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Could Proterozoic Positive Carbon Isotope Excursions be Tracking Intense Methane Fluxes to the Atmosphere? An Analogue-Based Hypothesis
Cadeau P, Ader M, Jézéquel D, Fouilland E, Le Floch' E, Sarazin G, Bernard C & Leboulanger C
Cadeau P, Ader M, Jézéquel D, Fouilland E, Le Floch' E, Sarazin G, Bernard C & Leboulanger C (2020) Goldschmidt Abstracts, 2020 299
09c: Room 3, View in program
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Very exciting findings. Do you think that these results can be scaled up to much larger marine systems? Or, more specifically, do you think that the small reservoir of carbon in this lake is easier to isotopically perturb from atmospheric equilibrium?
Thanks! Yes they can definitively be. In fact we think we have an example in the Neoproterozoic: a 400 km wide restricted basin. It is just published by Caetano-Filho et al 2020 in Geoscience frontiers. We also have a sulfur story on that system presented by Caetano-Filho in this virtual Goldchmidt.
Also, considering the lake is volcanic, is some of the methane of a high-temperature origin rather than from methanogenesis, and would have a different isotopic composition?
Good question! We have C and H isotope signatures of dissolved methane and it is definitively biogenic. Only a few locations where CO2 magmatic gaz bubble through the water column have shown very low methane contents with a high temperature isotope signature. All these values are published in Milesi et al 2020 in ACS_Earth_and_Space_Chemistry.
Very thought provoking presentation. Several questions. 1). under your Methanogenesis versus respiration(%) vs. 13C plot, was the organic carbon burial rate too high? If we pick 50% on the x-axis, that requires 50% of the CO2 removed from the atmosphere, which is way too high compare with the 20 or 25% value Dick Holland had proposed. As I am quite convinced by the atmosphere disequilibrium idea, I guess my really question here is whether that percentage is a measurable index in that particular lake. 2). Under the same plot, what does the 15%, 55% CH4 oxidation really means? Does if refers to the percentage of oxidized organic carbon, or refers to the percentage of methane that released from the organic carbon get oxidized in the atmosphere? The later scenario really stretches my head, as methane get destroyed by OH very rapidly, so its atmospheric lifetime is very,very short compared with O2 and CO2.
as to point 2: if refers to the percentage of methane that has been produced in the lake, that get oxidized in the water column and therefore does not make it to the atmosphere.
Thanks for these questions and comments. I realize I went way to fact when presenting this slide. 1) In our curves, the organic carbon burial rate varied from 1.4% (blue curves, this is the measured value in the lake) to 7% of the primary productivity (black curve). As you can see, the effect of organic carbon burial is really small in this system. As to the 50% in the X axis, it in fact represents the percentage of primary productivity (that we have quantified) that is mineralized by methanogenesis. It does not relate to CO2 taken up from atmosphere. We were not able to measure the respective rates of organic matter mineralization by respiration and methanogenesis in the lake. I will answer point 2 soon!
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