Dr. Maxwell Lechte - Ironstones of Yukon: insights into marine iron cycling in the early Neoproterozoic

The early Neoproterozoic Tonian Period (1000–720 Ma) witnessed the rise of eukaryote-dominated ecosystems, which may have resulted from changes in global biogeochemical cycles. However, paleoenvironmental information from Tonian strata are scarce. Iron-rich chemical sediments deposited in shallow marine settings, known as ironstones, are important tracers of ancient ocean chemistry. Typically characterised by the presence of ooids composed of iron oxides or ferrous iron silicates, ironstones can potentially offer valuable insights into the redox conditions of shallow seawater and porewaters. However, the use of ironstone geochemistry as a paleoredox proxy is limited due to conflicting interpretations regarding the genesis of these enigmatic deposits. Here we describe ca. 850 Ma ironstones from the Katherine Group (Yukon), the first described example of an ironstone from the Tonian Period. These ironstones are hosted by quartz arenites and siltstones of the Katherine Group, interpreted to have been deposited in a peritidal setting characterised by migrating sandbars. The Katherine ironstones are well-preserved and feature iron ooids with concentric coatings of interlaminated hematite and berthierine. Iron isotope evidence indicates that the hematite is the product of partial oxidation of ferrous iron. We suggest that these ironstones formed by the variable precipitation of iron oxyhydroxides and authigenic berthierine precursor gels from ferruginous seawater near a redox boundary between suboxic and anoxic conditions, with vigorous iron cycling. This evidence for ferruginous conditions and partial iron oxidation in shallow seawater suggests extremely low atmospheric oxygen levels during the middle Tonian, which may have exerted an environmental control on organismal physiology and ecological complexity during this time. A subsequent rise in atmospheric oxygen in the mid-late Tonian may have paved the way for the first emergence of complex multicellularity in eukaryotes.

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