Reversals in the Earth’s magnetic field are believed to result from instability that develops within the outer core geodynamo. To understand this process and to predict how the Earth system would respond to a future reversal, it is critical to map out a detailed description of the global geomagnetic field that occurs during the transition between long-lived, relatively stable, polarity states.
After decades of study, the geometric structure, timing, and duration of reversals remain elusive, principally because they are short-lived phenomena and thus high-resolution recordings are sparse and difficult to date. The Matuyama-Brunhes (M-B) reversal is the most studied because it has been identified in dozens of marine sediment cores, exposed sedimentary successions and a few lava flow sequences. It occurred roughly 780,000 years ago and is believed to be the most recent.
The vast majority of M-B transition records come from marine sediment cores however sedimentary archives can be degraded with respect to magnetic fidelity and temporal resolution and because of this inferences about the reversing field are controversial. Key issues that may complicate sedimentary records include depositional processes, low deposition rates, weak magnetization, and, in some cases, remagnetization.
Singer, B., Jicha, B., Mochizuki, N., Coe, R. (2019). Synchronizing volcanic, sedimentary, and ice core records of Earth’s ;ast magnetic polarity reversal. Science Advances, 5(8). https://advances.sciencemag.org/content/5/8/eaaw4621