Paul R. Bierman

ESTIMATING RATES OF DENUDATION USING COSMOGENIC ISOTOPE ABUNDANCES IN SEDIMENT

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.

Understanding Earth’s eroding surface with 10Be

For more than a century, geologists have sought to measure the distribution of erosion rates on Earth’s dynamic surface. Since the mid-1980s, measurements of in situ 10Be, a cosmogenic radionuclide, have been used to estimate outcrop and basin-scale erosion rates at 87 sites around the world. Here, we compile, normalize, and compare published 10Be erosion rate data (n = 1599) in order to understand how, on a global scale, geologic erosion rates integrated over 103 to 106 years vary between climate zones, tectonic settings, and different rock types. Drainage basins erode more quickly (mean = 218 m Myr−1; median = 54 m Myr−1) than outcrops (mean = 12 m Myr−1; median = 5.4 m Myr−1), likely reflecting the acceleration of rock weathering rates under soil. Drainage basin and outcrop erosion rates both vary by climate zone, rock type, and tectonic setting. On the global scale, environmental parameters (latitude, elevation, relief, mean annual precipitation and temperature, seismicity, basin slope and area, and percent basin cover by vegetation) explain erosion rate variation better when they are combined in multiple regression analyses than when considered in bivariate relationships. Drainage basin erosion rates are explained well by considering these environmental parameters (R2 = 0.60); mean basin slope is the most powerful regressor. Outcrop erosion rates are less well explained (R2 = 0.32), and no one parameter dominates. The variance of erosion rates is better explained when subpopulations of the global data are analyzed. While our compilation is global, the grouped spatial distribution of cosmogenic studies introduces a bias that will only be addressed by research in under-sampled regions.

Mid-Pleistocene cosmogenic minimum-age limits for pre-Wisconsinan glacial surfaces in southwestern Minnesota and southern Baffin Island: a multiple nuclide approach

Abstract Paired 10 Be and 26 Al analyses (n=14) indicate that pre-Wisconsinan, glaciated bedrock surfaces near the northern (Baffin Island) and southern (Minnesota) paleo-margins of the Laurentide Ice Sheet have long and complex histories of cosmic-ray exposure, including significant periods of partial or complete shielding from cosmic rays. Using the ratio, 26 Al / 10 Be , we calculate that striated outcrops of Sioux Quartzite in southwestern Minnesota (southern margin) were last overrun by ice at least 500,000 years ago. Weathered bedrock tors on the once-glaciated uplands of Baffin Island (northern margin) are eroding no faster than 1.1 m Myr−1, the equivalent of at least 450,000 years of surface and near-surface exposure. Our data demonstrate that exposure ages and erosion rates calculated from single nuclides can underestimate surface stability dramatically because any intermittent burial, and the resultant lowering of nuclide production rates and nuclide abundances, will remain undetected.

Using in situ produced cosmogenic isotopes to estimate rates of landscape evolution: A review from the geomorphic perspective

The application of in-situ produced cosmogenic isotopes to problems in geomorphology has increased rapidly over the past decade. At least 57 papers and numerous abstracts have been published since the mid-1980s when the first mass-spectrometric measurements of terrestrially produced cosmogenic isotopes were made. Taken at face value, these studies provide quantitative information about rates of landscape evolution and landform age; however, the significance of calculated erosion rates and exposure ages depends strongly on the models used to interpret isotopic data, the validity of assumptions inherent to these models, and the geologic surroundings in which the samples were collected. This paper attempts to place cosmogenic isotope studies in a geomorphic context by reviewing fundamentals of the method and evaluating the validity of assumptions under which these data have been interpreted. At present, the establishment of high-precision, cosmogenically based glacial and alluvial chronologies is stymied by the evolution of geomorphic surfaces, the erosion of rock from sampled boulders, the potential for isotope inheritance from previous exposure, and the uncertainty of isotopic measurements. Uncertainties in isotope production rates and the observed variability of exposure ages on individual geomorphic surfaces limit the confidence with which cosmogenic ages can be correlated reliably with those obtained by other techniques. Estimation of erosion rates at single points on the landscape gives useful small-scale information. Extrapolation of these rates over longer time and larger spatial scales is less sure and most likely biased toward lower erosion rates by die inadvertent selection of resistant sample sites. However, because erosion rates are so poorly constrained at present, even estimates to within a factor of 2 may be of significant value to geomorphologists and tectonicists.

Millennial-scale storminess variability in the northeastern United States during the Holocene epoch

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.

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.

