Oligocene paleoceanographic changes based on an interbasinal comparison of Cibicidoides spp. δ18O records and a new compilation of data
Abstract Oligocene paleoceanographic changes are useful for assessing future changes in the warming world because the levels of atmospheric pCO2 in the Oligocene were similar to those expected within the next century. However, the overall Oligocene history is still unclear even though our understanding has been improved by previous studies. In this paper, we review the preexisting hypotheses for the Eocene–Oligocene and Oligocene–Miocene changes as well as the long-term global changes. Based on newly compiled Cibicidoides spp. δ18O (δ18OCIB) records, we divide the Oligocene paleoceanographic changes into five phases: an abrupt increase in δ18OCIB values during the Eocene–Oligocene transition (EOT) (Phase 1), a gradual decrease in the early Oligocene (Phase 2), a long-term slow increase in the early to middle Oligocene (Phase 3), a gradual decrease in the late Oligocene (Phase 4) and an abrupt increase near the Oligocene–Miocene transition (OMT) (Phase 5). These recurring sequential changes in the global-scale δ18OCIB profile following the EOT are likely not directly connected to the strengthening and stratification of ocean currents during the Oligocene as a result of the unidirectional strengthening of the Antarctic circumpolar current (ACC) and the northern component water (NCW). Instead, we suggest that the strengthened ocean currents contributed significantly to interbasinal differences in δ18OCIB values. For a more robust explanation of Oligocene long-term δ18OCIB changes following the EOT, future studies will require high-resolution and more reproducible records of changes in the volume of Antarctic ice and atmospheric pCO2, as well as information about the tectonic history.