Sedimentology and tephrochronology of last-glacial and holocene lake sediments and peat from south Westland, New Zealand

Abstract

A high resolution study of lake sediments and peat from three ice marginal sites in South Westland was carried out with the aim of determining the nature and timing of abrupt climate change during the last glaciation and the glacial-interglacial transition. The flux and composition of aeolian dust were used as proxies for the extent of exposed outwash surfaces and hence the extent of ice advance in the vicinity of the core sites. Cores were collected from Skiffington Swamp, Galway Tarn, and Okarito Pakihi in South Westland and the sediments characterised by measuring magnetic susceptibility (MS), combustible organic carbon (COC), grain size and mineralogy. Previously analysed pollen assemblages (Vandergoes, 2000; Vandergoes and Fitzsimons, 2003; Vandergoes et al., 2005; M. J. Vandergoes, unpublished data) were correlated with the data from this study to interpret the relationship between climate and glacial extent. Cores from Skiffington Swamp and Galway Tarn contained a visible tephra layer identified as the Kawakawa Tephra (c. 26,500 cal. years B.P.) based on stratigraphic position and glass shard composition. This layer provided an excellent chronostratigraphic reference point. The results from this study show that between 58,000 and 38,000 cal. years B.P., the climate was cool and variable in South Westland. Minor glacier advances occurred within a few kilometres of the Southern Alps. More significant glacier advance was recorded by MS and COC data at Skiffington Swamp after 38,000 cal. years B.P., which implies that glacial advance occurred earlier than the climate cooling recorded by pollen studies. Data from the most distal site (Galway Tarn, located approximately 10 km to the west of Skiffington Swamp) show that ice advanced near to that site at 32,000 cal. years B.P. This marks the start of maximum post-MIS 4 glacier extent. MS values were lowered at Skiffington Swamp during the interval from 30,000 to 22,000 cal. years B.P., indicating that the glacier margins and outwash plains were located well to the west. A decline in MS values at Galway Tarn and a peak in MS values at Skiffington Swamp occurred at 20,000 cal. years B.P., as glaciers retreated and outwash areas moved closer to the Southern Alps. There is no clear evidence at these three sites for a late glacial ice advance. Any such advance had limited impact 10-20 km from the present ice position.