Zinc, carbon, and oxygen isotopic variations associated with the Marinoan deglaciation

Author(s): Maxwell M. Thiemens, Robina Shaheen, Carina T. Gerritzen, Ildiko Gyollai, Kenneth Chong, Friedrich Popp, Christian Koeberl, Mark H. Thiemens, Frédéric Moynier
Abstract: The "Snowball Earths" were cataclysmic events during the late Neoproterozoic's Cryogenian period (720-635 Ma) in which most, if not all, of Earth’s surface was covered in ice. Paleoenvironmental reconstructions of these events utilize isotopic systems, such as Δ¹⁷O and barium isotopes of barites. Other isotopic systems, such as zinc (Zn), can reflect seawater composition or environmental conditions (e.g., temperature changes) and biological productivity. We report here a multi-isotopic C, O, and Zn data set for carbonates deposited immediately after the Marinoan glaciation (635 Ma) from the Otavi Group in northern Namibia. In this study, we chemically separated calcite and non-calcitic carbonate phases, finding isotopically distinct carbon and oxygen isotopes. These could reflect changes in the source seawater composition and conditions during carbonate formation. Our key finding is largescale Zn isotopic variations over the oldest parts of the distal foreslope cap carbonate sections. The magnitude of variation is larger than any found throughout post-snowball cap carbonates to date, and in a far shorter sequence. This shows a heretofore undiscovered difficulty for Zn isotopic interpretations. The primary Zn sources are likely to be aeolian or alluvial, associated with the massive deglaciation related run-off from the thawing continent and a greater exposed surface for atmospheric aerosol entrainment. The samples with the lightest Zn isotopic compositions (δ⁶⁶Zn < 0.3 ‰) potentially reflect hydrothermally sourced Zn dominating the carbonates’ Zn budget. This finding is likely unique to the oldest carbonates, when the meltwater lid was thinnest and surface waters most prone to upwelling of hydrothermally dominated Snowball Earth brine. On the other hand, local variations could be related to bioproductivity affecting the Zn isotopic composition of the seawater. Similarly, fluctuations in sea-level could bring the depositional site below and above a redoxcline, causing isotopic variations. These variations in Zn isotope ratios preclude the estimation of a global Zn isotopic signature, potentially indicating localized resumption of export production.
Organisation(s): Department of Geology, Department of Lithospheric Research
External organisation(s): University of Edinburgh, Vrije Universiteit Brussel, University of California, San Diego, Université de Paris, University of Vienna
Journal: Mineralogy and Petrology
Volume: 117
Pages: 373-386
No. of pages: 14
ISSN: 0930-0708
DOI: https://doi.org/10.1007/s00710-023-00827-0
Publication date: 06-2023
Peer reviewed: Yes
Austrian Fields of Science 2012: 105105 Geochemistry
Keywords
ASJC Scopus subject areas: Geophysics, Geochemistry and Petrology
Portal url: https://ucrisportal.univie.ac.at/en/publications/b9152022-5b1a-4685-98ec-8ab5786616e4

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