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The Ca and Mg isotope record of the Cryogenian Trezona carbon isotope excursion

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dc.contributor.author Ahm, Anne-Sofie C.
dc.contributor.author Bjerrum, Christian J.
dc.contributor.author Hoffman, Paul F.
dc.contributor.author Macdonald, Francis A.
dc.contributor.author Maloof, Adam C.
dc.contributor.author Rose, Catherine V.
dc.contributor.author Strauss, Justin V.
dc.contributor.author Higgins, John A.
dc.date.accessioned 2021-06-03T20:30:57Z
dc.date.available 2021-06-03T20:30:57Z
dc.date.copyright 2021 en_US
dc.date.issued 2021
dc.identifier.citation Ahm, A. C., Bjerrum, C. J., Hoffman, P. F., Macdonald, F. A., Maloof, A. C., Rose, C. V., … Higgins, J. A. (2021). The Ca and Mg isotope record of the Cryogenian Trezona carbon isotope excursion. Earth and Planetary Science Letters, 568, 1-13. https://doi.org/10.1016/j.epsl.2021.117002. en_US
dc.identifier.uri https://doi.org/10.1016/j.epsl.2021.117002
dc.identifier.uri http://hdl.handle.net/1828/13013
dc.description.abstract The Trezona carbon isotope excursion is recorded on five different continents in platform carbonates deposited prior to the end-Cryogenian Marinoan glaciation (>635 Ma) and represents a change in carbon isotope values of 16–18‰. Based on the spatial and temporal reproducibility, the excursion previously has been interpreted as tracking the carbon isotopic composition of dissolved inorganic carbon in the global ocean before the descent into a snowball Earth. However, in modern restricted shallow marine and freshwater settings, carbon isotope values have a similarly large range, which is mostly independent from open ocean chemistry and instead reflects local processes. In this study, we combine calcium, magnesium, and strontium isotope geochemistry with a numerical model of carbonate diagenesis to disentangle the degree to which the Trezona excursion reflects changes in global seawater chemistry versus local shallow-water platform environments. Our analysis demonstrates that the most extreme carbon isotope values (∼-10‰ versus +10‰) are preserved in former platform aragonite that was neomorphosed to calcite during sediment-buffered conditions and record the primary carbon isotope composition of platform-top surface waters. In contrast, the downturn and recovery of the Trezona excursion are recorded in carbonates that were altered during early fluid-buffered diagenesis and commonly are dolomitized. We also find that the nadir of the Trezona excursion is associated with a fractional increase in siliciclastic sediments, whereas the recovery from the excursion correlates with a relative increase in carbonate. This relationship suggests that the extreme negative isotopic shift in platform aragonite occurred in concert with periods of increased input of siliciclastic sediments, changes in water depth, and possibly nutrients to platform environments. Although the process for generating extremely negative carbon isotope values in Neoproterozoic platform carbonates remains enigmatic, we speculate that these excursions reflect kinetic isotope effects associated with CO2 invasion in platform waters during periods of intense primary productivity. en_US
dc.description.sponsorship This work was supported by a grant from the Simons Foundation (SCOL 611878, ASCA) and the Carlsberg Foun-dation to ASCA. ASCA and CJB also acknowledge support from the Danish National Research Foundation (Grant No. DNRF53). ACM and CVR acknowledge support from NSF (EAR-0842946) for fund-ing fieldwork on the Trezona Formation in South Australia. JAH acknowledges support from NSF (IES-1410317) and from NSF OCE CAREER Grant (1654571). en_US
dc.language.iso en en_US
dc.publisher Earth and Planetary Science Letters en_US
dc.subject Trezona excursion en_US
dc.subject Snowball Earth en_US
dc.subject Ca and Mg isotopes en_US
dc.subject diagenesis en_US
dc.subject carbon cycle en_US
dc.title The Ca and Mg isotope record of the Cryogenian Trezona carbon isotope excursion en_US
dc.type Article en_US
dc.description.scholarlevel Faculty en_US
dc.description.reviewstatus Reviewed en_US


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