(2020) Diagenesis of Benthic Foraminifera: Fluid Penetration and Isotopic Exchange Visualized with NanoSIMS
Cisneros-Lazaro D, Adams A, Guo J, Baumgartner L, Bernard S, Daval D, Baronnet A, Grauby O, Vennemann T, Stolarski J & Meibom A
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14b: Plenary Hall, Monday 22nd June 22:12 - 22:15
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Dear Deyanira, Thank you for your talk and the beautiful NanoSIMS images! You say that there is a species-specific susceptibility to fluid penetration but the role of organics is still unclear. I think, the treatment with H2O2 and the impact on the shell/skeleton is also species-specific and dependent on the density of suture zones and organic layers. In A. lessonii, there are many more suture zones that are leached by H2O2 and then you have much more mineral surface available for isotopic exchange. But despite an obvious species-specific susceptibility to fluid penetration, would you say that oxidative erosion similar to H2O2 treatment is a significant process in marine porewaters and with that exclude species like A. lessonii per se for future analyses? Thank you for your reply!
Thank you for your detailed questions Vanessa. I think you are correct in thinking that there will also be a species-specific response to oxidative erosion in nature or oxidative cleaning in the lab, not only because of the varying densities of suture zones and organic linings, which will lead to a greater or lesser surface area with which to exchange, but additionally because organic linings across species can have a lower or greater susceptibility to degradation. For example the organic lining of Ammonia tepida has been reported to be especially robust (Fhlaithearta et al., 2003) and might further contribute to the lack of differentiation in isotopic enrichment between the two cleaning procedures in our study. I don’t believe this means we should exclude certain species high surface area species, like A. lessonii, from palaeoceanographic analyses, but that we should be aware that different species, because of their specific ultrastructural construction, might be biased to different extents and further experiments are needed to quantify this.
Very interesting results! I wanted to ask if you have any hypotheses for why the suture zones seem to spatially concentrate the 18O enrichments and what the process is surrounding the surface area penetration. Do you think there are higher surface area to volume ratios here so that it is easier to diffuse more into the foram? Thanks!
Thank you for your question Kimberley. From the TEM images, we have shown that the suture zones represent domain boundaries between single crystal domains. The misorientation between these domains is around 8 degrees and one possible explanation we are exploring is that the increased strain-energy between these domains might provide some additional driving force for concentrating isotopic enrichments there, or it is possible that as you say, along these suture zones it is indeed the very high surface area along which it is easy to exchange isotopically. As to the process, these experiments lasted 6 days, and the speed and extent of exchange is leading us to consider two processes through which this can occur, dissolution-precipitation or solid-state diffusion along grain boundaries.
Excellent presentation. Have your or anyone else thought about conducting exchange experiments at different times and temperatures to try to develop an Arrhenius equation that could be used to determine exchange rates at seafloor temperatures?
To respond briefly to your question Ethan, yes, we have already started with similar experiments conducted at lower temperatures as well as for different lengths of time.
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