Abstract Details
(2020) Stable Ca and Sr Isotopes Indicate Biocalcification Crisis during OAE1a
Wang J, Jacobson A, Sageman B & Hurtgen M
https://doi.org/10.46427/gold2020.2734
The author has not provided any additional details.
14d: Plenary Hall, Tuesday 23rd June 00:42 - 00:45
Jiuyuan Wang
View abstracts at 3 conferences in series
Andrew Jacobson View all 5 abstracts at Goldschmidt2020 View abstracts at 4 conferences in series
Bradley Sageman View abstracts at 8 conferences in series
Matthew Hurtgen View all 2 abstracts at Goldschmidt2020 View abstracts at 12 conferences in series
Andrew Jacobson View all 5 abstracts at Goldschmidt2020 View abstracts at 4 conferences in series
Bradley Sageman View abstracts at 8 conferences in series
Matthew Hurtgen View all 2 abstracts at Goldschmidt2020 View abstracts at 12 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 Boehm Florian on Monday 22nd June 15:54
The correlations between calcium and strontium isotopes is really remarkable! As far as I know, this correlation was so far only observed in calcite. The material you measured is preserved as calcite, of course, but being shallow water deposits at least some part of it may originally have been precipitated as aragonite. Do you think the rate effects reflect the rate of recrystallization to calcite or is it really primary precipitation of calcite phases? What did you sample, ooids, marine cements, biogenic shells...? Did you do microsampling or measure bulk samples?
Dear Dr Böhm, thank you for your insightful questions. We think the kinetic effect reflects the primary precipitation rate. This is because the precipitation rate of recrystallized calcite is usually very slow (at/near equilibrium) and the stable Ca isotope fractionation during recrystallization are very close to ~0 (e.g., Fantle and Depaolo, 2007; Jacobson et al., 2008). One of your papers (Böhm et al 2012) also suggests that Sr isotope fractionation would be very close to zero at equilibrium. Recrystallization would also lose Sr. If our samples are partially recrystallized, then it should represent a mixing signal between seawater (recrystallized phase) and primary calcite (fractionationed phase), and we may miss the correlations with the “kinetic effect” slopes in both d44Ca vs Sr and d44Ca and d88Sr. If our samples are completely recrystallized, then it would represent seawater signals, which is also unfeasible given the magnitude of Ca and Sr isotope variations. Interestingly, we found that diagenesis in our K-Pg bivalve samples shifts d44Ca in another direction, yet they also exhibit a similar secular pattern. Our samples are dominated by packstone-wackstones (with lots of skeletal fragments). And an oolitic grainstone layer coincides with the negative d44Ca and d88Sr excursions. We didn’t perform micro-drilling but measured bulk samples. In fact, our results show that the kinetic effect of Ca and Sr fractionations that you and other researchers discovered through experiments could also be observed in natural settings, which I think is pretty cool. Again thank you! And let me know if you have further questions. You can reach me at jwang@earth.northwestern.edu
The correlations between calcium and strontium isotopes is really remarkable! As far as I know, this correlation was so far only observed in calcite. The material you measured is preserved as calcite, of course, but being shallow water deposits at least some part of it may originally have been precipitated as aragonite. Do you think the rate effects reflect the rate of recrystallization to calcite or is it really primary precipitation of calcite phases? What did you sample, ooids, marine cements, biogenic shells...? Did you do microsampling or measure bulk samples?
Dear Dr Böhm, thank you for your insightful questions. We think the kinetic effect reflects the primary precipitation rate. This is because the precipitation rate of recrystallized calcite is usually very slow (at/near equilibrium) and the stable Ca isotope fractionation during recrystallization are very close to ~0 (e.g., Fantle and Depaolo, 2007; Jacobson et al., 2008). One of your papers (Böhm et al 2012) also suggests that Sr isotope fractionation would be very close to zero at equilibrium. Recrystallization would also lose Sr. If our samples are partially recrystallized, then it should represent a mixing signal between seawater (recrystallized phase) and primary calcite (fractionationed phase), and we may miss the correlations with the “kinetic effect” slopes in both d44Ca vs Sr and d44Ca and d88Sr. If our samples are completely recrystallized, then it would represent seawater signals, which is also unfeasible given the magnitude of Ca and Sr isotope variations. Interestingly, we found that diagenesis in our K-Pg bivalve samples shifts d44Ca in another direction, yet they also exhibit a similar secular pattern. Our samples are dominated by packstone-wackstones (with lots of skeletal fragments). And an oolitic grainstone layer coincides with the negative d44Ca and d88Sr excursions. We didn’t perform micro-drilling but measured bulk samples. In fact, our results show that the kinetic effect of Ca and Sr fractionations that you and other researchers discovered through experiments could also be observed in natural settings, which I think is pretty cool. Again thank you! And let me know if you have further questions. You can reach me at jwang@earth.northwestern.edu
Submitted by Yuexiao Shao on Monday 22nd June 23:36
Why is d88/86Sr less affected by diagenesis than d44/Ca? I assume it's because the mass of Sr is much bigger than Ca... So how much can diagenesis affect d88/86Sr?
Dear Yuexiao, Great question! You are right that Sr is much heavier than Ca, so the fractionation of Sr is much smaller than that of Ca. And also… Unlike Ca, Sr is a trace element in calcite. Recrystallizing sediments often experience net Sr loss. Sr isotope equilibrium with pore fluids occurs during recrystallization, but this effect only shifts carbonate 87Sr/86Sr by 10-50ppm (e.g. Richter and DePaolo, 1988; Richter and Liang, 1993). The d88/86Sr contrast between pore fluid and carbonate is relatively small (because of smaller fractionation than Ca isotopes). I did some simple mixing calculations: with a 50ppm 87Sr/86Sr shift in sediment, the d88/86Sr of bulk carbonate only varies on the order of ~0.01 permil, which is below the precision of our measurements. Similar field observation has been reported by Voigt et al., 2015. Again, thank you for your question! Please let me know if I can help you with any more questions. You can reach me at jwang@earth.northwestern.edu
Why is d88/86Sr less affected by diagenesis than d44/Ca? I assume it's because the mass of Sr is much bigger than Ca... So how much can diagenesis affect d88/86Sr?
Dear Yuexiao, Great question! You are right that Sr is much heavier than Ca, so the fractionation of Sr is much smaller than that of Ca. And also… Unlike Ca, Sr is a trace element in calcite. Recrystallizing sediments often experience net Sr loss. Sr isotope equilibrium with pore fluids occurs during recrystallization, but this effect only shifts carbonate 87Sr/86Sr by 10-50ppm (e.g. Richter and DePaolo, 1988; Richter and Liang, 1993). The d88/86Sr contrast between pore fluid and carbonate is relatively small (because of smaller fractionation than Ca isotopes). I did some simple mixing calculations: with a 50ppm 87Sr/86Sr shift in sediment, the d88/86Sr of bulk carbonate only varies on the order of ~0.01 permil, which is below the precision of our measurements. Similar field observation has been reported by Voigt et al., 2015. Again, thank you for your question! Please let me know if I can help you with any more questions. You can reach me at jwang@earth.northwestern.edu
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