Insights from the Paleogene tropical Pacific: Foraminiferal stable isotope and elemental results from Site 1209, Shatsky Rise

Stable isotope and elemental measurements were conducted on foraminifera from a sequence of calcareous pelagic ooze at Ocean Drilling Program Site 1209 to document the thermal evolution of the North Pacific water column over Shatsky Rise and to address long‐standing questions about the nature of oceanic circulation in the early Paleogene. A major change in seasonality and water column structure in the early Eocene marks a change in tropical Pacific climate and circulation just prior to the early Eocene climatic optimum. Subsequent long‐term cooling is interrupted by a transient positive δ 18 O excursion identified in planktonic and benthic foraminifera during the late early Eocene that is interpreted as evidence for ephemeral formation of deep water from a low‐latitude, saline source. Paired analysis of benthic Mg/Ca and δ 18 O was undertaken to assess the relative contribution of temperature and seawater oxygen isotope composition to the benthic δ 18 O record. Stratigraphic trends of benthic δ 18 O and Mg/Ca decouple during two intervals in the early Eocene and early middle Eocene. Although variable seawater δ 18 O is the most likely candidate to explain decoupling of benthic δ 18 O and Mg/Ca, it is difficult to argue for substantial change in continental ice volume during a presumably ice‐free interval of time.