Changes in Earth's magnetic field during the Cretaceous Normal Superchron (CNS) spanning ∼121 Ma to ∼84 Ma hold important clues about the geodynamo evolution. Canonical models predict a persistently strong geomagnetic field with low variability during CNS, which, however, has not been observed in the available absolute paleointensity data and seafloor marine magnetic anomaly (MMA) records. The lack of relative paleointensity (RPI) data across CNS further impedes tests of model predictions. Here, we present a ∼9-Myr (∼94–∼85 Ma) RPI record from a Turonian to Santonian hemipelagic succession from IODP Site U1512 offshore southern Australia. Detailed paleomagnetic and rock magnetic analyses demonstrate that the ratio of natural remanent magnetization (NRM) demagnetized at 20 mT over magnetic susceptibility (MS), that is, NRM_20mT/MS, as a reliable proxy for the RPI of the Upper Cretaceous succession. The new RPI record shows marked changes in both intensity and variability at ∼90.8 Ma. Also, the 6 Myr-long (∼94–∼88 Ma), near-continuous, ∼1.2 kyr-resolution RPI record exhibits a strong positive correlation between field intensity and variability. Assuming this correlation holds for the entire CNS, an extrapolated RPI curve for the entire CNS is obtained by integrating the positive correlation with field variability estimates from the MMA data. The extrapolated RPI curve shows a strong and highly variable field in the middle CNS but a weak and stable field at its beginning and ending. These features imply a much more dynamic geodynamo than previously thought, and provide crucial benchmarks for unraveling the geodynamo evolution during CNS.

中文翻译：

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白垩纪正常超纪元（CNS）末期地球磁场的显着变化

白垩纪正常超纪元（CNS）期间地球磁场的变化（跨越～121Ma至～84Ma）为地球发电机演化提供了重要线索。规范模型预测 CNS 期间会出现持续强且低变异性的地磁场，然而，在现有的绝对古强度数据和海底海洋磁异常 (MMA) 记录中尚未观察到这种情况。中枢神经系统相对古强度（RPI）数据的缺乏进一步阻碍了模型预测的测试。在这里，我们展示了来自澳大利亚南部近海 IODP 站点 U1512 的土伦阶到桑托阶半远洋层序的~9-Myr (~94–~85 Ma) RPI 记录。详细的古地磁和岩石磁分析表明，20 mT 时退磁的自然剩磁 (NRM) 与磁化率 (MS) 的比率，即 NRM _20mT /MS，可以作为上白垩统 RPI 的可靠代表。新的 RPI 记录显示~90.8 Ma 的强度和变异性均发生显着变化。此外，6 Myr 长（∼94–∼88 Ma）、近连续、∼1.2 kyr 分辨率的 RPI 记录在场强和变异性之间表现出很强的正相关性。假设这种相关性适用于整个 CNS，则通过将正相关性与 MMA 数据的场变异性估计相结合，获得整个 CNS 的外推 RPI 曲线。推断的 RPI 曲线显示中枢神经系统中部有一个强且高度可变的场，但在其开始和结束处有一个弱且稳定的场。这些特征意味着地球发电机比之前想象的更加动态，并为揭示中枢神经系统期间地球发电机的演化提供了重要的基准。