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Palaeomagnetic evidence for stationary sources of geomagnetic secular variation
Physics of the Earth and Planetary Interiors, 35 (1984) 223—226 Elsevier Science Publishers B.V., Amsterdam — Printed in The Netherlands
223
Letter Section Palaeomagnetic evidence for stationary sources of geomagnetic secular variation M.E. Evans Institute of Earth and Planetary Physics, University ofAlberta, Edmonton, A Iberia (Canada) (Received April 2, 1984; revision accepted May 17, 1984)
Evans, M.E., 1984. Palaeomagnetic evidence for stationary sources of geomagnetic secular variation. Phys. Earth Planet. Inter., 35: 223—226. Asymmetrical curvilinear patterns of secular variation with approximately superposed “outward” and “return” trajectories can be attributed to stationary magnetic sources in the core. Some appropriate palaeomagnetic examples are presented here.
Since Halley’s original proposal in the late 17th century, there has been a persistent feeling that the long-term changes in the geomagnetic field (the so-called secular variation) involve some form of “westward drift”—but just what drifts and why has been widely debated. From a comparison of magnetic charts for 1907 and 1945 Bullard et al. (1950) concluded that it is the non-dipole part of the field which drifts westwards—by 0.20 annually. More recently Yukutake and Tachinaka (1969) have argued that the non-dipole field itself is more complex—some features seem to drift (the drifting field), while others remain fixed (the standing field). The slowness of these changes compared to the brevity of the historic record provides a strong incentive to seek relevant evidence in the palaeomagnetic data to see if standing/ drifting duality has been a persistent feature of the geomagnetic field. What might we expect to observe? Basically, a drifting source will cause the total magnetic force at a site to sweep out a more-or-less elliptical ioop (Runcorn, 1959),
whereas a standing source will produce a more linear pattern (Creer and Tucholka, 1982, 1983; Creer, 1983). Of course, more complex patterns are to be expected if both standing and drifting sources are present, or if two or more standing sources are close enough to affect the observation site in question. Secular variation loops interpreted in terms of drifting sources are already well documented in palaeomagnetic (Skiles, 1970; Turner and Thompson, 1981) and historic (MaIm and Bullard, 1981) records. In addition, Creer (1981) has correlated limnomagnetic records from Europe and North America and argues :that they favour a westward motion of the drifting part of the non-dipole field for the last 12000 y. Thus there is a considerable body of data, from diverse sources, supporting the validity of both a present-day and a palaeo-westward drift. On the other hand, support for the existence in earlier times of stationary sources has been less forthcoming. In his correlation between Europe and North America, Creer (1981) considered only the inclination records. Subsequently,
Present address: Centre Geologique et Geophysique, IJniversité des Sciences et Techniques du Languedoc, Place Eugene Bataillon, 34060, Montpellier, France.
Creer and Tucholka (1982) pointed out that the corresponding declination logs do not fit such a simple mOdel, and were obliged to invoke 5 sources
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0031-9201/84/$03.00
© 1984 ElsevierScience Publishers B.V.
224
(2 drifting, 3 standing) for the time interval 0—4760 y ago, and 4 sources (2 drifting, 2 standing) for the interval 4760—8520 y ago. In a later paper Creer and Tucholka (1983) stressed that these models were provisional and that “the paradox will only be resolved satisfactorily with additional data from other northern hemisphere sites which occupy a range of longitudes”. In the meantime therefore it is worthwhile to seek other, hopefully more compelling, evidence for the existence of standing sources. In a recent paper, Turner et a!. (1982) reported that a sequence of sediments deposited in western Canada between — 30000 and 20000 y ago yielded evidence of a recurxing asymmetrical perturbation towards shallow inclinations and easterly declinations, leading to an overall bias of 60 in declination and 30 in inclination from the direction of a geocentricaxial dipole (Fig. 1 a, b). As they pointed out, the observed pattern cannot be explained by a simple westward (or eastward) drifting source. However, a stationary outward-pointing source DEC o
DEC 1
1
C ,.-.~?
