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 field strength is fundamental to understanding the
characteristics and behavior of the geomagnetic field. Questions remain
about the strength of the average field, and the rela-tionship between
strength and reversal frequency. The dispersion of data from key
timeintervals 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 Murcielago Islands, Costa Rica and
new and revised 40Ar/39Ar ages along with biostratigraphic age
constraints from previous studies that indicate ages from 141 to 112 Ma.
One site with a 40Ar/39Ar age of 135+\-1.5Ma (2σ) gave a
reliable intensity result of 34+\-μT (equivalent to a
paleomagnetic dipole moment, PDM, value of 88+\-20 ZAm2),
while three sites between 121 and 112 vary from 21+\- to
34+\-4 μT (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 thatthe strength of the geomagnetic field was
about the same before and after the onset ofthe CNS. Therefore, the data
do not support a strict correlation between polarity interval length and
the strength of the magnetic field.