This project seeks to answer a simple question: how old are potholes and related geomorphic features found in the uplands of the McMurdo Dry Valleys, Antarctica? Some research suggests that they are over ten million years old and date the growth of the East Antarctic Ice Sheet, the world?s largest. However, some evidence suggests that these are young, erosional features that continuing to evolve to this day. This project uses cosmogenic nuclide dating to determine the age of the pothole floors. The results are important for determining the ice sheet?s history and interpreting the O-isotope record from the marine sediment cores, key records of global climate. Broader impacts include K12 outreach and incorporation of outcomes into university courses.
Large pothole and channels (~15 m deep) cut into sandstone bedrock within the Dry Valleys of Antarctica have been used to infer catastrophic subglacial flooding beneath a massively expanded East Antarctic Ice Sheet that overran the Transantarctic Mountains during the middle Miocene (~ 14 million years ago). Preservation of Miocene age features and sediments is attributed to very low erosion rates under a consistent polar desert climate. 21Ne and 10Be are isotopes that are formed when rocks are exposed to cosmic rays at the Earth’s surface. The concentrations of these two isotopes in the rocks are determined by the total duration of exposure to cosmic rays and the rate at which the rocks are eroding. Hence, measurements of these two isotopes are ideally suited to test the hypothesis that the potholes and channels were formed by subglacial floods 14 million years ago and these features have undergone low erosion rates because of the polar desert environment. 21Ne and 10Be were measured in nineteen samples collected along vertical transects of pothole and channel walls as well as from intervening benches of Sessrumnir Valley in the Western Asgard Range and Battleship Promontory in the Convoy Range. The 21Ne concentrations are significantly lower than that predicted for slowly eroding 14 million year old bedrock surfaces. Rather, average erosion rates appear to be high enough that the channels and potholes could be formed through subaerial erosive processes that operated for 14 million years, rather than from a single catastrophic flooding event in the Miocene. The combined 21Ne and 10Be data also suggest either a dramatic increase in erosion rates over the past 2 million years or at least one prolonged period of ice cover. A possible mechanism for an increase in the erosion rate within the past 2 million years could be increased moisture from snow melt during diurnal freeze-thaw cycles in Quaternary interglacial periods. A prolonged period of ice cover in the channels and potholes could have resulted from an expansion of cold based alpine glaciers during the warmer Pliocene period. During periods of decreased sea ice extent and open waters in McMurdo Sound, such as during the warmer climate of the Pliocene period, precipitation may have increased in the western Dry Valleys. Increased inland snowfall could have led to the expansion of cold based glaciers in Sessrumnir Valley and Battleship Promontory. Recent work in the Taylor Glacier indicates that it reached a highstand in the Pliocene that was ~200 meters higher than present day in a region only 20 km to the southwest of Sessrumnir Valley. Hence, increased snowfall during the Pliocene appears to be a very plausible explanation for our observations. If so, it suggests that the Antarctic Dry Valleys may not have been as dry as present day and our research suggests a more dynamic environment in the Dry Valleys than a static hyper arid climate. Our field work, an integral part of our research activities, is described in a blog (www.epsinantarctica.blogspot.com/) that was continually updated from the field. The blog acknowledges NSF support for the project and has additional links to scientific resources pertaining to Antarctica. In addition, we have communicated the results from our study in public talks at Harvard's Earth and Planetary Science Department, at the Harvard Foundation's Albert Einstein Science Conference, themed 'Grooming Leaders for the Green Revolution' intended for school students aged 9-16, and at a talk titled 'Antarctica: Tracking the World's Largest Glaciers' hosted by the Harvard Museum of Natural History for elementary and middle school children and their parents.
