THE MID-PLEISTOCENE CLIMATE TRANSITION IN NEW ZEALAND

The Mid-Pleistocene transition (MPT) at 900-600ka marks a fundamental reorganisation of the climate system from a world that for the preceding 33 million years had been dominated by 41ka oscillations in polar ice volume, to a 100ka climatic beat coincident with the development of much larger northern hemisphere ice sheets. The New Zealand continent sits astride a collisional sector of the Australia-Pacific plate boundary in the mid-latitude southwestern Pacific Ocean, and affords a range of high-quality, deep-ocean and shallow-marine sedimentary archives spanning the MPT. The high-resolution record of ODP 1123, located 1100 km offshore from eastern New Zealand, displays marked variations in warm- and cold-climate indicator species, in both vegetation and planktonic foraminiferal records at Milankovitch-scale periodicities. Time series analysis indicates that the vegetation record is covariant with marine climate proxies and is strongly coherent at the 40 and 100 ka orbital frequency. A pronounced increase in amplitude and a coeval decrease in frequency of sedimentary cycles from 40 to 100 ka occurs between 0.92 and 0.62 Ma. The dominance of subantarctic planktonic foraminiferal taxa during glacial phases implies periodic expansion of Antarctic surface water and/or Antarctic intermediate water over site 1123, which is coeval with intensified deep-water inflow to the Pacific Ocean reported by Hall et al. (2001 – Nature). Eustatic sea-level changes across the MPT are recorded by 10 shallow marine cyclothems or depositional sequences in onshore Wanganui Basin, that correspond to 41 and 100ka sea-level cycles during Oxygen Isotope Stages 32-10. Each sea-level cycle is represented by an individual depositional sequence comprising transgressive, highstand, and regressive systems tracts. In general, major sedimentary facies within the basin fill were deposited in a range of shoreface, and shelf marine environments during the rise, highstand and falling part of each glacio-eustatic cycle. 100ka sequences above the Bruhnes-Matuyama boundary (0.78 Ma) are thicker overall, display larger amplitude water depth changes, and thicker interglacial condensed intervals, than underlying 41ka-duration sequences.