Fast Directional Changes during Geomagnetic Transitions: Global Reversals or Local Fluctuations?

Abstract

Paleomagnetic investigations from sediments in Central and Southern Italy found directional changes of the order of 10∘ per year during the last geomagnetic field reversal (which took place about 780,000 years ago). These values are orders of magnitudes larger than what is expected from the estimated millennial timescales for geomagnetic field reversals. It is yet unclear whether these extreme changes define the timescale of global dipolar change or whether they indicate a rapid, but spatially localised feature that is not indicative of global variations. Here, we address this issue by calculating the minimum amount of kinetic energy that flows at the top of the core required to instantaneously reproduce these two scenarios. We found that optimised flow structures compatible with the global-scale interpretation of directional change require about one order of magnitude more energy than those that reproduce local change. In particular, we found that the most recently reported directional variations from the Sulmona Basin, in Central Italy, can be reproduced by a core-surface flow with rms values comparable to, or significantly lower than, present-day estimates of about 8 to 22 km/y. Conversely, interpreting the observations as global changes requires rms flow values in excess of 77 km/y, with pointwise maximal velocities of 127 km/y, which we deem improbable. We therefore concluded that the extreme variations reported for the Sulmona Basin were likely caused by a local, transient feature during a longer transition.

Characteristics of the Matuyama‐Brunhes Magnetic Field Reversal Based on a Global Data Compilation

“…Our results suggest that the field in the Mediterranean region has transitional behavior between 792 and 781 ka, which is longer than the duration documented in different individual places in this region (Just et al, 2019;Macrì et al, 2018;Sagnotti et al, 2014). We note here the Maffei et al (2021) deduction that the extreme variations recorded in the Sulmona site (Sagnotti et al, 2014) are spatially localized and only represent a transient feature during a longer transition.…”

“…We note here the Maffei et al. (2021) deduction that the extreme variations recorded in the Sulmona site (Sagnotti et al., 2014) are spatially localized and only represent a transient feature during a longer transition.…”

Rapid Changes in Strength and Direction of Earth's Magnetic Field Over the Past 100,000 Years

“…In this paper we have identified spatial and temporal geomagnetic field variations over the past 100 ky that are not present in the modern field spanning the most recent few hundred years. Previous studies have treated changes in direction and intensity separately (Davies & Constable, 2017, 2020; Korte & Constable, 2018; Maffei et al., 2021) and have generally focused on specific events such as the Matuyama‐Brunhes reversal (Sagnotti et al., 2014) or the LIAA (Ben‐Yosef et al., 2017; Osete et al., 2020; Shaar et al., 2016). Here we have synthesized the temporal and latitudinal variations in rapid magnetic field changes, exploiting the recent advances provided by global time‐dependent representations of the field over various portions of the last 100 ky. Over this time period we find that the fastest changes for direction and intensity occur under different circumstances.…”

“…Over the past 400 years the rates of intensity change, ∂B⁄∂t, display modest peak values of ∼150 nT yr 1 , while directional changes ∂ B/∂t yield maximum values less than 0.4°yr 1 (Finlay et al, 2020;Jackson et al, 2000). By contrast, intensity changes exceeding 750 nT yr 1 have been reported in the Levantine region around 1000 years BCE (Ben-Yosef et al, 2017;Shaar et al, 2016), while localized directional changes of 1°yr 1 or more have been recovered in a number of mid-latitude regions, often (but not always) coeval with the most recent excursions and reversals as summarized in Figure 1 of Maffei et al (2021). The spatial scale over which such rapid changes can be supported is unclear, but it must be fairly broad in order for the signal to originate from the core.…”