¹ºBe Surface-Exposure Chronology of the Left-Lateral Moraines of the Former Pukaki Glacier Lobe in the Mackenzie Region, South Island, New Zealand

During the Last Glacial Maximum (LGM), a lobe fed by tributary glaciers draining the Main Divide of New Zealand\u27s Southern Alps, flowed southward into the Mackenzie Region to fill the depression now occupied by Lake Pukaki. This former Pukaki glacier lobe deposited one of the most complete LGM moraine sequence in New Zealand. Centered at 43°S, 170°E. The well-preserved left lateral portion of this sequence was the focus of the study reported here. A glacial chronology consisting of sixty-five ages was constructed for the northern segment of the Pukaki glacier left-lateral moraines using 10Be exposure dating of boulders located on moraine crests. The resulting dates range from 70.5±1.5 ka on the most outboard moraines to 16.8±0.3 ka from a sampled boulder located 30 meters from the shore of Lake Pukaki. The dates can be categorized into six separate moraine-building events. Boulders from the outermost moraines afforded ages of 70.5±1.5 ka, 62.7±1.5 ka, and 59.8±1.2 ka. Dates derived from twelve boulders from a single prominent crest give an average age of 40.8±0.9 ka. Boulders from prominent ridges on a topographic divide record four moraine-building events with average ages of: 36.2±0.6 (1 date), 28.5±0.8 ka (4 dates), 25.4±0.6 ka (6 dates), and 20.4±0.6 ka (15 dates). These moraines and associated ages signify the peak of the LGM. Dates ranging from 19.7±0.8 ka to 16.8±0.3 ka derived from samples on moraine fragments and hummocky terrain, closest to the lake, are the youngest in this study. These dates record recession of the ice margin. The exposure chronology for this recession is in agreement with a minimum-limiting radiocarbon date of from organic material on a now-submerged moraine near the head of the lake of 16000±800 years (Moar, 1980). The 10Be chronology derived from the Pukaki left-lateral moraine system demonstrates a high degree of similarity to that of local SST records from nearby marine cores MD97-2120 and S0136-GC3 (Barrows et. al, 2007). This similarity between records indicates that a common climate driver influences both SST along the subtropical convergence as well as the temperature-driven mountain glaciers in the Southern Alps. In addition, the Pukaki moraine chronology shows striking similarity with the isotopic record from Antarctic ice cores, specifically the Antarctic climate signal as expressed in the Byrd ice core (Petit et. al, 1999). Located near the Subtropical Convergence at the outer margin of the Southern Ocean, the Pukaki moraine sequence is one of the northernmost terrestrial expressions of the Antarctic climate signal through the LGM and into the last termination. When compared to the overhead summertime insolation intensity, the exposure dates indicate that moraine-building events occurred nearly throughout an entire insolation cycle. The fact that some of these moraines were constructed during the global LGM was a problem, originally identified by Mercer (1984), and is what he called the fly in the ointment of the Milankovitch theory . The chronology from this study demonstrates that glaciers in the Southern Alps maintained a full-glacial configuration despite varying direct overhead insolation intensity. This implies that a climate driver other than of overhead insolation intensity influenced ice-volume fluctuations during the past glacial period in the Southern Alps of New Zealand than in the Northern Hemisphere