Geology topics
Publications and source records attributed to R. Middleton.
A new method of dating the surface exposure of rocks from in situ production of 10 Be and 26 Al has been applied to determine the age of Meteor Crater, Arizona. A lower bound on the crater age of 49,200 ± 1,700 years has been obtained by this method.
10 Be concentration, total carbon and grain-size were measured in cores collected in undisturbed estuarine sediments of three tributaries of the Chesapeake Bay. These cores were previously studied by Davis [1] and Brush [2,3] for pollen content, age and sedimentation rate. In this work, we compare the results obtained for these various analyses. In the cores, we observed two increases in 10 Be concentration concomitant with two major changes in the pollen composition of the sediments. These two pollen changes each correspond to well-dated agricultural horizons reflecting different stages in the introduction of European farming techniques [2]. In the Chesapeake Bay area, the agricultural development, associated with forest clearing, appears to have triggered the erosion, transport, and sedimentation into the river mouths of large quantities of 10 Be-rich soils. This phenomenon explains the observed rise in the sedimentation rate associated with increases in agricultural land-use.
The distribution and residence time of cosmogenic 10 Be in clay-rich soil horizons is fundamental to understanding and modelling the migration of 10 Be on terrestrial sediments and in groundwater solutions. We have analyzed seven profiles of clay-rich soils developed from terrace sediments of the Merced River, California. The terraces and soils of increasing age are used to compare the 10 Be inventory with a simple model of accumulation, decay and erosion. The data show that the distribution of 10 Be varies with soil horizon clay content, that the residence time of 10 Be in these horizons exceeds 10 5 years, and that to a rough approximation the inventory of 10 Be in a thoroughly sampled soil profile fits the equation: N = ( q − Em )(1 − e − λι )/ λ where q is delivery rate, E is erosion rate, m is the concentration of 10 Be in the eroding surface layer, λ is the decay constant, and t is the age of the depositional unit from which the soil has developed. The general applicability of this model is uncertain and warrants further testing in well-calibrated terrace sequences.
We have measured the concentration of the cosmogenic isotope 10 Be in soil samples from various horizons at six sites, including three independently dated Rappahannock River terraces and a previously undated Piedmont soil to which we have assigned an age. All of the incident 10 Be can be accounted for in one of these soils and a second is within a factor of two. In three soils, whose concentrations vary widely with depth, a significant fraction of the incident 10 Be cannot be accounted for. Incomplete sampling, and enhanced Be mobility caused by organic components, are the probable reasons for the low inventory of Be from these three soils. Overall, the data from these six sites indicate that 10 Be accumulation could be used to assign ages to soils if Be is not mobilized and lost from the soil profile.