Palaeomagnetic field intensity variations suggest Mesoproterozoic inner-core nucleation

Abstract

The Earth's inner core grows by the freezing of liquid iron at its surface. The point in history at which this process initiated marks a step-change in the thermal evolution of the planet. Recent computational and experimental studies have presented radically differing estimates of the thermal conductivity of the Earth's core, resulting in estimates of the timing of inner-core nucleation ranging from less than half a billion to nearly two billion years ago. Recent inner-core nucleation (high thermal conductivity) requires high outer-core temperatures in the early Earth that complicate models of thermal evolution. The nucleation of the core leads to a different convective regime and potentially different magnetic field structures that produce an observable signal in the palaeomagnetic record and allow the date of inner-core nucleation to be estimated directly. Previous studies searching for this signature have been hampered by the paucity of palaeomagnetic intensity measurements, by the lack of an effective means of assessing their reliability, and by shorter-timescale geomagnetic variations. Here we examine results from an expanded Precambrian database of palaeomagnetic intensity measurements selected using a new set of reliability criteria. Our analysis provides intensity-based support for the dominant dipolarity of the time-averaged Precambrian field, a crucial requirement for palaeomagnetic reconstructions of continents. We also present firm evidence for the existence of very long-term variations in geomagnetic strength. The most prominent and robust transition in the record is an increase in both average field strength and variability that is observed to occur between a billion and 1.5 billion years ago. This observation is most readily explained by the nucleation of the inner core occurring during this interval; the timing would tend to favour a modest value of core thermal conductivity and supports a simple thermal evolution model for the Earth.

Origin of microbial biomineralization and magnetotaxis during the Archean

“…Previous observations that magnetofossil concentrations in marine sediments plummet during weak-field intervals surrounding geomagnetic reversals implies that magnetotaxis confers a selective advantage in field strengths of ∼6 μT or greater (43). Recent analysis of reliable Archean paleointensity data suggests that field strengths of 20-50 μT are observed (44), indicating that the Archean geodynamo was sufficient to support magnetotaxis. An Archean origin of magnetotaxis, and its persistence in multiple lineages since their divergence during Archean time, implies both temporal continuity of Earth's dynamo and persistent environmental stratification.…”

Experimental determination of the electrical resistivity of iron at Earth’s core conditions

“…Biggin et al 11 found that the intensity and variability of the geomagnetic field were enhanced around 1.3 billion years ago and attributed this enhancement to the beginning of nucleation of the solid inner core. Such a remarkable change in palaeomagnetic field, however, may have been caused by another effect such as a change in spatial variation in the core-mantle boundary heat flux 27 .…”

The Role of Geomagnetic Field Intensity in Late Quaternary Evolution of Humans and Large Mammals

“…The demise of the Martian magnetic field, several billion years ago, is widely believed to have been the root cause for the near disappearance of the Martian atmosphere and the resulting dramatic change in the Martian environment from one featuring surface water and aqueous sedimentation to its present relative inactivity and sterility. The explosion of life in the Early Cambrian period at ~530 Ma has been associated with growth of Earth's inner core, the supposed strengthening of the dipole geomagnetic field, and the resulting thickening of Earth's atmosphere (Doglioni et al, ), although there is little evidence for strengthening of the geomagnetic field at this time (e.g., Biggin et al, ). On the other hand, the Late Ediacaran and Early Cambrian periods (~550 and ~530 Ma, respectively) may have been times of unusually high polarity reversal frequency (Bazhenov et al, ; Pavlov & Gallet, ), although precise estimates of reversal frequency are elusive due to poorly constrained age control in stratigraphic sections where the reversals were recorded.…”