A test of the cosmogenic 3He production rate in the south‐west Pacific (39°S)

Calculation of surface exposure ages, using in situ cosmogenic nuclides, requires accurate knowledge of local production rates. Here, we report the first attempt to calibrate cosmogenic He-3 production in the south-west Pacific region. We present a new radiocarbon chronology that precisely dates the emplacement of the Murimotu Formation, a large debris avalanche deposit in central North Island, New Zealand (ca. 830m asl; 39 degrees S), which occurred 10.4-10.6cal ka BP. Measurements of cosmogenic He-3 in pyroxene separated from large andesitic blocks exposed during this event yield a sea-level high-latitude production rate of 120 +/- 12 atoms g(-1) a(-1) (Lm' scaling). This is consistent with a recent global compilation, comprised predominantly of calibration sites located in the Northern Hemisphere. Thus, we conclude that the globally compiled cosmogenic He-3 production rate is valid in the south-west Pacific. Using independent, proximal calibrations of cosmogenic isotopes Be-10 and C-14 from quartz in New Zealand, we derive cosmogenic He-3/Be-10 and He-3/C-14 production ratios of 32.2 +/- 3.2 and 10.6 +/- 1.6, respectively.