(2020) Eustatic Control on Superheavy δ34S Pyrite Trends from Late Ediacaran-early Cambrian Carbonate Successions of the West Gondwana: Sulfate Distillation Cycles in Shallow Water Platforms?
Caetano-Filho S, Paula-Santos G, Sansjofre P, Cartigny P, Ader M, Guacaneme C, Babisnki M, Kuchenbecker M, Reis H & Trindade R
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14b: Plenary Hall, Monday 22nd June 22:36 - 22:39
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I'm wondering if the environmental hypothesis you propose at the end is testable with the data you have? Are there other predictions the hypothesis would make that might help to verify (or falsify) this hypothesis?
Dear Elizabeth, if you refer to the interpretation of a paleodepositional signature for superheavy pyrites, I think the striking coincidence between the isotope trends coupled to regressive stages in both basins, respecting different sedimentary thicknesses (i.e. sedimentary rates), is a strong evidence for a primary control over superheavy trends. If you concern about the hypothesis of sulfate limitation in these basins controlled by eustatic variation, unfortunately, to obtain constraints over sulfate concentrations in these rocks is a challenge, considering that CAS cannot be used as a direct measurement of primary sulfate availability, especially for these rocks which experienced a long geological evolution. Nevertheless, it is true that we recovered more CAS always from deposits of the transgressive systems tract. I strongly recommend to check two presentations from Cadeau et al. in the session 9c. They studied a potential modern analog for these conditions - the Dziani Dzaka lake, which present very low sulfate concentrations and superhavy pyrite values coupled to equally enriched carbon isotope values in both carbonates and organic carbon. The later condition would be a result of enhanced methanogenesis and methane escape to the atmosphere in this anoxic and sulfate-depleted environment, producing a condition of disequilibria between DIC and atmosphere, driven by 13C-enriched methanogenic CO2 input in the water column. Important to stress that equally enriched d13C values also occur in the Bambuí Group (>+14permil for carbonates and -14 for organic carbon; Caetano-Filho et al., 2020) after the superheavy pyrite trend in the basal sequence. In my opinion this is the best way to test this hypothesis, by comparing with modern analogues.
Thanks for interesting results and interpretation. Would you argue for a case of extreme paleogeography unique from today's oceans that resulted in this variability, low sulfate concentrations in the oceans, or both?
Dear Kimberly, I think both of the options. In this geotectonic setting of the Precambrian-Cambrian transition, large continental masses (supercontinents) implied in different circulation patterns - I do not believe we can consider open ocean circulation even for marginal basins at this time. On the other hand, several geochemical evidences, including sulfur isotope geochemistry, suggest that at this time maybe the Earth's atmosphere was not fully oxigenated. Addiationally, considering continental landmasses free of vegetation would imply in an inefficient transfer of humidity to continental interior and decreased chemical weathering regime which probably could result in lower sulfate income to the basins, compared with modern environments. Thanks for your interest.
Dear Sergio, interesting study! I was wondering what was so unique about these shallow water environments that resulted in this unique composition in otherwise not so unusual depositional environments/processes? Is this related to the specific Neoproterozoic seawater composition or metabolic processes of organisms restricted to this era?
Dear Georgina, thank you for your interest and question. I think the most important in this scenario is the continental assemble of very large masses resulting in restricted circulation patterns and inefficient bottom ventilation within these basins. In a time of probably decreased oxygenation and chemical weathering, sulfate marine reservoir would be probably equally decreased, which would be represented by the higher isotope sulfur variability preceding the Phanerozoic Eon. This scenario allied to restricted basins driven by supercontinent assembly would be more favorable to sulfate exhaustion through microbial sulfate reduction and pyrite burial in stratified water columns. Actually, I am not confortable to say that these data represent Neoproterozoic oceans, maybe we have a bias in the geological record represented by these intracontinental basins and shallow water platforms under inefficient circulation patterns.
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