Jurassic dipole low defined from land and sea data

Abstract

Estimates of the Earth's magnetic dipole moment through geologic time have been obtained using paleo‐intensities determined from igneous rocks. These measurements, normalized by conversion to virtual dipole moments (VDMs), were used by Prévot et al. [1990] to suggest there was an extended period named the ‘Mesozoic Dipole Low’ (MDL) from about 180 Ma to about 135 Ma. Subsequent reviews of the paleo‐intensity data appeared to indicate a minimum of about one‐third of the Cenozoic value occurred between about 180 and 120 Ma [Kono and Tanaka, 1995; Perrin and Shcherbakov, 1997].Recently there has been considerable debate regarding the existence of the MDL from those who have both made and reviewed more recent paleo‐intensity determinations [Selkin and Tauxe, 2000; Heller et al., 2002; Goguitchaichvili et al., 2002; Biggin and Thomas, 2003; Thomas and Biggin, 2003]. None of these authors has used or discussed the evidence available from marine magnetic anomalies, although McElhinny and McFadden [2000] pointed out that the minimum in this proposed MDL could be seen also in the marine magnetic anomaly record from the Jurassic Quiet Zone (JQZ). The JQZ is defined by a factor‐of‐four decrease in anomaly amplitudes with increasing age from about anomaly M19 (˜144 Ma) in the Pacific [Larson and Hilde, 1975; Cande et al., 1978].