The pulse of continental crust production and the structure of the galaxy

The pock-marked surface of the Moon provides a stark reminder of the impact flux endured by the early Earth. Notwithstanding, the role of exogenic (impact-driven) processes in the generation and evolution of Earth’s continental crust has attracted relatively little attention compared to endogenic processes driven by loss of heat from the planet’s interior. Here we explore various isotope time series inferred to track crust production within the context of changing local mass density for the Solar System over the duration of its orbit through the Milky Way galaxy. Using a global dataset of zircon Hf isotopes during the Archean, we find an enhanced probability of a step change in composition during entry into the galactic spiral arms, on a periodicity of ∼190 Myr. Fluctuations in zircon oxygen isotopes between normal and non-normal distributions also reveal periods of less normality corresponding to spiral arm entry, implying the production of a greater volume of buoyant lithosphere due to an enhanced flux of energetic impacts. Additionally, the age distributions of post-Archean terrestrial hypervelocity impact craters and lunar impact-melt clasts show elevated probabilities during the predicted phases of spiral-arm crossing. For a Sun–spiral-arm recurrence interval of ∼190 Myr, the local Galactic rotation model predicts a radial epicyclic period of approximately ∼150 Myr, which is also resolved in the zircon Hf change-point record for the ancient Earth. Both frequencies have been related to periodic disturbance of the Oort cloud and modifications to the impact flux in the inner Solar System. Together, these correlations suggest that some episodes of production and reworking of continental crust during the Archean were triggered by large impacts, some of which were probably comets. That there seems to be a fundamental connection between events on Earth and the galactic tide supports a role for periods of catastrophism through Earth’s history.