A mechanism for reversals?
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OLR (1987) 34 (9) D. Submarine Geology and Geophysics 763 infrequently, therefore sample size effects may be large relative to the faunal patterns reported. Examination of 10 carefully compiled large data sets (>1000 occurrences each) reveals that, if the materials used in assembling a large data set are resampled, as many as 25% of the species will not be found a second time even if both samples are of the same size. If the samples are of unequal size, then the larger sample may have as many as 70% unique species and the smaller no unique species. The implications of these values are important to studies of species richness, origination, and extinction patterns, and biogeographic phenomena such as endemism or province boundaries. Dept. of Geol. Sci., Old Dominion Univ., Norfolk, VA 23508, USA. D200. Gravity, geodesy, magnetism 87:5076 Fuller, Mike, 1987. A mechanism for reversals? [Report.] Nature, Lond., 326(6109):132-133. The model proposed by Bloxham and Gubbins (1985) to explain the magnetic reversal process has stimulated renewed interest in secular variations and reversals. Gubbins now (p.167 of this issue) reports that the decay of the dipole results from a localized reversed flux in the Southern Hemisphere which involves an 'expulsion of toroidal flux by fluid upwelling, analogous to the processes that give rise to sunspot formation and suggests that the growth and poleward migration of such reverse flux also provides a mechanism for reversals.' The topography or temperature at the base of the mantle determines the locus of fluid upwelling. The model is supported by new research on recent field reversals, which appear to occur within a relatively short interval (a few thousand years) with a field intensity roughly 10% of the intensity prevailing before the reversal. Dept. of Geol. Sci., Univ. of Calif., Santa Barbara, CA 93106, USA. (hbf) 87:5077 Gubbins, David, 1987. Mechanism for geomagnetic polarity reversals. Nature, Lond., 326(6109): 167- 169. A patch of flux of opposite sign to that expected to a dipole field occurs beneath southern Africa. It has intensified throughout the twentieth century, by a process that is believed to be flux expulsion asso- ciated with a particularly hot part of the lowermost mantle, and is drifting westwards. A similar patch lies beneath South America. The present fall in the dipole moment is directly related to the intensifi- cation and southward movement of these patches; and it is hypothesized that such a fall occasionally leads to polarity reversal. The almost linear increase in the frequency of polarity reversals that has occurred since the Cretaceous quiet interval is attributed to the growth of the hot patch of mantle presently under southern Africa. Bullard Lab., Dept. of Earth Sci., Madingley Rd., Cambridge, CB3 0EZ, UK. 87:5078 Hilton, John, 1987. A simple model for the inter- pretation of magnetic records in lacustrine and ocean sediments. Quat. Res., 27(2):160-166. A model based on low-frequency magnetic suscep- tibility measurements is presented which predicts the most probable sources of magnetic minerals found in sedimentary environments where specific magnetic sources influence the magnetic record. Authigenic magnetic minerals are typical of estuaries and eutrophic lakes; cosmic and eolian particles and magnetic bacteria are important in low-accumula- tion-rate deep-ocean sediments; catchment derived materials are the only likely sole source magnetics in high-accumulation, low-productivity lakes; and fly ash tends to dominate very-low-accumulation lake and ocean sediments. Freshwater Biol. Assoc., The Ferry House, Ambleside, Cumbria LA22 0LP, UK. (hbf) 87:5079 Roperch, Pierrick and Annick Chauvin, 1987. Tran- sitional geomagnetic field behavior: volcanic records from French Polynesia. Geophys. Res. Lefts, 14(2):151-154. Lab. de Geophys. Interne, Univ. de Rennes 2, 35042 Rennes Cedex, France. 87:5080 Sandwell, D.T., 1987. Biharmonic spline interpolation of GEOS-3 and Seasat altimeter data. Geophys. Res. Letts, 14(2): 139-142. Center for Space Res., Univ. of Texas, Austin, TX, USA. 87:5081 Zhang, Minghan and Mingde Gao, 1986. Paleo- magnetic study of some sediment cores from the Changjiang Delta. Stud. mar. sin., 27:137-151. (In Chinese, English abstract.) Inst. of Oceanol., Acad. Sinica, People's Republic of China. D210. Heat flow 87:5082 Van Gool, Mathijs, W.J. Huson, Rachmat Prawira- sasra and T.R. Owen, 1987. Heat flow and
Transcript of A mechanism for reversals?
