Millennial‐Scale Instability in the Geomagnetic Field Prior to the Matuyama‐Brunhes Reversal

Abstract

Changes in the Earth's magnetic field have global significance that reach from the outer core extending out to the uppermost atmosphere. Paleomagnetic records derived from sedimentary and volcanic sequences provide important insights into the geodynamo processes that govern the largest geomagnetic changes (polarity reversals), but dating uncertainties have hindered progress in this understanding. Here, we report a paleomagnetic record from multiple lava flows on Tahiti that bracket the Matuyama‐Brunhes (M‐B) polarity reversal ∼771,000 years ago. Our high‐precision 40Ar/39Ar ages constrain several rapid and short‐lived changes in field orientation up to 33,000 years prior to the M‐B reversal. These changes are similar to ones identified in other less well‐dated lava flows in Maui, Chile, and La Palma that occurred during an extended period of reduced field strength recorded in sediments. We use a simple stochastic model to show that these rapid polarity changes are highly attenuated in sediment records with low sedimentation rates. This prolonged 33,000 year period of reduced field strength and increased geomagnetic instability supports models that show frequent centennial‐to‐millennial‐scale polarity changes in the presence of a strongly weakened dipole field.

Constraining in situ cosmogenic nuclide paleo-production rates using sequential lava flows during a paleomagnetic field strength low

“…This represents a field strength reduction of up to ~86% compared to modern-day (Thébault et al, 2015). The flows vary in age from 771 to 805 ka and cover both the precursor and transitional phases of the M-B polarity reversal (Balbas et al, 2018).…”

“…A series of lava flows in the Punaruu valley of Tahiti-Nui meet all of the above criteria (Balbas et al, 2018;Mochizuki et al, 2011). The polarity orientation of these flows ranges through normal, reversed, and transitional, and record field intensity as low as ~5 µT during the time of formation (Mochizuki et al, 2011).…”

“…This study focuses on hand-samples taken from four lava flows that span ~20 m horizontally and >6 m vertically (Balbas et al, 2018;Mochizuki et al, 2011). The series of flows are located at 186 m elevation and the contacts between flows are clear with no discernable erosion or ash beds (Figure 2).…”

“…The thickness of overlying flows (~4 m) was sufficient to ensure no significant post-burial production of 3 He. This sequence was exposed in This study focuses on contacts between lava flows B1-B2 and B3-B4 (Balbas et al, 2018;Mochizuki et al, 2011). Flow B1 was exposed during a field strength low shown in the PISO1500 (Channell et al, 2009) and PADM2M (Ziegler et al, 2011) paleo-intensity stacks (Figure 1) at ~800-790 ka.…”

“…Both lava flows would have been exposed to surface radiation during a time-interval of reduced paleomagnetic field intensity known as the M-B precursor event (Channell et al, 2009;Hartl and Tauxe, 1996;Raisbeck et al, 2006;Singer et al, 2005). Flow B1 has an 40 Ar/ 39 Ar age of 801.7 ± 2.9 ka (2σ; n=6) and B3 has an age of 792.2 ± 4.8 ka (n=4) (Balbas et al, 2018). The overlying flows B2 and B4 have ages of 793.7 ± 4.1 (n=5) and 781.3 ± 3.9 ka (n=4) respectively (Balbas et al, 2018).…”