Cosmogenic exposure and erosion history of Australian bedrock landforms

Exceptionally high activities of in situ-produced 1 0 Be and 2 6 Al indicate that Australian bedrock surfaces are some of the most stable in the world. These cosmogenic nuclides demonstrate that Australian granite, exposed and sampled at 10 sites along a transect from the Eyre Peninsula (33°S) to the Kakadu area in the Northern Territory (14°S), has in some places eroded only decimeters over the past million years. However, nuclide activities are not saturated in any of the 61 exposed samples we analyzed, mandating that the surfaces we sampled are losing mass, albeit slowly, over time. Such mass loss indicates that inselbergs are dynamic landforms, confounding the assignment of discreet ages to inselberg surfaces. Nevertheless, the differential between slow rates of erosion measured on the very stable uppermost parts of these landforms and the more rapid rates measured below their flanks and below the adjacent grus-covered planes suggests that the forms of many large inselbergs, which stand meters to tens of meters high, must be at least Tertiary in age and possibly much older. Activities of 1 0 Be and 2 6 Al in 61 exposed Australian samples range from 5.1 ′ 0.1 × 10 6 to 5.3 ′ 0.6 x 10 5 atoms.g - 1 and 2.1 ′ 0.1 x 10 7 to 3.4 ′ 0.5 x 10 6 atoms.g - 1 , respectively (normalized to sea level, at latitudes of >60°, using only neutron scaling). Activities of 1 0 Be and 2 6 Al in seven shielded samples (3.5-13 m below the surface) are much lower, ranging from 3.4 ′ 0.2 x 10 4 to 7.0 ′ 0.1 x 10 4 atoms.g - 1 and 1.6 ′ 0.1 x 10 5 to 2.8 ′ 0.2 x 10 5 atoms.g - 1 , respectively. 2 6 Al/ 1 0 Be ratios of exposed samples range from 3.89 ′ 0.19 to 6.29 ′ 0.82. If 2σ uncertainty ranges and a nominal sea-level, high-latitude 1 0 Be-production rate of 5.17 atoms.g - 1 .yr - 1 are considered, ratio data for all but one surface sample are consistent with continuous surface or near-surface exposure and/or erosion. Rock surfaces farther north in Australia, where the climate is more humid, have lower nuclide activities; thus, we infer that surface stability is inversely related to mean annual precipitation. From 61 exposed samples, we calculated maximum limiting, single-nuclide, model erosion rates of 0.3 ′ 0.1 to 5.7 ′ 1.0 m/m.y. and minimum limiting, single-nuclide, model exposure ages of 105 ′ 16 to 1310 ′ 190 ka (by assuming a sea-level, high-latitude 1 0 Be-production rate of 5.17 atoms.g - 1 yr - 1 ). Seven shielded samples, in which the nuclide inventory is muon-generated, also have nuclide activities consistent with similarly low rates of erosion but over even longer time scales. It appears that the Eyre Peninsula inselbergs have been and are eroding so slowly that they may well be direct descendents of Cenozoic and perhaps even Mesozoic forms.

Last Glacial Maximum ice sheet dynamics in Arctic Canada inferred from young erratics perched on ancient tors

Along-standing debate regarding the reconstruction of former ice sheets revolves around the use of relative weathering of landscapes, i.e., the assumption that highly weathered landscapes have not been recently glaciated. New cosmogenic isotope measurements from upland bedrock surfaces and erratics along the northeastern margin of the Laurentide Ice Sheet (LIS) shed light on this debate. 10 Be and 26 Al concentrations from three perched erratics, yielding cosmogenic exposure ages of 17–11 ka, are much lower than those measured in two unmodified, highly weathered tors upon which they lie, which yield cosmogenic exposure ages of >60 ka. These findings suggest that non-erosive ice covered weathered upland surfaces along the northeastern margin of the LIS during the last glacial maximum. These data challenge the use of relative weathering to define the margins of Pleistocene ice sheets. The juxtaposition of non-erosive ice over upland plateaus and erosive ice in adjacent fiords requires strong gradients in basal thermal regimes, suggestive of an ice-stream mode of glaciation. r 2003 Elsevier Science Ltd. All rights reserved.

Sediment yield exceeds sediment production in arid region drainage basins

We use 10 Be and 26 Al to determine long-term sediment generation rates, identify significant sediment sources, and test for landscape steady state in Nahal Yael, an extensively studied, hyperarid drainage basin in southern Israel. Comparing a 33 yr sediment budget with 33 paired 10 Be and 26 Al analyses indicates that short-term sediment yield (113–138 t· km –2 · yr –1 ) exceeds long-term sediment production (74 ± 16 t· km –2 · yr –1 ) by 53%–86%. The difference suggests that the basin is not in steady state, but is currently evacuating sediment accumulated during periods of more rapid sediment generation and lower sediment yield. Nuclide data indicate that (1) sediment leaving the basin is derived primarily from hillslope colluvium, (2) bedrock weathers more rapidly beneath a cover of colluvium than when exposed, and (3) long-term erosion rates of granite, schist, and amphibolite are similar.

Cosmogenic Ages for Earthquake Recurrence Intervals and Debris Flow Fan Deposition, Owens Valley, California

Model exposure ages (beryllium-10, aluminum-26) of boulders on an offset debris flow fan yield an earthquake recurrence interval between 5800 and 8000 10Be:26Al years for a strand of the Owens Valley fault in California, which last ruptured in an earthquake of moment magnitude >7.5 in 1872. Cosmogenic age estimates for this and several nearby fan surfaces flanking the eastern Sierra Nevada are consistent with stratigraphic relations and suggest that these surfaces were abandoned after 1000, 8000, and 21,000 10Be:26Al years ago. The wide scatter and nonconcordance of 10Be:26Al ages on an older fan surface suggest that boulder erosion and lowering of the fan surface there have influenced apparent exposure ages.