Palaeoclimatic implications of isotopic data from modern and early Holocene shells of the freshwater snail Melanoides tuberculata, from lakes in the Ethiopian Rift Valley

Carbon and oxygen isotope ratios in the shells of the freshwater snail Mela noides tuberculata yield information on the isotopic composition of the wat er in which the shell was formed, which in turn relates to climatic conditi ons prevailing during the snails' life span. Melanoides is particularly imp ortant because it is widespread in Quaternary deposits throughout Africa an d Asia and is ubiquitous in both fresh and highly evaporated lakes. Whole-s hell and incremental growth data were collected from modern and fossil shel ls from two lakes in the Ethiopian Rift Valley. delta(18)O values in the mo dern shells from Lake Awassa are in equilibrium with modern waters, while d elta(18)O values in subfossil shells from the margins of Lake Tilo indicate high rainfall during the early Holocene. Sequential analysis along the gro wth spiral of the shell provides information on seasonal or shorter-term va riability of lake water during the lifetime of the organism.