UVM Theses and Dissertations
Format:
Print
Author:
Trodick, Charles David
Title:
Dept./Program:
Geology
Year:
2011
Degree:
MS
Abstract:
This thesis reports and interprets in situ and meteoric ¹⁰Be concentrations of sediment collected from the Potomac River Basin, a major source of sediment for the Chesapeake Bay. It includes data for 62 fluvial sediment samples with measurements of both in situ produced and meteoric ¹⁰Be along with an additional eight samples for which only meteoric ¹⁰Be was measured. I sampled three sites on the main-stem Potomac River, and one site on the Shenandoah River (basin areas of 29796, 24851, 2254, and 4136 km²). Ten Potomac River samples, including the four above, came from USGS gaging station sites at which suspended sediment was or is monitored (basin area 23 to 29796 km²). Sixty samples, including the eight with only meteoric ¹⁰Be analyses, came from tributaries of the Potomac River (5 km² to 64 km²).
Basin-scale erosion rates, based on interpretation of in situ ¹⁰Be concentrations, range from 3 to 39 m My⁻¹ with a mean and median erosion/rate ,12 m My⁻¹. Basins in the Coastal Plain (n=8) have the lowest erosion rates, 4 to 39 m Mil with a mean of 10 m Mil and a median of 6 m Mil. Basins in the Appalachian Phiteau and Blue Ridge (n=6, n=8; mean erosion rates of 13 and 12 m My⁻¹) range from 9 to 18 m My⁻¹ and have a median of 13 m My⁻¹. Twenty samples from the Piedmont have erosion rates that range from 3 to 21 m My⁻¹ and a mean and median of 12 m My⁻¹. Seventeen samples from the Valley and Ridge have a range from 3 to 29 m My⁻¹, a mean of 11 m My⁻¹ and median of 9 m My⁻¹. Past studies using in situ ¹⁰Be erosion rates have found similar erosion rates measured in and near the Appalachian Mountains (3-70 m My⁻¹).
Erosion rates show no correlation with slope (R²=0.06, p>0.05) and only a weak correlation with elevation (R²=0.09, p=0.01). The range of erosion rates change little with basin area (R²=0.02, p=0.23). Using ANOVA testing, erosion rates in the Piedmont, Blue Ridge, Valley and Ridge and Appalachian Plateau physiographic provinces are not statistically separable (p=0.25), but the Coastal Plain has much lower rates than the other provinces with a median half that of almost all of the other provinces (p<0.01) when an outlier is removed from the Coastal Plain. A comparison of meteoric and in situ ¹⁰Be in 62 basins shows less meteoric ¹⁰Be in the sediments than expected given the in situ ¹⁰Be concentrations. The lower meteoric ¹⁰Be concentrations most likely reflect lack of knowledge of where meteoric ¹⁰Be resides in many of my basins.
United States Geologic Survey sediment yield data (n=l0) suggest that rates of sediment export are similar to rates of sediment generation in the Potomac Basin. Brown et al. (1988) calculated erosion indexes that compare the amount of meteoric ¹⁰Be entering the basin and the amount of meteoric ¹⁰Be leaving the basin. Brown et al.'s (1988) erosion indexes from the Chesapeake Bay Watershed show two thirds, 29 of 45, of their basins had more meteoric ¹⁰Be entering the basin than leaving. Their average erosion index was 0.87. Of the ten gauged basins I analyzed, eight had more meteoric ¹⁰Be entering the basin than leaving. My average erosion index was 0.65. Meteoric ¹⁰Be concentrations used for the erosion index calculations were updated with modern standards and delivery rates.
Basin-scale erosion rates, based on interpretation of in situ ¹⁰Be concentrations, range from 3 to 39 m My⁻¹ with a mean and median erosion/rate ,12 m My⁻¹. Basins in the Coastal Plain (n=8) have the lowest erosion rates, 4 to 39 m Mil with a mean of 10 m Mil and a median of 6 m Mil. Basins in the Appalachian Phiteau and Blue Ridge (n=6, n=8; mean erosion rates of 13 and 12 m My⁻¹) range from 9 to 18 m My⁻¹ and have a median of 13 m My⁻¹. Twenty samples from the Piedmont have erosion rates that range from 3 to 21 m My⁻¹ and a mean and median of 12 m My⁻¹. Seventeen samples from the Valley and Ridge have a range from 3 to 29 m My⁻¹, a mean of 11 m My⁻¹ and median of 9 m My⁻¹. Past studies using in situ ¹⁰Be erosion rates have found similar erosion rates measured in and near the Appalachian Mountains (3-70 m My⁻¹).
Erosion rates show no correlation with slope (R²=0.06, p>0.05) and only a weak correlation with elevation (R²=0.09, p=0.01). The range of erosion rates change little with basin area (R²=0.02, p=0.23). Using ANOVA testing, erosion rates in the Piedmont, Blue Ridge, Valley and Ridge and Appalachian Plateau physiographic provinces are not statistically separable (p=0.25), but the Coastal Plain has much lower rates than the other provinces with a median half that of almost all of the other provinces (p<0.01) when an outlier is removed from the Coastal Plain. A comparison of meteoric and in situ ¹⁰Be in 62 basins shows less meteoric ¹⁰Be in the sediments than expected given the in situ ¹⁰Be concentrations. The lower meteoric ¹⁰Be concentrations most likely reflect lack of knowledge of where meteoric ¹⁰Be resides in many of my basins.
United States Geologic Survey sediment yield data (n=l0) suggest that rates of sediment export are similar to rates of sediment generation in the Potomac Basin. Brown et al. (1988) calculated erosion indexes that compare the amount of meteoric ¹⁰Be entering the basin and the amount of meteoric ¹⁰Be leaving the basin. Brown et al.'s (1988) erosion indexes from the Chesapeake Bay Watershed show two thirds, 29 of 45, of their basins had more meteoric ¹⁰Be entering the basin than leaving. Their average erosion index was 0.87. Of the ten gauged basins I analyzed, eight had more meteoric ¹⁰Be entering the basin than leaving. My average erosion index was 0.65. Meteoric ¹⁰Be concentrations used for the erosion index calculations were updated with modern standards and delivery rates.