More stable yet bimodal geodynamo during the Cretaceous superchron?

Abstract

We report palaeomagnetic and 40Ar/39Ar dating results from two sequences of basaltic lava flows deposited at the same locality in western China, yet separated in time by ~50 Myr: one set lies within the Cretaceous normal superchron at 112–115 Ma and a second at 59–70 Ma spanning the Cretaceous‐Palaeogene boundary. We find that magnetic field directions during the superchron exhibit bimodal populations: one with inclinations representative of a dipolar field and another with shallow inclinations that could reflect a more complex, multipolar field. However, the time‐dependent variability in field directions was 50% lower during the superchron than after, which implies greater field stability during the superchron. Our results suggest that episodes of less dipolar field behavior occurred within the Cretaceous superchron and raise the question whether a second, more multipolar, field state is more persistent than previously thought.

Refining the chronostratigraphy of the Karoo Basin, South Africa: magnetostratigraphic constraints support an early Permian age for the Ecca Group

“…Typically inclination shallowing expected from sedimentary rocks which have undergone compaction is recognized by an E-W directed elongation of the paleomagnetic dataset (Tauxe and Kent, 2004). This section lies within a geomagnetic superchron, and the exact behaviour of the Earth's magnetic field during these types of events is still poorly understood (Granot et al, 2012;Hounslow, 2016;Lhuillier et al, 2016). Contributions from a non-zero non-GAD (Geocentric Axial Dipole) field can change the distribution of directions, where a non-zero axial octupole of the same sign as the axial dipole will enhance the N-S elongation of the observed directional data resulting in a shallow polarity bias (Tauxe and Kent, 2004).…”

Paleomagnetism of Paleocene‐Maastrichtian (60–70 Ma) Lava Flows From Tian Shan (Central Asia): Directional Analysis and Paleointensities

“…In the most recent paleomagnetic study in this area, Lhuillier et al. (2016) sampled a 550‐m‐thick sequence (P section, 59–70 Ma) spanning the Cretaceous‐Paleocene boundary and consisting of 76 lava flows and 5 intercalated layers of baked sediments. They also sampled two possibly distinct Early Cretaceous sections dated 112–115 Ma: the 100‐m‐thick K‐section (22 lava flows), and the 200‐m‐thick L‐section (35 lava flows).…”

“…In order to get a fully consistent data set, we first reinterpreted the demagnetization diagrams from section P (Lhuillier et al., 2016) and chose to only retain the most robust sites for which the vector endpoints of the ChRM component unambiguously went to the origin. A small secondary component was usually removed by 10–20 mT or 200–300°C.…”

“…(d) Field photograph of section B (left of the road) and section A (right of the road). (e)–(f) Field photographs of section P (Lhuillier et al., 2016) and section C. (g)–(h) Examples of baked sedimentary layers considered for absolute paleointensity determinations.…”

Correlated Relative and Absolute Geomagnetic Paleointensities From a Pliocene N‐R Polarity Reversal Record in Basaltic Lava Flows on Kauai, Hawaii

“…The goal of the work reported here was to construct a record of relative paleointensity through the Olokele transition and tie it to most reliable of the absolute paleointensity results obtained previously. Motivating this effort were recent studies (De Groot et al, ; Lhuillier et al, ; Paterson et al, ) employing the pseudo‐Thellier technique (Tauxe et al, ) on volcanic rocks to reconstruct relative changes in the ancient field intensity. The technique, originally developed for sedimentary materials, uses the acquisition of a standard anhysteretic remanent magnetization (ARM) as a proxy for thermoremanent magnetization (TRM) susceptibility.…”