Widespread seafloor anoxia during generation of the Ediacaran Shuram carbon isotope excursion

Chadlin M. Ostrander*, Christian J. Bjerrum, Anne Sofie C. Ahm, Simon R. Stenger, Kristin D. Bergmann, Mohamed A.K. El-Ghali, Abdul R. Harthi, Zayana Aisri, Sune G. Nielsen

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

4 Citations (Scopus)

Abstract

Reconstructing the oxygenation history of Earth's oceans during the Ediacaran period (635 to 539 million years ago) has been challenging, and this has led to a polarizing debate about the environmental conditions that played host to the rise of animals. One focal point of this debate is the largest negative inorganic C-isotope excursion recognized in the geologic record, the Shuram excursion, and whether this relic tracks the global-scale oxygenation of Earth's deep oceans. To help inform this debate, we conducted a detailed geochemical investigation of two siliciclastic-dominated successions from Oman deposited through the Shuram Formation. Iron speciation data from both successions indicate formation beneath an intermittently anoxic local water column. Authigenic thallium (Tl) isotopic compositions leached from both successions are indistinguishable from bulk upper continental crust (ε205TlA ≈ −2) and, by analogy with modern equivalents, likely representative of the ancient seawater ε205Tl value. A crustal seawater ε205Tl value requires limited manganese (Mn) oxide burial on the ancient seafloor, and by extension widely distributed anoxic sediment porewaters. This inference is supported by muted redox-sensitive element enrichments (V, Mo, and U) and consistent with some combination of widespread (a) bottom water anoxia and (b) high sedimentary organic matter loading. Contrary to a classical hypothesis, our interpretations place the Shuram excursion, and any coeval animal evolutionary events, in a predominantly anoxic global ocean.

Original languageEnglish
Pages (from-to)556-570
Number of pages15
JournalGeobiology
Volume21
Issue number5
DOIs
Publication statusPublished - Sept 2023

Keywords

  • Animals
  • Carbon Isotopes/analysis
  • Geologic Sediments/chemistry
  • Seawater/chemistry
  • Oceans and Seas
  • Hypoxia
  • Water
  • Oxygen/analysis

ASJC Scopus subject areas

  • Ecology, Evolution, Behavior and Systematics
  • General Environmental Science
  • General Earth and Planetary Sciences

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