Cosmogenic 36Cl production rates from Ca spallation in Iceland

Cosmogenic 36Cl concentrations were measured in whole-rock basalt samples from twenty-one surface sites on four radiocarbon-dated postglacial lava flows in Iceland. The new 36Cl data are derived from splits of rock from the same suite of calibration sites used to determine cosmogenic 3He production rates in recent work, and thus allow a direct co-calibration of production rates for these two widely used terrestrial cosmogenic nuclides. Ca spallation is the dominant 36Cl production reaction in our samples; hence the calibration exercise is focused on this production pathway. The four calibration flows yield mean Ca-spallation 36Cl production rates that agree within one standard deviation. The grand mean Ca spallation production rate from all four calibration flows in Iceland is 57 ± 5 atoms 36Cl (g Ca)- 1 yr- 1 (± 1σ; normalized to sea level at high latitudes with the standard atmosphere) and falls within the range of values reported in previous 36Cl calibration studies. We contend that the Ca-spallation 36Cl production rates in Iceland are 17% higher than normalized values calibrated in western U.S., which is equivalent in direction and magnitude to the previously reported offset in calibrated 3He production rates between these two regions. Relatively high terrestrial cosmogenic nuclide production rates in Iceland are interpreted to reflect the time-integrated impact of persistent low atmospheric pressure associated with the Icelandic Low. The new 36Cl production rate calibrations contribute significantly to an understanding of the production systematics of this nuclide and enable accurate 36Cl surface exposure dating in Iceland.