Milankovitch cycles in Banded Iron Formations: An early Paleoproterozoic window into Earth's climate and Solar System evolution

Abstract

The astronomical theory of climate change resulting from quasi-periodic variations in the Earth’s orbit and spin axis (Milankovitch forcing) is now widely accepted for the Phanerozoic, the most recent ~0.5 billion years of Earth history. However, much older, Precambrian intervals remain largely unexplored. In this thesis, we investigated the influence of Milankovitch forcing on banded iron formations (BIFs) deposited during the earliest Paleoproterozoic, ~2.48 - 2.46 billion years ago. We show, based on cyclostratigraphic analysis combined with high-precision uranium-lead dating, that regular variations in the Kuruman Iron Formation (IF) in South Africa are related to the long (405 kyr) and very long (~1.2 - 1.6 Myr) eccentricity cycles of Earth’s orbit. Using the same approach, we identify the expression of short (~100 kyr) eccentricity and climatic precession in the slightly younger Joffre Member of the Brockman IF in Western Australia. The precession-related cycles are subsequently targeted for high-resolution geochemical analysis and modelling to unravel their origin. As such, we find evidence for dynamical redox changes in the BIF-forming marine environment, which may have been caused by monsoonal-driven changes in oxygenic photosynthesis, before the onset of atmospheric oxygenation ~2.4 billion years ago. Using the ratio between the eccentricity- and precession-related cycles, we further arrive at a reconstruction of the precession frequency, Earth-Moon distance and length-of-day at that time. Finally, we attempt to establish an astrochronological framework for the Kuruman IF and time-equivalent Dales Gorge Member of the Brockman IF.