OLR (1987) 34 (9) D. Submarine Geology and Geophysics 763
infrequently, therefore sample size effects may be large relative to the faunal patterns reported. Examination of 10 carefully compiled large data sets (>1000 occurrences each) reveals that, if the materials used in assembling a large data set are resampled, as many as 25% of the species will not be found a second time even if both samples are of the same size. If the samples are of unequal size, then the larger sample may have as many as 70% unique species and the smaller no unique species. The implications of these values are important to studies of species richness, origination, and extinction patterns, and biogeographic phenomena such as endemism or province boundaries. Dept. of Geol. Sci., Old Dominion Univ., Norfolk, VA 23508, USA.
D200. Gravity, geodesy, magnetism 87:5076
Fuller, Mike, 1987. A mechanism for reversals? [Report.] Nature, Lond., 326(6109):132-133.
The model proposed by Bloxham and Gubbins (1985) to explain the magnetic reversal process has stimulated renewed interest in secular variations and reversals. Gubbins now (p.167 of this issue) reports that the decay of the dipole results from a localized reversed flux in the Southern Hemisphere which involves an 'expulsion of toroidal flux by fluid upwelling, analogous to the processes that give rise to sunspot formation and suggests that the growth and poleward migration of such reverse flux also provides a mechanism for reversals.' The topography or temperature at the base of the mantle determines the locus of fluid upwelling. The model is supported by new research on recent field reversals, which appear to occur within a relatively short interval (a few thousand years) with a field intensity roughly 10% of the intensity prevailing before the reversal. Dept. of Geol. Sci., Univ. of Calif., Santa Barbara, CA 93106, USA. (hbf)
87:5077 Gubbins, David, 1987. Mechanism for geomagnetic
polarity reversals. Nature, Lond., 326(6109): 167- 169.
A patch of flux of opposite sign to that expected to a dipole field occurs beneath southern Africa. It has intensified throughout the twentieth century, by a process that is believed to be flux expulsion asso- ciated with a particularly hot part of the lowermost mantle, and is drifting westwards. A similar patch lies beneath South America. The present fall in the dipole moment is directly related to the intensifi- cation and southward movement of these patches;
and it is hypothesized that such a fall occasionally leads to polarity reversal. The almost linear increase in the frequency of polarity reversals that has occurred since the Cretaceous quiet interval is attributed to the growth of the hot patch of mantle presently under southern Africa. Bullard Lab., Dept. of Earth Sci., Madingley Rd., Cambridge, CB3 0EZ, UK.
87:5078 Hilton, John, 1987. A simple model for the inter-
pretation of magnetic records in lacustrine and ocean sediments. Quat. Res., 27(2):160-166.
A model based on low-frequency magnetic suscep- tibility measurements is presented which predicts the most probable sources of magnetic minerals found in sedimentary environments where specific magnetic sources influence the magnetic record. Authigenic magnetic minerals are typical of estuaries and eutrophic lakes; cosmic and eolian particles and magnetic bacteria are important in low-accumula- tion-rate deep-ocean sediments; catchment derived materials are the only likely sole source magnetics in high-accumulation, low-productivity lakes; and fly ash tends to dominate very-low-accumulation lake and ocean sediments. Freshwater Biol. Assoc., The Ferry House, Ambleside, Cumbria LA22 0LP, UK. (hbf)
87:5079 Roperch, Pierrick and Annick Chauvin, 1987. Tran-
sitional geomagnetic field behavior: volcanic records from French Polynesia. Geophys. Res. Lefts, 14(2):151-154. Lab. de Geophys. Interne, Univ. de Rennes 2, 35042 Rennes Cedex, France.
87:5080 Sandwell, D.T., 1987. Biharmonic spline interpolation
of GEOS-3 and Seasat altimeter data. Geophys. Res. Letts, 14(2): 139-142. Center for Space Res., Univ. of Texas, Austin, TX, USA.
87:5081 Zhang, Minghan and Mingde Gao, 1986. Paleo-
magnetic study of some sediment cores from the Changjiang Delta. Stud. mar. sin., 27:137-151. (In Chinese, English abstract.) Inst. of Oceanol., Acad. Sinica, People's Republic of China.
D210. Heat flow 87:5082
Van Gool, Mathijs, W.J. Huson, Rachmat Prawira- sasra and T.R. Owen, 1987. Heat flow and
