TY - JOUR

T1 - Low oxygen seawater and major shifts in the paleoenvironment towards the terminal Ediacaran

T2 - Insights from the Portfjeld Formation, North Greenland

AU - Rugen, Elias J.

AU - Ineson, Jon R.

AU - Frei, Robert

N1 - Publisher Copyright: © 2022

PY - 2022

Y1 - 2022

N2 - 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.

AB - 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.

KW - Carbon isotope excursion

KW - Chromium and strontium isotopes

KW - Detrital correction model

KW - Ediacaran embryos

KW - Positive cerium anomaly

KW - Rare earth elements

U2 - 10.1016/j.precamres.2022.106781

DO - 10.1016/j.precamres.2022.106781

M3 - Journal article

AN - SCOPUS:85136066222

VL - 379

JO - Precambrian Research

JF - Precambrian Research

SN - 0301-9268

M1 - 106781

ER -