Clay formation and nutrient cycling across the Sturtian Snowball Earth Glaciation

Al Bahri, Jalila 2025. Clay formation and nutrient cycling across the Sturtian Snowball Earth Glaciation. PhD Thesis, Cardiff University. Item availability restricted.

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Abstract

It has been proposed that the physical weathering of Earth’s continents by thawing glaciers during the Neoproterozoic Sturtian Snowball Earth glaciation, ~ 720–660 million years ago, produced glacial till that fertilised post-Snowball oceans with nutrients, particularly phosphorus. This flux of nutrients into seawater is thought to have triggered a surge in primary production, contributing to ocean-atmosphere oxygenation and potentially driving a step change in biological evolution. However, the precise mechanism by which these nutrients were transported from land to the ocean and eventually released in a bioavailable form for primary production remains poorly understood. This thesis seeks to address this gap through detailed mineralogical and geochemical analysis of shallow marine sedimentary rocks deposited before, during and after the global Cryogenian Sturtian Snowball Earth glaciation. In addition to unravelling specific pathways by which nutrients were delivered to Cryogenian seawater following the glaciation, this study explores the interplay between redox dynamics and nutrient cycling during this transformative period in Earth's history. Advanced mineralogical X-ray diffraction (XRD) and Scanning Electron Microscopy (SEM) analyses, alongside quantitative chemical extraction of Fe-bound P minerals, revealed a strong correlation between detrital clay mineral production by the melting Sturtian Snowball ice sheets and a short-lived increase in dissolved seawater phosphate content by at least 20-fold. These observations suggest that interactions between physical and chemical weathering of glacial till under the post-Snowball CO2–rich atmosphere were critical in releasing nutrients from the continents to seawater. Clay minerals, acting as carriers, facilitated the transport of both detrital and dissolved nutrients. Upon entering the ocean, the changes in salinity and pH triggered the clays to release the bound elements, which fuelled biological productivity. Seawater redox conditions explored using multiple redox proxies (d238UCAU, FeT/Al and iron speciation), reveal a transient oxygenation of the ocean margins immediately after the Sturtian Snowball glaciation. The results show that the rise and fall in seawater oxygenation matched nutrient maximum and minimum concentrations, respectively, supporting the proposition that Proterozoic seawater nutrient bioavailability, particularly dissolved phosphate content, regulated marine primary productivity and ocean-atmosphere oxygenation dynamics. These findings underscore the pivotal role of clay mineral production and their involvement in nutrient delivery during post-Sturtian Snowball glacial runoff. Acting as nutrient vectors, clay minerals facilitated the transport and release of key nutrients, highlighted by phosphate, from the continents into the ocean. It is therefore proposed that clay mineral formation played a central role in driving Neoproterozoic primary production and promoting ocean-atmosphere oxygenation in the aftermath of the Sturtian Snowball Earth glaciation and probably throughout Earth’s history.

Item Type:	Thesis (PhD)
Date Type:	Completion
Status:	Unpublished
Schools:	Schools > Earth and Environmental Sciences
Subjects:	Q Science > QE Geology
Date of First Compliant Deposit:	4 August 2025
Last Modified:	05 Aug 2025 11:48
URI:	https://orca.cardiff.ac.uk/id/eprint/180220

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