New Production Rate Estimates for In Situ Cosmogenic 14C From Lake Bonneville, Utah and Northwest Scotland

Accurate and precise quantification of the relationship between past climatic changes and surficial processes is necessary to yield a better understanding of environmental responses to current climatic change. Terrestrial in situ cosmogenic nuclide (TCN) analytical techniques have enabled broad advances in Quaternary geologic and surficial process research over the past two decades, yet the lack of a commonly accepted framework for TCN measurements and spatial and temporal production rate scaling has limited the ability to draw robust comparisons on a global scale. The CRONUS-Earth and CRONUS-EU projects seek to remedy this situation through a multidisciplinary investigation of TCN production and measurement systematics. Under the CRONUS-Earth project, we aim to improve the calibration of in situ cosmogenic 14C (in situ 14C) production rates. In situ 14C is a particularly useful nuclide for late Quaternary surficial process studies by virtue of its short half-life (5.73 ka) - unique among commonly measured TCNs. Lifton et al. (2001, GCA 65, p. 1953) estimated the in situ 14C production rate in quartz based on measurements of wave-cut quartzite bedrock benches associated with the highstand of Pleistocene Lake Bonneville, Utah (17.4 ± 0.3 cal ka). To allow direct comparison of production rates for commonly measured TCNs in the same samples, CRONUS-Earth resampled the Lake Bonneville site in 2005, and sampled Younger-Dryas-age (11.6 cal ka) glacial and landslide deposits in northwest Scotland in 2006, with additional sites pending. Recent stepwise etching experiments have suggested the need for a more robust quartz pretreatment protocol than was employed by Lifton et al. (2001). When combined with improved extraction procedures (e.g., Miller et al., 2006, Quat. Geochron. 1, p. 74), a downward revision of the Lifton et al. (2001) time-integrated site production rate estimate of up to 15.3% may be required, from 52.9 ± 1.7 to 44.8 ± 2.9 14C at g-1 yr-1. Initial data from a new Lake Bonneville sample yields a time-integrated site production rate of 46.7 ± 0.8 14C at g-1 yr-1 - consistent with the revised estimate. We plan to present and compare additional measurements from the Lake Bonneville and Scotland samples as well.