Last glacial jetting of cold waters through the Subtropical Convergence zone in the Southwest Pacific off eastern New Zealand, and some geological implications

Recent evidence suggests that the Subtropical Convergence (STC) zone east o f New Zealand shifted little from its modern position along Chatham Rise du ring the last glaciation, and that offshore surface waters north of the STC zone cooled only slightly. However, at nearshore core site P69 (2195 m dep th), 115 km off the east coast of North Island and ca 300 km north of the m odern STC zone, planktonic foraminiferal species, transfer function data an d stable oxygen and carbon isotope records suggest that surface waters were colder by up to 6 degrees C during the late last glacial period compared t o the Holocene, and included a strong upwelling signature. Presently sire P 69 is bathed by south-flowing subtropical waters in the East Cape Current, The nearshore western end of Chatham Rise supports a major bathymetric depr ession, the Mernoo Saddle, through which some exchange between northern sub tropical and southern subantarctic water presently occurs. It is proposed t hat as a result of much intensified current flows south of the Rise during the last glaciation, a consequence of more compressed subantarctic water ma sses, lowered sea level, and an expanded and stronger Westerly Wind system, there was accelerated leakage northwards of both Australasian Subantarctic Water and upwelled Antarctic Intermediate Water over Mernoo Saddle in a mo dified and intensified Southland Current. The expanded cold water masses di splaced the south-flowing warm East Cape Current off southeastern North Isl and, and offshore divergence was accompanied by wind-assisted upwelling of nutrient-rich waters in the vicinity of P69. A comparable kind of inshore c old water jetting possibly characterised most glacial periods since the lat est Miocene, and may account for the occasional occurrence of subantarctic marine fossils in onland late Cenozoic deposits north of the STC zone, rath er than invoking wholesale major oscillations of the oceanic STC itself. (C ) 2000 Elsevier Science B.V. All rights reserved.