University of Oregon, United States

The St. Stephens Quarry, Alabama (SSQ) outcrop and corehole offer a unique opportunity to evaluate the largest global climate cooling event of the Cenozoic ( nearly equal 33.8-33.5 Ma; latest Eocene-earliest Oligocene) in which warm, high CO2 greenhouse conditions gave way to the icehouse climates that still prevail today. We integrate upper Eocene-lower Oligocene lithostratigraphic, magnetostratigraphic, biostratigraphic, stable isotopic, Mg/Ca, benthic foraminiferal faunal, downhole log, and sequence stratigraphic studies from the SSQ corehole, that provides a superior record to the adjacent outcrop because of limited weathering. The SSQ succession is dissected by hiatuses associated with sequence boundaries: North Twistwood Creek-Cocoa (35.4-35.9 Ma), mid-Pachuta (33.9-35.0 Ma), Shubuta-Bumpnose (lowermost Oligocene; nearly equal 33.6 Ma), Mint Spring-Red Bluff (33.0 Ma), Byram-Glendon (30.5-31.7 Ma), and Bucatunna-Chickasawhay (the famed mid-Oligocene fall; nearly equal 30.2 Ma). Here, we integrate three proxies (ae18O, Mg/Ca, sequence stratigraphy) from SSQ with deep-sea Sites 522 (South Atlantic) and 1218, (Pacific) ae18O and Mg/Ca records to delineate for the first time the three components of the greenhouse-to-icehouse transition (cooling, ice-volume increase, and sea-level fall). A nearly equal 1 0/00 ae18O increase in the SSQ corehole is correlated to the global earliest Oligocene (Oi1) event using magnetobiostratigraphy; this increase is associated with the Shubuta-Bumpnose contact, an erosional surface, and a biofacies shift in the corehole, providing a first-order correlation between ice growth and a sequence boundary that indicates a sea-level fall. A precursor ae18O increase of 0.50/00 (33.8 Ma, mid-Chron C13r) at SSQ correlates with a 0.50/00 increase in the deep Pacific and Atlantic Ocean. Our comparisons reveal that the Eocene-Oligocene transition occurred in two to three steps, with increasing influence by ice volume relative to cooling. In total, ice sheets grew to nearly equal 125% larger than they are today, with an associated nearly equal 105 m relative sea-level ( nearly equal 67 m eustatic) fall. Our study establishes the relationships among ice volume, ae18O, and sequences: a latest Eocene cooling and minor ice-volume event was followed by an earliest Oligocene ice-volume and cooling event that lowered sea-level and formed a sequence boundary during the early stages of eustatic fall.