Cosmic ray production rates of ¹⁰Be and ²⁶Al in quartz from glacially polished rocks

Abstract

We have studied the concentrations of 10Be and 26Al in quartz crystals extracted from glacially polished granitic surfaces from the Sierra Nevada range. These surfaces were identified with the glacial advance during the Tioga period ∼11,000 years ago. Our measurements yield the most accurate estimates to date for the absolute production rates of these nuclides in SiO2 due to cosmic ray nucleons and muons for geomagnetic latitudes 43.8°–44.6°N and altitudes 2.1–3.6 km. The estimates are relatively free from uncertainties in snow cover since we studied a suite of rock surfaces inclined 0°–75° with respect to the horizontal. The principal uncertainty arises due to the lack of a precise date for the glacial retreat event, about ±10%. The 26Al/10Be ratio at production (6.0) is determined more accurately since the exposure age of the samples is 2 orders of magnitude smaller than the mean lives of the two nuclides. Production rates of 10Be and 26Al in quartz at other latitudes and altitudes in the troposphere can be determined from the present measurements by scaling, using the known altitude and latitude dependence of cosmic ray fluxes of nucleons and negative muons. Knowledge of the production rates of these nuclides is a prerequisite for their application in erosion and geomorphological studies.

Prospecting Glacial Ages and Paleoclimatic Reconstructions Northeastward of Nevado Coropuna (16° S, 73° W, 6377 m), Arid Tropical Andes

“…Bromley et al [37] defined a lower moraine group (~5000-4900 m), extending to the lower area of the glacial valleys, as CI and a higher moraine group (~5500 m), deposited in the middle part of the Santiago valley, as CII. For each group of moraines they found two age ranges, depending on the scaling model used: Lm [70] or Li [71][72][73]: -CI moraines:~25-15 ka (Lm) and~21-12 ka (Li), with outlier ages~47 and~31 ka (Li) and~61 and~37 ka (Lm). -CII moraines:~11-12 ka (Lm) and~11-8 ka (Li).…”

Crustal–lithospheric structure and continental extrusion of Tibet

“…The cosmogenic isotope technique is based upon the fact that cosmogenic nuclides accumulate in situ from exposure of, for example, quartz grains to cosmic rays at or near the land surface (e.g. Nishiizumi et al 1986Nishiizumi et al , 1989. Therefore it is possible to date the time at which an alluvial fan containing the measured sample was abandoned and left exposed, and, in principal, if a fan offset is measurable in that fan as it crosses the fault, then a slip rate can be determined.…”

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.…”