Low oxygen seawater and major shifts in the paleoenvironment towards the terminal Ediacaran: Insights from the Portfjeld Formation, North Greenland

Earth's environment during the diversification of metazoans in the Late Ediacaran was characterised, amongst other factors, by changing oxygen levels and an extreme negative carbon isotope (δ¹³C) excursion, the Shuram Excursion. The Portfjeld Formation, North Greenland, deposited in the Franklinian Basin of eastern Laurentia, presents a unique opportunity to investigate a previously geochemically unstudied succession of Ediacaran marine carbonates to better understand the environment at this time. The succession encompasses a δ¹³C excursion (from +4 to −8‰) comparable to the Shuram Excursion, as well as the Portfjeld Biota, fossils akin to the earliest putative metazoan life forms, the Weng'an Biota. We present a multiproxy geochemical study of 44 samples including rare earth element (and yttrium; REE + Y), chromium isotope (δ⁵³Cr) and strontium isotope (⁸⁷Sr/⁸⁶Sr) compositions. The Portfjeld Formation reveals that low oxygen waters existed beneath oxic surface waters in a stratified early Franklinian Basin. Samples from an interval including bituminous cherty dolomites are enriched in cerium relative to its neighbouring REE+Y's, expressed as positive cerium anomalies (Ce/Ce* = 1.15–1.38), as well the redox sensitive element molybdenum (EF(Mo) = 23–179). This indicates deposition in waters with active metal oxyhydroxide dissolution, which necessitates dissolved oxygen levels as low as <10 μM. This has potential implications for the habitable environment of early putative metazoan forms, as the Portfjeld Biota fossils are found in this interval. The δ⁵³Cr composition of carbonates can be used to track redox related paleoenvironmental changes. A stratigraphic increase in the estimated authigenic δ⁵³Cr composition (from +0.75 to +1.28‰) occurs throughout the nadir and recovery of the succession's δ¹³C carbon excursion. This δ⁵³Cr shift coincides with an increase in detrital element concentration and ⁸⁷Sr/⁸⁶Sr values. Therefore, in this instance the δ⁵³Cr composition of the carbonates of the Portfjeld Formation appears to have responded to elevated on-land weathering, and thus nutrient input, which may have induced further fractionation associated with surface water Cr (VI) back reduction by primary producers, in line with modern ocean observations. These geochemical changes indicate that Ediacaran carbon isotope excursions may be tied to environmental shifts that eventually led to the dramatic increase in metazoan complexity that ensued.