Toward a better understanding of the Cretaceous geomagnetic polarity superchron via paleosecular variation studies at the Earth's equator

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The Earth’s geomagnetic reversal record is marked by two periods when the field remained in a single polarity state for an exceptionally long time. One of these periods, called the Cretaceous normal superchron (CNS), lasted roughly 40 million years. Just why the field remained so stable is the subject of active debate, yet little data exists to address this first-order problem that can ultimately place constraints on how the geodynamo works. One way to answer the problem is to study secular variation. Secular variation records can be broken down into asymmetric and symmetric field coefficients, called primary and secondary geodynamo families, with the secondary part being independent of latitude. The contribution of the primary geodynamo family varies as a function of latitude, being null at the geomagnetic equator. Thus, the secondary component serves as a baseline value of secular variation, and a study of secular variation at the geomagnetic equator serves as one of the most diagnostic parameters to understand field behavior. To this end, we collected 446 samples from marine sections in Peru that were deposited at the equator during the CNS. A 1.5 km-thick sedimentary sequence yielded excellent quality data that should be subjected to further sampling and analyses. Two volcanic sites were also sampled. Both yielded reliable data, but they represent too little time to adequately compare with the sedimentary rocks. We are confident that further sampling and analyses will result in an incomparable study that will set the benchmark for secular variation studies and advance our understanding of the nature and cause of polarity superchrons.