U
S~
so
a
0
s 60 L2? 0
Fig. 1. Declination—Inclination plots of four palaeomagnetic secular variation perturbations. The square with the inscribed diagonal cross represents the field direction of a geocentric axial dipole at each site. (a) Besette Creek sediments, Canada; interval C—D of Turner et al. (1982) (b) Bessette Creek sedsments, Canada; interval B—F (c) British lake sediments; interval i—l of Turner and Thompson (1982) (d) Ninole Volcanic Series, Hawaii; flows 56—75 of Doell and Cox (1965).
situated to the west of the site would produce the general pattern observed if it gradually increased from zero up to a maximum and then decayed back to zero (rather like a half-rectified sine wave). Such a source will produce a series of field directions confined to the plane determined by the fixed directions of the main dipole field and the perturbation field of the standing source. On the stereographic projections usually employed in palaeomagnetism an arc of a great circle will be traced out, first in one direction as the source intensifies, then in the return sense as it wanes. Figure ic, d illustrates two other examples from British lake sediments (Turner and Thompson, 1982) and Hawaiian lava flows (Doell and Cox, 1965). Estimates of the timing and duration of these perturbation pulses based on the available dating control are summarized in Table I, which also lists the magnitudes of the perturbing fields necessary to account for the observed extrema. These estimates indicate that lifetimes of one thousand to several thousand years, and local anomalous fields of the order of 10000—20000 nT are typical. In view of what is known about the time scale of secular variations (— iO~—iO~ y, see e.g., Barton, 1982), the free-decay of current loops in the core (~ iO’~y, see e.g., Stacey, 1977), and the magnitude of the present non-dipole field (up to ±15000nT), these values are not unreasonable. As pointed out above, more complex patterns will result if two or more nearby sources simultaneously affect a single site. The example illustrated in Fig. 2a represents the lowest of three features reported by Turner et al. (1982), the upper two being those shown in Fig. la, b.whose The pattern can be attributed to two sources phase and amplitude relationships are such that a partial ellipse is produced by their superposition. Such a proposal is rather contrived, but a recent study by Rosenblaum and Larson (1983) of a sequence of sediments in Colorado lend it credence. This can be gleaned from the separate declination and inclination logs they present, but is rendered much clearer if their data is replotted on a combined declination—inclination plot (Fig. 2b). The reality of this pattern is further supported by an extremely good correlation between the data of Fig. 2b and those from a second core collected in a
225
TABLE I Timing and magnitude estimates of the perturbations shown in Fig. 1 Figure Id 1c lb la
Timing (10~y)
L
8
D’, I’
A
B’(pT)
8.2—6.1
2.1
—
—
24° 44° 14° 23°
048, 127, 052, 058,
55° 22° 50° 50°
21.2 24.8 12.1 19.5
20.5—19.9 24.9—23.1
0.6 1.8
—36 —79 —29 —38
Timing is given in I0~y before present. In the case of Hawaii (Fig. lc) dating control is poor. On the basis of estimated lava accumulation rates Doell and Cox (1965) claimed that the entire section, of which these data represent a part, spans <20000 y, sometime during the Brunhes epoch. L is the duration of each perturbation in l0~y. 8 is the maximum angle of divergence from a geocentric axial dipole field. D’, I’ are the declination and inclination of the minimum anomalous field capable of producing the divergence 8. 2 (1 ~zT = l000y). B’ is the corresponding magnitude of the minimum anomalous field, assuming a dipole moment of 8 x 1022 Am A is the site latitude.
