The strength of the Mesoproterozoic geomagnetic field: new absolute paleointensity estimates from 1.1 billion-year-old Midcontinent Rift volcanics

Paleomagnetic data from Precambrian rocks are one of the few lines of observational data that can be brought to bear on the evolution of Earth's core. A recent compilation of paleointensity estimates from throughout Earth history has been interpreted to indicate that Earth's magnetic field strength increased in the Mesoproterozoic (between 1.6 and 1.0 billion years ago), with this increase interpreted to mark the onset of inner core nucleation (Biggin et al., 2015). However, much of the data within the Precambrian paleointensity database are from Thellier experiments with non-ideal results that can manifest as two-sloped Arai diagrams. Choices made when interpreting these data may significantly change conclusions about the long-term trends of Earth's geomagnetic field, even calling into question the interpretation of a Mesoproterozoic increase (e.g. Smirnov et al., 2016). Two-sloped Arai diagrams are difficult to interpret and can yield two paleointensity estimates, even for the same directional component, often yielding a higher paleointensity estimate from the low-temperature portion and a lower paleointensity estimate from the high-temperature portion. The question of which estimate, if any, represents the past magnetic field strength is actively debated and both can pass certain commonly utilized quality criteria. In many cases, data with such behavior may be better considered to provide minimum and maximum, rather than absolute, paleointensity estimates. Here we present new paleointensity experimental results from volcanics of the 1.1 billion-year-old North American Midcontinent Rift. While most of the results exhibit non-ideal two-slope behavior in Arai plots, some flows have more ideal single slope behavior leading to estimates that may be some of the best constraints on the strength of Earth's field for this time. Data from the Midcontinent Rift dominate the late-Mesoproterozoic paleointensity database. As a result, these data strongly affect interpretations of trends in magnetic field strength and dynamics of the deep interior through this critical interval.