Young inner core inferred from Ediacaran ultra-low geomagnetic field intensity

An enduring mystery about Earth has been the age of its solid inner core. Plausible yet contrasting core thermal conductivity values lead to inner core growth initiation ages that span 2 billion years, from similar to 0.5 to >2.5 billion years ago. Palaeomagnetic data provide a direct probe of past core conditions, but heretofore field strength data were lacking for the youngest predicted inner core onset ages. Here we present palaeointensity data from the Ediacaran (similar to 565 million years old) Sept-Iles intrusive suite measured on single plagioclase and clinopyroxene crystals that hosted single-domain magnetic inclusions. These data indicate a time-averaged dipole moment of similar to 0.7 x 10(22) A m(2), the lowest value yet reported for the geodynamo from extant rocks and more than ten times smaller than the strength of the present-day field. Palaeomagnetic directional studies of these crystals define two polarities with an unusually high angular dispersion (S = similar to 26 degrees) at a low latitude. Together with 14 other directional data sets that suggest a hyper-reversal frequency, these extraordinary low field strengths suggest an anomalous field behaviour, consistent with predictions of geodynamo simulations, high thermal conductivities and an Ediacaran onset age of inner core growth.