New constraints on mid-Proterozoic ocean redox from stable thallium isotope systematics of black shales
Loading...
Date
2021-12-15
Authors
Li, Zijian
Cole, Devon B.
Newby, Sean M.
Owens, Jeremy
Kendall, Brian
Reinhard, Christopher
Advisor
Journal Title
Journal ISSN
Volume Title
Publisher
Elsevier
Abstract
Stable thallium (Tl) isotope data from organic-rich siliciclastic sedimentary rocks have the potential to track ocean redox
state on a broad scale. Here, we report new Tl isotope data from the Mesoproterozoic Velkerri Formation (Roper Group)
and the Paleoproterozoic Wollogorang Formation (Tawallah Group), McArthur Basin, Northern Territory, Australia, and
interpret these in the context of rhenium-osmium (Re-Os) geochronometry on the same sample suite. Previous work has shown
that marine black shales from the Velkerri Formation provide evidence for closed-system Re-Os systematics, yielding a precise
isochron with an age of 1361 ± 21 Ma that agrees well with independent age constraints for the unit. The isotopic composition
of authigenic Tl in euxinic black shales from the upper Velkerri Formation (ε205Tl = -2.4 ± 0.8, 2SD) indicates that the Tl
isotope composition of local seawater at 1.36 Ga was within a plausible range for Tl inputs to the ocean. Isotope mass balance
modeling of the Tl isotope system within a Monte Carlo framework suggests that the Tl isotopic composition of seawater
at 1.36 Ga was homogenous on a global scale and that the burial of Mn-oxides exerted minimal isotopic leverage on the
Tl isotope composition of seawater at 1.36 Ga. Taken together with existing Mo, Cr, and U isotope data from the same samples,
these observations are consistent with a low-O2 ocean-atmosphere system during this period of the Mesoproterozoic.
Previous work has shown that the Re-Os systematics of black shales from the older (1.73 Ga) Wollogorang Formation are
scattered and yield an erroneously young isochron age of 1359 ± 150 Ma, which has been attributed to post-depositional
hydrothermal alteration at ~1640 Ma. We observe no systematic relationship between stable Tl isotope compositions and
the extent of alteration as gauged by open-system Re-Os behavior (-4.7 ± 1.4 for the upper Wollogorang Formation and
-4.8 ± 0.4 for the lower Wollogorang Formation), in marked contrast to previous observations for the molybdenum (Mo)
and uranium (U) isotope systems. The invariant signature of the Tl isotope data suggests the Tl isotope system was largely
unperturbed during hydrothermal alteration. However, it remains difficult to definitively rule out the possibility that authigenic
Tl isotope signatures have been overprinted by later localized hydrothermal fluid alteration in the Wollogorang Formation
shales. These observations highlight the potential insights afforded by evaluating open-system behavior via radiogenic isotope
systems together with other stable isotope tracers in efforts to reconstruct the redox landscape of Earth’s oceans over time.
Description
The final publication is available at Elsevier via https://doi.org/10.1016/j.gca.2021.09.006. © 2021. This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/
Keywords
ocean oxygenation, proterozoic redox, manganese cycle