New composite bio- and isotope stratigraphies spanning the Middle Eocene Climatic Optimum at tropical ODP Site 865 in the Pacific Ocean

Abstract. The Middle Eocene Climatic Optimum (MECO) at ca. 40 Ma is one of the largest of the transient Eocene global warming events. However, it is relatively poorly known from tropical settings since few sites span the entirety of the MECO event and/or host calcareous microfossils, which are the dominant proxy carrier for palaeoceanographic reconstructions. Ocean Drilling Program (ODP) Pacific Ocean Site 865 in the low-latitude North Pacific (Allison Guyot) has the potential to provide a useful tropical MECO reference, but detailed stratigraphic and chronological constraints needed to evaluate its completeness were previously lacking. We have addressed this deficit by generating new high-resolution biostratigraphic, stable isotope, and X-ray fluorescence (XRF) records spanning the MECO interval (∼38.0–43.0 Ma) in two holes drilled at Site 865. XRF-derived strontium ∕ calcium (Sr∕Ca) and barium ∕ strontium (Ba∕Sr) ratios and Fe count records allow correlation between holes and reveal pronounced rhythmicity, enabling us to develop the first composite section for Holes 865B and 865C and a preliminary cyclostratigraphy for the MECO. Using this new framework, the sedimentary record is interpreted to be continuous across the event, as identified by a pronounced transient benthic foraminiferal δ18O shift of ∼0.8 ‰. Calcareous microfossil biostratigraphic events from widely used zonation schemes are recognized, with generally good agreement between the two holes, highlighting the robustness of the new composite section and allowing us to identify planktic foraminiferal Zones E10–E15 and calcareous nannofossil Zones NP15–18. However, discrepancies in the relative position and ordering of several primary and secondary bioevents with respect to published schemes are noted. Specifically, the stratigraphic highest occurrences of planktic foraminifera, Acarinina bullbrooki, Guembelitrioides nuttalli, and Morozovella aragonensis, and calcareous nannofossils, Chiasmolithus solitus and Sphenolithus furcatolithoides, and the lowest occurrence of Reticulofenestra reticulata all appear higher in the section than would be predicted relative to other bioevents. We also note conspicuous reworking of older microfossils (from planktic foraminiferal Zones E5–E9 and E13) into younger sediments (planktic foraminiferal Zones E14–15) within our study interval consistent with reworking above the MECO interval. Regardless of reworking, the high-quality XRF records enable decimetre-scale correlation between holes and highlight the potential of Site 865 for constraining tropical environmental and biotic changes, not just across the MECO but also throughout the Palaeocene and early-to-middle Eocene interval.