I realized that I haven't updated my faithful readers on my dissertation topic quest since April. I've honed in on the terrestrial silica biogeochemical cycle as the topic. Namely, I'm interested in silica deposition, storage, and turnover rates for various soil environments (past and present). With that in mind, here is a new list of possible parts (or chapters) of my dissertation.
1. The effect of sediment mass accumulation rates on phytolith accumulation rates. I’ll compare times of pedogenesis to times of drought. There may actually be more phytoliths entrained in droughty times, due to rapid burial. I've already blogged about this one, and you can see my grant proposal on this subtopic here. Thus, I won't beat a dead horse.
2. Quantify silica (Si) dissolution rates in paleosols. The Si dissolution rate for any soil is pretty easy to calculate: it’s simply the total Si deposition rate (from plant litterfall) minus that which is stored in the soil long-term as phytoliths, and that which is exported out of the soil (by various means). The amount stored in a paleosol is easy to calculate: it is simply the amount present now. The total which was deposited originally is a bit harder to estimate. To do this, I’ll use a modern analog. In other words, I'll try to determine what the conditions were like when the paleosol was formed, and then find an area which has similar conditions today. To determine the modern analogs, I’ll use pollen reconstructions. I’ll need to get two things from the pollen reconstructions: 1) the paleoclimate, which will help me zero in on modern areas with similar climate; and 2) the local vegetation composition. I can also use the phytolith assemblages to determine vegetation composition. This one is important, because species can have differing phytolith production rates. Once the modern analogs are selected, I can use them to measure net primary productivity (NPP) and the phytolith production rate. This only leaves phytolith export to determine, which should be doable as well. Overall, if the dissolution rate can be determined for a paleosol, it should make paleoreconstructions that much more accurate.
3. The effect on soil particle size on Si accumulation/dissolution rates (sand vs. silt vs. clay). There may be more leaching of Si in sandy soils due to higher rates of water infiltration. The dissolution rate may be higher due to this as well. Thus, the Si cycle may be much faster in sandy soils.
4. The effect of soil grain size also has broader implications on the Si turnover rate: does vegetation growing in Si poor soils have a lower phytolith production rate? This one seems kind of obvious, but it may be that most of the Si is simply entrained in the biomass. That which is deposited as litterfall may be quickly recycled back into the biomass. Thus, areas with less overall Si in the terrestrial Si pool (soil, litterfall, and biomass) may have quicker turnover rates.