Pothole and channel system formation in the McMurdo Dry Valleys of Antarctica: New insights from cosmogenic nuclides

Abstract

Large pothole and channel features (∼15m deep, ∼30m wide) carved into the Beacon Sandstone in the upland Dry Valleys of Antarctica have been used to infer catastrophic subglacial flooding beneath an expanded East Antarctic Ice Sheet that overran the Transantarctic Mountains during the mid-Miocene. Though the age and erosion rates of these geomorphic features have not been quantified, preservation of the potholes and channels has been attributed to negligible erosion under consistent polar desert conditions since the retreat of the ice sheet at ∼14Ma. We present cosmogenic 21Ne and 10Be data from samples collected along vertical transects of pothole and channel walls, as well as from intervening benches, within Battleship Promontory in the Convoy Range and within Sessrumnir Valley in the Western Asgard Range to constrain their exposure history. Measurements of fissiogenic 136Xe are used to estimate a nucleogenic 21Ne concentration in the Beacon Sandstone of 7.7±2.4×106atomsg−1 and to correct our 21Ne data for this component. Sample concentrations of cosmogenic 21Ne and 10Be are significantly lower than previously measured in the regional bedrock and reveal steady state erosion rates ranging from 99 to 171cmMa−1 in Battleship Promontory and from 59 to 383cmMa−1 in Sessrumnir Valley. Continuous exposure at such erosion rates would remove 8–54m of bedrock over a 14Ma period, a length scale similar to the features themselves, and suggests that these systems could have formed primarily through subaerial erosive processes. Alternatively, if the features formed subglacially in the Miocene, then a complex erosion and exposure history must have occurred to prevent the accumulation of cosmogenic nuclides to levels higher than those observed. Either prolonged and extensive ice cover of these features prior to 2Ma, or a threefold increase in erosion rates during the Plio–Pleistocene could produce the 21Ne and 10Be concentrations measured here. Ultimately, all of these scenarios imply that the upland Dry Valleys landscape has experienced greater changes over the last 14Ma than previously considered.