Jennifer Larsen

Temporally and spatially uniform rates of erosion in the southern Appalachian Great Smoky Mountains

We measured 1 0 Be in fluvial sediment samples (n = 27) from eight Great Smoky Mountain drainages (1-330 km 2 ). Results suggest spatially homogeneous sediment generation (on the 10 4 -10 5 yr time scale and >100 km 2 spatial scale) at 73 ′ 11 t km - 2 yr - 1 , equivalent to 27 ′ 4 m/m.y. of bedrock erosion. This rate is consistent with rates derived from fission-track, long-term sediment budget, and sediment yield data, all of which indicate that the Great Smoky Mountains and the southern Appalachians eroded during the Mesozoic and Cenozoic at ∼30 m/m.y. In contrast, unroofing rates during the Paleozoic orogenic events that formed the Appalachian Mountains were higher (≥10 2 m/m.y.). Erosion rates decreased after termination of tectonically driven uplift, enabling the survival of this ancient mountain belt with its deep crustal root as an isostatically maintained feature in the contemporary landscape.

Cosmogenically enabled sediment budgeting

We used 10 Be and 26 Al to constrain the millennial-scale sedi- ment and nuclide budget for a common, long-studied, but poorly understood landform in arid regions, the desert piedmont. We sampled the Chemehuevi Mountain piedmont, a complex multi- surfaced landform in the Mojave Desert, western United States. The nuclide data indicate that sediment is produced more rapidly (1.1 3 10 5 kg·yr 21 ·km 22 ) in steep mountain source basins than on the low-gradient pediment (4.0 3 10 4 kg·yr 21 ·km 22 ) or the intra- piedmont mountain range (2.5 3 10 4 kg·yr 21 ·km 22 ). However, the bulk of the sediment in transport is derived from erosion of the large abandoned alluvial surface (3.9 3 10 4 kg·yr 21 ·km 22 ). The combination of mass and nuclide budgeting suggests that sediment transport speeds decrease downslope from tens of meters per year in confined channels on the proximal pediment to decimeters per year in unconfined distributaries on distal wash surfaces. The sed- iment and nuclide budgeting approach we use is particularly valu- able in arid regions where geomorphically significant events are infrequent and dating control is poor, thus confounding traditional sediment-budgeting techniques.

Testing sugarbush soils : effects of sample storage and drying

Abstract The need for an assessment of sampling effects on forest surface soils has followed the increased interest in the fertility status of northern hardwood forests, especially sugar maple stands. We sampled soils from a fertility research site in north‐central Vermont and measured the effects of drying (55°C) and moist, low temperature storage (4°C) on extractable aluminum (Al), calcium (Ca), potassium (K), magnesium (Mg), phosphorus (P), manganese (Mn), zinc (Zn), boron (B), iron (Fe), and pH. Drying induced large, significant increases in ammonium acetate (pH 4.8) extracted P, Mn, B, and Fe. Smaller, but statistically significant, changes were found in pH, Al, Ca, and Zn. Moist storage for 10 weeks resulted in small significant changes in only pH and K. The magnitude of these changes indicate that samples can be stored at low temperature for up to 10 weeks with little effect on soil test results. Drying should be avoided because of its drastic effect on the extractability of many soil test elements.

Long-Term Sediment Generation Rates for the Upper Río Chagres Basin

In situ-produced cosmogenic 10Be was measured in 17 sediment samples to estimate the rate and distribution of sediment generation in the upper Rio Chagres basin over the last 10 to 20 kyr. Results indicate that the upper Rio Chagres basin is generating sediment uniformly. Nuclide activities suggest basin-wide sediment generation rates of 143 and 354 tons/km/yr (avg. = 234 ± 74 tons/km/yr; n = 7) for small tributary basins and 248 to 281 tons/km/yr (avg. = 267 ± 97 tons/km/yr; n = 3) for large tributary basins. The weighted average of all tributaries is 269 ± 63 tons/km/yr; n = 10). A sample collected upstream of Lago Alhajuela suggests that the entire basin is exporting sediment at a rate of 275 ± 62 tons/km/yr. These cosmogenic nuclide measurements all suggest that the upper Rio Chagres basin (when considered on scales 350 km2) is generating sediment at ∼270 tons/km/yr. This long-term (1 −20 kyr) sediment generation rate that is equivalent to the estimate derived from suspended sediment yield measured below the upper Rio Chagres- Rio Chico confluence from 1981–96 (289 tons tons/km/yr). Such similarity implies that decadal and millennial sediment yields are similar. Thus, short-term sediment yields and long-term sediment generations are in balance, implying steady landscape behavior over time. The background sediment yield suggests that it would take ∼3,600 years to completely fill Lago Alhajuela, the reservoir for the Panama Canal. Taking into account the present day 2- to 3- fold increase in sediment yields for adjacent human-impacted Rio Boqueron and Rio Pequeni basins, the filling time is reduced to ∼2,000 years. However, it would only take between 250 to 600 years to reduce the reservoir capacity (69% of maximum) enough to drain the entire reservoir for precipitation conditions similar to the 1982 El Nino event. Such models highlight the importance of proper watershed management in order to reduce the sedimentation of Lago Alhajuela.