Late Glacial to Holocene glacial history of the Merrie Range, South Central Fiordland, New Zealand

Abstract

Adjacent to Lake Roe on the western flanks of the Merrie Range, South Central Fiordland, are two suites of glacial moraines that reflect the history of deglaciation during the Late Glacial and Holocene. Terrestrial cosmogenic nuclide dating using 10Be from quartz provided exposure ages ranging from 8,790 ± 3,310 to 13,380 ± 630 years BP for the Lake Roe site, and from 11,050 ± 510 to 11,610 ± 520 years BP for the North Roe site. These exposure ages, along with the closely spaced moraine ridges, indicate a phased retreat of the glaciers beginning approximately 13,400 years ago, encompassing the Antarctic Cold Reversal (ACR) (14,700 – 12,700 cal yr BP) and the Younger Dryas Chronozone (YDC) (12,500 – 11,700 cal yr BP). Schmidt hammer R-values measured on erratic boulders from 13 moraines spanning both study sites indicate that the moraine formations are closely spaced in time, as the mean R-values from each moraine overlap within error and cannot be differentiated. Paleo-equilibrium line altitudes (ELAs) were reconstructed using multiple reconstruction methods. The reconstructed Lake Roe glacier displayed an increase in ELA from 1140 m at 13,380 ± 630 years BP to 1230 m at 8,900 ± 350 years BP. Similar ELA results were obtained for the North Roe glacier, with a rise in ELA from 1222 m at 11,610 ± 520 years BP to 1234 m at 11,050 ± 510 years ago. The combined use of pollen and elemental TOC and TN analyses enabled the reconstruction of the Mid – Late Holocene vegetation and climatic history of the area. The vegetation history of the Lake Roe area mirrors that of the general southern New Zealand region, in respect to the nature of vegetation change. Although the TCN dataset is sparse, there is sufficient age control on the timing of moraine formation to indicate that glacial advance/retreat phases occurred during both the ACR and YDC at the Lake Roe and North Roe sites. The ACR is generally considered to be 2-3°C cooler than present, whilst the YDC was characterised by warming in southern New Zealand and typically a period of glacier retreat. The pattern emerging in the Merrie Range is of cooling/temperature-driven advances during the ACR and of advances coeval with the YDC driven by increased precipitation, possibly associated with strengthening westerlies over Fiordland. However, further dating of the moraine sequence is required to assess this model.