separate sedimentary basin some 100 km away (see Rosenbaum and Larson, 1983, fig. 12). The similarity of the two features shown in Fig. 2a, b suggests that they are actually correlative, but this conflicts with the available dating control, The Canadian feature (Fig. 2a) occurred about 28000 y ago (based on the evidence of 5 radiocarbon dates), whereas the Colorado feature seems to be — 10000 y younger (based on 9 radiocarbon dates). Thus very large errors must be attributed to the radiocarbon results if the two magnetic features are to be equated, if not then it appears that they represent two separate, but highly similar, perturbations, possibly involving the same sources. All the above examples suggest perturbations
DEC
towards inclinations shallower than the corresponding geocentric axial dipole values—this is consistent with Cox’s (1975) explanation of the long-term existence of a zonal quadrupole (g~) moment. On the other hand, the declination bias, which is to the east in all six cases, is puzzling. However, it must be noted that since these examples basically represent only three sites—northwestern North America, Hawaii, and Britain—the observed declination bias is quite likely owing to chance. Much more data, with adequate global coverage, will be necessary before any firm conclusion can be drawn about possible bias in declination and inclination. In the meantime, evidence for stationary pulsating sources can be sought in secular variation patterns consisting of highly asymmetrical curvilinear perturbations with approxi-
D 0C
mately superposed outward and return trajectories. This type of feature can be recognised on
b
a
~0
60
‘~
separate declination and inclination logs, but is clination (Bauer) plots, and it is hoped that in
~::
~0:~r~~ss.
Fig. 2. Declination—Inclination plots of two further palaeomagnetic secular variation perturbations. (a) Bessette Creek sediments, Canada; interval A—B of Turner et al. (1982). (b) Devhns Park core sediments, Colorado; interval 12.01—8.78 m of Rosenbaum and Larson (1983). The points plotted were hand-digitized at 1 mm intervals from their fig. 9.
field will include such dia-
References .
.
Barton,. C.E., 1982. Spectral . analysis of paleomagnetic time . senes and the geomagnetic field. Philos. Trans. R. Soc., London, Ser A: 306: 203—209.
226 Bullard, E.C., Freedman, C., Gellman, H. and Nixon, J., 1950. The westward drift of the Earth’s magnetic field. Philos. Trans. R. Soc. London, Ser A: 243: 67—92. Cox, A., 1975. The frequency of geomagnetic reversals and the symmetry of the non-dipole field. Rev. Geophys. Space Phys., 13: 35—51. Creer, K.M., 1981. Long-period geomagnetic secular variation since 12,000 yr BP. Nature, 292: 208—212. Creer, K.M., 1983. Computer synthesis of geomagnetic palaeosecular variations. Nature, 304: 695—699. Creer, K.M. and Tucholka, P., 1982. The shape of the geomagnetic field through the last 8,500 years over part of the northern hemisphere. J. Geophys., 51: 188—198. Creer, K.M. and Tuchoilca, P., 1983. In: K.M. Creer, P. Tucholka and C.E. Barton (Editors), “Geomagnetism of Baked Clays and Recent Sediments”. Elsevier, pp. 273—305. Doell, R.R. and Cox, A., 1965. Paleomagnetism of Hawaiian Lava Flows. J. Geophys. Res., 70: 3377—3405. Malin, S.R.C. and Bullard, E.C., 1981. The direction of the Earth’s magnetic field at London, 1570—1975. Philos. Trans. R. Soc. London, Ser A: 299: 357~—423. Rosenbaum, J.G. and Larson, E.E., 1983. Paleomagnetism of two late Pleistocene lake basins in Colorado: an evaluation
of detrital remanent magnetization as a recorder of the geomagnetic field. J. Geophys. Res., 80: 10611—10624. Runcorn, S.K., 1959. On the theory of geomagnetic secular variation. Ann. Geophys., 15: 87—92. Skiles, D.D., 1970. A method of inferring the direction of drift of the geomagnetic field from paleomagnetic data. J. Geomagn. Geoelectr., 22: 441—462. Stacey, F.D., 1977. Physics of the Earth. Wiley, New York, 2nd edn, 414 pp. Turner, G.M. and Thompson, R., 1981. Lake sediment record of the geomagnetic secular variation in Britain during Holocene times. Geophys. JR. Astron. Soc., 65: 703—725. Turner, G.M. and Thompson, R., 1982. Detransformation of the British geomagnetic secular variation record for Holocene times. Geophys. J.R. Astron. Soc., 70: 789—792. Turner, G.M., Evans, M.E. and Hussin, I.B., 1982. A geomagnetic secular variation study (31,000—19,500 bp) in western Canada. Geophys. J.R. Astron. Soc., 71: 159—171. Yukutake, T. and Tachinaka, H., 1969. Separation of the Earth’s magnetic field into drifting and standing parts. Bull. Earthquake Res. Inst., 47: 65—97.
