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 t o a low of similar to 6 degrees C in the Early Oligocene, of a magnitu de similar to that of open-ocean isotopic studies from similar latitud es. However, the brachiopod-derived paleotemperatures are similar to 1 0 degrees C lower than those inferred from occurrences of warm-water f auna and flora (e.g.. large foraminifera, mangrove and coconut palms), which suggest a subtropical (20-23 degrees C) climate throughout most of the New Zealand Oligocene. We propose that a proto-Subtropical Con vergence (pSTC), with cool subantarctic water flowing along the easter n coast of the South Island, first developed in the Early Oligocene. T he subtropical fauna and flora distribution patterns are consistent wi th this hypothesis. Warm-water biota (e.g., larger foraminifera, and m angrove trees) occur primarily on the west and north coasts of New Zea land in warm subtropical water characteristic of water north of the pS TC, similar to the modern Subtropical Convergence, We explain the occu rrence of occasional subtropical taxa in Early Oligocene strata of the east coast of the South Island as a result of the north-south migrati on of the pSTC or the colonization of relict Surtseyan-type seamounts in local waters warmed by volcanic eruptions. (C) 1998 Elsevier Scienc e B.V.