Astrochronology of the Miocene Climatic Optimum record from Ocean Drilling Program Site 959 in the eastern equatorial Atlantic.

The Miocene Climatic Optimum (MCO; ~16.9–14.7 Ma) was a relatively warm interval which interrupted the Cenozoic cooling trend and bears analogies with projected near-future climate change. Evidence for MCO warming and climatic variability is dominantly based on studies from mid- to high-latitude regions and deep ocean benthic foraminiferal oxygen isotope reconstructions, whereas studies from tropical latitudes are needed to resolve latitudinal temperature gradients and ocean nutrient cycling. Sedimentary cores retrieved at Ocean Drilling Program (ODP) Site 959 (Leg 159) in the eastern equatorial Atlantic Ocean offer a near-continuous, low-latitude record spanning the Early to Middle Miocene, but age constraints were limited. To achieve an orbitally resolved age model, we generated new calcareous nannofossil and diatom biostratigraphy as well as high-resolution bulk carbonate stable carbon (δ13C) and oxygen isotope (δ18O) ratios, magnetic susceptibility (MS), weight percent CaCO3 and mean greyscale records. We record several diagnostic biostratigraphic markers and identify the well-dated onset of the MCO, Monterey Excursion (ME), Carbon Maxima (CM) events and peak warming in the bulk carbonate isotope records. An orbital age model is realized by tuning the bulk carbonate δ13C record to eccentricity extracted from the Laskar et al. (2004) astronomical solution that is consistent with the bio- and chemostratigraphic constraints. We conclude that the studied sediment record spans the Early to Middle Miocene interval between ~18.2 and 15 Ma and includes a hiatus directly prior to the onset of MCO of maximally ~700 kyr. All records reveal dominant eccentricity- paced variability, while prominent precession and obliquity paced variability is observed between ~16.9–16.1 Ma; this interval corresponds to a node in the long ~2.4 Myr eccentricity cycle. Moreover, a major shift from bio-siliceous-dominated to carbonate-rich sediments is found across the onset of the MCO (~16.9 Ma). Both lithologies represent intervals of relatively high productivity, likely associated with upwelling. Ultimately, our high-resolution record from Site 959 can provide an important opportunity for reconstructing a tropical paleoclimate record at precession-to-eccentricity resolution during the MCO.