The role of paleoclimatic dynamics in hominin evolution is not well understood, largely due to the difficulty of generating relevant terrestrial paleoclimate records. A stable isotope aridity index can be used to estimate water deficit (WD) using stable oxygen isotopes in mammalian tooth enamel. WD is a function of potential evapotranspiration (PET) and mean annual rainfall, and PET is a function of temperature and latitude. The index is based on a comparison of species that are categorized as evaporation insensitive (EI) and evaporation sensitive (ES), based on regressions between WD and the isotopic enrichment between oxygen isotope values in tooth enamel and local meteoric water (ɛenamel-mw
). Taxa for which ɛenamel-mw
does not change with WD are classified as EI, and reflect local meteoric water. Taxa for which ɛenamel-mw
is sensitive to aridity are classified as ES, and reflect evaporative water sources (i.e. leaf or body water). Isotopic enrichment between EI and ES taxa increases with aridity. We present a revised calibration for the aridity index, based on a large compilation of previously published and new oxygen isotope values in tooth enamel (n > 1100 from 53 species of large mammals representing 34 locations in eastern and central Africa), that includes updated PET estimates and represents a substantially expanded range of WD values (>3700 mm/year).
The aridity index is used to estimate water deficit in the early Pleistocene of the Lake Victoria Basin, western Kenya. We present oxygen isotope values in fossil mammalian tooth enamel from the Homa Peninsula localities Kanjera South (ca. 2.0 Ma) and Nyayanga (1.78-2.3 Ma). We find that paleo-aridity estimates fall within the range of WD values in modern eastern African savanna biomes, and indicate that Kanjera South was less arid than Nyayanga. While paleo-aridity estimates must be interpreted with caution due to challenges relating to sample size, classification of EI and ES taxa, and geological context, these results demonstrate that this index can be used to reconstruct terrestrial paleoclimatic variation relevant to hominin evolution. Characterizing extant EI and ES taxa in other regions will expand the utility of this approach for examining animal water-use and terrestrial aridity in additional paleontological, archaeological, and modern systems.