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Research and Education Activities:

            In 2009 I was funded to investigate source to sink processes by using cosmogenic nuclide concentrations in different alluvial grain size fractions. The idea was to explore the geomorphic processes responsible for transporting sediments from source area to depocenter and determine the geomorphic controls on cosmogenic nuclide concentrations among variable grain size fractions in alluvial sediment. In addition, I felt that analyzing the cosmogenic nuclide concentrations in different grain size fractions would help better understand the systematics of cosmogenic nuclide accumulations, which could have important implications for sampling strategy and dating alluvial landforms.

            The project has involved three participants to date including, PI Frankel, Patrick Belmont from Utah State, and a female M.S. student, Tina Marstellar. The project got off to a relatively slow start because it took longer than expected to recruit a graduate student. All three participants participated in field work and sample analysis. Frankel and Marstellar were responsible for sample processing. Throughout the project, Frankel has supervised Marstellar in field work and sample processing and analysis.

            To date, PI Frankel, M.S. student Marstellar, and collaborator Belmont have spent approximately two weeks in the field collecting sand, pebble, and cobble size quartzite an carbonate clasts to measure Be-10 concentrations. We collected 21 samples 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 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 the near future.

            In addition, for comparison between a tectonically active, arid setting and a humid, tectonically inactive setting, we collected 15 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 now been processed by Marstellar and are currently awaiting analysis at LLNL-CAMS.    

Findings:

            Based on the initial sample collection, processing and analysis efforts, several interesting results have come of our research to date. First, and most importantly, the relationship between Beryllium-10 concentrations and grain size in alluvial sediment from the Red Wall Canyon study site in Death Valley has important implications for source to sink processes. The key result thus far is that there does not appear to be any significant difference between Beryllium-10 concentrations in different grain size samples collected from active stream channels in this tectonically active, arid setting.

            For example, we collected both sand (250 to 500 micron) and cobble (4 to 8 cm) size clasts from four separate sites in the active channel at the Red Wall Canyon fan. In all cases, Beryllium-10 concentrations are similar between different clast sizes. The cosmogenic nuclide concentrations are also similar those measured in 1 to 2 cm-sized clasts from deep within the Red Wall Canyon fan deposit. This suggests that the denudation processes are similar for a wide range of relatively fine grain sizes. In other words, the processes acting to erode and transport 250 micron to 8 cm-sized grains are similar.

            More importantly, the processes responsible for eroding and transporting this relatively fine grained material appears to be different that the processes delivering coarse cobbles and small boulders to the alluvial depocenter at the study location. Cosmogenic nuclide concentrations for fine and coarse grained material on the alluvial fan surface are significantly different. Boulders tend to have significantly lower nuclide concentrations compared with finer grained material, suggesting much different erosion and transport histories. In this case, our preliminary interpretations suggest that large cobbles and small boulders are eroded from hillslopes and transported quickly through the fluvial system from source area to depocenter. In contrast, finer grained material moves more slowly off of hillslopes and through the fluvial system, thereby accumulating higher cosmogenic nuclide concentrations during erosion and transport. These results will be presented at the 2010 Geological Society of America Annual Meeting and the 2010 Fall American Geophysical Union Meeting.

            Based on these findings, we are planning a second field season in Death Valley during March, 2011. Field work during this time will focus on additional sample collection of multiple grain sizes from both active channels and fan deposits. In addition, we plan to expand our sampling efforts to other drainages beyond that feeding the Red Wall Canyon fan.

Training and Development:

            Although initially this project did not have a graduate student, the funds are now partially supporting a female M.S. candidate, Tina Marstellar. Marstellar is being trained in both field and laboratory techniques related to cosmogenic nuclide geochronology, geomorphology, sedimentology, stratigraphy, and tectonics. Her expected graduation date is May, 2011. Following graduation, Marstellar intends to remain at Georgia Tech as a technician/manager in the cosmogenic nuclide geochronology laboratory.

            In addition to training an M.S. student, the project has fostered collaboration between Utah State University (collaborator Belmont) and Georgia Tech. More importantly, the project has supported the continued operation and development of the Georgia Tech cosmogenic nuclide geochronology laboratory. This laboratory is the only one of its kind is the southeast U.S. and is beginning to act a regional cosmogenic nuclide sample processing facility.

Broader Impacts/Outreach:

            PI Frankel is 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 a spring break field course for Georgia Tech graduate students and undergraduates in March, 2010.