The stratigraphic section located at Toadstool Park, northwestern Nebraska, preserves a detailed sedimentological and fossil record of the Eocene-Oligocene boundary, yet chronologic control is limited to sparse ash dates and magnetostratigraphy. To improve the chronologic control of the Toadstool Park sediments, we examined the fossil collection at the University of Nebraska State Museum and recorded the stratigraphic level of some important biochronologic events. All stratigraphic levels were determined relative to the Upper Purplish White layer (UPW), a distinctive ash that is chemically and mineralogically correlated to an ash dated at 34.6 ± 0.1 Ma. The small artiodactyls Leptomeryx speciosus, Leptomeryx mammifer, and Leptomeryx yoderi co-occur at ‒34 ± 2 m, implying that the early Chadronian–middle Chadronian boundary (35.7 Ma) is located at this level. The first occurrence of L. mammifer (i.e., the middle Chadronian–late Chadronian boundary, 34.8 Ma) is located at ‒18 ± 2 m, and the first occurrence of the camel Poebrotherium wilsoni (34.2 Ma) is located at 0 ± 2 m. The first occurrences of two other artiodactyls (Hypertragulus calcaratus and Leptomeryx evansi), which define the Chadronian-Orellan boundary (33.9 Ma), are located approximately at the UPW, ~5 m lower than in the nearby type sections of Wyoming. This apparent diachroneity may reflect sampling bias in Nebraska, i.e., loose material washed down from higher outcrops. The first occurrences of the dwarfed oreodont Miniochoerus affinis and of the rodent Eumys elegans (i.e., late early Orellan, 33.6 Ma) are located at +8 ± 2 m and +11 m, respectively. Finally, the first occurrence of M. gracilis (i.e., early late Orellan, 33.2 Ma) is located at +13 ± 2 m. These time points, along with magnetostratigraphy and 40 Ar/ 39 Ar dates obtained for this section, were used to refine the age model for a previously published stable isotope record of fossil bone CO 3 . The record is augmented here with 59 bone samples from federal lands near Toadstool Park spanning the critical time period of 33.8–33.4 Ma. The δ 18 O values in the complete data set show a step to higher values (from 23.0‰ ± 0.2‰ to 24.5‰ ± 0.2‰) at ca. 34 Ma, synchronous with the marine record. In combination with unchanged enamel compositions, this increase in bone CO 3 δ 18 O translates into a mean annual temperature drop of 7.1 ± 3.1 °C across the boundary. The time of onset of the climate change coincides with an increase in sedimentation rate from 19 to 37 m/m.y., confirming the strong role of climate on continental erosion and sediment yield.

The greenhouse-icehouse change across the Eocene-Oligocene transition and associated Oi-1 glaciation event is the most profound climatic change in Earth’s recent geological history. Marine reconstructions of the Oi-1 glaciation using foraminiferal δ 18 O isotopic compositions suggest that much of the change was associated with Antarctic ice growth rather than climatic change. Nonetheless, some cooling is expected to have occurred on land in addition to drier conditions associated with water tied up in the polar ice caps, and some recent results based on stable isotope analyses of bones support this viewpoint. Nonmarine paleoclimatic conditions (mean annual temperature, mean annual precipitation) may be quantitatively reconstructed using paleosols preserved in continental successions to test this general model. Results from Oregon and Nebraska suggest moderate drying and cooling, not as a stepwise change at the time of the Oi-1 glaciation, but as part of a long-term aridification and cooling event associated in part with emplacement of the Cascade Range. In contrast, intermontane Montana’s paleoprecipitation and paleotemperatures fluctuated on short-term (i.e., Milankovitch) time scales but on balance were both essentially unchanged by the Oi-1 glaciation. Results from Europe (UK, Spain) suggest a different pattern characterized by stable (i.e., unchanging) paleotemperatures in both localities and increasingly wet conditions in the UK. Taken together, these results indicate that (1) strongly regionalized climatic change was associated with the Oi-1 glaciation, (2) physiographic position with respect to orographic features played a key role in determining those regional climatic responses to the global event, and (3) there was little or no cooling on land associated with the Oi-1 glaciation.