LATE GLACIAL ENVIRONMENTAL CHANGE RECORDED IN A NORTHERN NEW ZEALAND MAAR CRATER LAKE

Onepoto maar crater is situated in Auckland City, northern New Zealand, and contains an almost continuous and high-resolution record of lacustrine sedimentation spanning to at least 250 ka. The section of interest here covers the period from ca. 28 to 8 ¹⁴C kyr BP during which ca 4 m of laminated organic-rich sediments accumulated in the deep paleolake. Tephra from the central North Island are present throughout the sequence, and provide a chronostratigraphic framework underpinned by AMS ¹⁴C ages. Environmental changes within the lake and catchment are interpreted from variations in a range of physical, chemical, and biological proxy indicators including: grain size, major oxide and trace element geochemistry, total organic matter content, elemental carbon/nitrogen ratios, carbon and nitrogen isotopes, magnetic susceptibility, diatom assemblages and biogenic silica content. These proxies have allowed us to infer changes in the extent and productivity of vegetation in the lake and catchment, as well as changes in the source of sediments.

Elements of the bulk organic matter and geochemical content of the sediments preserved a high-resolution record of vegetation response to climate perturbations, even during climatic events that were probably insufficient to induce a major changes in the vegetation cover. There are indications of an interval of low plant productivity and erosion commensurate with cool and dry conditions from ca 27 to 14.7 ¹⁴C ka BP; following which climate amelioration with increasing plant productivity (both aquatic and terrestrial) was accompanied by reduced input of terrigenous material. A subsequent marked decline in biomass productivity and enhanced terrestrial input after ca 11.4 ¹⁴C ka BP suggests the resumption of cooler and/or drier conditions between ca. 11.4 to 10 ¹⁴C ka BP. Finally the onset of early Holocene warm, wet conditions was marked by high aquatic and terrestrial productivity that is displayed in most of the proxies. The details of the paleoenvironmental interpretation will be discussed as will the implications for late glacial paleoclimate in the SW Pacific.