TY - JOUR
T1 - Uranium isotope evidence for temporary ocean oxygenation in the aftermath of the Sturtian Snowball Earth
AU - Lau, Kimberly V.
AU - Macdonald, Francis A.
AU - Maher, Kate
AU - Payne, Jonathan L.
N1 - Funding Information:
We thank Claire Miles, Karrie Weaver, Guangchao Li, Dave Mucciarone, Monique Foung, and Nicole O'Keefe for laboratory assistance; Erik Sperling, members of the Payne Paleobiology group, Thomas Bristow, David Johnston, and Uyanga Bold for helpful discussion; and Charlie Langmuir for preliminary elemental data. We gratefully acknowledge the constructive comments by Morten Andersen, Tais Dahl, and an anonymous reviewer that improved the manuscript. This research was supported by an American Chemical Society Petroleum Research Fund Grant ( 53241-ND2 ) to KM, NASA Astrobiology Institute (MIT node) support to FAM, and an Achievement Rewards for College Scientists ( ARCS ) Foundation Fellowship to KVL. The R code for our box model can be accessed at https://purl.stanford.edu/gx919xw2942 .
PY - 2017/1/15
Y1 - 2017/1/15
N2 - The appearance and radiation of animals are commonly attributed to Neoproterozoic oceanic oxygenation, yet independent geochemical evidence for such an event remains equivocal. Strata deposited between the Sturtian and Marinoan Snowball Earth glaciations (660 to 640 Ma) contain the earliest animal biomarkers and possible body fossils. To quantify the extent of seafloor oxygenation during this critical interval, we present uranium isotope ratios (238U/235U denoted as δ238U) from limestone of the Taishir Formation in Mongolia through two stratigraphic sections that are separated by ∼75 km within the same depositional basin. Above the Sturtian glacial deposits, through ∼150 m of stratigraphy, δ238U compositions have a mean value of −0.47‰. This interval is followed by a ∼0.3‰ decrease in δ238U, coincident with the Taishir negative carbon isotope excursion. Thereafter, δ238U values remain relatively low until the erosional unconformity at the base of the Marinoan glacial deposits. Using a box model, we show that the best explanation for the higher δ238U values of the post-Sturtian limestones is extensive—but temporary—oxygenation of the seafloor, and is inconsistent with a scenario involving only increased delivery of uranium to the oceans due to post-Snowball weathering. The decline in δ238U in overlying strata, coincident with the Taishir negative δ13C excursion, indicates a subsequent decrease in seafloor oxygenation. The U isotopic data, combined with modeling results, challenge the notion of a simple, unidirectional oxygenation of Neoproterozoic oceans.
AB - The appearance and radiation of animals are commonly attributed to Neoproterozoic oceanic oxygenation, yet independent geochemical evidence for such an event remains equivocal. Strata deposited between the Sturtian and Marinoan Snowball Earth glaciations (660 to 640 Ma) contain the earliest animal biomarkers and possible body fossils. To quantify the extent of seafloor oxygenation during this critical interval, we present uranium isotope ratios (238U/235U denoted as δ238U) from limestone of the Taishir Formation in Mongolia through two stratigraphic sections that are separated by ∼75 km within the same depositional basin. Above the Sturtian glacial deposits, through ∼150 m of stratigraphy, δ238U compositions have a mean value of −0.47‰. This interval is followed by a ∼0.3‰ decrease in δ238U, coincident with the Taishir negative carbon isotope excursion. Thereafter, δ238U values remain relatively low until the erosional unconformity at the base of the Marinoan glacial deposits. Using a box model, we show that the best explanation for the higher δ238U values of the post-Sturtian limestones is extensive—but temporary—oxygenation of the seafloor, and is inconsistent with a scenario involving only increased delivery of uranium to the oceans due to post-Snowball weathering. The decline in δ238U in overlying strata, coincident with the Taishir negative δ13C excursion, indicates a subsequent decrease in seafloor oxygenation. The U isotopic data, combined with modeling results, challenge the notion of a simple, unidirectional oxygenation of Neoproterozoic oceans.
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U2 - 10.1016/j.epsl.2016.10.043
DO - 10.1016/j.epsl.2016.10.043
M3 - Article
AN - SCOPUS:85003597930
VL - 458
SP - 282
EP - 292
JO - Earth and Planetary Science Letters
JF - Earth and Planetary Science Letters
SN - 0012-821X
ER -