PALEOSOL ISOTOPIC RECORD OF REGIONAL CLIMATIC AND GLOBAL ATMOSPHERIC CHANGES ACROSS THE EOCENE-OLIGOCENE BOUNDARY IN THE NORTHERN GREAT PLAINS, U.S.A

From the late Eocene to the early Oligocene (ca. 35-30 Ma), the Earth system transitioned to a glacial icehouse state for the first time since the Permian. Rapid growth of the Antarctic ice sheet during the earliest Oligocene to almost modern volume (Oi-1 event) is recorded by glaciomarine sediments and a persistent +1‰ shift in the oxygen isotope composition (δ¹⁸O) of benthic forams. The onset of Oi-1 coincides with a +0.7‰ excursion in the carbon isotope composition (δ¹³C) of benthic forams that suggests increased burial of organic carbon. We use the δ¹⁸O and δ¹³C values of 139 paleosol carbonates in five measured sections of the White River Group (WRG) and superposed Sharps Formation (SF) in Toadstool Park, NE and the Big Badlands, SD to examine the terrestrial record of this transition in mid-continental North America. Based on a new stratigraphic framework, our composite section provides a mostly complete record from the latest Eocene to the early Oligocene. Carbonate δ¹⁸O values do not change abruptly and permanently correlative with Oi-1 but instead increase steadily up-section. Mean values increase in the WRG from 20.4±1.71‰ in the latest Eocene to 22.4±1.53‰ in the earliest Oligocene to 24.8±0.72‰ in the SF. Cooling by ca. 5° C and evaporative enrichment of soil water by ca. 3.5‰ due to increasing aridity can explain the δ¹⁸O trend. This result is consistent with other paleoclimatic data for the region but distinct from those on other continents. Increased aridity may reflect surface uplift to the west causing the initiation/intensification of the modern monsoonal regime over western and central North America in which most moisture is delivered to the northern Great Plains by regional anti-cyclonic air flow during summer. Carbonate δ¹³C values increase from -7.5‰ at the base of our section to -5.9‰ coincident with the Oi-1 carbon isotope excursion in the earliest Oligocene and then decrease to a stable value of ca. -8.4‰ in the upper WRG and SF. The early and late values suggest C₃ biomass and changes in aridity and atmospheric pCO₂ that are consistent with our δ¹⁸O data and the marine record. Timing of the δ¹³C peak indicates the terrestrial biosphere and marine reservoirs of the global carbon cycle were linked via atmospheric CO₂ and supports a role for greenhouse radiative forcing in the onset of the Cenozoic icehouse.