WHEN AND WHERE IS MY CRITICAL ZONE, AND WHAT IS THE ROCK RECORD REALLY RECORDING?

The Eocene-Oligocene Transition (EOT) marks a global shift from hothouse to icehouse conditions at approximately 33.7 Ma. On the Great Plains, vertical changes in lithology, paleosols, stable isotopes, and faunal communities within the terrestrial White River Sequence of Wyoming, Nebraska, and South Dakota have commonly been used as evidence in support of this shift. At any given location, paleosols change up section to progressively open conditions, sediments change from mudstone to siltstone dominated, fluvial systems change from meandering to braided and ephemeral streams, and sedimentation rates increase up section; but these changes are not synchronous across the region. They appear to step through time from southwest to northeast. This raises the question of causality and the need to differentiate between local to regional controls on sedimentation, which can masquerade as climate change, versus climatically induced changes. In order to address this question, vertical changes in potentially climate-sensitive features across several key exposures of the Eocene-Oligocene White River Sequence are plotted and correlated with respect to recently revised radiometric dates from volcanic ashes in Wyoming and Nebraska to produce a spatiotemporal framework of climate and environmental signatures. Comparisons of stratigraphic intervals and individual paleolandscapes across several hundred kilometers from southwest to northeast between and along volcanic tuff markers show dramatic differences in paleo-CLORPT conditions. Sedimentation rates are higher to the southwest, with overall degrees of pedogenesis greater to the northeast in association with relatively “wetter” soil conditions along isochronous paleolandscapes. Stable isotopes from fossil bones suggest greater water stress and cooler temperatures from southwest to northeast within defined time intervals, and biostratigraphic zonations are diachronous across this region. In order to fully understand the dynamics of the terrestrial EOT across the Great Plains, or any deep time critical zone, the potential spatiotemporal variability of, and influences on, climatically sensitive features must be documented.