Lineated marine magnetic anomalies represent a continuous record of geomagnetic field behavior and have been used to map the age of most oceanic crust, typically with a temporal resolution on the order of 105-106 years. The role of polarity reversals in generating magnetization contrasts is well established. The pattern of these reversals as observed in sea surface magnetic anomaly profiles forms the basis of the geomagnetic polarity timescale. To the extent crustal magnetization is a thermoremanence that accurately preserves field reversals, a corresponding record of geomagnetic intensity must also be present. These geomagnetic intensity fluctuations ultimately may provide a much finer temporal framework than the reversal sequence alone.

Detailed near-bottom magnetic anomaly surveys provide the best means to document this geomagnetic intensity signal in the ocean crustal magnetization. Results from a near-bottom survey of ~10-11 Ma crust in the North Pacific (Fig. 1) demonstrate the coherence of short wavelength anomalies over more than 100 km. Comparison with sea surface data from a faster spreading ridge some 7000 km away indicates that these features are global. Results from a similar survey of the Brunhes (0-0.78 Ma; Fig. 2) illustrate a second way to establish the importance of geomagnetic intensity fluctuations, namely comparison of the anomaly fluctuations with independent records of past intensity variations (e.g. from sedimentary sequences).

Fig. 1 Representative near-bottom anomaly profiles from anomaly 5 in the North Pacific and a stack of sea surface anomaly profiles from the faster spreading South Pacific. Short wavelength anomalies are coherent on near-bottom profiles as much as 100 km apart, indicating that these features are geomagnetic in origin (and not, for example, related to the crustal accretionary process). The similarity of the near-bottom stack (red) to sea surface profiles from the South Pacific indicates that the signal is global.


click to view full image

Fig. 2 Comparison of near-bottom anomaly profiles from the ultrafast-spreading (75 mm/yr half rate) southern East Pacific Rise and independent estimates of geomagnetic intensity variations in the Brunhes (0-0.78 Ma) derived from sediments. Coherent fluctuations in these two very different kinds of recording media provide a compelling case that the common features reflect geomagnetic intensity.




Related references:
Gee, J., Cande, S.C., Hildebrand, J.A., Donnelly, K. and R.L. Parker, 2000, Geomagnetic intensity variations over the past 780 kyr obtained from near-seafloor magnetic anomalies, Nature, 408, 827-832.

Bowers, N.E., Cande, S.C., Gee, J., Hildebrand, J.A. and R.L. Parker, 2001, Fluctuations of the paleomagnetic field during chron C5 as recorded in near bottom marine magnetic anomaly data, J. Geophys. Res., 106: 26,379-26,396.

Recent and ongoing research projects:

1. A paleointensity record from the ocean crust
2. Mapping sea surface magnetic anomalies
3. Characterizing the time-averaged magnetic field
4. Long term variations in geomagnetic intensity
5. Magnetization of meteorites and lunar samples
6. Applications of magnetic data to fabric development in cumulates