Milankovitch cycles in 2.5-Ga iron formations as archive of the early Earth and Earth-Moon system

Regular stratigraphic alternations in lower Paleoproterozoic iron formations (IFs) from South-Africa and Western Australia were recently linked to Milankovitch forcing (1, 2). Hence, valuable information may potentially be obtained from these ancient marine deposits about early Solar System dynamics and astronomical-forced climate change when Earth System was operating in fundamentally different ways compared to present-day and the Phanerozoic. In particular, the dominant imprint of long-period eccentricity observed in the Kuruman IF from South-Africa (2) implied a primary influence of climatic precession, while clear precession-scale variations were unfortunately not encountered in this unit. A clear and consistent expression of precession and eccentricity, however, is essential to investigate the climatic-environmental response to the precessional forcing directly, and to determine the precession frequency as to potentially constrain past Earth-Moon dynamics. Here we report results of cyclostratigraphic analysis and high-precision U-Pb dating of the 2.46-Ga Joffre Member of the Brockman IF, Western Australia, revealing exceptionally regular precession- and eccentricity-scale alternations identified in both outcrop and core. Based on the thickness ratio between the precession- and short eccentricity-related alternations seen in outcrop, we estimate a significantly shorter precession period at the time of deposition of the Joffre Member, translating to a shorter Earth-Moon distance and length-of-day (3). In addition, based on detailed geochemical analysis and modelling of the precession-related cycles identified from core, we present a first-order climate interpretation with possible implications for the redox evolution of the ocean-atmosphere (4).

(1) de Oliveira Rodriguez et al. 2019; (2-4) Lantink et al. 2019; 2022; 2023.