WAS THE CRETACEOUS NORMAL SUPERCHRON AN INTERVAL OF RELATIVELY HIGH PALEOINTENSITY?

The fact that the natural remanent magnetization (NRM) intensity of MORB samples shows systematic variations as a function of age has long been recognized: maximum as well as average intensities are generally high for very young samples, falling off rather rapidly to less than half the recent values in samples between 10 and 30 Ma, whereupon they slowly rise in the early Tertiary and Cretaceous to values that approach those for the very young samples. NRM intensities measured in this study follow the same trends as those observed in previous publications. In this study we take a statistical approach and examine whether this pattern can be explained by variations in one or more of all previously proposed mechanisms: chemical composition of the magnetic minerals, abundance of these magnetization carriers, vectorial superposition of parallel or antiparallel components of magnetization, magnetic grain or domain size patterns, low-temperature oxidation to titanomaghemite, or geomagnetic field behavior. We find that the samples do not show any compositional, petrological, rock-magnetic or paleomagnetic patterns that can explain the trends. Geomagnetic field intensity is the only effect which can not be directly tested on the same samples, but shows a similar pattern as our measured NRM intensities. We therefore conclude that the geomagnetic field strength was, on-average, significantly greater during the Cretaceous than during the Oligocene and Miocene, although it was perhaps rather variable on a short time scale.