Improving in Situ Cosmogenic Chronometers

Abstract

New radiocarbon ages for Sierra Nevada deglaciation, the first 10 Be measurements from the Laurentide terminal moraine, and calculations based on paleomagnetic field strength have the potential to substantially improve the accuracy of cosmogenic age estimates. Specifically, three new constraints apply to the interpretation of measured abundances of in situ produced cosmogenic 10Be and 26Al: (1) A suite of minimum-limiting radiocarbon dates indicates that the Sierra Nevada was deglaciated at least several thousand years earlier than assumed when Nishiizumi et al. (1989) first calibrated 10Be and 26 Al production rates based on polished bedrock surfaces in the range, with retreat beginning by 18,000 cal yr B.P. and completed by 13,000 cal yr B.P. (2) Concentrations of 10Be in moraine boulders and glacier-polished bedrock in New Jersey show little variance (10%, 1σ) and can be used to calculate a preliminary 10Be production rate (integrated over the past 21,000-22,000 cal yr B.P. at 41°, 200-300 m altitude) that is about 20% lower than currently accepted. (3) Calculations of the effect of past geomagnetic field-strength variations on production rates suggest that the use of temporally averaged production rates may generate age errors of >20%; however, cosmogenic exposure ages can be corrected for this effect, although the corrections currently are imprecise. Many previously reported late-Pleistocene 10Be and 26Al exposure ages are probably too young and are less accurate and less precise than implied by reported uncertainties. The discrepancy between accepted production rates and those calculated from Laurentide exposures, when considered together with the Sierran deglacial chronology and the model results, suggest that correlations between cosmogenic and other numerical ages, especially for brief events like the Younger Dryas and Heinrich events, will not be robust until temporal variations and the altitude/latitude scaling of production rates are fully understood and quantified at levels comparable to current analytic uncertainties (∼3%).

New constraints on the late Cenozoic incision history of the New River, Virginia

“…The chosen production rate of 5.55 atoms/g•qtz is ~7% higher than short-term rates calibrated from samples ≤ 13 ka old and ~7% lower than modeled and calibrated rates for timescales of millions of years, so is considered appropriate for timescales of tens to few millions of years (Perg et al, 2001;Clark et al, 1995). Empirically, choosing a higher or lower production rate from among accepted values changes resulting model ages by no more than about 10%.…”

“…10 Be production rates are constrained by models (Masarik and Reedy, 1995), using measured neutron and muon fluxes (Dunai, 2000), by direct measurement in artificial targets (Nishiizumi et al, 1996), and by calibration against deposits with independent age constraint (Nishiizumi et al, 1986;Brown et al, 1991;Clark et al, 1995). The production rate at any given location fluctuates due to changes in solar output, geomagnetic field configuration, and atmospheric density (Dunai, 2000;Bierman, 1994).…”

“…Rates of CRN production are calibrated based on measurements from surfaces of known age (e.g. Lal, 1991;Clark et al, 1995).…”

Irregular earthquake cycle along the southern Tianshan front, Aksu area, China

“…Zero-erosion model ages were calculated using a sea level high-latitude (SLHL) 10 Be production rate of 5.06 atoms/g quartz/yr. This production rate is based on measurements of glacial surfaces in the Sierra Nevada by Nishiizumi et al [1989] recalculated using the revised 13,000 years glacial retreat ages reported by Clark et al [1995], and rescaled for latitude and altitude using the coefficients of Lal [1991], as described by Owen et al [2002]. An uncertainty of 6% on the production rates [Stone, 2000] is considered.…”

Late Quaternary erosion in southeastern Australia: a field example using cosmogenic nuclides

“…These production rates were used for consistency with previous reports of erosion rates or exposure ages, despite the growing debate over production rates (e.g. Clark et al, 1995;Nishiizumi et al, 1996;Stone et al, 1998a;Dunai, 2000) and the potential contribution of muons to nuclide concentrations under moderate and high erosion rates (e.g. Brown et al, 1995;Stone et al, 1998b;Granger and Smith, in press).…”