USING TERRESTRIAL COSMOGENIC NUCLIDES TO CONSTRAIN TIMING OF PENULTIMATE ICE ADVANCE IN THE OGILVIE MOUNTAINS, YUKON TERRITORY

Throughout the Pleistocene epoch, Yukon Territory was repeatedly influenced by glacial ice originating from the Cordilleran ice sheet (CIS) and independent alpine glaciers. The penultimate limit in Yukon has garnered controversy in recent years, as moraines in the central region of the territory (MIS 6) were found to predate supposedly correlative moraines in the southwest territory and Alaska (MIS 4). The Ogilvie Mountains, located east of Dawson City, near the northern limit of the CIS, have proven especially problematic for absolute dating studies. The penultimate limit here has been considered to be MIS 6, MIS 4 and even the maximum extent of MIS 2, with the mapped MIS 2 limit being just being a recessional moraine. With studies focused at the Chapman Lake moraine, this research will attempt to elucidate the age of the penultimate glaciation. We use a cosmogenic ³⁶Cl depth versus concentration profile in penultimate outwash gravels to date this glaciation. The age calculated by the depth profile will integrate with stratigraphy of sections using radiocarbon ages, macrofossil samples, and optically stimulated luminescence ages as well as boulder ages. This research will determine whether MIS 4 or MIS 6 provided the conditions necessary for ice nucleation in the Ogilvie Mountains during the Penultimate Glaciation.

Cosmogenic data is now available and depth age models are being calculated. This knowledge will have substantial impact in determining the effects of climate forcing in this isolated region and its relationship to the existing glacial framework of the territory. Two radiocarbon ages of ~45 ka, which are likely non-finite, at the top of a glaciolacustrine sequence overlying a till in the Chapman Lake area imply the till to be at least MIS 4, but likely MIS 6. Two TCN boulder samples taken on a penultimate surface south of the study area have yielded minimum ages which correspond to MIS 6. We expect that upon completion of the depth profile age calculations, a MIS 6 age can be assigned to the penultimate advance in the Ogilvie Mountains.