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| Titel |
The unstable geomagnetic field during the last glacial |
| VerfasserIn |
Norbert Nowaczyk, Ute Frank, Jessica Kind, Birgit Plessen, Helge Arz |
| Konferenz |
EGU General Assembly 2013
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 15 (2013) |
| Datensatznummer |
250074106
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| Zusammenfassung |
| Detailed stratigraphic analyses of a sediment composite record from three different sites in
the southeastern Black Sea yielded a high-resolution, well-dated paleomagnetic record of the
past 14 to 68 ka. Age constraints are provided by 16 AMS 14C ages, identification
of the Campanian Ignimbrite tephra (39.28±0.11 ka), and by detailed tuning of
sedimentologic parameters of the Black Sea sediments to the oxygen isotope record
from the Greenland NGRIP ice core. Dansgaard-Oeschger events 3 through 18 are
very well expressed in the Black Sea sedimentary records of Ca-content, oxygen
isotopes as well as in records of ice-rafted detritus. Though hampered by some
larger hiatusses at one site, and patchy contaminations by diagenetically formed
greigite, the paleomagnetic composite record obtained from the preserved primary
detrital magnetite phase reflects a highly dynamic geomagnetic field during the last
glacial period. Relative variations of paleointensity inferred from the sediments’
magnetisations were converted into a record of the virtual axial dipole moment
(VADM). Thus, the Black Sea paleomagnetic record comprises evidence for the
Norwegian-Greenland-Sea excursion at 64.5 ka (VADM = 1.5Ã1022Â Am2), a full reversal of
the geomagnetic field during the Laschamp excursion at 41 ka and several subsequent
excursions with low northern virtual geomagnetic pole (VGP) latitudes, including the
Mono Lake excursion at 34.5 ka (VADM = 3.0Ã1022Â Am2). According to the
derived age model, VGP positions during the Laschamp excursion persisted at high
southern latitudes in Antarctica for an estimated 440 years, making the Laschamp
excursion a short-lived event with fully reversed polarity directions. Recorded field
reversals of the Laschamp excursion, lasting only an estimated ~250 years, are
characterized by very low paleointensities with VADMs as low as 0.50Ã1022Â Am2. The
reversed phase of the Laschamp excursion is associated with a significant field
recovery with a VADM of 2.0Ã1022Â Am2, which is about 25% of the present day field
(2010 dipole moment = 7.745Ã1022Â Am2). The central, fully reversed phase of the
Laschamp excursion is bracketed by VGP excursions to the Sargasso Sea (~41.9 ka)
and to the Labrador Sea (~39.6 ka). Paleomagnetic results from the Black Sea
are in excellent agreement with VGP data from the French type locality which
facilitates the chronological ordering of the non-superposed lavas that crop out at
Laschamp-Olby. Rates of change calculated from the Black Sea VADM record also give
some information on how to assess the global decay of the present-day geomagnetic
field, which is significantly enhanced in the area of the South Atlantic Anomaly. |
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