Multi-proxy records of Holocene climate and vegetation change from Ethiopian crater lakes

The sediments of Ethiopian crater lakes have differentially sensitive palaeoclimate indicators that vary with time. Lake Tilo, in the south-central Rift Valley, shows a 10,000-year diatom and oxygen-isotope sequence that may be interpreted in terms of hydrochemical and hydrological responses to century-scale climate changes. The diatom record of lake salinity became sensitive to climate variability only after a sharp reduction in hydrothermal inflow at 5500 ¹⁴C yrs BP. In contrast, the oxygen isotope composition of the lake as recorded by authigenic calcite varied in response to early Holocene climatic change despite the hydrothermal influence and became especially sensitive to climate variability after hydrothermal flow diminished. Pollen data from the same core show savanna vegetation throughout the Holocene, indicating that strong rainfall seasonality has long been characteristic of the region. Deposits of aragonite varves in Lake Hora, a crater lake on the western margin of the Rift Valley, may reveal climate variability at annual to decadal timescales. Because the aragonite is precipitated during dry-season mixing, aragonite δ¹⁸O values for individual white laminae reflect the composition of the entire lake integrated over its water-residence time of about ten years. Nevertheless, the high-resolution varve chronology presents an opportunity for calibrating the isotopic record against instrumental climate data, provided that the climatic controls on the isotopic composition of these groundwater-fed lakes are fully understood.