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            In 2009, PI Frankel was funded to investigate source to sink processes by using cosmogenic nuclide concentrations in different alluvial grain size fractions. The research involves exploring the geomorphic processes responsible for transporting sediments from source area to depocenter and determining the geomorphic controls on cosmogenic nuclide concentrations among variable grain size fractions in alluvial sediment. In addition, analyzing the cosmogenic nuclide concentrations in different grain size fractions would provide a better understanding of the systematics of cosmogenic nuclide accumulations, which could have important implications for sampling strategy and dating alluvial landforms.

            The project has involved three participants including, PI Frankel, Patrick Belmont from Utah State, and a female M.S. student, Tina Marstellar.

            In 2009, PI Frankel, M.S. student Marstellar, and collaborator Belmont collected 21 samples comprised of sand, pebble, and cobble size quartzite and carbonate clasts to measure Be-10 concentrations from active channels and older alluvial fan deposits at the Red Wall Canyon alluvial fan in northern Death Valley, California. Nineteen of the 21 samples were processed in the Georgia Tech Cosmogenic Nuclide Geochronology (GT-CNG) laboratory by Marstellar under PI Frankel’s supervision and analyzed at the Lawrence Livermore National Laboratory Center for Accelerator Mass Spectrometry (LLNL-CAMS). The two additional samples are carbonates, which cannot be processed at Georgia Tech. We will process these samples at Purdue University.

            In 2010, Pi Frankel and M.S. student Marstellar collected 15 quartzite samples (10 sand sized and 5 cobble sized grains) from active streams draining the Blue Ridge Mountains in Georgia and North Carolina. We felt that this would make for an interesting data set for comparison of source to sink processes in these two drastically different climatic and tectonic settings. These samples have been processed by Marstellar and analyzed at LLNL-CAMS.  

            In 2011, Pi Frankel and collaborator Belmont collected an additional 18 samples of multiple grain sizes from both active channels and fan deposits from the Red Wall Canyon alluvial fan. These samples are currently being processed by Marstellar in the GT_CNG laboratory.

Findings:

            Several insights are provided by our first two sample collection and analysis efforts. First, our Red Wall Canyon study site provides the relationship between Beryllium-10 concentrations and grain size in alluvial sediment appears to change systematically with distance down channel/fan. We collected both sand (250 to 500 micron) and pebble (4 to 8 cm) size clasts from sites in the active channel. At the upstream sites (8-10 km above the fan), sand exhibits higher ¹⁰Be concentrations, whereas pebbles exhibit lower concentrations. At the lower sites (near the apex of the fan), sand exhibits lower concentrations and gravel exhibits a relatively higher concentration.  We have one site that is intermediate between the upstream and downstream sites, which exhibits similar ¹⁰Be concentration for both sand and gravel. Another observation is that ¹⁰Be concentrations in sand decrease monotonically with distance downstream, changing more than two-fold over the distance of our samples. 

Taken together, these two observations suggest that erosion rates and processes of Red Wall Canyon are in a transient state.  We interpret the upstream-to-downstream decrease in ¹⁰Be concentrations in sand to represent a recent slowing of erosion, with upstream samples closely reflecting modern erosion rates and downstream samples reflecting some bias toward higher erosion rates of the past.  We interpret the reversal in the relationships between ¹⁰Be concentration and grain size to indicate some combination of transience in erosion rates over time as well as differential transport of the two grain sizes. The 2011 additional samples were collected to further investigate these phenomena.

We analyzed one pebble sample collected near the location of our downstream samples. It was collected from 520 cm below the fan surface. This sample was deposited on the fan prior to 70 ka, and reflects the exposure history of pebbles at the time of deposition. This sample contains the lowest ¹⁰Be concentration measured to date for pebble samples, suggesting a somewhat faster erosion/transport rate for pebbles 70,000 years ago, when Death Valley experienced a much wetter climate.

            The Blue Ridge samples were collected from ten different basins.  While an upstream and downstream comparison is not available, the cobble (4 to 8 cm) size samples provided, on average, higher ¹⁰Be concentrations than the sand (250 to 500 micron) samples. Basin morphometric analyses are currently being conducted to understand how watershed form relates to rates and processes delivering coarse and fine sediment.

The results from the Red Wall Canyon samples were presented at the 2010 Geological Society of America Annual Meeting and the 2010 Fall American Geophysical Union Meeting. The results for the Blue Ridge samples will be presented at the 2011 Fall American Geophysical Union Meeting.

Training and Development:

            The funds partially supported a female M.S. candidate, Tina Marstellar. Marstellar continues to work in the GT-CNG laboratory and has become a skilled scientist and lab technician as a result. She has been trained in both field and laboratory techniques related to cosmogenic nuclide geochronology, geomorphology, sedimentology, stratigraphy, and tectonics. Her expected graduation date is December, 2011. Following graduation, Marstellar intends to remain at Georgia Tech as a technician/manager in the GT-CNG laboratory.

            The project has fostered collaboration between Utah State University (collaborator Belmont) and Georgia Tech and helped support the continued operation and development of the GT-CNG laboratory. This laboratory is the only one of its kind is the southeast U.S. and is currently serving as a regional cosmogenic nuclide sample processing facility.

Broader Impacts/Outreach:

           PI Frankel was continuing his outreach efforts with Death Valley National Park to relay research results to Park visitors. In addition, Frankel used the Death Valley study site as a location to discuss erosion and sediment dynamics during spring break field courses for Georgia Tech graduate students and undergraduates in March, 2010 and 2011.  PI Frankel was tragically lost in an accident in July 2011. New PI Andrew Newman, M.S. student Marstellar, and collaborator Belmont feel it is crucial to PI Frankel’s memory to bring this project to completion and are committed to doing so by August 2012.