The geomagnetic polarity time scale (GPTS) is based on the marine magnetic anomalies, the striping pattern of strong and weak magnetic signals recorded by the ocean crust. Strong signals correspond to normal polarity and weak signals to reverse polarity. Adjacent normal and reverse stripes are numbered, backward in time from C1 (top = today) to C34, of which the normal portion is a long period of normal polarity in the Mid Cretaceous. Older magnetic stripes are referred to as the M-sequence (Mesozoic sequence) starting with M0 below the very long C34 anomaly.
The current GPTS extends from today backward in time down to magnetic anomaly M29 (approximately 157 million years ago). Older oceanic crust is rare and typically has subdued magnetic anomaly patterns that are difficult to correlate. Thus, pre-M29 time scales remain controversial because the marine magnetic anomaly data is of rather poor quality. This complicates analysis of important features of the geomagnetic field: the reversal frequency and the expression of the Mesozoic dipole low (also termed Jurassic Quiet Zone).
Tominaga et al.  extend the geomagnetic polarity time scale down to the Mid Jurassic (M44, about 170 million years ago) based on a composite of the Japanese lineation set they published previously, and a new, highly detailed, multiscale magnetic anomaly profile of the Hawaiian lineation set, both from the western Pacific Ocean.
The weak anomaly portion M41-M39 is best expressed in the Japanese profile and argued to represent the onset and maximum expression of the Mesozoic dipole low or the core of the Jurassic Quiet Zone, an important long-lasting low-intensity feature of the geomagnetic field. From the midwater reference profile, the Jurassic reversal frequency in the M29-M44 time span (157-170 million years ago) appears to have been about 19 reversals per million years, i.e. extraordinarily high, and double the previous estimates for that period.
Citation: Tominaga, M., Tivey, M. A., & Sager, W. W. . A new middle to Late Jurassic Geomagnetic Polarity Time Scale (GPTS) from a multiscale marine magnetic anomaly survey of the Pacific Jurassic Quiet Zone. Journal of Geophysical Research: Solid Earth, 126, e2020JB021136. https://doi.org/10.1029/2020JB021136
―Mark Dekkers, Associate Editor, JGR: Solid Earth