Erosion of the eastern United States observed with ¹⁰Be

Abstract

The problem of identifying areas of accelerated erosion in a dynamic landscape is complicated. The limited history of sediment yield measurements makes this task difficult even if geomorphic evidence is available. Beryllium-10, a cosmogenic isotope produced by cosmic rays interacting with the earth's atmosphere and surface, has chemical and physical properties that make it useful as a tracer for erosion and sediment transport processes. The rarity of the stable isotope, 9Be, allows "Be to be detected with accelerator mass spectrometry in natural materials at extremely low levels. Backgrounds for rocks and sediments below lo5 atom per g are now attainable, a value to be compared with an average deposition rate of 1.3 x lo6 atom cm-2 yr-l. The affinity of Be for the components of soil and sediment is sufficiently high that it is effectively immobilized on contact, thereby allowing "Be to function as a tracer of sediment transport. TO a good approximation all the "Be transport out of a drainage basin is on the sediment leaving it. The number of "Be atoms passing the gauging station can be determined by measuring the concentration of the isotope in the sediment, if the annual sediment load is known. The ratio of the "Be carried from the basin by the sediment to that incident upon it, called the erosion index, has been determined for 48 drainage basins within the same physiographic province, which allows them to be reasonably compared, all of whieh have sediment yield data. Basins located in the Atlantic coastal plain have an average index of 0.3 with the maximum observed being 0.9. Basins located between the fall line and the mountains, a region called the Piedmont, have an average value of 2.2 with individual values ranging from 0.6 to 6.7; this marked difference is thought to result from two centuries of farming on land of moderate gradient. Basins in the highland regions reflect local conditions with low indices for those in grass and timber and high indices associated with destructive land use. The data allow an estimate of the erosion index for the pre-colonial Piedmont, which then allows the precolonial sediment yield to be calculated. A number of basins have also been examined world wide with similar conclusions derived. An important deviation from the rule is noted for rivers that erode large regions of loess, such as the Mississippi, Hwang Ho, and Yangtze. Large aeolian deposits were laid down during the ice age in these basins, deposits that brought inherited "Be with them and that are easily eroded.. . ,

The dependence of meteoric 10Be concentrations on particle size in Amazon River bed sediment and the extraction of reactive 10Be/9Be ratios

“…Another important application is the use of meteoric 10 Be adsorbed on sediment as an erosion rate meter in large watersheds. Pioneered by Brown et al (1988) and You et al (1988), concentrations of meteoric 10 Be measured in sediment at the outlet of a catchment should, similar to in situ-produced 10 Be, be representative of the denudation rate of the catchment. However, these advantages are compromised by some major, yet largely unexplored, challenges: first, the atmospheric 10 Be deposition rate must be known on a basin-wide scale.…”

Earth surface erosion and weathering from the 10Be (meteoric)/9Be ratio

“…This method, explored first by Lal and Peters (1967), was established as a monitor of basin-wide erosion in two pioneering studies of river sediment by L. Brown et al (1988) and You et al (1988), but was partially sidelined in the years that followed by the astounding success of the "sister nuclide", in situ-produced 10 Be measured in quartz to determine exposure ages and rates of denudation (Lal, 1991). The advantages of the meteoric variety of 10 Be over the in situ-produced nuclide lie in this isotopes' higher concentrations, requiring smaller sample amounts, its applicability to quartz-free lithologies, and the possibility to determine denudation rate time series in fine-grained sedimentary deposits.…”

Boundary scavenging in the Pacific Ocean: a comparison of10Be and231Pa

“…The 1°Be concentrations of river sediments range world-wide from I to 4 x 108 at g-1 [50], much too low to have contributed significantly to the l°Be content of the sediments at the two margin sites off South America (3-5 × 109 at g-1; Table 2). These margin sediments are about 80% marine biogenic material [51], further reducing the contribution that continental detritus could make to the 1°Be content of sediments at these sites.…”