Milankovitch Cycles in Paleoproterozoic Banded Iron Formation

Banded iron formations (BIFs) were widely deposited from the Neoarchean to early Paleoproterozoic oceans, between 2.8 and 2.4 billion years ago. So far, they have been mainly linked to hydrothermal plume activity¹, continental growth² and the rise of oxygen in the ocean and atmosphere³. In contrast, very little attention has been paid to climate variability and its potential role in the formation of BIFs. Climate oscillations on the 10⁴ - 10⁶ year scale known as Milankovitch forcing must have been operative at that time⁴ and may explain rhythmic layering observed in BIFs^5-6. However, this hypothesis has never been fully tested, partially due to uncertainties associated with BIF depositional rates. For this reason, we have carried out cyclostratigraphic analysis on the Paleoproterozoic Kuruman BIF, South Africa. We have combined this with high-precision, high-accuracy TIMS U-Pb zircon dating of four ash intervals interbedded in the BIF. In field exposures of the Kuruman BIF, we identified characteristic, rhythmic alternations in the weathering profile, which are laterally consistent over 250 km. Based on our spectral analysis results and precise U-Pb ages, we hypothesize these patterns are related to orbital forcing.

(1) Isley, A. E. Hydrothermal plumes and the delivery of iron to banded iron formation. J. Geol. 103, 169-186 (1995); (2)Trendall, A. F. The significance of iron-formation in the Precambrian stratigraphic record. Speci. Publs int. Ass. Sediment. 33-66 (2002); (3) Lyons, T. W., Reinhard, C. T. & Planavsky, N. J. The rise of oxygen in Earth's early ocean and atmosphere. Nature 506, 307-315 (2014); (4) Laskar, J. et al. A long-term numerical solution for the insolation quantities of the Earth. Astron. Astrophys. 428, 261-285 (2004); (5) Trendall, A. F. & Blockley, J. B. The iron formations of the Precambrian Hamersley Group, Western Australia with special reference to the associated crocidolite. Geological Survey of Western Australia 119, (1970); (6) Beukes, N. J. Lithofacies and stratigraphy of the Kuruman and Griquatown iron-formations, northern Cape Province, South Africa. Trans. Geol. Soc. South Africa 83, 69-86 (1980).