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Paper No. 12
Presentation Time: 10:45 AM

LOW RATES OF BEDROCK OUTCROP EROSION IN THE CENTRAL APPALACHIAN MOUNTAINS INFERRED FROM IN SITU 10BE CONCENTRATIONS


PORTENGA, Eric W.1, BIERMAN, Paul2, TRODICK Jr, Charles D.2 and ROOD, Dylan H.3, (1)Department of Environment & Geography, Macquarie University, North Ryde, 2109, Australia, (2)Department of Geology, University of Vermont, Delehanty Hall, 180 Colchester Ave, Burlington, VT 05405, (3)Center for Accelerator Mass Spectrometry, Lawrence Livermore National Laboratory, Livermore, CA 94550, eric.portenga@students.mq.edu.au

We collected 72 samples from spur-ridge and ridge-top bedrock outcrops within the Potomac and Susquehanna River basins and calculated erosion rates utilizing in situ 10Be concentrations measured in quartz. Over one hundred basin-scale erosion rates have been estimated cosmogenically in this region (Reuter, 2005; Duxbury, 2008; Trodick, this meeting); however, only 17 outcrop erosion rates have been measured until now.

Outcrop erosion rates in the Potomac River basin (n=46) range from 1.0±0.11 to 66±4.8 m/My, average 15±1 m/My, and have a median of 7.1±0.6 m/My; outcrop erosion rates in the Susquehanna River basin (n=26) range from 1.8±0.2 to 28±2 m/My, average 10±0.7 m/My, and have a median of 8.3±0.7 m/My. The average bedrock outcrop erosion rate for the region is 13±1 m/My which is greater than the few other erosion rates measured on bedrock outcrops in the region (4-7 m/My, Reuter, 2005, Hancock and Kirwin, 2007; Duxbury, 2008). Outcrop erosion rates are significantly indistinguishable from the average basin rate of 12 m/My for 62 Potomac River sites (Trodick, this meeting) and significantly lower than the average basin rate of 20 m/My for 79 Susquehanna River sites (Reuter, 2005). Similar rates of erosion for bedrock outcrops and drainage basins suggest that the Central Appalachians as a whole have reached a general state of equilibrium. This is in contrast to some studies which suggest an increase in relief near our study sites (Reuter, 2005; Hancock and Kirwin, 2007).

We observe a weak, positive correlation between bedrock outcrop erosion rates and relief in meters within a 5km radius (R2=0.24; p<0.0001), and an even weaker positive correlation between bedrock outcrop erosion rates and elevation (R2=0.11; p=0.0042). A weak, negative correlation is observed between bedrock outcrop erosion rates and latitude (R2=0.14; p=0.0013). Bedrock outcrop erosion rates were analyzed against mean annual precipitation and temperature but no correlations were significant.

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