Source of Oceanic Magnetic Anomalies and the Geomagnetic Polarity Timescale

J. S. Gee, D. V. Kent

Research output: Chapter in Book/Report/Conference proceedingChapter

7 Scopus citations

Abstract

Marine magnetic anomalies provide the framework for the geomagnetic polarity timescale for the Late Jurassic to Recent (since 160. Ma). Magnetostratigraphic records confirm that the polarity reversal sequence interpreted from magnetic anomalies is complete to a resolution of better than 30. ky. In addition to this record of polarity reversals, magnetic anomalies also appear to preserve information on geomagnetic intensity fluctuations. The correspondence of coherent near-bottom anomaly variations with independent estimates of field intensity provides strong evidence that geomagnetic intensity modulates the magnetization of the oceanic crust. Indeed, many short-wavelength anomaly variations in sea-surface magnetic profiles over fast-spreading ridges are likely attributable to geomagnetic intensity variations. Although longer-term geomagnetic field behavior may also be reflected in anomaly amplitudes, documenting such a signal requires a better understanding of time-dependent changes in the magnetic source (e.g., from low-temperature alteration) that may also affect magnetic anomalies.The extrusive layer, with an average remanence of 5Am-1, is the largest contributor to magnetic anomalies. However, enhanced sampling of oceanic gabbros (average remanence of 1Am-1) and, to a lesser extent, dikes (average remanence of 2Am-1) reveals that these deeper (and thicker) layers likely generate anomalies comparable to those from the lavas. Lava accumulation at intermediate- and fast-spreading ridges typically occurs over a narrow (1-3km) region, and dike emplacement is even more narrowly confined, resulting in a relatively high-fidelity record of geomagnetic field behavior. The slow cooling of the gabbroic layer, however, results in gently dipping polarity boundaries that significantly affect the skewness of the resulting anomalies, which is also a sensitive measure of net rotations of the source layer(s). The magnetizations of the dikes and gabbros are characterized by high stability and are not expected to significantly change with time, although there are insufficient data to confirm this. The lavas, however, typically show evidence of low-temperature alteration, which has been long regarded as a process that progressively reduces the magnetization (and degrades the geomagnetic signal) in the extrusive layer and reduces the amplitude of magnetic anomalies. Sufficient data have become available to examine this conventional wisdom. There is a substantial (4×) reduction in magnetization from on-axis samples to immediately off-axis drill sites (0.5My) but little further change in half-dozen or so deep crustal sites to 160Ma. High paleointensity that characterizes the last few thousand years may contribute significantly to the high on-axis magnetization. The task of evaluating changes in remanence of the extrusive layer is made more difficult by substantial cooling-rate-dependent changes in magnetic properties and the systematic variation in remanence with iron content (magnetic telechemistry). The commonly cited magnetic anomaly amplitude envelope is in fact not systematically observed - the Central Anomaly is elevated at slow-spreading ridges but is not as prominent at faster-spreading rates. Nonetheless, magnetic anomaly amplitudes are consistent when magnetization change is poorly constrained. Direct determinations of the degree of low-temperature oxidation reveal the presence of highly oxidized titanomagnetite in samples less than 1My old, suggesting a short (105 years) time constant though the effects of low-temperature oxidation are quite heterogeneous. While low-temperature oxidation does have some effect on lava magnetization and anomaly amplitudes, there is increasing evidence that marine magnetic anomalies are capable of recording a broad spectrum of geomagnetic field behavior, ranging from millennial-scale paleointensity variations, to polarity reversals, to apparent polar wander, to, more speculatively, long-term changes in average field strength. Several emerging tools and approaches - autonomous vehicles, oriented samples, absolute paleointensity of near-ridge lavas, and measurements of the vector anomalous field - are therefore likely to significantly advance our understanding of the geomagnetic signal recorded in the oceanic crust, as well as our ability to utilize this information in addressing outstanding problems in crustal accretion processes.

Original languageEnglish (US)
Title of host publicationGeomagnetism
PublisherElsevier Inc.
Pages419-460
Number of pages42
Volume5
ISBN (Electronic)9780444538031
ISBN (Print)9780444538024
DOIs
StatePublished - Jan 1 2015

All Science Journal Classification (ASJC) codes

  • Physics and Astronomy(all)

Keywords

  • Anomaly amplitude envelope
  • Anomaly skewness
  • C-sequence
  • CAMH
  • Central Anomaly
  • Cretaceous quiet zone
  • Cryptochrons
  • Excursions
  • Gabbros
  • Geomagnetic polarity timescale
  • Jurassic quiet zone
  • Low-temperature oxidation
  • M-sequence
  • Maghemitization
  • Magnetite
  • Marine magnetic anomalies
  • Mid-ocean ridge basalts
  • NRM
  • Paleointensity fluctuations
  • Peridotites
  • Serpentinization
  • Sheeted dikes
  • Source layers
  • TRM
  • Telechemistry
  • Titanomagnetite

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