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