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 δ 18 O reconstructions are less clear, and there are few high‐resolution δ 18 O records. We present a new detailed record of benthic foraminiferal δ 18 O from Site 1209 that exhibits variations (Δ δ 18 O benthic ) of 0.6‰–1.3‰. Different approaches have previously been used to interpret Δ δ 18 O benthic , 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 Δ δ 18 O benthic ; or (3) assuming temperature and seawater δ 18 O contributions can be partitioned through comparison with benthic Mg/Ca. Using assumption 1, the record from Site 1209 indicates changes in seawater δ 18 O of 0.3‰–0.7‰, equivalent to ∼33–72 m (m) of ASL (assuming mean ice δ 18 O of ∼−45‰). Using assumption 2 and two different end‐member calibrations, the δ 18 O benthic record implies changes in ASL of 23–50 m or 50–108 m. The third approach yields changes in seawater δ 18 O 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.