175 The Neoproterozoic Earth and Life Co-evolution, Part I

This session aims to enhance discussions and foster interdisciplinary collaborations to advance understanding of the Neoproterozoic Earth and life co-evolution. Contributions from paleontologists, geochemists, sedimentologists, Earth system modelers, and related fields are highly encouraged.

Tuesday, 21 October 2025
8:00 AM - 12:00 PM
HBGCC, 304A

A low oxygen habitat for Earth’s first animal ecosystems: A thallium isotope record of the Avalon Assemblage at Mistaken Point.

Paper number: 175-10
Program: Technical Sessions
Time: 10:25 AM – 10:40 AM

Authors

Abstract

Animals have only been present on Earth for the last ~0.5 Ga, a mere 10% of Earth’s history. Identifying the environmental or biological constraints on this late emergence remains a fundamental issue critical to understanding the rise of large complex life.

Increased global deep marine oxygenation is hypothesized to have sparked the emergence of animal life during the Ediacaran Period. However, Ediacaran paleoredox proxy records do not yield a straightforward signal of increased deep marine oxygenation at or around the rise of animals. Clarifying the link between deep ocean oxygenation and the appearance of animal life requires a proxy sensitive to dissolved oxygen (O2) accumulation in global marine bottom waters. We apply such a proxy, thallium (Tl) isotopes, to the Ediacaran deep-water stratigraphy at Mistaken Point Ecological Reserve (MPER), Newfoundland, Canada. These strata host the oldest known diverse animal ecosystems and are an ideal target to test the hypothesized coeval rise of animals and oxygen.

Thallium isotopes in seawater are strongly fractionated during seafloor Mn oxide burial, which is highly sensitive to bottom water O2. The residence time of Tl in the ocean exceeds ocean circulation timescales, so Tl isotopes ratios are globally homogenous. Marine sediments from modern unrestricted anoxic settings, in which local Mn oxide burial is absent, capture today’s globally homogenous seawater Tl isotope ratio. Ancient sedimentary rocks formed under similar conditions should do the same, providing information about past bottom water O2.

Pilot Tl isotope data from stratigraphically below and throughout the range of animal fossils at MPER show no fractionation from crustal compositions. MPER Tl isotope data are consistent with contemporaneous Tl isotope records from Oman and Northwest Canada and contribute to a global picture of low Mn oxide burial and therefore limited O2 accumulation in global marine bottom waters during the Ediacaran Period. Enrichment factors of redox-sensitive trace metals (V, Mo, U, Co, Mn) in MPER samples indicate oxic conditions. Taken together, Tl isotope and trace metal data suggest transiently oxygenated or low-oxygen habitats of the earliest animal ecosystems and argue against increased environmental oxygenation as a strong trigger on the emergence of animal life on Earth.

Geological Society of America Abstracts with Programs. Vol. 57, No. 6

doi: 10.1130/abs/2025AM-413184

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