Last Glacial Maximum to Holocene environmental history of the Lake Tennyson area, North Canterbury, New Zealand

Abstract

The once extensively glaciated Southern Alps of New Zealand are an important location for testing hypotheses surrounding regional and hemispheric climate synchronicity, particularly during the last glacial cycle. A key means of addressing such hypotheses has been the construction of high resolution glacial chronologies using the relatively new surface exposure dating (SED) technique. The existing chronologies provide good constraint on the timing of glacial fluctuations however, they are largely limited to the central Southern Alps. This thesis extends the geographic coverage of the SED catalogue by constructing a glacial chronology from two interrelated moraine suites to the south and southwest of Lake Tennyson, North Canterbury. Cosmogenic nuclide exposure ages and field mapping suggest that ice abandoned its Last Glacial Maximum (LGM) margins ca. 18.73 ± 0.36 kyr B.P. Paleo-equilibrium-line altitudes were reconstructed for each of the dated moraines, comparing toe-to-headwall altitude ratios of 0.35, 0.4, and 0.5. During the local LGM, ELAs were between 610 and 740 m lower than present and the associated local LGM temperatures were between 3.66 and 4.45ºC cooler. Chemical and palynological evidence from selected sediment cores extracted from interand intra-moraine peat bogs revealed that a warming trend occurred following the removal of glacial ice whereby alpine grassland was colonised by shrubs. By 1,020 ± 26 cal. yr B.P., beech forest (Nothofagus) had migrated from LGM refuges into the shrubland to become the dominant plant cover. Sub-bottom acoustic profiling and coring of Lake Tennyson revealed heavily disturbed sediments attributed to one or more large historic earthquakes. Radiocarbon dating of organic material within and above deformed layers constrains the timing of the most recent deformation event to between 626 ± 37 and 113 ± 82 cal. yr B.P while folded coseismic turbidite layers provide evidence for more than one earthquake prior to the deformation. A hummocky slump deposit within the lake is linked to the seismic deformation. The source of this slump can be traced to head scarp through which the trace of the Awatere Fault passes.