Climate change response to astronomical forcing during the Oligocene-Miocene transition in the equatorial Atlantic (ODP Site 926)

The Oligocene-Miocene transition period was characterized by a decrease in global CO2 levels, expansion of polar ice sheet, fall in global sea-level, etc. However, the reasons for, and mechanisms of, this global, extreme-cold climate change event ( Mi-1) still remain controversial. Our samples from the core of the Ocean Drilling Program ( ODP) Leg 154, Site 926, located in the equatorial Atlantic, mainly consist of light-gray, nannofossil chalk with foraminifers interbedded with greenish-gray, clayey, nannofossil chalk sediments. Color variation from light-gray layers ( up to 80% carbonate content) to dark layers (similar to 60% carbonate content) was observed to occur cyclically at the meter scale. Therefore, we chose color reflectance lightness ( L*) data as the paleoclimate proxy on which to perform cyclostratigraphic analysis because it could reflect carbonate content changes. Based on the recognition of the 405 kyr long eccentricity and similar to 40 kyr obliquity cycles of the L* series, we tuned the series to establish an absolute astronomical time scale using the published age of the Oligocene-Miocene boundary ( OMB) as the anchor for an absolute age control point. The power spectra of the tuned L* series showed that the long eccentricity signals became significantly weak, while the obliquity signals became strong, from the Late Oligocene to the Early Miocene. The 405 kyr long eccentricity minimum coincided with the 1.2 Myr obliquity node at the OMB, and similar convergences might be closely related to other extreme-cold events in Earth's history. In addition, the sedimentation accumulation rate, oxygen isotopes of benthonic foraminifers, and rodents' per-taxon turnover rate from Central Spain showed the same similar to 2 Myr cyclicity, which indicates the significant influence of Earth-orbital forcing on the Earth system and ecological evolution on the million-year time scale.