Source: Journal of Geophysical Research: Solid Earth
About 70 years of paleomagnetism studies have taught us that the geomagnetic field behavior, although on average dipolar and oriented along the spin axis of the Earth, is intricate and very erratic. Rocks become magnetized over a range of timescales, from just a few minutes to several years—a geological instant. Consequently, when we drill a core into a rock for paleomagnetic analysis, we capture only a single snapshot of an unpredictable and rapidly changing field, which we refer to as a “spot-reading.” Constraining such motion requires a time-lapse movie with as many spot-readings as possible. Yet, paleomagnetists face a dilemma: while they strive for quantity, they must also ensure the quality of each individual reading.
To mitigate errors and biased measurements, paleomagnetic sampling protocols recommend collecting multiple samples (more than 5) from each rock that represents a single spot reading of the magnetic field, such as a lava flow or sedimentary layer. However, the practical limitations of time and resources often force a compromise between quantity and precision: the clash in the field many times comes with the question: should I stay, or should I go (to another place to sample)?
Sapienza et al.  have proved mathematically that more sites with less samples give a better view of the field compared to less but more precise site sampling. The results support favoring even the collection of one sample per site, even in the presence of many spurious observations.
While it may be disheartening to discover that our results might not reach their full potential, there’s ample room for improvement. The authors provide a mathematical tool for estimating the optimal sampling approach in paleomagnetic studies, invaluable to new paleomagnetic studies. We, as paleomagnetists, may have generated remarkable snapshots of the geomagnetic field, but it’s time to move from photo albums to movies that capture its true essence.
Citation: Sapienza, F., Gallo, L. C., Zhang, Y., Vaes, B., Domeier, M., & Swanson-Hysell, N. L. (2023). Quantitative Analysis of Paleomagnetic Sampling Strategies. Journal of Geophysical Research: Solid Earth, 128, e2023JB027211. https://doi.org/10.1029/2023JB027211
—Daniel Pastor-Galán, Associate Editor, JGR: Solid Earth