Speaker: Dr. Nicholas Tosca (University of Oxford, UK)

Abstract:

Despite iron’s role in the chemical and biological evolution of the Precambrian Earth, its linkages with other biogeochemical cycles remain obscure. The interaction between iron and silica is one important example; persistent anoxia and the lack of a skeletal silica sink through the Precambrian would have promoted a variety of reactions between these two components through much of Earth’s early history. Although both iron and silica have each left clear fingerprints in the Precambrian record, evidence for their interaction, and the attendant biogeochemical consequences, is cryptic. Experimental geochemistry provides one avenue to explore how these interactions are expressed in the rock record. Here I will discuss work from our laboratory showing that the presence of both Fe²⁺ and SiO₂(aq), either in the water column or in sediment pore waters, commonly leads to the rapid nucleation of Fe-serpentine nanoparticles which aggregate to form greenalite at 25^oC. This non-classical crystal growth pathway is consistent with greenalite’s unique crystal structure and with its nearly primary origin in iron formation. Establishing a mechanistic underpinning for greenalite precipitation also permits new constraints on the chemistry of ferruginous Precambrian waters. For example, greenalite may have nucleated from waters with a pH as high as 7.7-8.3, implicating alkalinity as a key trigger in coupling Fe and Si during the anoxic deposition of several Late Archean and Paleoproterozoic iron formations. The common association of greenalite with deeper water iron formation facies (i.e., below storm and/or fair weather wave base) supports the hypothesis that the upwelling of alkaline water masses drove precipitation events that were at times basin wide. Beyond iron formation, greenalite also occurs in some Al-bearing siliciclastics yet cannot precipitate in the presence of dissolved or particulate Al; these occurrences probably reflect crystallization that took place in the water column, explaining why greenalite is essentially absent from modern sediments. Extended more broadly, our results provide an essential puzzle piece that allows us to re-evaluate the inorganic reactions acting as upper limits on water column Fe²⁺ concentrations in non-sulfidic basins. For example, experimental work in mixed silicate-carbonate systems shows that kinetic competition would have set a ceiling for dissolved Fe²⁺ that was sensitive to pH, and higher than previously estimated. Together, a better understanding of greenalite's distribution in chemical and siliciclastic sediments should help disentangle the co-evolution of redox and acid-base chemistries in many Precambrian basins, and will help to construct a record based on a robust new proxy. That aqueous phenomena are recorded with such fidelity in the structure of an ancient sedimentary component offers a rare opportunity to sharpen our view of Earth’s early biogeochemical evolution.