Ice-thickness variation during marine oxygen isotope stage 4–2 glaciation determined from kame terraces in the Rangitata Valley, New Zealand

Abstract

ABSTRACT An extensive kame-terrace sequence in the middle Rangitata Valley reveals ice-volume fluctuations spanning the last (Otiran) glaciation. Stratigraphic and sedimentologic characteristics document lateral ice-marginal processes and provide context for luminescence dating. The sequence provides novel and complementary data on glacier ice thickness, which fluctuated substantially throughout the Otiran glaciation. Thick ice constructed one of the highest kame terraces (540 m above the valley floor) ca. 68 ka and thinned nearly 500 m to the valley floor by ca. 53 ka. Following an episode of ice thickening to an unknown elevation, ice again thinned to the valley floor by ca. 44 ka. Ice thickened to its greatest late marine oxygen isotope stage (MIS) 3 extent of 480 m by ca. 37 ka, and thinned to 230 m by ca. 31 ka. The final ice expansion, to 260 m, occurred by ca. 25.5 ka, and the ice fluctuated and thinned to 240 m at ca. 22–20 ka and to 170 m at ca. 21–17 ka. Published cosmogenic radionuclide (CRN) ages indicate surface stabilization near the valley floor (55 m) by ca. 18 ka. This ice-thickness chronology provides an independently derived ice-volume record that is consistent with local and regional glacial chronologies. The site, lying between the Mackenzie Basin and the northern Canterbury Plains drainages, displays a chronology with advances correlative in part with each of those regions. Maximum ice extent occurred 70–65 ka in the Rangitata Valley and the Mackenzie Basin, while the subsequent ice expansion ca. 37 ka is similar in timing to chronologies in both the Rakaia Valley to the north and the Mackenzie Basin to the south.