A NEW ESTIMATE OF THE SPALLOGENIC PRODUCTION RATE OF IN SITU COSMOGENIC 10BE FROM LAKE BONNEVILLE SHORELINE FEATURES, PROMONTORY POINT, UTAH

In situ cosmogenic ¹⁰Be is the most widely used terrestrial cosmogenic nuclide (TCN) in surficial process and Quaternary geologic studies. As measurement capabilities improve and more sophisticated applications are developed, an accurate production rate estimate is critical for interpreting ¹⁰Be measurements in terrestrial materials. A recent study by Balco et al. (2009, Quat. Geochron. 4, 93) suggests differences on the order of 10% between ¹⁰Be production rate estimates from New England and more commonly used global values.

The CRONUS-Earth project has been working to better characterize production rate systematics for all commonly measured TCNs. As part of this effort, we sampled a well-dated wave-cut quartzite shoreline bench associated with the high stand of Pleistocene Lake Bonneville (Bonneville level) at Promontory Point, Utah. The Bonneville shoreline formed during a period of overflow into southern Idaho, and was abandoned essentially instantaneously during the catastrophic Bonneville flood. The timing of shoreline formation is constrained by over 20 ¹⁴C ages to between ca. 18.3 and 17.4 cal ka. Field and GIS analyses suggest that the sampling locations were covered by ca. 25 m of water during overflow, and that approximately 80-100 m of overburden was removed during formation of the ca. 300-m-wide bedrock bench. We take 17.4±0.2 cal ka as the exposure age for our samples, as no evidence of prior occupation of this shoreline level has been recognized.

Samples were thoroughly homogenized, and aliquots distributed to four laboratories for ¹⁰Be processing. Aliquots from three labs were analyzed for ¹⁰Be at LLNL-CAMS, while the fourth was analyzed at PRIME Lab. The mean ¹⁰Be concentration at the site is (2.54±0.10)x10⁵ at g^-1 (07KNSTD, n=24), yielding a site production rate of approximately 14.7±0.6 ¹⁰Be at g^-1 a^-1. Corresponding sea level, high latitude (SLHL) production rates were calculated with a chi-squared minimization routine, using algorithms and assumptions from the CRONUS Online Calculator (Balco et al., 2008, Quat. Geochron. 3, 174). These SLHL values (4.2 to 4.7 at g^-1 a^-1, depending on scaling model) are consistent with those of Balco et al. (2009) and the previous global estimates, but with lower scatter (< 4% 1σ).