Reconstruction of the pre-Pleistocene climate of continental interiors has been hampered by a lack of climatic proxies and limited temporal resolution. We developed a new approach based on the oxygen isotope composition (δ18O) of hematite, which occurs as a coating on fossil vertebrates, and used this method to study climatic change across the Paleocene-Eocene boundary in the Bighorn basin, Wyoming. Hematite coatings form in soils or shallow sediments, and therefore may be used to monitor near-surface conditions.
Hematite δ18O values are measured by combining selective leaching, laser fluorination, and mass-balance calculation. These values, in conjunction with δ18O values for co-occurring carbonates, are used to assess the plausibility of published hematite-water oxygen isotope fractionation relations. Though uncertainty remains as to the appropriate fractionation, all are relatively insensitive to temperature variations. Thus, differences in hematite δ18O values most likely reflect shifts in surface-water δ18O values.
Analysis of hematites spanning the Paleocene-Eocene transition reveals a roughly 4‰ decrease in the δ18O of surface water in very early Eocene time. This episode probably reflects a cooling event (perhaps as great as 6 °C) that started 0.7 m.y. after the terminal Paleocene δ13C excursion, which corresponds to a pulse of extreme marine warming. Following the cooling, which lasted ∼0.6 m.y., temperatures rebounded to values close to those for late Paleocene time. The cooling episode coincides remarkably well with other indicators of environmental and climatic change from the basin, such as mammalian turnover events and mean annual temperatures estimated from leaf physiognomy.