Elevation dependence of cosmogenic 36Cl production in Hawaiian lava flows

We measured an elevation profile of cosmogenic Cl-36 in two well-preserved lava flows on Mauna Kea, Hawaii (19.8 degrees N, 155.5 degrees W) in order to directly constrain the elevation dependence of cosmogenic nuclide production rates. The flows are vertically-extensive hawaiites erupted at 40.1 +/- 0.6 and 62.2 +/- 1.0 ka from point-vents on the upper flanks of Mauna Kea. The average paleo cutoff rigidity (a measure of geomagnetic shielding of cosmic rays) for these flows is 11 GV and their paleo-elevation range is 2100-3700 m. Production of Cl-36 is dominated by neutron reactions, with the high-energy K-39(n,x) and Ca-40(n,x) mechanisms accounting for nearly half of the Cl-36 production and the low-energy reaction Cl-35(n,gamma) responsible for the remaining half. Production by negative muons is small at the elevations of our samples, accounting for less than 2% of the total production in the lowest elevation samples. The elevation dependence of Cl-36 production measured in these lava flows is described by an effective attenuation length of 138 +/- 5 g cm(-2). This result is close to the value of 140 g cm(-2) determined from neutron monitor surveys of high-energy nucleon fluxes, but significantly below the value of 149 g cm(-2) determined from measurements of low-energy neutrons. The predicted atmospheric attenuation length for these lava flows, incorporating both high- and low-energy mechanisms, is 144 g cm(-2). The good agreement between the Cl-36 elevation profile and cosmic-ray surveys validates the use of neutron flux measurements to scale Cl-36 production rates when production by muons is negligible. (c) 2006 Elsevier B.V All rights reserved.