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(2020) Enhancement of Carbonate, Silicate, and Sulfide Weathering via Fluvial Sediment Abrasion

Scheingross J, Bufe A, Hemingway J, Hovius N, Schleicher A & Goldberg T


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10f: Plenary Hall, Thursday 25th June 23:45 - 23:48

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Submitted by Jotis Baronas on Saturday 20th June 14:42
Hi Joel, really cool stuff, nice to finally see some chem data from your experiments! A couple questions: 1. Which solutes specifically contribute to the increase in TDS? Do you know if this contribution is mostly in the colloidal fraction, or "truly" dissolved (say, <0.02µm)? If it's not colloidal, you could calculate weathering rates for your sediments. How do these compare to field- or batch-experiment dissolution rates for these types of rocks (or main minerals)? Seems like you are seeing super high rates! 2. Based on your abrasion vs diameter plot, <300µm particles don't abrade. I assume this depends on the flow velocity as well to some degree? In any case, does this mean we don't have to worry about abrasion in rivers where majority of sediment is already in the silt-fine sand fraction? Is there any data out there on abrasion in real rivers? 3. Would love to hear more about the carbonate weathering in the shale experiments! Cheers, Jotis
Hi Jotis - Thanks for your interest and thoughtful questions. Some answers below. 1a) All of our dissolved load measurements were on water filtered at 0.22 µm, so the measurements reflect the colloidal and 'truly' dissolved fractions. There's plenty of sample left, so we could go back and filter some samples at 0.02 µm to try to tease this out. 1b) In terms of which solutes are specifically contributing here, it varies a bit with lithology, but in for basalt experiments we're seeing mostly Ca + Mg (from trace carbonate I think) and a bit of Si, for granite, the trends seem to be driving by Si production, and for shale mostly Ca + Mg (from carbonates) and some sulfate. TDS reaches ~80-100 ppm in basalt and granite experiments, and ~140 ppm in shale experiments. 2a) Yes - the grain size where particles become viscously damped (i.e., the grain size at which they stop abrading) depends on the kinetic energy of particle impact (so both on the grain size, and the particle velocity, which, for suspended sediment, should approximately equal the flow velocity + turbulent fluctuations). The plot I showed in the talk is for a fixed flow velocity. Particles tend to be partly viscously damped for Stokes numbers (St) < ~100, and probably fully viscously damped for St < 20ish (these numbers are rough and I don't think the community has reached full agreement on them). A 300 micron particle in suspension in a river with velocity = 1 m/s will have St ~ 90, so it will be partly damped (meaning that it will still abrade a little bit, but viscous dampening softens the impact), whereas a ~60 micron particle would have a St ~ 20 and would probably not abrade at all. If all your suspended load is in the silt fraction, then for most rivers that is not likely to contribute too much to abrasion, but in a fast moving river fine sand may still be contributing. 2b) Data on abrasion in real rivers? None that I know of. Most of the geomorph community has thought about this in terms of the evolution of the bedload, so models/data of particle abrasion and downstream fining tend to focus on how bedload size is reduced moving downstream. I assume this is because the fine-grained abrasion products enter the wash load, and thus don't influence the physical evolution of the system. Of course, for those of us interested in the geochemistry, this is where all the action is. The best data published data that I can't point you to at the moment is that of Lizzy Trower (CU Boulder), she has a few (very cool!) papers on the balance between chemical precipitation and particle abrasion in setting ooid grain size (lab experiments and some field work in the Bahamas too). 3) Carbonate weathering in shale experiments - I'm still working on this and the data is a bit messy. I think what's happening is that there's a huge spike in carbonate weathering at the start of the experiment when sediment is placed in water and begins abrading. Within a day or less system then gets saturated with respect to CaCO3. However, sulfuric acid continues to be produced (there's lots of trace pyrite in the shale), and this allows for continued carbonate weathering at a lower rate for the rest of the experiment (i.e., sulfide oxidation is the rate limiting step for further carbonate weathering after the system becomes saturated).

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