Milankovitch cycles in REE+Y: A new geochemical tool for cyclostratigraphic analysis of BIFs

Abstract

Spectral analysis was performed on the UU BH-1 core containing the Kuruman banded iron formation (BIF) in order to identify the presence of Milankovitch cycles. We analysed the Fe/Mn which has been found to capture Milankovitch cycles, as well as the rare-earth element + Yttrium (REE+Y) record of a ~100m section of the core. Several recurring cycles were found across all proxies examined in the form of ~3.8m, ~5.2m, ~8.0m and ~19.5m cycles. The ~8.0m cycle was found to be a double ~3.8m cycle and together with the ~5.2m cycle explains nearly all elemental variations found throughout the stratigraphy. The ~19.5m cycle most likely functions as a modulation on the ~5.2m and ~8.0m cycles as the ~8.0m cycles were found to be most dominant with a weak ~19.5m cycle in the middle of the stratigraphy. REE+Y were able to capture a distinct signal throughout the entire stratigraphy, even in sections where Fe/Mn was incapable of capturing clear cycles. The REE+Y showed an antiphase relationship with Fe/Mn while Y/Ho and La were nearly in-phase with it. Y/Ho exhibited an exceedingly high coherence (0.71-0.91) with Fe/Mn at the period considered and was found to most directly respond to changes in continental input and thus climate. Due to the agreement between the cycles found in the Fe/Mn and REE+Y record we conclude that there is a direct climatological control on REE+Y in BIFs.