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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 δ18 O 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. [ABSTRACT FROM AUTHOR]

While the Atlantic Ocean is ventilated by high-latitude deep water formation and exhibits a pole-to-pole overturning circulation, the Pacific Ocean does not. This asymmetric global overturning pattern has persisted for the past 2-3 million years, with evidence for different ventilation modes in the deeper past. In the current climate, the Atlantic-Pacific asymmetry occurs because the Atlantic is more saline, enabling deep convection. To what extent the salinity contrast between the two basins is dominated by atmospheric processes (larger net evaporation over the Atlantic) or oceanic processes (salinity transport into the Atlantic) remains an outstanding question. Numerical simulations have provided support for both mechanisms; observations of the present climate support a strong role for atmospheric processes as well as some modulation by oceanic processes. A major avenue for future work is the quantification of the various processes at play to identify which mechanisms are primary in different climate states. [ABSTRACT FROM AUTHOR]

The ocean depth at which the rate of calcium carbonate input from surface waters equals the rate of dissolution is termed the calcite compensation depth. At present, this depth is~4,500?m, with some variation between and within ocean basins. The calcite compensation depth is linked to ocean acidity, which is in turn linked to atmospheric carbon dioxide concentrations and hence global climate. Geological records of changes in the calcite compensation depth show a prominent deepening of more than 1?km near the Eocene/Oligocene boundary (~34 million years ago) when significant permanent ice sheets first appeared on Antarctica, but the relationship between these two events is poorly understood. Here we present ocean sediment records of calcium carbonate content as well as carbon and oxygen isotopic compositions from the tropical Pacific Ocean that cover the Eocene/Oligocene boundary. We find that the deepening of the calcite compensation depth was more rapid than previously documented and occurred in two jumps of about 40,000?years each, synchronous with the stepwise onset of Antarctic ice-sheet growth. The glaciation was initiated, after climatic preconditioning, by an interval when the Earth's orbit of the Sun favoured cool summers. The changes in oxygen-isotope composition across the Eocene/Oligocene boundary are too large to be explained by Antarctic ice-sheet growth alone and must therefore also indicate contemporaneous global cooling and/or Northern Hemisphere glaciation. [ABSTRACT FROM AUTHOR]

A 13-million-year continuous record of Oligocene climate from the equatorial Pacific reveals a pronounced "heartbeat" in the global carbon cycle and periodicity of glaciations. This heartbeat consists of 405,000-, 127,000-, and 96,000-year eccentricity cycles and 1.2-million-year obliquity cycles in periodically recurring glacial and carbon cycle events. That climate system response to intricate orbital variations suggests a fundamental interaction of the carbon cycle, solar forcing, and glacial events. Box modeling shows that the interaction of the carbon cycle and solar forcing modulates deep ocean acidity as well as the production and burial of global biomass. The pronounced 405,000-year eccentricity cycle is amplified by the long residence time of carbon in the oceans.