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(2020) The Role of Seafloor-Hydrothermal Activity as a Driver of Marine Anoxia

Davis Barnes B, Slack J, Hannington M, Planavsky N & Kump L


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14e: Plenary Hall, Wednesday 24th June 01:15 - 01:18

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 Benjamin Tutolo on Tuesday 23rd June 04:22
This is very, very cool! I know you said in your conclusions that you're going to be working on comparing this with the VMS rock record in the future, but I was wondering if you could give an educated guess as to what you expect for, say, Neoproterozoic time or other important intervals?
As I mentioned briefly, the record of VMS deposits extends back to the Archean, and generally the greatest cumulative mass is found in Paleozoic and Proterozoic units, so applying our model approach to deeper time is certainly an important focus. On one hand, the smaller reservoir of oxidants (O2 and SO4) means that even moderate VMS fluxes could have a significant, broad impact on the surface ocean – this was illustrated in the figure comparing surface ocean O2 for the same flux, but different atmospheric levels of O2. On the other hand, the most abundant reduced species in our modern VMS fluid estimate is H2S, and seafloor hydrothermal activity forming in low-SO4 oceans likely generated lower fluxes of H2S. So, whereas I'd hypothesize that VMS deposits will have a greater impact on the ocean redox state as we move to more ancient, low-O2 time periods (early Paleozoic, Neoproterozoic), we'll need to test certain assumptions about hydrothermal fluid composition in our model. Thanks for your question!

Submitted by Kenneth Bolster on Tuesday 23rd June 20:54
This is very neat stuff. If you were to consume all the oxygen in a particular region, the remaining sulfide could still be oxidized by nitrate in the water column. If you included that oxidation process as well, do you think you could still get sulfide accumulating in the same way?
This is an excellent question. Although not shown in the presentation, in cGENIE we can specify that once all of the local O2 is consumed in redox reactions, that next local NO3 and SO4 will be consumed to oxidize our modeled reductants. The modeled nitrate reservoir is relatively smaller than either oxygen or sulfate, but it can mitigate some of the sulfide in the water column. However, the modern level of nitrate alone is unlikely to oxidize all of the accumulating sulfide that our more extreme scenarios predict. Because of this balance, future model runs will focus on testing the sensitivity of developing water column anoxia and euxinia to nitrate reservoirs of various sizes, since the nitrogen cycle has evolved significantly over Earth history. Thanks for your question!

Submitted by Drew Syverson on Tuesday 23rd June 21:46
Great modeling! How would changes in seawater composition, such as seawater-SO4, have an effect on subseafloor sulfide precipitation and the overall flux of the reduced metals and gases into the overlying water column?
Seawater chemistry likely plays a significant role in modulating the impact of VMS fluxes. We've explored some of these relationships in GENIE results: for example, the SO4 pool will be important, because hydrothermal fluids injecting into low-SO4 oceans likely had lower concentrations of H2S. Conversely, for much of Earth's history, SO4 represents the largest oxidant pool in the ocean, and so fluxes of reductants in low-SO4 conditions could have a broader impact on the ocean redox state. As mentioned above, the nitrate chemistry of seawater will impact the oxidation of H2S, so making predictions about nutrient levels in the ancient ocean and the strength of the biological pump will also be vital to constrain the development of shallow-water anoxia related to these VMS fluxes. As a final example, seawater in Precambrian times may have had elevated iron content, which will certainly impact hydrothermal fluid scavenging. There are numerous geochemical factors which will limit the impact of our modeled VMS fluxes, but generally speaking, we predict that they will have a more significant effect on seawater redox state as we simulate more ancient conditions.

Submitted by Mingsong Li on Tuesday 23rd June 23:05
Hi Ben, great job! With VMS fluxes at only one site, ocean anoxia may be restricted to a single basin. Did you try to model multiple sites with VMS fluxes (which could happen :-) ) to produce a "global" anoxia event? That might be very fun.
Excellent question! So far, we've only displayed scenarios in which the benthic flux of reductants occurs at one grid square. What is more probable, especially when VMS deposits form in arc and back-arc settings, is that many smaller fluxes occurred over some boundary. Our cGENIE simulations are limited in terms of spatial resolution, but in future runs we will emplace several reduced fluxes along a continental margin and compare the associated dysoxia with the single-site simulations we presented here. I hypothesize that more spread out, smaller VMS fluxes will have a broader effect on modeled seawater O2 levels than a single large flux.

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