We propose, as a testable hypothesis, a basin-scale approach for interpreting the abundance of in situ produced cosmogenic isotopes, an approach which considers explicitly both the isotope and sediment flux through a drainage basin. Unlike most existing models, which are appropriate for evaluating in situ produced cosmogenic isotope abundance at discrete points on Earth's surface, our model is designed for interpreting isotope abundance in sediment. Because sediment is a mixture of materials, in favourable cases derived from throughout a drainage basin, we suggest that measured isotope abundances may reflect spatially averaged rates of erosion. We investigate the assumptions and behaviour of our model and conclude that it could provide geomorphologists with a relatively simple means by which to constrain the rate of landscape evolution if a basin is in isotopic steady state and if sampled sediments are well mixed.

No abstract

The Bolivian Andes flank one of Earth's major topographic features and dominate sediment input into the Amazon Basin. Millennial-scale erosion rates and dominant controls on erosion patterns in this range are poorly known. To define these patterns, we present 48 erosion rate estimates, derived from analysis of in situ 10 Be in quartz-bearing alluvium collected from the Upper Beni River basin.Erosion rates, corrected for the non-uniform distribution of quartz in the sample basins, range from 0·04 mm a ). Hence, our data do not record any significant variation in erosion rate over the last several million years. Mean and modal short-term erosion rates for the Andes are an order of magnitude lower than rates in the Ganges River headwaters in the High Himalaya and an order of magnitude greater than rates typical of the European Alps.In the Upper Beni River region of the Bolivian Andes, short-term, basin-averaged erosion rates correlate with normalized channel steepness index, a metric of relative channel gradient corrected for drainage area. Neither normalized channel steepness index nor basin-averaged erosion rate shows strong correlation with mean basin hillslope gradient or mean basin local relief because many hillslopes in the Upper Beni River region are at threshold values of slope and local relief. Patterns of normalized channel steepness index appear primarily to reflect tectonic patterns and transient adjustment to those patterns by channel networks. Climate and lithology do not appear to exert first-order controls on patterns of basin-averaged erosion rates in the Bolivian Andes.

No abstract

Slow erosion has characterized the Namib Desert, the Namibian escarpment, and the adjacent Namibian highlands over the Pleistocene. Paired analyses (n‫)66؍‬ of in-situ-produced 10 Be and 26 Al in quartz-bearing samples of bedrock primarily from inselbergs, of sediment from dry river and stream channels, and of clasts from desert surfaces reveal large inventories of these cosmogenic nuclides indicating significant landscape stability over at least the past million years.Bedrock samples (n ‫؍‬ 47) collected in three transects from the coast, across the escarpment, and into the highlands, show no spatial pattern in elevation-normalized nuclide abundance despite a difference in mean annual precipitation (MAP) between sample sites at the coast (MAP <25 mm yr ؊1 ) and those in the highlands (MAP >400 mm yr ؊1 ). Average model erosion rates inland of the escarpment (3.2 ؎ 1.5, n ‫؍‬ 9) are indistinguishable from average rates seaward of the escarpment (3.6 ؎ 1.9, n ‫؍‬ 38) indicating that rock on the pedimented coastal plain is eroding at the same rate as rock in the highlands. Sediment samples (n ‫؍‬ 3) from small streams suggest that the landscape as a whole is eroding more rapidly than the bedrock outcrops and that a basin in the steep escarpment zone is eroding several times faster (16 m my ؊1 ) than either a basin in the highlands (5 m my ؊1 ) or a basin in the coastal plain (8 m my ؊1 ). Data from large rivers (n ‫؍‬ 4) constrain erosion rates, averaged over 10 5 yrs and 10 4 to 10 5 km 2 , between 3 and 9 m my ؊1 . Small quartz clasts (n ‫؍‬ 12) collected from four desert surfaces record extraordinarily long, variable, and in some cases complex exposure histories. Simple 10 Be model ages are as high as 1.8 my; some minimum total histories, considering both 10 Be and 26 Al and including both burial and exposure, exceed 2.7 my. As a group, the Namibian cosmogenic data do not support the model of significant and on-going escarpment retreat.The similarity of erosion rates calculated from 10 Be analysis of fluvial sediments and longer-term (10 7 yr), average mass removal rates estimated by others using fission track analysis of rock suggests that Namibian erosion rates have reached a steady state and are changing little over time. At outcrop scales, the concordance of 10 Be and 26 Al in most bedrock samples suggests that the model of steady, uniform bedrock erosion is valid; there is no indication of intermittent burial, shedding of thick rock slabs, or stripping of previous cover. At an intermediate scale, a transect of bedrock samples north of Gobabeb demonstrates that the northern boundary of the massive Namib Sand Sea has been steady and unshifting. Similarly low cosmogenically estimated erosion rates across west and central Namibia suggest that the landscape is in geomorphic steady state, its overall appearance changing only slowly through time.

No abstract

No abstract

For the purpose of detecting the effects of human activities on climate change, it is important to document natural change in past climate. In this context, it has proved particularly difficult to study the variability in the occurrence of extreme climate events, such as storms with exceptional rainfall. Previous investigations have established storm chronologies using sediment cores from single lakes, but such studies can be susceptible to local environmental bias. Here we date terrigenous inwash layers in cores from 13 lakes, which show that the frequency of storm-related floods in the northeastern United States has varied in regular cycles during the past 13,000 years (13 kyr), with a characteristic period of about 3 kyr. Our data show four peaks in storminess during the past 14 kyr, approximately 2.6, 5.8, 9.1 and 11.9 kyr ago. This pattern is consistent with long-term changes in the average sign of the Arctic Oscillation, suggesting that modulation of this dominant atmospheric mode may account for a significant fraction of Holocene climate variability in North America and Europe.