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Abstract Details

(2020) Magnesium and Strontium Isotope Fractionation during Microbial Dolomite Formation

Tatzel M, Paytan A, DiLoreto Z, Dittrich M, Bontognali T & Sanchez-Román M

https://doi.org/10.46427/gold2020.2568

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14b: Plenary Hall, View in program

Listed below are questions that have been submitted by the community that the author will try and cover in their presentation. To submit a question, ensure you are signed in to the website. Authors or session conveners approve questions before they are displayed here.

Submitted by Vanessa Fichtner on
Dear Michael, Thank you for sharing your intriguing idea about mixed clay and dolomite ?26/24Mg values! I have two questions about your talk: Were you able to extract and specify certain clay minerals from the cores? And do you exclude the presence/dissolution of calcite when you attribute a Mg/Ca = 1 in the leachate to f (dol) = 1? Thank you for your replies!
Hi Vanessa, thanks for these important questions. I could extract clay minerals, but I were not yet able to identify them. My assumption for the Mg/Ca of (proto)dolomite is 0.8 to 0.6 as most dolomite is Ca-rich. Prior to this analysis, Ca-carbonates were removed by leaching with EDTA.

Submitted by Stephen Romaniello on
Hi Michael, Great talk! I was surprised that different strains of bacteria show such a large difference in how they fractionate d26Mg. In terms of mechanism, how do you think these microbes are interacting with the Mg to produce the observed isotopic fractionation? Thanks!
Dear Stephen, that’s a very good question. My working hypothesis is that microbially produced organic compounds attract Mg, which locally attains high concentrations and induces kinetic effects. Variations in magnitude of Mg isotope fractionation might thus relate to variable degrees of supersaturation and to variable net fractionation factors.

Submitted by Stephen Romaniello on
Hi Michael, Great talk! I was surprised that different strains of bacteria show such a large difference in how they fractionate d26Mg. In terms of mechanism, how do you think these microbes are interacting with the Mg to produce the observed isotopic fractionation? Thanks!


Submitted by Kimberly Lau on
Thanks for the interesting talk! I am also curious about the mineralogy of the clays from these cores, and also whether you have any ideas about whether they are Mg-clays or sorbing Mg from porewaters.
Hi Kim, thanks for the great question. I do not know the mineralogy of the clays yet. They appear to form authigenically and likely sorb Mg from pore water. This is suggested by the strong depletion in 26Mg in the water overlying the sediments in location KAAS-1 (see slide 12 of my presentation).

Submitted by Georgina Lukoczki on
Hi Michael, thanks for your great talk! Even though you mostly say "dolomite," I assume what you really mean is protodolomite, is that so? This is a really exciting study, and the objective to develop a method to differentiate microbial dolomite from non-microbial dolomite is very ambitious. In addition to the wide range of values you observed, I think a further difficulty would be that often microbially formed dolomites are overprinted by other dolomitization processes and I wonder if it was possible to differentiate these various types of dolomites if they are intimately overgrown? Another thing I wonder about is how progressive recrystallization of protodolomite into more stoichiometric dolomite might fractionate Mg isotopes potentially eliminating but at least obscuring the microbial signature in ancient dolomites. Thanks!
Dear Gina, thanks a lot for raising these important points. Correct, the predominant carbonate mineral is Ca-rich dolomite/protodolomite. It is well possible that early diagenetic recrystallization can obliterate signatures obtained in the primary dolomite forming process. This may depend also on water/rock ratios and the Mg mass balance in these sediments. I think that a separation of intergrown dolomite generations is hardly possible, especially in these sediments that also contain clay minerals as major Mg compartment.

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