Late Quaternary paleolimnology of Onepoto maar, Auckland, New Zealand: Implications for the drivers of regional paleoclimate

摘要

A high-resolution record of lacustrine environments spanning ca. 30-9 cal ka BP was obtained from Onepoto maar, northern North Island, New Zealand. The multi-proxy record of environmental change is constrained by tephrochronology and AMS ~(14)C ages and provides evidence for episodes of rapid environmental change during the Last Glacial Coldest Period (LGCP: 28.5-18 cal kaBP) and Late Glacial-Interglacial Transition (LGIT) from northern New Zealand. The Onepoto pollen record indicates that the LGCP was cold, dry and windy in the Auckland region with vegetation dominated by herbs and grasses in a beech forest mosaic. At the same time the diatom record indicates oligotrophic conditions with low lakes levels and turbulence whilst cladocerans indicate low water temperatures. The inference of cold, dry and windy conditions during the LGCP is supported by geochemical evidence for increased sediment influx, charcoal and CO_2 limiting conditions for terrestrial macrophytes. Rapid climate amelioration and forest expansion after ca. 18 cal ka BP corresponds with reduced sediment influx, diatom and cladoceran-inferred higher lake levels indicating increasing moisture availability and temperature. Diatom flora indicates that an oligotrophic, circumneutral lake was becoming established during the LGIT but conditions were still variable. Between ca. 13.8 and 12.5 cal ka BP two brief drier and possibly cooler episodes are apparent (ca. 13.8-13.2 and 13-12.5 cal ka BP) followed by a warm phase combined with generally stable high lake levels between ca. 12.5 and 10.5 cal ka BP. Subsequently the lake water chemistry became more alkaline and eutrophic, possibly because sea level had risen high enough by this time to influence ground water chemistry.