Mathematics – Logic
Scientific paper
Aug 1982
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1982rspta.306..211m&link_type=abstract
Philosophical Transactions of the Royal Society of London. Series A, Mathematical and Physical Sciences, Volume 306, Issue 1492,
Mathematics
Logic
27
Scientific paper
New palaeomagnetic data from 106 14C-dated lava flows ranging in age from 200 to 31 000 years B.P. yield an estimated angular dispersion value of 9.5 degrees. These data and other new geological information permit a more precise estimate of the time interval recorded by lava flow sequences previously used to measure palaeosecular variation in Hawaii. When weighted according to revised estimates of recording interval, the combined Brunhes lava sequences yield an angular dispersion of 11.2-0.8+0.9 degrees, still lower than that predicted by global models of the secular variation. Several of the lava flow sequences previously thought to have recorded quiet intervals of geomagnetic behaviour actually record only very short time intervals. Recent analyses by Duncan (1975), McElhinny & Merrill (1975) and Coe et al. (1978) have suggested that geomagnetic secular variation reported previously from Hawaii by Doell & Cox (1965, 1971, 1972) and Doell (1972a, b) was unduly biased toward low values. This suggestion was made on the grounds that the Puna and Kau series data, which made up nearly half of the original data set used by Doell & Cox, record a very short time interval. With the addition of new 14C and geological data, it is now clear that the grounds for this suggestion were well founded, as the best estimates now available indicate that the sequences in question accumulated over time intervals of 500 years or less. However, new data from 14C-dated flows, when combined with the previously available data, yield dispersion values consistent with the original dispersion estimate of 10.8 degrees by Doell & Cox (1972). This is true regardless of whether the `suspect' Kau and Hamakua series data are included with their original weighting, or with a more reasonable attenuated weighting, as discussed above. The Puna series are not included in an overall estimate of dispersion, as they represent a very short time interval that is adequately covered by the 14C series data set. Having recognized that some lava sequences in Hawaii probably record only a very short time interval, it seems reasonable to reconsider the concept of `quiet' and `noisy' behaviour of the geomagnetic field, as proposed by Cox (1975). For Brunhes-age lavas, three sequences had suggested `quiet' behaviour and two of these (Puna and Kau) can be shown to have artificially low angular dispersion values resulting from a sampling bias. The remaining Hamakua series data are more equivocal, since there is no direct evidence available to constrain the interval of their accumulation. However, it is equally likely that the Hamakua series also represents a short interval. If this is indeed correct, then the `quiet' and `noisy' subdivision would be an artificial one, at least on a 103 to 104 year timescale. However, as noted by Kawai et al. (1973), quiet and noisy behaviour on a timescale of 2-3 × 105 years may be possible. The apparently low overall dispersion from Brunhes-age lavas in the Hawaiian Islands may reflect such behaviour.
Champion Duane E.
Holcomb Robin T.
McWilliams Michael O.
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