THE EARLY TO MIDDLE PLEISTOCENE TRANSITION IN NORTHWESTERN CANADA AND POTENTIAL RESPONSE OF THE CORDILLERAN ICE SHEET

One of the most remarkable features of the northern Cordilleran Ice Sheet is the record of repeated extensive glaciation in the latest Pliocene and Early Pleistocene (ca. 2.7–0.78 Ma) relative to more restricted ice during the Middle and Late Pleistocene (<780 ka). And further, the most extensive Cordilleran glaciation was also the earliest (on the basis of North Pacific ice rafting) in the late Gauss Chron (ca. 2.7–2.58 Ma). Several models have been suggested to explain these differences, including changes in atmospheric circulation and Laurentide Ice Sheet feedbacks, but the most common explanation is the influence of an uplifting Alaska Range and St. Elias Mountains to the south, forming a progressive barrier to Pacific moisture. This explanation provides a means to limit moisture to the interior during the Quaternary, but it is difficult to invoke given that the early record of permafrost (presence of periglacial structures) and multiple meteoric water isotope proxies suggest that much of the continentality of interior Alaska and Yukon was present by at least the Middle Pliocene. The most striking difference in the stratigraphic record that seems to coincide with these changes, however, is the evidence of deep weathering recorded by paleosols on drift surfaces. Paleo-Luvisols with textural B horizons (~ 20% clay) exceeding 1 m in thickness are present on multiple drift surfaces through central Yukon. These largely date to the Matuyama Chron (>0.78 Ma). Weathering of primary minerals in these relict paleosols has depleted mobile elements (Ca, Na, K) to a greater extent than in soils formed during Middle and Late Pleistocene interglacials. Other proxies, including the persistence of relict permafrost through interglacials and development of the steppe fauna, suggest more broad environmental changes across the Early to Middle Pleistocene transition across this region. The advance of the Cordilleran Ice Sheet on more deeply weathered surfaces in the late Pliocene and Early Pleistocene could have provided a means of enhancing lubrication of the bed leading to more extensive ice sheets.