223
Letter Section Palaeomagnetic evidence for stationary sources of geomagnetic secular variation M.E. Evans Institute of Earth and Planetary Physics, University ofAlberta, Edmonton, A Iberia (Canada) (Received April 2, 1984; revision accepted May 17, 1984)
Evans, M.E., 1984. Palaeomagnetic evidence for stationary sources of geomagnetic secular variation. Phys. Earth Planet. Inter., 35: 223—226. Asymmetrical curvilinear patterns of secular variation with approximately superposed “outward” and “return” trajectories can be attributed to stationary magnetic sources in the core. Some appropriate palaeomagnetic examples are presented here.
Since Halley’s original proposal in the late 17th century, there has been a persistent feeling that the long-term changes in the geomagnetic field (the so-called secular variation) involve some form of “westward drift”—but just what drifts and why has been widely debated. From a comparison of magnetic charts for 1907 and 1945 Bullard et al. (1950) concluded that it is the non-dipole part of the field which drifts westwards—by 0.20 annually. More recently Yukutake and Tachinaka (1969) have argued that the non-dipole field itself is more complex—some features seem to drift (the drifting field), while others remain fixed (the standing field). The slowness of these changes compared to the brevity of the historic record provides a strong incentive to seek relevant evidence in the palaeomagnetic data to see if standing/ drifting duality has been a persistent feature of the geomagnetic field. What might we expect to observe? Basically, a drifting source will cause the total magnetic force at a site to sweep out a more-or-less elliptical ioop (Runcorn, 1959),
whereas a standing source will produce a more linear pattern (Creer and Tucholka, 1982, 1983; Creer, 1983). Of course, more complex patterns are to be expected if both standing and drifting sources are present, or if two or more standing sources are close enough to affect the observation site in question. Secular variation loops interpreted in terms of drifting sources are already well documented in palaeomagnetic (Skiles, 1970; Turner and Thompson, 1981) and historic (MaIm and Bullard, 1981) records. In addition, Creer (1981) has correlated limnomagnetic records from Europe and North America and argues :that they favour a westward motion of the drifting part of the non-dipole field for the last 12000 y. Thus there is a considerable body of data, from diverse sources, supporting the validity of both a present-day and a palaeo-westward drift. On the other hand, support for the existence in earlier times of stationary sources has been less forthcoming. In his correlation between Europe and North America, Creer (1981) considered only the inclination records. Subsequently,
Present address: Centre Geologique et Geophysique, IJniversité des Sciences et Techniques du Languedoc, Place Eugene Bataillon, 34060, Montpellier, France.
Creer and Tucholka (1982) pointed out that the corresponding declination logs do not fit such a simple mOdel, and were obliged to invoke 5 sources
-~
0031-9201/84/$03.00
© 1984 ElsevierScience Publishers B.V.
224
(2 drifting, 3 standing) for the time interval 0—4760 y ago, and 4 sources (2 drifting, 2 standing) for the interval 4760—8520 y ago. In a later paper Creer and Tucholka (1983) stressed that these models were provisional and that “the paradox will only be resolved satisfactorily with additional data from other northern hemisphere sites which occupy a range of longitudes”. In the meantime therefore it is worthwhile to seek other, hopefully more compelling, evidence for the existence of standing sources. In a recent paper, Turner et a!. (1982) reported that a sequence of sediments deposited in western Canada between — 30000 and 20000 y ago yielded evidence of a recurxing asymmetrical perturbation towards shallow inclinations and easterly declinations, leading to an overall bias of 60 in declination and 30 in inclination from the direction of a geocentricaxial dipole (Fig. 1 a, b). As they pointed out, the observed pattern cannot be explained by a simple westward (or eastward) drifting source. However, a stationary outward-pointing source DEC o
DEC 1
1
C ,.-.~?
