1. Uranium isotope evidence for limited euxinia in mid-Proterozoic oceans.
- Author
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Gilleaudeau, Geoffrey J., Romaniello, Stephen J., Luo, Genming, Kaufman, Alan J., Zhang, Feifei, Klaebe, Robert M., Kah, Linda C., Azmy, Karem, Bartley, Julie K., Zheng, Wang, Knoll, Andrew H., and Anbar, Ariel D.
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URANIUM isotopes , *CARBONATE rocks , *ATMOSPHERIC oxygen , *SHALE , *TRACE metals , *OCEAN - Abstract
Reconstructing Earth's oxygenation history is key to deciphering environmental controls on early biospheric evolution. During the mid-Proterozoic Eon, low (but potentially variable) atmospheric p O 2 led to highly heterogeneous marine redox conditions, with most studies indicating a relatively shallow depth of oxygen penetration. The relative proportion of oxic, anoxic and iron-rich (ferruginous), and anoxic and sulfide-rich (euxinic) conditions on the global seafloor is difficult to quantify, however, due to a general reliance on local redox proxies applied to the temporally discontinuous black shale record. It is particularly important to constrain the global prevalence of euxinic bottom waters because sulfide toxicity has been implicated as a primary constraint on evolution in mid-Proterozoic oceans and limits the solubility of bioessential trace metals such as copper, zinc, and molybdenum. Here, we present a suite of new uranium (U) isotope data from marine carbonate rocks that span the entire mid-Proterozoic interval (∼1.8 to 0.8 Ga). U-isotopes in well-preserved carbonate rocks represent a powerful new proxy that can be used to quantitatively constrain the global extent of marine euxinia. The median δ 238 U value for mid-Proterozoic carbonate samples is −0.43‰, which is lower than the median value for modern post-depositional carbonate sediments from the Bahamas, yet significantly higher than carbonate δ 238 U values recorded during other times of expanded anoxia in Earth history, such as during the end-Permian mass extinction. When paired with a three-sink isotope mass balance model, our data indicate that no more than 7% of the global seafloor was euxinic during the mid-Proterozoic Eon, although transient pulses of expanded euxinia are recorded. Although these results challenge early expectations of a sulfide-rich mid-Proterozoic ocean, they are consistent with more recent inferences from the black shale trace metal record. Evidence therefore indicates that euxinic bottom waters were relatively limited in their spatial extent in the mid-Proterozoic oceans. • Uranium isotopes in marine carbonate constrain Proterozoic seafloor euxinia. • Median Proterozoic δ 238U (−0.43‰) lower than modern Bahamian carbonate. • Median Proterozoic δ 238U higher than end-Ediacaran and Permo-Triassic carbonate. • Mass balance model suggests that at maximum 7% of seafloor was euxinic. • Mid-Proterozoic euxinia was less prevalent than previously envisaged. [ABSTRACT FROM AUTHOR]
- Published
- 2019
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