(2020) Transient Ocean Oxygenation at 850Ma Recorded in δ98Mo of Microbial Carbonates
O'Sullivan E, Nägler T, Wille M, Kamber B & Turner E
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09f: Room 3, Thursday 25th June 23:45 - 23:48
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Can you specify your method of extracting Mo isotope signals from carbonate rocks? Do you think it is possible that the differences of Mo isotope values between carbonate rocks and shales are derived from experiments? Thanks!
Thank you for your question! I did not have time to explain this in my talk. The method I used was bulk digestion. After petrographic examination of the microbialites, I did not see any signs of Fe or Mn oxides, and XRD analysis showed that it was pretty much pure calcite with minor dolomite and quartz, and some organic matter (1-2%TOC range max, graphite). So I used an overnight hot HCl digestion on powdered sample, to leach all of the Mo from any remaining insolubles (none was obvious apart from graphite), and I then did a separate analysis of the same powder for major elements, trace elements and TOC so that I could monitor the Mo content of bulk rock vs Al content etc. My idea was that I would model and correct for detrital contamination. Unfortunately my models didn't work at all! I then repeated many of the samples and found that there were huge differences in Mo content and isotope composition despite being thoroughly milled (by hand to avoid contamination). I conclude that there is a mixture of different contaminants that may not have been visible under a microscope. I then tried to use these repeated samples to model the end members of the individual samples, as the direction of alteration to the composition was always roughly in the same direction. I found that the some of the pure carbonate end member calculations converged on around 1.8 permil or higher at a lower Mo content, but the second end member had a very high range of compositions, so it was likely a mixture of several sources. This is pretty rough work, I haven't had the time to work on it any more. The important point was that the Mo isotopes are always shifted towards lighter isotopes. I am not aware of a mechanism that favours concentration of heavy isotopes in the solid phase. Therefore the isotopically heaviest sample is the closest to coeval seawater, but is likely s minimum estimate. As for the shales, I do not trust the data due to the trace element characteristics, even though they agree well with previous estimates of proterozoic ocean water. As there has been evidence of an offset between euxinic black shale and overlying seawater in the black sea and in various other cases, I don't doubt the offset that I see between the black shales and the microbialites. My next step is to try a reductive dissolution of the microbialites to isolate the pure carbonate signature, and to analyse more black shales for Mo isotope compositions as well as Fe speciation, to better understand the environment. If you have any other questions, or if you have any suggestions, I welcome further email correspondence!
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