A team led by Professor Yongxiang Li from the School of Earth Sciences and Engineering at Nanjing University has made new progress in studying the geomagnetic field intensity during the Cretaceous Normal Superchron (CNS). On April 15, the relevant findings were published in the Journal of Geophysical Research: Solid Earth.
During geological history, the Earth's magnetic poles underwent multiple polarity reversals. However, during the mid-Cretaceous period, spanning approximately 121 to 84 million years ago, the geomagnetic field remained almost consistently positive, a period known as the CNS. Although simulation studies suggested that the geomagnetic field intensity was high and stable during the CNS, this inference had not been verified by paleomagnetic results recorded in rocks.
Researchers conducted a relative paleointensity study on sedimentary rocks from the Upper Cretaceous strata recovered from Site U1512 in the southern seas of Australia during International Ocean Discovery Program Expedition 369. Their aim was to examine whether the paleomagnetic field intensity during the CNS behaved as predicted by simulation studies, showing a "high and stable" state.
Through stepwise demagnetization experiments and detailed rock magnetic and mineralogical analyses, researchers eliminated the influence of later secondary remanence and environmental changes on the remanent magnetization intensity, establishing an index indicating relative paleomagnetic field intensity variations (RPI) — NRM20mT/x. Combining biostratigraphic age constraints, they obtained a nearly continuous, high-resolution record of relative paleointensity from the late CNS, around 88 to 94 million years ago.
The record revealed that the geomagnetic field intensity did not exhibit a "high and stable" state but showed fluctuating characteristics. Moreover, the overall trend of fluctuating variations was consistent with the fluctuations observed in the Mid-Atlantic Ridge magnetic anomaly (MMA) data, indicating that the paleomagnetic field intensity during the late CNS was not stable but rather variable. Further analysis revealed a positive correlation between relative paleointensity and its fluctuation magnitude.
Based on the analysis of U1512 RPI and Mid-Atlantic MMA data, researchers inferred the relative paleointensity variations during the CNS: stronger during the mid-CNS and weaker during the early and late CNS, demonstrating that throughout the CNS period, although the polarity of the geomagnetic field was stable, its intensity was not in a "high and stable" state but rather dynamically changing. Accordingly, the geomagnetic dynamo behavior during the CNS period was not as previously thought to be steady-state but rather dynamic and variable. These research findings are of significant importance for understanding the magnetic dynamo behavior of the ancient geomagnetic field during polarity superchrons and deep Earth processes.
Additionally, previous studies determined the age of the low-intensity anomaly Q1 in the late CNS MMA to be ~92 million years ago. The records obtained in this study also contain a minimum value "Q1," corresponding to Q1 in the MMA data, and more precisely constrain its age to be 90.8 million years ago.
Based on this, the research team reconstructed a model of crustal expansion on the African side of the Atlantic. This model highly resembles the kinematic characteristics of the recent African plate movement, further confirming the validity of the age of Q1 as 90.8 million years ago. Therefore, the research team proposed that the low-intensity paleomagnetic anomaly Q1, occurring at 90.8 million years ago during the late CNS, could serve as an important time boundary or anchor point for constraining the age of strata during the CNS period.
Doctoral student Xinyu Liu from Nanjing University is the first author of the paper, with Yongxiang Li as the corresponding author, and the research co-author is Professor Carl Richter from Louisiana State University.
Related paper information: https://doi.org/10.1029/2023JB028104
