The strength of the Earth's magnetic field and the Cretaceous Normal Superchron: New data from Costa Rica

Abstract

Constraining the long-term variability and average of the Earth's magnetic fi eld strength is fundamental to understanding the characteristics and behavior of the geomagnetic field. Questions remain about the strength of the average field, and the relationship between strength and reversal frequency. The dispersion of data from key time intervals reflects the complexity in obtaining absolute paleointensity values. Here, we focus on the Cretaceous Normal Superchron (CNS; 121-84 Ma), during which there were no reversals. We present new results from 42 submarine basaltic glass (SBG) sites collected on the Nicoya Peninsula and Murci elago Islands, Costa Rica and new and revised 40Ar/39Ar ages along with biostratigraphic age constraints from previous studies that indicate ages ranging from 141 to 112 Ma. One site with a 40Ar/39Ar age of 135 +/-1.5 Ma (2 ) gave a reliable intensity result of 34 +/- 8 uT (equivalent to a paleomagnetic dipole moment, PDM, value of 88 +/- 20 ZAm2), while three sites between 121 and 112 vary from 21 +/- 1 to 34 +/- 4 uT (53 +/- 3 to 87 +/- 10 ZAm2) spanning the onset of the CNS. These results from the CNS are all higher than the long-term average of 42 ZAm2 and similar to data from Suhongtu (46-53 ZAm2) and the Troodos Ophiolite (81 ZAm2, reinterpreted using the same criteria of this study). Together with the reinterpreted data, the new Costa Rica results suggest that the strength of the geomagnetic eld was about the same before and after the onset of the CNS. Therefore, the data do not support a strict correlation between polarity interval length and the strength of the magnetic fi eld.