U
S~
so
a
0
s 60 L2? 0
Fig. 1. Declination—Inclination plots of four palaeomagnetic secular variation perturbations. The square with the inscribed diagonal cross represents the field direction of a geocentric axial dipole at each site. (a) Besette Creek sediments, Canada; interval C—D of Turner et al. (1982) (b) Bessette Creek sedsments, Canada; interval B—F (c) British lake sediments; interval i—l of Turner and Thompson (1982) (d) Ninole Volcanic Series, Hawaii; flows 56—75 of Doell and Cox (1965).
situated to the west of the site would produce the general pattern observed if it gradually increased from zero up to a maximum and then decayed back to zero (rather like a half-rectified sine wave). Such a source will produce a series of field directions confined to the plane determined by the fixed directions of the main dipole field and the perturbation field of the standing source. On the stereographic projections usually employed in palaeomagnetism an arc of a great circle will be traced out, first in one direction as the source intensifies, then in the return sense as it wanes. Figure ic, d illustrates two other examples from British lake sediments (Turner and Thompson, 1982) and Hawaiian lava flows (Doell and Cox, 1965). Estimates of the timing and duration of these perturbation pulses based on the available dating control are summarized in Table I, which also lists the magnitudes of the perturbing fields necessary to account for the observed extrema. These estimates indicate that lifetimes of one thousand to several thousand years, and local anomalous fields of the order of 10000—20000 nT are typical. In view of what is known about the time scale of secular variations (— iO~—iO~ y, see e.g., Barton, 1982), the free-decay of current loops in the core (~ iO’~y, see e.g., Stacey, 1977), and the magnitude of the present non-dipole field (up to ±15000nT), these values are not unreasonable. As pointed out above, more complex patterns will result if two or more nearby sources simultaneously affect a single site. The example illustrated in Fig. 2a represents the lowest of three features reported by Turner et al. (1982), the upper two being those shown in Fig. la, b.whose The pattern can be attributed to two sources phase and amplitude relationships are such that a partial ellipse is produced by their superposition. Such a proposal is rather contrived, but a recent study by Rosenblaum and Larson (1983) of a sequence of sediments in Colorado lend it credence. This can be gleaned from the separate declination and inclination logs they present, but is rendered much clearer if their data is replotted on a combined declination—inclination plot (Fig. 2b). The reality of this pattern is further supported by an extremely good correlation between the data of Fig. 2b and those from a second core collected in a
225
TABLE I Timing and magnitude estimates of the perturbations shown in Fig. 1 Figure Id 1c lb la
Timing (10~y)
L
8
D’, I’
A
B’(pT)
8.2—6.1
2.1
—
—
24° 44° 14° 23°
048, 127, 052, 058,
55° 22° 50° 50°
21.2 24.8 12.1 19.5
20.5—19.9 24.9—23.1
0.6 1.8
—36 —79 —29 —38
Timing is given in I0~y before present. In the case of Hawaii (Fig. lc) dating control is poor. On the basis of estimated lava accumulation rates Doell and Cox (1965) claimed that the entire section, of which these data represent a part, spans <20000 y, sometime during the Brunhes epoch. L is the duration of each perturbation in l0~y. 8 is the maximum angle of divergence from a geocentric axial dipole field. D’, I’ are the declination and inclination of the minimum anomalous field capable of producing the divergence 8. 2 (1 ~zT = l000y). B’ is the corresponding magnitude of the minimum anomalous field, assuming a dipole moment of 8 x 1022 Am A is the site latitude.
separate sedimentary basin some 100 km away (see Rosenbaum and Larson, 1983, fig. 12). The similarity of the two features shown in Fig. 2a, b suggests that they are actually correlative, but this conflicts with the available dating control, The Canadian feature (Fig. 2a) occurred about 28000 y ago (based on the evidence of 5 radiocarbon dates), whereas the Colorado feature seems to be — 10000 y younger (based on 9 radiocarbon dates). Thus very large errors must be attributed to the radiocarbon results if the two magnetic features are to be equated, if not then it appears that they represent two separate, but highly similar, perturbations, possibly involving the same sources. All the above examples suggest perturbations
DEC
towards inclinations shallower than the corresponding geocentric axial dipole values—this is consistent with Cox’s (1975) explanation of the long-term existence of a zonal quadrupole (g~) moment. On the other hand, the declination bias, which is to the east in all six cases, is puzzling. However, it must be noted that since these examples basically represent only three sites—northwestern North America, Hawaii, and Britain—the observed declination bias is quite likely owing to chance. Much more data, with adequate global coverage, will be necessary before any firm conclusion can be drawn about possible bias in declination and inclination. In the meantime, evidence for stationary pulsating sources can be sought in secular variation patterns consisting of highly asymmetrical curvilinear perturbations with approxi-
D 0C
mately superposed outward and return trajectories. This type of feature can be recognised on
b
a
~0
60
‘~
separate declination and inclination logs, but is clination (Bauer) plots, and it is hoped that in
~::
~0:~r~~ss.
