Mio-pliocene seasonality on the snake river plain: comparison of faunal and oxygen isotopic evidence

Abstract

Analysis of fish faunas and oxygen isotopic composition of a fish otolith from lacustrine deposits of southwestern Idaho provide a means of evaluating regional Miocene and Pliocene climates. A disharmonious assemblage consisting of coldwater salmon and trout and warmwater sunfish and catfish from the Chalk Hills Formation of the Snake River Plain indicates that the climate of the late Miocene was warm and moist with cool summers and mild winters. Colonization of the lake by deepwater sculpins and whitefish in the Pliocence indicates that the climate was moist and equable, but with summers cooler than either the Miocene or Quaternary. Oxygen isotopic variation among seasonal growth rings in an aragonitic otolith of a Pliocene littoral sunfish suggests a seasonal range of temperatures locally more equable than at present. Extremely depleted values of [delta]18O in carbonates suggest that the lake was maintained by tributaries from high-elevation watersheds, with locally low evaporation, rather than high precipitation.