Geomagnetic long-term secular variations in Italian Lower Cretaceous shallow-water carbonates

Details Geomagnetic long-term secular variations in Italian Lower Cretaceous shallow-water carbonates Geomagnetic long-term secular variations in Italian Lower Cretaceous shallow-water carbonates

Jun 1999

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999geoji.137..713t&link_type=abstract

Geophysical Journal International, Volume 137, Issue 3, pp. 713-722.

Astronomy and Astrophysics

Astronomy

1

Carbonates, Geomagnetism, Italy, Lower Cretaceous, Secular Variation

Scientific paper

The remanent magnetic properties of an 88 m bore core are unrelated to either the dolomite content or the sedimentological textures and are considered to be carried primarily by biogenetic magnetite that was cemented in during very early diagenesis. Individual readings represent time intervals of c. 720+/-32 yr and, after 40 mT partial demagnetization, they provide an almost continuous record of averaged geomagnetic secular variations over a period of some 3.17 Myr. The magnitude of directional secular variation is twice that of the present day, despite being smoothed, and the secular variations appear to grade into polarity transitions, suggesting no difference in their mechanisms. The rates of change in direction between subjacent levels in the core have a log-normal distribution which extends smoothly beyond 90 deg and has a median value of 13 deg/700 yr, the same as for unsmoothed European secular variation during the last 2000 yr. The intensity of remanence, after 40 mT partial demagnetization, appears to provide a reasonable approximation to geomagnetic field intensity. This tends to be weaker when the direction is moving faster, reflecting averaging, but is unrelated to the distance of the vector from the mean direction; that is, it depends on the rate of change and not on the virtual pole latitude. The virtual poles, after correction for tectonic rotations about horizontal and vertical axes, have latitudes that form a strongly platykurtic Fisherian distribution, while their longitudes have a circular distribution on which are superimposed two Gaussian peaks, 180 deg apart. This bore core thus provides detailed information of smoothed geomagnetic secular variation in the Lower Cretaceous (127+/-3 Ma) which shows clear regularities in behaviour, some related to changes in the Earth's orbital parameters.

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