Silica, diatoms, and grasses

Dear Phinches, Dear All,

I must confess to be astounded (and very pleased) that my little article was noticed, even more so that it was read all the way to the end! I hope this doesn't mean somebody will notice that the latest PalAss newsletter contains pretty much the identical text... Anyhow, I've thought a little bit about the role of plants in general, phytoliths and grasses in particular, in the silica cycle, and am only too happy to take the chance to share my thoughts and ask for your feedback. The short answer is that I think grassland expansion might change the delivery mode of silica to the oceans from a "steady trickle" to a "pulsed" pattern. I am not convinced that grassland expansion could actually increase the steady-state (or time-averaged) supply rate to the ocean. Let me explain why:

Plants can increase silicate weathering locally, and thus the supply of dissolved silica to the oceans, by a number of processes -- mechanical disaggregation of rock by roots, carbon dioxide pumping to the soils, acidity from humic and other organic acids, and so on. This is a pretty well established idea, and Berner used this in his GEOCARB model of carbon dioxide over time to argue for a decrease in atmospheric carbon dioxide with the rise of land plants in the Devonian. An important point to note, though, is that the silicate weathering rate over geological timescales is controlled (as far as we know) by the Walker feedback -- the temperature-silicate weathering-carbon dioxide feedback -- and plants can only really affect the weathering rate *constant*, not the rate itself. If you like, the feedback stabilizes pushes and pulls on a steady state -- the rate constant (cue plants) affects the tightness of the spring which couples push or pull to the system. If you haven't read the Walker paper, I highly recommend it (Walker, Hays, and Kasting, 1981, JGR vol. 86, 9776-9782). The key point, though, is that this has nothing to do with grasses and phytoliths -- as far as I know, there is no palpable difference between grasses and other plants in the way they disaggregate rock, produce humic acid, etc. to affect silicate weathering rates.

Now, the part that singles out grasses in the silica cycle is indeed the fact that grasses make phytoliths. Some folks have proposed that the evolutionary rise of grasslands to ecological dominance has thus increased the supply of silicic acid to the oceans; an example is the paper on modern plankton evolution by Falkowski and peeps (2004, Science vol. 305, 354-360). The idea behind it all is that grasses accumulate huge amounts of silica (up to around 10% dry weight) in various tissues, a lot of it as phytoliths (little amorphous silica shapes), much more than most other plants today (although some, like the horsetails, accumulate even more). This is, of course, fascinating for its own reasons from the perspective of vertebrate grazer evolution (or coevolution), which Phil can no doubt confirm, but that's beside the point here. The point that is frequently made regarding the silica cycle is that this phytolith silica is of a more soluble form than the silicate minerals from which silicate weathering supplies silicic acid (in part mediated by the processes listed at the start of the last paragraph). This is true -- amorphous silica is at least an order of magnitude more soluble than most silicates. If plants are producing this more soluble form, the dissolution of phytoliths will supply more silicic acid to the oceans (all the time via rivers, of course). However, I have a problem extrapolating from the solubility observation to the idea that the rate of supply can increase. Consider the following simple depiction of the system we're looking at:



Here, the rate of supply to the oceans is F[out]. You can immediately see that, unless the masses of the soil water or phytolith reservoirs change (i.e. violating steady state), F[out] must equal F[weathering]. Now, the size of the global phytolith reservoir is not known, but my own ball park estimate is that it's on the order of 1000 to 10,000 Tmol. Currently, F[out] is around 5 Tmol per year. I take the scenario in which phytoliths formation increases dissolved silica supply to the oceans to mean that F[out]>>F[Fweathering]. If you imagine, for argument's sake, the extreme case where F[weathering]=0, you would empty the phytolith reservoir in, at most, 2,000 years... Clearly, this can't be a prolonged source of extra silica.

What *could* happen, of course, is that the phytolith reservoir acts to modulate (here's where my word choice in the article comes in!) F[out]. What I mean here is best explained by analogy -- imagine a river with a dam (in this analogy, water=silica!). At steady state, with the sluice gates wide open, the river flux above the dam (F[weathering]) is the same as the flux out of dam(F[out]). Now, if you close the dam, either completely for a short time, or a little bit for a longer time, water will build up behind the dam. Eventually, you'll reach another steady state -- either when the water spills over the top of the dam (if it's all closed up), or if the higher water level is pushing the water out faster. Either way, you'll be back to F[weathering]=F[out]. Now imagine suddenly opening the dam back up: for some time, you will have F[out] >> F[weathering], but only until Lake Meade has run dry. Jumping back from the analogy to the silica world now, you could imagine this sort of thing happening on glacial-interglacial timescales (this is Conley's idea now, from his 2002 paper in GBC vol. 16 section 68). Entering into a glacial, silica held in biomass might be released as the climate cools, resulting in a pulse of silica to the oceans, and a period of direct inputs from F[weathering] without going through the phytolith reservoir. In interglacials, inputs might be more indirect. As a lush, grassy vegetation builds up a large phytolith reservoir, most of the F[out] will be sourced from phytolith dissolution, but F[out] would actually have to be smaller than F[weathering] during the accumulation of the reservoir.

Does any of this make sense? I would love to hear your input, feedback, and ideas, and really appreciate your interest!

Best wishes from sunny Cambridge (MA),
- Ben.