A Hyperactive Geomagnetic Field in the Late Visean (Early Carboniferous) From the Late Asbian Stratotype Section in Northwest England, UK

Nearly synchronous global changes in geomagnetic polarity give both a detailed irregular pacing to geological time and provide a glimpse into heat transfer processes across the core-mantle boundary which drives the Earth's geodynamo. Although the Late Carboniferous is characterized by some well-studied reversals, details of the tempo of polarity changes in the Early Carboniferous are unknown. This work addresses this by providing a detailed record of polarity changes over a similar to 2 million year interval at around 334.5-332.5 million years ago-from the Trowbarrow Quarry section in NW England. We demonstrate that these limestones likely preserve magnetization from close to their time of formation and record at least 31 polarity reversals. These observations support the idea that the Earth's dynamo was in a hyperactive reversing state similar to those sustained for tens of Myr in the Late Jurassic, parts of the Cambrian and the Late Ediacaran. It further corroborates a similar to 200 Myr cyclicity in paleomagnetic field behavior since the Precambrian, potentially linked to variable core heat flow forced by mantle convection. The pattern of geomagnetic polarity changes during the Early Carboniferous (Mississippian) is not known in detail. This information sparsity is addressed by determining a magnetostratigraphy from the late Asbian (late Visean at similar to 333 Ma) in Trowbarrow Quarry, UK. This is the stratotype section of the late Asbian and has a detailed foraminiferal zonation based on the same set of paleomagnetic samples, establishing a detailed biostratigraphy. The 195 m-thick section was sampled at an average spacing of 1.1 m, yielding a detailed magnetostratigraphy comprising nine major magnetozone couplets, and seven submagnetozones. The section dataset has a 78% bias to normal polarity determined from 177 sampling levels. The magnetization is carried by a mixture of hematite and detrital magnetite, with 68% of specimens dominated by hematite magnetizations. The primary magnetization passes a fold test showing its age was prior to the latest Carboniferous. The hematite is inferred to be largely of detrital, eolian origin, although some reddened levels are associated with emergent surfaces, suggesting that a small fraction of hematite is associated with platform emergence. The Mississippian age magnetization is partly overprinted with Kiaman Superchron-age and Brunhes-age magnetizations. Using the duration of the section based on astrochronology indicates a reversal frequency of 15.7 +/- 0.75 Myr-1, indicating that the geodynamo was in a hyperactive reversing state between 335 and 333 Ma. A 195 m section, representing similar to 2 Myr, yields a magnetostratigraphy comprizing nine major magnetozone couplets and seven submagnetozones Primary magnetizations are predominantly carried by hematite with lesser important detrital magnetite. Data pass regional fold tests A reversal frequency of 15.7 +/- 0.75 Myr-1 is calculated, suggesting that the geodynamo was in a hyperactive state at similar to 333 Ma