Constraints on glaciation in the middle Eocene (46–37 Ma) from Ocean Drilling Program (ODP) Site 1209 in the tropical Pacific Ocean

Abstract

The presence of glacial ice in the late middle Eocene has been vigorously debated. Recently published sedimentary data from the high latitudes is suggestive of episodic cooling events and near‐freezing sea surface temperatures during parts of the late middle Eocene. Constraints on ice volumes and the significance of excursions in open ocean foraminifera and seawater δ18O reconstructions are less clear, and there are few high‐resolution δ18O records. We present a new detailed record of benthic foraminiferal δ18O from Site 1209 that exhibits variations (Δδ18Obenthic) of 0.6‰–1.3‰. Different approaches have previously been used to interpret Δδ18Obenthic, including (1) an a priori assumption of a 50% contribution of temperature, similar to what is reconstructed for the Last Glacial Maximum–recent change; (2) applying Oligocene calibrations between apparent sea level (ASL) and Δδ18Obenthic; or (3) assuming temperature and seawater δ18O contributions can be partitioned through comparison with benthic Mg/Ca. Using assumption 1, the record from Site 1209 indicates changes in seawater δ18O of 0.3‰–0.7‰, equivalent to ∼33–72 m (m) of ASL (assuming mean ice δ18O of ∼−45‰). Using assumption 2 and two different end‐member calibrations, the δ18Obenthic record implies changes in ASL of 23–50 m or 50–108 m. The third approach yields changes in seawater δ18O of up to 0.6‰ to 1.4‰. We explore the compatibility of the results of each of these approaches with other studies that discuss evidence for ephemeral glaciations during the middle Eocene with variable ice storage at one or both poles.