Paleointensities on 8 ka obsidian from Mayor Island, New Zealand

Abstract

Abstract. The 8 ka BP (6050 BCE) pantelleritic obsidian flow on Mayor Island, Bay of Plenty, New Zealand, has been investigated using 30 samples from two sites. Due to a very high paramagnetic/ferromagnetic ratio, it was not possible to determine the remanence carriers. This is despite the fact that the samples were studied intensively at low, room, and high temperatures. We infer that a stable remanence within the samples is carried by single- or close to single-domain particles. Experiments to determine the anisotropy of thermoremanence tensor and the dependency on cooling rate were hampered due to alteration resulting from the repeated heating of the samples to temperatures just below the glass transition. Nonetheless, a well-defined mean paleointensity of 57.0 ± 1.0 μT, based on individual high quality paleointensity determinations, was obtained. This field value compares very well to a paleointensity of 58.1 ± 2.9 μT, which Tanaka et al. (2009) obtained for 5500 BCE at a site 100 km distant. Agreement with geomagnetic field models, however, is poor. Thus, gathering more high-quality paleointensity data for the Pacific region and for the southern hemisphere in general to better constrain global field models is very important.