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Earth History Through the Lens of Carbonate Diagenesis
Ahm A-S & Higgins J
Ahm A-S & Higgins J (2020) Goldschmidt Abstracts, 2020 21
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14b: Plenary Hall, View in program
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Dear Anne-Sophie, Thank you very much for this interesting talk! Do you think the deeper sediments in site 1003 never experienced this early diagenesis with increasing ?44/40Ca values and lower Sr/Ca ratios? Or did they go through the same early, fluid buffered diagenesis and then incorporated the lower ?44/40Ca values and the higher Sr/Ca later maybe from younger sediments above? My second question is: You showed the figure from Swart and Eberli (2005) where decreasing ?13C values were attributed to successive recrystallization of surface aragonite to more stable carbonate phases. Can that be applied to the Neoproterozoic carbonate samples from your data set so that the two aragonite end-members should have ?13C values of +15‰ and -‰ rather than +10 and -10‰? Thank you for your replies!
I think that the deeper sediments experienced less fluid-buffered early diagenesis and never experienced the same degree of resetting as the top of the sediment column does today (i.e. there has been a change in the degree of fluid-flow since the Miocene). Instead these older sediments recrystallized more slowly in rock-buffered settings and preserved their primary chemistry. Mainly because it takes a lot of fluid bringing in new Ca to alter Ca-isotopes of carbonates and that kind of fluid-flow is not observed deep in the sediment-column. For the second question: Swart and Eberli (2005) show a cross-plot of aragonite % versus d13C values. Interestingly, if you look closely at the figure, you will see that there are a bunch of samples with 0% aragonite that have d13C value of 5 per mill. These are essentially the sediment-buffered end-members that have completely recrystallized but kept their original d13C values. We interpret the Neoproterozoic carbonates similarly, i.e. the aragonite end-members that are sediment-buffered have retained their primary chemistry of approximately +10 and -10 per mill.
Hi Anne-Sofie, thank you for this great talk! I was wondering how well this proxy can be applied to successions that have undergone a more complex diagenetic history? Both in terms of early, near surface diagenesis, when the same sediment package might be placed into a fluid and sediment buffered diagenetic environment multiple times with changes in relative sea level, and also in terms of later, burial diagenesis, with ancient successions that have gone through multiple episodes of burial and uplift. Are these isotopic fractionation processes only significant enough when there is also a change in mineralogy, as in aragonite to calcite/dolomite or calcite to dolomite? So, as long as there is no mineralogical change, the coupling between the carbon and calcium isotopes along with Sr/Ca ratios can be used to identify fluid vs. sediment buffered diagenesis even in successions with complex diagenetic history?
I try to think about the diagenetic history in terms of the composition of the pore-fluid. For early marine diagenesis, Ca-isotope are heavy because seawater is heavy (0 per mill), however, for later stage diagenesis you may expect the pore-fluid composition to be different from seawater because there have been a ton of reaction with the wall rock in the subsurface. These two scenarios (early- vs late) are similar to the fluid- vs sediment-buffered end-members. In other words, the sediment-buffered carbonates have still recrystallized (no aragonite left) but they likely recrystallized later/deeper with less fluid-flow and retained their Ca-isotopes values of the precursor carbonates. For the second part of the question: It is easier for aragonite neomorphism because the range in d44Ca and Sr/Ca values is much larger than, for example, calcite-to-calcite. However, there is still a significant change in the Ca-fractionation during diagenetic calcite recrystallization and this framework therefore also works for other diagenetic reactions (e.g., Fantle and DePaolo, 2007). It can also be very informative to include Mg-isotope in this framework as an extra dimension to identify fluid- vs sediment-buffered end-members for dolomites.
I really enjoyed your talk, Anne-Sofie! My question is about fluid- vs. rock-buffered groupings. Do you see correlations with lithogenic characteristics with each type of alteration? Do you think there is a way to distinguish between temporal (e.g., time scale of fluid flow) versus transport-related factors that result in different "types" of diagenesis?
For the Trezona excursion, there seem to be a broad correlation between litho-facies and the groupings with deeper water deposition (thinner carbonate beds within siliciclastics) being more sediment-buffered. However, there is no obvious correlation between the physical characteristic of the rocks and the groups (e.g., they do not look more banged up when it’s fluid-buffered). In general, you need a lot of fluid-flow to reset Ca-isotopes during early diagenesis, whereas later stage resetting probably has less potential to alter Ca-isotopes (rock-buffered). These differences between early- vs late diagenesis will also be different for other isotope systems, such as d18O, that is very susceptible to later stage alteration.
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