NEW OSL GLACIAL CHRONOLOGY FROM THE RAKAIA VALLEY, NEW ZEALAND

Chronologic data emerging from glacial landforms and sediments in the Rakaia Valley, a major drainage on the eastern flank of the Southern Alps, document a complex OIS 6-2 glacial succession. Data reported here were determined through optically stimulated luminescence dating of sediments exposed in thick (>100 m) stratigraphic sections in the inner Rakaia Valley and will be augmented by cosmogenic radionuclide exposure age dating of moraine surface boulders. The thick stratigraphic sequence exposed in valley exposures in the middle Rakaia valley contains lodgement till, basal meltout till, ablation till, glacial lacustrine sediments, and thick aggradational outwash gravels. Basal glacial-lacustrine sediments exposed at the Rakaia Gorge and at Acheron Bank/Whitecliffs yielded OSL ages of ca. 180 ka, while pond sediments in basal till at Montrose station yielded an age of ca. 130 ka. These results document glacial activity both very early and very late during OIS 6. The preservation of these sediments in this highly active fluvial and glacial environment is surprising. A stratigraphically higher section at Montrose station yielded an OSL age of 77 ka, documenting OIS 4 glaciation. The only previous numerical ages pertaining to Rakaia glaciation, from Lyndon Stream, document two glacial advances around the Northern Hemisphere ice sheet maximum( (LGM), shortly before and shortly after 18-23 ka. While this preliminary chronology documents glacial advances during major episodes of Northern Hemisphere ice sheet glaciation, dramatic cooling is not necessarily implied to drive these advances. In fact, new paleoecological data, from fossil beetles and chironomids at the Lyndon Stream site document minimal cooling, of -2 deg. C to 0 deg. C versus modern during the LGM. Minimal cooling in the hyperhumid central ranges of the Southern Alps would greatly enhance snow accumulation, and thus could be a major driver of ice buildup during times of sustained westerly atmospheric flow.