Abstract Details
(2020) Pore Water Conditions Drive Calcium Carbonate Dissolution in Reef Sands
Kessler A, Rogers A, Cyronak T, Bourke M, Hasler-Sheetal H, Glud R, Greening C, Meysman F, Eyre B & Cook P
https://doi.org/10.46427/gold2020.1284
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12c: Plenary Hall, Wednesday 24th June 22:12 - 22:15
Adam J. Kessler
View all 4 abstracts at Goldschmidt2020
Angus Rogers View abstracts at 3 conferences in series
Tyler Cyronak View abstracts at 2 conferences in series
Michael Bourke View abstracts at 2 conferences in series
Harald Hasler-Sheetal View abstracts at 2 conferences in series
Ronnie Glud View abstracts at 3 conferences in series
Chris Greening View all 5 abstracts at Goldschmidt2020 View abstracts at 2 conferences in series
Filip Meysman View abstracts at 7 conferences in series
Bradley Eyre View abstracts at 4 conferences in series
Perran Cook View abstracts at 4 conferences in series
Angus Rogers View abstracts at 3 conferences in series
Tyler Cyronak View abstracts at 2 conferences in series
Michael Bourke View abstracts at 2 conferences in series
Harald Hasler-Sheetal View abstracts at 2 conferences in series
Ronnie Glud View abstracts at 3 conferences in series
Chris Greening View all 5 abstracts at Goldschmidt2020 View abstracts at 2 conferences in series
Filip Meysman View abstracts at 7 conferences in series
Bradley Eyre View abstracts at 4 conferences in series
Perran Cook View abstracts at 4 conferences in series
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 Christopher Sabine on Tuesday 16th June 20:58
Nice talk...another factor that would influence the dissolution is the mineralogy of the sediments. Did you characterize the contributions of calcite, aragonite, magnesian calcite, etc. present in you sand? Was it the same for all your samples? You mentioned the importance of bacterial activity in micro-niches. If you sterilized the sediments before running your experiment, would you no longer have dissolution?
Hi Christopher Great questions. The sand at this site has been characterised a few times, but not in this study. The samples all came from one spot, so this wasn't really an issue. In general, it's about 2/3 aragonite and 1/3 HMC. The expected changes over the course of this experiment (hours) are negligible, so we would cannot monitor dissolution (or tell what is dissolving) this way. Similarly, the changes in Ca2+ and Mg2+ in the pore water are too small to measure. I would expect not to have dissolution if the sediments were sterilised, but we did not do this. This would be a nice experiment to do, though. I'll have to go back to Heron Island to give it a try :-)
Nice talk...another factor that would influence the dissolution is the mineralogy of the sediments. Did you characterize the contributions of calcite, aragonite, magnesian calcite, etc. present in you sand? Was it the same for all your samples? You mentioned the importance of bacterial activity in micro-niches. If you sterilized the sediments before running your experiment, would you no longer have dissolution?
Hi Christopher Great questions. The sand at this site has been characterised a few times, but not in this study. The samples all came from one spot, so this wasn't really an issue. In general, it's about 2/3 aragonite and 1/3 HMC. The expected changes over the course of this experiment (hours) are negligible, so we would cannot monitor dissolution (or tell what is dissolving) this way. Similarly, the changes in Ca2+ and Mg2+ in the pore water are too small to measure. I would expect not to have dissolution if the sediments were sterilised, but we did not do this. This would be a nice experiment to do, though. I'll have to go back to Heron Island to give it a try :-)
Submitted by Andreas Andersson on Tuesday 23rd June 03:36
Very nice and interesting talk! I have a couple of questions: 1) What is your reasoning that the transition from net calcification to net dissolution at Omega-Aragonite of ~2 does not represent the apparent solubility of the most soluble bulk mineral phase in the sediments opposed to dissolution within microenvironments in your reactor? 2) Under your anoxic experimental conditions, how did you distinguish between alkalinity produced from CaCO3 dissolution vs. alkalinity from anaerobic redox processes?
Hi Andreas, thanks for your questions. You are correct that the sediment contains a lot of high magnesium calcite (HMC), which is more soluble. In our paper, we also present these figures plotted against the HMC solubility, and it is generally at least 1.5 We measured Fe and S reduction to correct for these processes (their contribution is negligible under most circumstances: the Fe content of the sediment is low, and sulfate reduction takes at least a day of prolonged anoxia to kick in). For more information on this point, see Tess Hutchinson's talk (https://goldschmidt.info/2020/abstracts/abstractView?id=2020003679), or the paper associated with this presentation (doi.org/10.1016/j.gca.2020.04.001)
Very nice and interesting talk! I have a couple of questions: 1) What is your reasoning that the transition from net calcification to net dissolution at Omega-Aragonite of ~2 does not represent the apparent solubility of the most soluble bulk mineral phase in the sediments opposed to dissolution within microenvironments in your reactor? 2) Under your anoxic experimental conditions, how did you distinguish between alkalinity produced from CaCO3 dissolution vs. alkalinity from anaerobic redox processes?
Hi Andreas, thanks for your questions. You are correct that the sediment contains a lot of high magnesium calcite (HMC), which is more soluble. In our paper, we also present these figures plotted against the HMC solubility, and it is generally at least 1.5 We measured Fe and S reduction to correct for these processes (their contribution is negligible under most circumstances: the Fe content of the sediment is low, and sulfate reduction takes at least a day of prolonged anoxia to kick in). For more information on this point, see Tess Hutchinson's talk (https://goldschmidt.info/2020/abstracts/abstractView?id=2020003679), or the paper associated with this presentation (doi.org/10.1016/j.gca.2020.04.001)
Submitted by Abby Lunstrum on Thursday 25th June 19:45
Thank you for the great talk (and corresponding paper). I enjoyed them both! I had a question about your observation of no CA effect on dissolution. I wonder if this could be from other mechanisms, like the CA clumping together, adsorbing to something else, or otherwise not being able to make contact with the grains, that aren't necessarily related to transport affecting dissolution. It seems like we still have a lot to learn about CA:dissolution behavior in the environment!
Hi Abby; thanks for your question! What you suggest is - of course - possible. And I agree that we still have a LOT to learn. This is very much the first step towards approaching this pore-water scale question, and our interpretations are by no means the final word on this! The CA definitely dissolves (that's easy to see). We know that in well-mixed experiments, CA enhances CaCO3 dissolution. My personal favourite example is Adam Subhas's work, which I first saw presented at the Prague Goldschmidt in 2015 and which was what first got me thinking about this topic. As a start, check out his 2017 PNAS paper. Similarly, we don't think that it adsorbs, largely because otherwise *nobody* would see an effect!
Thank you for the great talk (and corresponding paper). I enjoyed them both! I had a question about your observation of no CA effect on dissolution. I wonder if this could be from other mechanisms, like the CA clumping together, adsorbing to something else, or otherwise not being able to make contact with the grains, that aren't necessarily related to transport affecting dissolution. It seems like we still have a lot to learn about CA:dissolution behavior in the environment!
Hi Abby; thanks for your question! What you suggest is - of course - possible. And I agree that we still have a LOT to learn. This is very much the first step towards approaching this pore-water scale question, and our interpretations are by no means the final word on this! The CA definitely dissolves (that's easy to see). We know that in well-mixed experiments, CA enhances CaCO3 dissolution. My personal favourite example is Adam Subhas's work, which I first saw presented at the Prague Goldschmidt in 2015 and which was what first got me thinking about this topic. As a start, check out his 2017 PNAS paper. Similarly, we don't think that it adsorbs, largely because otherwise *nobody* would see an effect!
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