Evidence for the Early Oligocene formation of a proto-Subtropical Convergence from oxygen isotope records of New Zealand Paleogene brachiopods

Using the oxygen isotope record in fossil brachiopods, we reconstruct Late Eocene through Early Miocene paleotemperatures of an eastern New Zealand shelf environment. Our results show a decline in temperature to a low of ∼6°C in the Early Oligocene, of a magnitude similar to that of open-ocean isotopic studies from similar latitudes. However, the brachiopod-derived paleotemperatures are ∼ 10°C lower than those inferred from occurrences of warm-water fauna and flora (e.g., large foraminifera, mangrove and coconut palms), which suggest a subtropical (20–23°C) climate throughout most of the New Zealand Oligocene. We propose that a proto-Subtropical Convergence (pSTC), with cool subantarctic water flowing along the eastern coast of the South Island, first developed in the Early Oligocene. The subtropical fauna and flora distribution patterns are consistent with this hypothesis. Warm-water biota (e.g., larger foraminifera, and mangrove trees) occur primarily on the west and north coasts of New Zealand in warm subtropical water characteristic of water north of the pSTC, similar to the modern Subtopical Convergence. We explain the occurrence of occasional subtropical taxa in Early Oligocene strata of the east coast of the South Island as a result of the north-south migration of the pSTC or the colonization of relict Surtseyan-type seamounts in local waters warmed by volcanic eruptions.