Fig. 2. Declination—Inclination plots of two further palaeomagnetic secular variation perturbations. (a) Bessette Creek sediments, Canada; interval A—B of Turner et al. (1982). (b) Devhns Park core sediments, Colorado; interval 12.01—8.78 m of Rosenbaum and Larson (1983). The points plotted were hand-digitized at 1 mm intervals from their fig. 9.
field will include such dia-
References .
.
Barton,. C.E., 1982. Spectral . analysis of paleomagnetic time . senes and the geomagnetic field. Philos. Trans. R. Soc., London, Ser A: 306: 203—209.
226 Bullard, E.C., Freedman, C., Gellman, H. and Nixon, J., 1950. The westward drift of the Earth’s magnetic field. Philos. Trans. R. Soc. London, Ser A: 243: 67—92. Cox, A., 1975. The frequency of geomagnetic reversals and the symmetry of the non-dipole field. Rev. Geophys. Space Phys., 13: 35—51. Creer, K.M., 1981. Long-period geomagnetic secular variation since 12,000 yr BP. Nature, 292: 208—212. Creer, K.M., 1983. Computer synthesis of geomagnetic palaeosecular variations. Nature, 304: 695—699. Creer, K.M. and Tucholka, P., 1982. The shape of the geomagnetic field through the last 8,500 years over part of the northern hemisphere. J. Geophys., 51: 188—198. Creer, K.M. and Tuchoilca, P., 1983. In: K.M. Creer, P. Tucholka and C.E. Barton (Editors), “Geomagnetism of Baked Clays and Recent Sediments”. Elsevier, pp. 273—305. Doell, R.R. and Cox, A., 1965. Paleomagnetism of Hawaiian Lava Flows. J. Geophys. Res., 70: 3377—3405. Malin, S.R.C. and Bullard, E.C., 1981. The direction of the Earth’s magnetic field at London, 1570—1975. Philos. Trans. R. Soc. London, Ser A: 299: 357~—423. Rosenbaum, J.G. and Larson, E.E., 1983. Paleomagnetism of two late Pleistocene lake basins in Colorado: an evaluation
of detrital remanent magnetization as a recorder of the geomagnetic field. J. Geophys. Res., 80: 10611—10624. Runcorn, S.K., 1959. On the theory of geomagnetic secular variation. Ann. Geophys., 15: 87—92. Skiles, D.D., 1970. A method of inferring the direction of drift of the geomagnetic field from paleomagnetic data. J. Geomagn. Geoelectr., 22: 441—462. Stacey, F.D., 1977. Physics of the Earth. Wiley, New York, 2nd edn, 414 pp. Turner, G.M. and Thompson, R., 1981. Lake sediment record of the geomagnetic secular variation in Britain during Holocene times. Geophys. JR. Astron. Soc., 65: 703—725. Turner, G.M. and Thompson, R., 1982. Detransformation of the British geomagnetic secular variation record for Holocene times. Geophys. J.R. Astron. Soc., 70: 789—792. Turner, G.M., Evans, M.E. and Hussin, I.B., 1982. A geomagnetic secular variation study (31,000—19,500 bp) in western Canada. Geophys. J.R. Astron. Soc., 71: 159—171. Yukutake, T. and Tachinaka, H., 1969. Separation of the Earth’s magnetic field into drifting and standing parts. Bull. Earthquake Res. Inst., 47: 65—97.