Dylan H. Rood

Uncertainties in slip-rate estimates for the Mission Creek strand of the southern San Andreas fault at Biskra Palms Oasis, southern California

This study focuses on uncertainties in estimates of the geologic slip rate along the Mission Creek strand of the southern San Andreas fault where it offsets an alluvial fan (T2) at Biskra Palms Oasis in southern California. We provide new estimates of the amount of fault offset of the T2 fan based on trench excavations and new cosmogenic 10Be age determinations from the tops of 12 boulders on the fan surface. We present three alternative fan offset models: a minimum, a maximum, and a preferred offset of 660 m, 980 m, and 770 m, respectively. We assign an age of between 45 and 54 ka to the T2 fan from the 10Be data, which is significantly older than previously reported but is consistent with both the degree of soil development associated with this surface, and with ages from U-series geochronology on pedogenic carbonate from T2, described in a companion paper by Fletcher et al. (this volume). These new constraints suggest a range of slip rates between ∼12 and 22 mm/yr with a preferred estimate of ∼14–17 mm/yr for the Mission Creek strand of the southern San Andreas fault. Previous studies suggested that the geologic and geodetic slip-rate estimates at Biskra Palms differed. We find, however, that considerable uncertainty affects both the geologic and geodetic slip-rate estimates, such that if a real discrepancy between these rates exists for the southern San Andreas fault at Biskra Palms, it cannot be demonstrated with available data.

Constant cosmogenic nuclide concentrations in sand supplied from the Nile River over the past 2.5 m.y.

Quartz sand in the eastern Mediterranean coastal plain is supplied through an extended transport system, which includes the Nile River, east Mediterranean longshore currents, and inland eolian transport. While the concentrations of cosmogenic nuclides ( 26 Al and 10 Be), and their ratio, in modern sand deposited along the coast of the eastern Mediterranean reflect the combined effect of exposure and burial during transport, the concentrations of these nuclides in buried sands are the result of decay of this initial dosing. Samples of modern exposed sand (n = 3) collected from the coastal plain of Israel yield an average 26 Al/ 10 Be ratio of 4.8 ± 0.2, significantly lower than the expected ratio of 6.8 for exposed quartz grains at the surface. A similar ratio of 4.5 ± 0.3 was measured in a late Pleistocene sand sample, indicating similar exposure-burial histories during transport in spite of the difference in climatic conditions. The results imply a steady, preburial cosmogenic nuclide ratio related to the Nile River9s ability, through storage and recycling, to buffer the effects of climatic and tectonic perturbations on cosmogenic nuclide concentrations in the transported quartz. All ancient and buried sand samples (n = 11) fall on a decay path that originates from the concentrations and ratio of 26 Al and 10 Be in modern sand, suggesting steady preburial concentrations of cosmogenic nuclides in quartz sand over the past 2.5 m.y.

Calibrating a long‐term meteoric 10Be accumulation rate in soil

[1] Using 13 samples collected from a 4.1 meter profile in a well-dated and stable New Zealand fluvial terrace, we present the first long-term accumulation rate for meteoric 10 Be in soil (1.68 to 1.72 × 10 6 at/(cM 2 ·yr)) integrated over the past ~18 ka. Site-specific accumulation data, such as these, are prerequisite to the application of meteoric 10 Be in surface process studies. Our data begin the process of calibrating long-term meteoric 10 Be delivery rates across latitude and precipitation gradients. Our integrated rate is lower than contemporary meteoric 10 Be fluxes measured in New Zealand rainfall, suggesting that long-term average precipitation, dust flux, or both, at this site were less than modern values. With accurately calibrated long-term delivery rates, such as this, meteoric 10 Be will be a powerful tool for studying rates of landscape change in environments where other cosmogenic nuclides, such as in situ 10 Be, cannot be used.

Retracted: Background rates of erosion and sediment generation in the Potomac River basin, USA, derived using in situ 10Be, meteoric 10Be, and 9Be

This article has been retracted by the authors. Beryllium isotopes are often used to estimate rates of landscape change, but results from different beryllium isotope systems have rarely been compared. Here, we combine measurements of in situ and meteoric 10 Be ( 10 Be i and 10 Be m , respectively) with the reactive and mineral phases of 9 Be ( 9 Be reac and 9 Be min , respectively) to elucidate short- and long-term rates of erosion and sediment transport in the Potomac River basin on the North American passive margin. Sixty-two measurements of 10 Be i in alluvium show that the Potomac watershed is eroding on average at 11 m m.y. −1 (∼30 Mg km −2 yr −1 ), which is consistent with regional erosion rate estimates. The 10 Be i erosion rates correlate with basin latitude, suggesting that periglacial weathering increased proximal to the Laurentide ice sheet. The average of 55 10 Be m / 9 Be reac -derived sediment generation rates (26.2 ± 18.3 Mg km −2 yr −1 ) is indistinguishable from the average of 62 10 Be i rates; however, 10 Be m / 9 Be reac - and 10 Be i -based sediment generation rates are uncorrelated for individual basins. The lack of correlation on a basin-by-basin basis suggests biogeochemical assumptions inherent to the 10 Be m / 9 Be reac technique are not valid everywhere. Contemporary sediment yields ( n = 10) are up to 10 times greater than 10 Be i - or 10 Be m -derived sediment generation rates. However, we find that benchmark levels set to manage sediment export into Chesapeake Bay are within the uncertainty of long-term sediment generation rates. Erosion indices derived from 10 Be m measurements range from 0.07 to 1.24, signifying that sediment retention occurs throughout the basin, except in the Appalachian Plateau. Paleo−erosion indices, calculated from the 150 k.y. Hybla Valley sediment core, suggest sediment excavation and storage under colder and warmer climate conditions, respectively.