| The extensive fluvial successions of the Bighorn basin (Wyoming, USA) have produced the most complete continental record of early Paleocene to early Eocene age. Much of what we know today about mammal turnover in North America and the terrestrial response to transient global warming events during this critical interval of Cenozoic climate evolution comes from studies of this area. At present age control is insufficient to address such key-issues as mammal evolution rates and duration of global warming events in detail due to lack of reliable tie-points. In principle sedimentary cycles related to astronomical (Milankovitch) climate forcing may provide the necessary temporal accuracy and resolution. The fluvial successions indeed reveal a distinct rhythmic bedding due to the repetitive intercalation of fossil soils (paleosols) exhibiting striking orange, red and purple colors. The consistent color alternations have been attributed to pulsating tectonics, autocyclic processes or climate change, but the origin of the rhythmicity remains unclear. However, the successions have never been subjected to a detailed cyclostratigraphic study, including time series analysis, with the aim to syste¬matically explore the potential role of astronomical climate forcing. Due to considerable progress in conventional dating, time is now perfectly right to carry out such a cyclo¬stratigraphic study. Moreover a careful pilot study yielded very promising results indicating that Milankovitch cycles are indeed present in these successions. We therefore propose to carry out a rigorous and very detailed cyclostratigraphic and integrated strati¬graphic investigation of the classic fluvial successions in the Upper Paleocene to Lower Eocene Willwood Formation of the Bighorn Basin in order to determine whether astronomical climate forcing on Milankovitch (and sub-Milankovitch) time scales is indeed responsible for the observed hierarchical vertical paleosol stacking patterns as suggested by the outcome of our pilot. In addition climate proxies will be applied to reconstruct climate change, determine phase relations with the orbital parameters and establish an astronomical tuning for the fluvial successions. The potential influence of astronomical driven climate change on fluvial sedimentation, soil formation and flora/fauna composition will be investigated and the results will be compared with the outcome of climate modeling experiments of orbital extremes. Finally, very accurate and high-resolution age models will be used to determine - possibly climate driven - rates in mammal evolution and migration, and to put very tight constraints on the age and duration of carbon-isotope excursions associated with the extreme greenhouse events of this critical interval of time. |
