UC San Diego

There has been an increasing effort toward the constraint of the average and long-term variability of the magnetic field strength, fundamental to better understand the characteristics and behaviour of the geomagnetic dipole field. Nonetheless, open questions remain about the value of the average dipole field, the relation between dipole strength and excursion reversal. Indeed, depending on the criteria adopted to analyse the current database, different long-term average values can be found, leading to different answers. The reason for the open debate can explained with the limited amount of data from key time intervals and geographical areas, due to both to complexities behind the method to obtain absolute paleointensities (several methods and experimental designs, selection criteria, high failure rate, etc..) and suitable materials.

Here, we focus on the Cretaceous Normal Superchron, a long period, from approximately 121 to 83 Ma, when the magnetic field was characterised by a stable polarity. Yet, few paleointensity data were available so far. In this study, we present new results from 48 Submarine Basaltic Glass sites from pillow lava margins, sampled on the upper crust sequence of the Costa Rica Ophiolite. Ar/Ar ages along with biostratigraphic age constraints from previous studies indicate ages ranging from from 139 to 94 Ma. After 473 samples were measured using the IZZI-Thellier protocol and analysed using strict selection criteria, 13 sites between 109 and 133 Ma gave reliable and robust results. Our new results from Costa Rica suggest that the strength of the Earth Magnetic field during CNS, 70.2 ± 21 ZAm²  are slightly lower than the pre-CNS and also lower than, for instance, at Troodos Ophiolite (81 ± 43 ZAm²; Tauxe and Staudigel 2004), consistent with the observations by Tauxe (2006) of an average dipole moment being substantially less than the present day value.