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Calibration of the production rates of cosmogenic 36C1 from potassium

Chlorine-36 production rates from potassium have been calibrated for the top 10m of the lithosphere, where "6C1 is produced by two main reaction channels: i) spallation, which dominates 3bCl production in the top 2 metres below the earth's surface, and ii) negative muon capture, which dominates at depths of between 2 and ~20 metres below the earth's surface. Rock samples were collected from two sites: i) rock surfaces exposed by the end of the Loch Lomond Readvance glaciation in Scotland 11,600 years ago, and ii) a 6.1m depth profile at a granite quarry in south eastern Australia, for which a steady state J6C1 concentration dictated by a history of constant erosion was assumed. In addition, the 36C1 measurements were made on pure K-feldspar mineral separates to remove the b0Cl contributions from calcium, iron and titanium, and to minimise the 36C1 contribution from chloride. The potassium spallation production rate was deduced from 18 Scottish Kfeldspar mineral separates to be 161 (9) atom (gK)'1 y r1 at sea level and high latitude. This value lies between two earlier calibrations after these had been adjusted to allow for 36C1 production by negative muon capture and low-energy neutron capture. The 36C1 yield per stopped negative muon was determined from 15 Kfeldspar mineral separates in the depth profile to be 5.80(0.48)xl0'2 atom per stopped p at sea level and high latitude. Assuming a negative muon stopping rate of 175 p g'1 yr'1, with a 10% uncertainty, the 36C1 production rate by negative muon capture on 39K is 10.2 (1.3) atom (gK)'1 yr'1 at sea level and high latitude. A calibration of the fast neutron flux just above the air/ground boundary was also obtained from 36C1 measurements on eight pure quartz mineral separates prepared from the Scottish samples. This quantity is required for determining the 36C1 production due to low-energy neutron capture on 3oCl in the near-surface. A value of 740 (63) n (g air)'1 yr'1 scaled to sea level and high latitude was determined, which is -35% higher than the previously published value.