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

• Published in: Precambrian Research
• Main Authors: Rugen, Elias J., Ineson, Jon R., Frei, Robert
• Format: Article in Journal/Newspaper
• Language: English
• Published: 2022
• Subjects: Carbon isotope excursion; Chromium and strontium isotopes; Detrital correction model; Ediacaran embryos; Positive cerium anomaly; Rare earth elements; Greenland; North Greenland
• Online Access: https://curis.ku.dk/portal/da/publications/low-oxygen-seawater-and-major-shifts-in-the-paleoenvironment-towards-the-terminal-ediacaran(c996d39d-cbee-49d2-8f73-5fd37c89c6ef).html; https://doi.org/10.1016/j.precamres.2022.106781; https://curis.ku.dk/ws/files/319797433/Low_oxygen_seawater_and_major_shifts.pdf

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 (δ 13 C) 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 δ 13 C 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 (δ 53 Cr) and strontium isotope ( 87 Sr/ 86 Sr) 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 δ 53 Cr composition of carbonates can be used to track redox related paleoenvironmental changes. A stratigraphic increase in the estimated authigenic δ 53 Cr composition (from +0.75 to +1.28‰) occurs throughout the nadir and recovery of the succession's δ 13 C carbon excursion. This δ 53 Cr shift coincides with an increase in detrital element concentration and 87 Sr/ 86 Sr values. Therefore, in this instance the δ 53 Cr composition of the carbonates of ...