Thallium isotope evidence for an anoxic global deep ocean ∼1.83 billion years ago
Abstract
he evolution of ocean redox conditions following the final global-scale interval of banded iron formation deposition ~1.83 billion years ago (Ga) has been a motivator of multiple key studies on mid-Proterozoic ocean chemistry. But few if any of these studies provide information about the extent of dissolved oxygen (O 2 ) accumulation in the deepest marine water masses that covered the greatest fraction of the global seafloor. We fill this knowledge gap with thallium (Tl) isotopes, an indirect tracer of deep ocean oxygenation levels via seafloor manganese (Mn) oxide burial. We apply Tl isotopes to ~1.83-billion-year-old shales from the Rove and Virginia formations from the Superior Province (Ontario, Canada, and Minnesota, USA). We analyzed shale samples from four cores separated by up to 250 km. These sedimentary rocks were deposited under variable, but predominantly reducing, local redox conditions. Most of our pyrite-bound authigenic Tl isotopic compositions (ε 205 Tl A ) are indistinguishable from the globally averaged seawater input value of -1.8‱ (or nearly so). If our averaged ε 205 Tl A value of -1.9‱ is a globally representative ancient seawater signal, shales from the Rove and Virginia formations formed at a time of widespread deep marine anoxia. Some areas of the sunlit surface ocean were mildly oxygenated at this time according to other lines of evidence, but deeper waters were not sufficiently oxygenated to stabilize Mn oxides over large areas of the seafloor and register a measurable effect on long-term seawater Tl isotope mass-balance. These data provide an independent, global perspective on the evolution of ocean redox conditions in the aftermath of the final major interval of global iron formation deposition at the start of the mid-Proterozoic.
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BibTeX
@article{ostrander2026thallium,
title = {{Thallium isotope evidence for an anoxic global deep ocean ∼1.83 billion years ago}},
author = {Chadlin M Ostrander and Andy W Heard and Donald E Canfield and Philip W Fralick and Sune G Nielsen and Jean N R Clemente and Simon W Poulton},
journal = {Earth and Planetary Science Letters},
year = {2026},
month = sep,
doi = {10.1016/j.epsl.2026.120348},
url = {https://hal.science/hal-05763103v1},
note = {CC BY 4.0},
}
RIS
TY - JOUR
TI - Thallium isotope evidence for an anoxic global deep ocean ∼1.83 billion years ago
AU - Chadlin M Ostrander
AU - Andy W Heard
AU - Donald E Canfield
AU - Philip W Fralick
AU - Sune G Nielsen
AU - Jean N R Clemente
AU - Simon W Poulton
PY - 2026
DA - 2026/09/24
JO - Earth and Planetary Science Letters
DO - 10.1016/j.epsl.2026.120348
UR - https://hal.science/hal-05763103v1
AB - he evolution of ocean redox conditions following the final global-scale interval of banded iron formation deposition ~1.83 billion years ago (Ga) has been a motivator of multiple key studies on mid-Proterozoic ocean chemistry. But few if any of these studies provide information about the extent of dissolved oxygen (O 2 ) accumulation in the deepest marine water masses that covered the greatest fraction of the global seafloor. We fill this knowledge gap with thallium (Tl) isotopes, an indirect tracer of deep ocean oxygenation levels via seafloor manganese (Mn) oxide burial. We apply Tl isotopes to ~1.83-billion-year-old shales from the Rove and Virginia formations from the Superior Province (Ontario, Canada, and Minnesota, USA). We analyzed shale samples from four cores separated by up to 250 km. These sedimentary rocks were deposited under variable, but predominantly reducing, local redox conditions. Most of our pyrite-bound authigenic Tl isotopic compositions (ε 205 Tl A ) are indistinguishable from the globally averaged seawater input value of -1.8‱ (or nearly so). If our averaged ε 205 Tl A value of -1.9‱ is a globally representative ancient seawater signal, shales from the Rove and Virginia formations formed at a time of widespread deep marine anoxia. Some areas of the sunlit surface ocean were mildly oxygenated at this time according to other lines of evidence, but deeper waters were not sufficiently oxygenated to stabilize Mn oxides over large areas of the seafloor and register a measurable effect on long-term seawater Tl isotope mass-balance. These data provide an independent, global perspective on the evolution of ocean redox conditions in the aftermath of the final major interval of global iron formation deposition at the start of the mid-Proterozoic.
SN - 0012-821X
ER -