Another remarkable record of the mid-Miocene climate transition has recently been reported from on land in the Transantarctic Mountains west of the McMurdo Dry Valleys (Lewis et al., 2007). Polar conditions since the mid-Miocene have preserved wet-based ice marginal sediments overlain by thin dry-based glacial deposits between 1,200 and 1,500 m above sea level. Both lithofacies include volcanic ash layers dating the transition at between 13.94 and 13.62 Ma. A different but related study involved dating similar volcanic ash layers in sediments interpreted as flood deposits in the Labyrinth, a set of spectacular anastomosing channels formed within a dolerite sill in the floor of upper Wright Valley nearby (Lewis et al., 2006). The ages constrain the hypothesized megaflood event(s) to within the period 12.4 and 14.4 Ma, and also support the concept developed by Denton and Sugden (2005) that megafloods may have had a significant role in the past behaviour and erosional effectiveness of the Antarctic Ice Sheet in earlier times. Miocene megadebris flow deposits up to 400 m thick and over 200 km in extent on the slope and rise off Wilkes Land (Donda et al., 2007) might also be related. Recent reports (Wingham et al., 2006) suggest that significant subglacial flows are still taking place beneath the Antarctic Ice Sheet, though perhaps not on that scale.
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