The major goal of this project is to determine if high-elevation alluvial fan sediments could record climate shifts along terrestrial margins of the East Antarctic Ice Sheet (Fig. 1).  This novel approach examined sediments along terrestrial margins rather than marine margins.  Marine margins are more commonly studied because sediment cores can be used to estimate climate and ice sheet behavior going back millions of years.  A major drawback, however, is that marine records are usually far from modern ice sheet margins and likely miss short-lived melting events.  In contrast, alluvial fans register melting adjacent to terrestrial ice margins.  The fans small size and easily identifiable meltwater sources mean that even a single-summer melt event should be apparent in the sedimentary record.

We studied alluvial fans in the Dry Valleys region of east Antarctica (Fig. 1).  The fans are perched on steep slopes situated 5 to 25 km from the modern ice margin.  Channels only a few 10’s of cm wide to a maximum of about 2 m funnel water to fans from permanent snow banks and small glaciers.  Today all of the studied fans and associated channels appear inactive.  Small trickles of water have been observed in channels, mostly from snow melting within the channels themselves, but flows are much too weak to move sediment.  Polygonal patterned ground and well-developed desert pavements on fan surfaces suggest no sediment has been carried to the fans for centuries to millennia.  By inference this means that margins of the nearby ice sheet have produced little to no meltwater for centuries to millennia.

Our results show that fans were primarily constructed from shallow sheet floods with only a few beds representing deposition from debris flows.  Sediment grain size and bedding indicate flows only 2 to 10 cm cm deep traveling at rates of only 1 to 1.4 m/s.  Deposition was highly episodic because well-developed desert pavements, comprised of wind-eroded and stained gravel, are buried within the fans (Fig. 2).  They separate each package of sediment delivered during a melt event and mark long periods of exposure before being buried in a subsequent event.

We used optically stimulated luminescence (OSL) dating of sediment grains to determine when melt events occurred.  An OSL date registers the last time sediments were exposed to solar radiation meaning it represents burial age.  The ages cluster in three groups centered at 2.1 ka (thousand years before present), 8.9 ka and 14.2 ka (Fig. 3).  Given past geologic and geomorphic studies, this is an unexpected result.  At elevations similar to those of the fans geomorphic features and fragile deposits of volcanic ash have been preserved for as long as 14 million years, which has been used to argue for widespread landscape stability.  This study, however, shows that the assumption of widespread inactivity for landscapes in the Dry Valleys is incorrect and that large local variations are probably common.

Based on these results, the study shows that high-elevation alluvial fans in the Dry Valleys provide a sensitive climate record that captures rare, small-scale melting events.  The next step is to establish causation for melting.  To do this we used GIS analyses of meltwater source regions, deployed data loggers for local temperature and humidity at meltwater sources (over two summers), and applied regional climatological modeling.  Our GIS results showed that individual geomorphic attributes, such as elevation, size and distance from the coast, are only weakly correlated to fan size.  The strongest correlation was with distance to the Ross Sea, meaning that larger and presumably more active fans tend to be those closest to the local source of warm, moist air.  Based on our temperature data...

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