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

地球的地磁反转记录以两个周期为标志，在这两个周期中，磁场在非常长的时间内保持单极状态。其中一个时期被称为白垩纪正常超时(CNS)，持续了大约4000万年。究竟为什么磁场保持如此稳定是活跃辩论的主题，然而，几乎没有数据来解决这个一阶问题，这个问题最终可能会限制地球发电机的工作方式。回答这个问题的一种方法是研究长期变化。长期变化记录可分为不对称和对称的场系数，称为初级和次级地发电机族，次级部分与纬度无关。初级地球发电机家族的贡献随着纬度的变化而变化，在地磁赤道为零。因此，二次分量作为长期变化的基准值，而研究地磁赤道的长期变化是理解磁场行为的最具诊断性的参数之一。为此，我们从秘鲁的海洋部分收集了446个样本，这些样本是在北冰洋期间沉积在赤道的。一个1.5公里厚的沉积层序产生了高质量的数据，应该进行进一步的采样和分析。还对两个火山遗址进行了采样。两者都提供了可靠的数据，但它们所代表的时间太少，无法与沉积岩进行充分的比较。我们相信，进一步的采样和分析将导致一项无与伦比的研究，这将为长期变化研究设定基准，并促进我们对极性超级时间的性质和原因的理解。

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.