Transient fertilization of a post-Sturtian Snowball ocean margin with dissolved phosphate by clay minerals

The Neoproterozoic Cryogenian period (720–635 million years ago) witnessed significant glaciations, known as Snowball Earth events. The Sturtian glaciation is one such event, and its aftermath is marked by a noticeable increase in marine phosphorus (P) levels in sedimentary rocks. This increase is linked to enhanced primary productivity, ocean-atmosphere oxygenation, and ultimately, the rise of animals. However, the precise mechanisms responsible for transporting bioavailable phosphate from continents to oceans remain unclear. Existing hypotheses suggest a broad association with mechanical weathering by melting ice sheets, but the specific processes of P sourcing and delivery are not well-understood. This study investigates the hypothesis that the production of clay minerals by the melting Sturtian Snowball ice sheets played a crucial role in increasing seawater phosphate bioavailability, thereby driving the observed changes in marine productivity and oxygenation. Understanding this process is critical for comprehending the evolution of Earth's biogeochemical cycles and the rise of complex life.

Previous research has established a correlation between the rise in phosphorus in Cryogenian marine sediments and post-Snowball Earth deglaciation. Studies on Fe-rich deposits and shales indicate significantly higher P concentrations compared to older sediments in similar depositional settings. These observations suggest higher dissolved seawater PO4 concentrations during and after Neoproterozoic glaciations. However, the mechanisms by which PO4 was sourced from the continents and transported to the oceans in bioavailable forms remain poorly understood. Modern analogs show that most PO4 is transferred from land to oceans via riverine-transported clay and metal oxide particles, rather than in solution. This suggests a potential role for detrital clay minerals in transporting PO4 from land during the Great Oxidation Event (GOE). The study builds on these existing insights by focusing on the specific role of clay minerals formed during the melting of Sturtian glaciers.

To investigate the causal relationship between continental erosion and the supply of dissolved PO4 to continental margin waters, the researchers collected marine sediments from the Isles of Islay and the Garvellachs in the Dalradian Supergroup, Scotland. These sediments span pre-glacial, deglacial, and post-glacial phases of the Sturtian glaciation. The study area comprises the Tonian Lossit Limestone Formation (LLF), the Port Askaig Tillite Formation (PATF), and the Bonahaven Dolomite Formation (BDF). The researchers employed a combination of geochemical techniques, including bulk facies chemostratigraphy, mineralogical analysis (XRD and SEM-EDS), elemental analysis (ICP-AES), iron speciation analysis, chemically extractable Fe-bound P analysis, and stable isotope analysis (δ13C, δ18O, δ56Fe). The iron speciation analysis used a sequential extraction protocol to quantify operationally defined Fe phases, enabling the assessment of reactive and unreactive Fe components. Chemically extractable Fe-bound P analysis helped determine P distribution among different Fe phases. Stable isotope data provided insights into carbon and oxygen cycling, while iron isotopes helped constrain redox conditions. Finally, a four-box ocean biogeochemical model was used to simulate PO4 dynamics and oxygenation changes.

The study reveals a strong correlation between clay mineral abundance and phosphate enrichment in post-Sturtian sediments. Specifically, the immediate post-Snowball interval shows a significant (at least 20-fold) increase in seawater phosphate bioavailability, linked to a dramatic rise in sheet silicate-bound P. This is followed by a shift in P sink from clays to magnetite, indicating a change in the dominant mechanism for P sequestration. Bulk sediment geochemical analysis revealed elevated P2O5, TiO2, Al2O3, Fe2O3, Cr, V, and Ni concentrations in the PATF and lower BDF. Elemental normalization to Ti highlights subtle enrichment patterns, suggesting co-regulated accumulation of these elements. The data indicate that bulk sediment P was primarily associated with non-calcium-bearing minerals (Fe-oxHR and unreactive detrital silicates), rather than apatite. Microbial recycling of organic-rich P played a limited role in overall P enrichment. Analysis of reactive and unreactive Fe phases reveals a decrease in Fe-oxHR (highly reactive iron oxyhydroxides) at the end-Snowball transition, coinciding with the increase in phosphate bioavailability. The distribution of leachable P within different Fe phases demonstrates a high P enrichment in association with sheet silicates in the tillites and immediate post-Snowball sediments. The increase in magnetite-bound P suggests that magnetite precipitation captured a snapshot of dissolved PO4 in seawater. Iron speciation analysis, combined with Fe/Al ratios, indicates a transition from anoxic to oxic conditions during the immediate post-Snowball period, followed by a return to anoxic conditions. The numerical model confirms the observed temporal rise and fall in seawater PO4 and the transient oxygenation event. The model supports the hypothesis that increased riverine P input caused by ice sheet meltwater was the primary driver of the phosphate fertilization.

The findings of this study provide compelling evidence for a direct causal link between the production of clay minerals during Sturtian deglaciation and a transient increase in seawater phosphate bioavailability. The release of phosphate from the newly formed clays, coupled with a reduction in the capacity of iron oxyhydroxides to scavenge phosphate, created a window of high phosphate availability. This led to a short-lived but significant increase in marine primary productivity and, consequently, transient oxygenation of the continental margin waters. The model simulations support these findings, demonstrating that a large, abrupt input of riverine P can indeed trigger these changes, highlighting the importance of glacial meltwater as a driver of biogeochemical change in the post-Snowball ocean. The study refines our understanding of the intricate interplay between geological processes and biogeochemical cycles in shaping the Earth's environment and its influence on early life.

This study demonstrates a causal relationship between glacial meltwater delivery of clay minerals and a transient increase in phosphorus bioavailability in a post-Sturtian Snowball ocean margin. The short-lived event of enhanced phosphate supply drove a temporary increase in marine primary productivity and oxygenation. Future research could investigate the spatial extent and duration of similar events across different regions. Analyzing the isotopic compositions of phosphorus in greater detail could also shed further light on the sources and transformations of phosphate during these crucial events.

The study is based on a single location and might not fully represent the global scale of the transient fertilization event. The numerical model is a simplified representation of complex biogeochemical processes, and there may be other factors influencing P cycling not fully captured by the model. The operational definitions of iron phases in the sequential extraction method may not precisely reflect the true mineralogical composition, potentially introducing uncertainties in the interpretation of the data.