Siliceous and Delicious

The New Schedule calls for working on the diatom diversity slash Eocene-Oligocene project today. What I really ought to do is provide the one-page chapter outline I said I’d write for Charles. I’ve also not heard back from Alroy yet, so perhaps I should resend that email to a different email address—I seem to remember finding several addresses for him.

So what is this diversity chapter going to be about? In the first instance, it’s about discovering the true diversity history of marine planktonic diatoms. It’s motivated by a desire to understand the history of diatom participation in the silica cycle, for which diatom diversity has been used as a proxy since Harper and Knoll (1975), see also Siever, Racki and Cordey, Lazarus et al. What needs to be addressed up front (I think) is that this is a very limited view of diatom participation in the silica cycle, because what matters in a biogeochemical sense is the export efficiency with which diatoms transfer silica from surface to deep waters, and more broadly, from ocean to sediment (see Treguer, 1995, or Sarmiento for example). This is going to be controlled by a number of factors, importantly the numerical abundance of diatoms, as well as their degree of silicification. Both of these factors could be quite plausibly decoupled from diversity—indeed silica export today is dominated by a small number of high-abundance, heavily silicified species living in the high latitude oceans of the southern hemisphere (need to check on reference for that). We need to therefore address both the degree of silicification and the abundance of diatoms separately. The former is something I am doing in my Chapter 4 projects, the latter needs to be addressed elsewhere (but I’m not sure where).

The diversity history of a clade is, of course, also interesting in and of itself as a key feature of its evolutionary history. But regardless for the motivation of examining it, interpreting raw tabulations of numbers of species known throughout geological time as a faithful record of actual diversity can, theoretically, be quite misleading, since differences in reported diversity can also be due to variations in sampling intensity or fossil preservation. A number of statistical methods have been applied to correct for such variations, beginning with simple rarefaction and culminating in more sophisticated methods such as those applied to the record of diatom diversity in Rabosky and Sorhannus (2009).

What I want to do is to apply Alroy’s new “shareholder quorum subsampling” to the diatom diversity data and compare the results to previously published subsampling exercises (Rabosky and Sorhannus, 2009).