Clay mineral geochemistry and paleoenvironmental reconstruction across the Cryogenian Sturtian Snowball glaciation

Abstract

Recent evidence from marine sedimentary rocks suggests that clay minerals were important in the supply of bioessential nutrients to seawater at the end of the Sturtian Snowball Earth glaciation that occurred ∼ 717–660 million years ago. However, little is known about the identity of these clay minerals, their abundance and distribution in the pre-glacial, glacial and post-glacial deposits. Here, high resolution petrographic scanning electron microscopy-energy dispersive x-ray spectroscopy, combined with bulk and < 2µ clay-size fraction X-ray diffraction mineralogical analysis and trace and major element geochemistry, point to dominant enrichment of illite in shallow marine platform sediments preserved across Sturtian Snowball Earth facies on Islay and the Garvellachs Island, Scotland. The glacial till is enriched with detrital chlorite, while pore-filling authigenic kaolinite is observed in the pre-glacial and post-glacial greenhouse facies, but scarce in the glaciogenic tillites. Discriminant binary plots of Th/Sc vs Zr/Sc ratios, indicative of insignificant modification of primary sediment composition by hydraulic sorting and recycling processes, together with chemical weathering indices, point to moderate to intense chemical weathering of the pre-glacial and post-glacial source rocks. Low Chemical Index of Alternation (CIA) values, coupled to the distinct chlorite composition of the glacial till, is consistent with mechanical tillage of continental landmass by the terminal Snowball melting ice sheets. The moderate to intense chemical weathering indices in the pre- and post-glacial deposits correspond with greenhouse conditions of elevated atmospheric CO₂ content and temperature. Overall, the predominantly detrital origin of the clay minerals implies enhanced post-glacial nutrient supply bound to clay minerals in lower pH river water during transportation, followed by release in higher pH saline seawater.