act as 'gigantic rain gauges', sensitive only to changes in precipitation/evaporation ratios.