First palaeointensity data from the cryogenian and their potential implications for inner core nucleation age

Abstract

Summary The timing of inner core nucleation is a hugely significant event in Earth's evolution and has been the subject of intense debate. Some of the most recent theoretical estimates for the age of nucleation fall throughout the Neoproterozoic era; much younger than previously thought. A young inner core requires faster recent core cooling rates and a likely hotter early core; knowledge of its age would be invaluable in understanding Earth's thermal history and total energy budget. Predictions generated by numerical dynamo models need to be tested against such data, but records are currently much too sparse to constrain the event to a precise period of time. Here, we present results from 720 Ma dolerite dykes (and one sill) from the Franklin Large Igneous Province, which fall within a crucial 300 Myr gap in palaeointensity records. This study uses three independent techniques on whole rocks from 11 sites spread across High Arctic Canada and Greenland, to produce virtual dipole moments ranging from 5 - 20 ZAm2 (mean 11 ZAm2); almost one order of magnitude lower than the present-day field. These weak-field results agree with recent ultralow palaeointensity data obtained from Ediacaran rocks formed ∼150 Myr later and may support that the dynamo was on the brink of collapse in the Neoproterozoic prior to a young inner core formation date.

Improvements to the Shaw-Type Absolute Palaeointensity Method

“…It has recently been noted that Slope T values may be dependent on hold duration (Lloyd et al, 2021); this is now further supported in more detail here. To analyse the cause of these latest observations, we subjected three of the specimens from site MD6 to a third TRM, heating to 610°C using a hold duration of 40 min and compared the AF demagnetisation spectra.…”

“…The inclusion of LTD treatment is optional with respect to the theory behind the DHT. Recent studies of rocks that were moderately to highly MD (15-28%), obtained very similar palaeointensity estimates from LTD and non-LTD sister specimens (Yamamoto et al, 2007;Lloyd et al, 2021) and other studies successfully use the method without LTD. (Thallner et al, 2021). However, the incorporation of LTD can be important, not only for removing MD-like remanence, but also for the additional information it provides.…”

“…The initial findings presented in this study, however, suggest that it may be possible for the DHT to allow alteration to occur undetected in certain samples and that slope T values could potentially be dependent on the hold duration used (c.f. Lloyd et al, 2021). This is highlighted in TRM and ARM can both be affected independently by alteration (Tsunakawa and Shaw, 1994;Tanaka and Komuro, 2009), which is demonstrated by the existence of non-unit slope T values.…”

“…A large dataset of measurements undertaken on 426 individual specimens was compiled from multiple Shaw-DHT experiments, mostly carried out at the University of Liverpool over the last 3 years. Full multi-method palaeointensities are now published on many of these sample sets (Lloyd et al, 2021;Thallner et al, 2021). The experiments were performed using a wide variety of varying input parameters, including differences in the laboratory TRM bias field, ARM bias field, hold durations, sample positions, oven used and number of measurement steps.…”

“…Summary of all samples used in the dataset. Included are specimens from studies byThallner et al, 2021 andLloyd et al, 2021 (and other submitted work). Base set of selection that is applied to the entire dataset, and in addition to each of the parameters tested in this study.…”