A NEW ZEALAND RECORD OF GLOBAL CLIMATE CHANGE SINCE MARINE ISOTOPE STAGE 6

Okarito Bog, Westland, New Zealand (NZ) occupies a deep basin formed between moraine sets that predate Stage 2 glacial advances from the Southern Alps, one of the major centres of Pleistocene ice accumulation in the Southern Hemisphere. Peat cores from this site provide a continuous record of environmental change extending from the present back to late Stage 6. Multiple cores from the bog show a consistent sequence of alternating organic/inorganic units, whilst the pollen record of vegetation and climate change concurs with marine isotope stratigraphy, suggesting that this terrestrial record of Southern Ocean climate is primarily a response to orbital forcing. At least at the last glacial termination, radiocarbon dating shows that deglaciation in NZ occurred largely in phase with northern hemisphere (and not local) insolation forcing. Further work at Okarito Bog aims to achieve more precise chronological control through AMS 14C dating of pollen concentrates, luminescence dating and micro-tephra analysis.

In addition to climate change at the Milankovitch scale, the Okarito Bog record shows shorter-lived abrupt climate changes, typically in the form of marked fluctuations between herb and shrub pollen during cool stages. These are interpreted as primarily temperature-driven fluctuations in regional altitudinal vegetation zones, including treeline, that alternately bring subalpine grassland communities closer to and further away from the core site at near present sea level. A broad correspondence between herb pollen peaks at Okarito and independently derived records of regional glacier advances during the last glaciation supports this assertion. Ongoing work at Okarito Bog, aimed at higher sampling resolution, further palaeocological and sedimentological analyses, and improved chronological control (see above), therefore is being undertaken in pursuit of two further goals: (1) to generate a reliable continuous proxy record for regional glaciation and abrupt climate change; and (2) to establish more precisely the timing of abrupt climate change and significant glacier movement in this region in relation to any northern hemisphere counterparts. In addition, the same methodology will be applied to several other sites in Westland, with records provisionally shown to span the last glaciation (Stages 4-2).