Technical note: A software framework for calculating compositionally dependent in situ 14C production rates

Abstract

Abstract. Over the last 30 years, in situ cosmogenic nuclides (CNs) have revolutionized surficial processes and Quaternary geologic studies. Commonly measured CNs extracted from common mineral quartz have long half-lives (e.g., 10Be, 26Al) and have been applied over timescales from a few hundred years to millions of years. However, their long half-lives also render them largely insensitive to complex histories of burial and exposure of less than ca. 100 kyr. On the other hand, in situ cosmogenic 14C (in situ 14C) is also produced in quartz, yet its 5.7 kyr half-life renders it very sensitive to complex exposure histories during the last ∼25 ka, a particularly unique and powerful tool when analyzed in concert with long-lived nuclides. In situ 14C measurements are currently limited to relatively coarse-grained (typically sand-sized or larger, crushed or sieved to sand) quartz-bearing rock types, but while such rocks are common, they are not ubiquitous. The ability to extract and interpret in situ 14C from quartz-poor and fine-grained rocks would thus open its unique applications to a broader array of landscape elements and environments. As a first step toward this goal, a robust means of interpreting in situ 14C concentrations derived from rocks and minerals spanning wider compositional and textural ranges will be crucial. We have thus developed a MATLAB®-based software framework to quantify spallogenic production of in situ 14C from a broad range of silicate rock and mineral compositions, including rocks too fine grained to achieve pure quartz separates. As expected from prior work, production from oxygen dominates the overall in situ 14C signal, accounting for >90 % of production for common silicate minerals and six different rock types at sea level and high latitudes (SLHL). This work confirms that Si, Al, and Mg are important targets but also predicts greater production from Na than from those elements. The compositionally dependent production rates for rock and mineral compositions investigated here are typically lower than that of quartz, although that predicted for albite is comparable to quartz, reflecting the significance of production from Na. Predicted production rates drop as compositions become more mafic (particularly Fe-rich). This framework should thus be a useful tool in efforts to broaden the utility of in situ 14C to quartz-poor and fine-grained rock types, but future improvements in measured and modeled excitation functions would be beneficial.