Archives for the Month of February 2006 on Ramblings of a Geologist

Climate Change: the role of Silicate weathering

Scientific papers are often torturously technical -- square and dry as toast -- so to understand what they're trying to say I often have to translate these texts into something I can see or imagine.

I have been slogging through several such papers this weekend in order to write a research proposal for my masters at Cambridge next year. One of my more important "imagination translations" (central to the questions we will be trying to answer) is described below:

I am standing outside in the rain, on a rocky coast, facing the ocean. The rocks beneath my feet are typical of Earth's crust, roughly granitic and containing lots of silica. For kicks let's call it anorthite (for you chemists, that's CaAl2Si2O8). I breathe out, adding some more carbon dioxide (CO2) to the air. This atmospheric CO2 is absorbed by the grasses around me, which produce organic acids. It is also absorbed by soil waters, although I'm not sure how, and becomes carbonic acid. The rain pounds down and a river next to me gushes into the bay ahead, washing acid from soil and grass into the ocean.

Or, if that's too artsy for you, just read the chemical equation:

2CO2 + 3H2O + CaAl2Si2O8 = Ca+2 + 2HCO3- + Al2Si2O5(OH)4

This reaction supposedly provides the feedback that regulates climate over geologic time and "maintains equable climatic conditions on Earth" (West, 2005). I have yet to truly understand why (I know it must have to do with regulating CO2 in the atmosphere, which influences global temperatures among other things), but I'm working on it.

This Si weathering feedback is not a new idea, and lots and lots of work has been done on it. I could probably explain most of the process to you in detail if I sat down and read papers for a week. What I may actually be working on at Cambridge has to do with improving our ability to use a relatively new proxy to understand past and present climate. This proxy (science lingo for "data that measures something indirectly") is a ratio of elements: Germanium and Silica (Ge and Si). As elements, they are very similar and Ge often substitutes for Si in mineral lattice sites (which means, they're about the same size/charge and sometimes you can switch one for the other, like apples and oranges). Ge and Si have similar cycles and distributions in the ocean, BUT there are certain forces/conditions/events that can upset their usual balance. Like ice ages. Previous studies have found that there is less Ge relative to Si in glacial oceans than interglacial oceans.

There are lots of different examples of deviations from the "normal" ratio (defined as amounts measured in the ocean today), and lots of ideas about WHY and HOW this "fractionation" happens. My project will probably investigate either weathering on the continent, or sedimentation on the seafloor. I'm interested in physical/chemical weathering mechanisms and how they impact the Ge/Si ratio because once we can quantify that, we'll be better able to understand climate regulation. I'm also interested in what happens to Ge and Si during deposition and burial, because that will help us understand the sedimentary record. It's great if we can put together a good story for how modern cycles work, but if we want to understand the past, we also have to know what happens to these sediments after they're deposited. Understanding modern cycles without knowing about post-depositional alterations is like knowing how to read but not being able to uncrumple a mashed piece of paper.

The oceanic sedimentary record is like a big mish-mashed encyclopedia written in a hundred different inter-connected languages all at once, with volumes stacked one on top of another for millenia. Essentially de-coding those volumes and teasing out the stories hidden within can tell us a lot about how the Earth worked in the past and how it will work in the future. Specifically, understanding the Ge/Si ratio might give us more insight into the role of weathering/erosion in global climate, and also help us predict how the Earth will respond to rising levels of atmospheric CO2.

To be honest, I'm pretty excited about it, although I still need to do a lot of work (and read a lot of toasty-dry papers) before I can put together a research plan.

Through the Mud ...

I have worked for about 5 months to pull information out of my Lake Erie sediment cores so carefully, methodically; now I am sitting here with a huge pile of data and I don't know what to do with it. I feel like a squirrel that's just worked its tail off gathering acorns and has just discovered that the whole pile won't fit in its cheeks at once. Maybe that's not the greatest analogy for it, but I've been studying Geology for the past 4 years, not English, so maybe I can get away with it.

In any case, I am trying to make sense of my data, alternating between analyzing little bite-sized pieces and stepping back, blurring my vision a bit, to get a feel for the ever-elusive "big picture." I've got a smorgasboard of data to mix and match: grainsize, magnetic susceptibility, water content, biogenic silica, carbonate content, stable isotopes, radiocarbon dates, ostracodes, fingernail clams, and diatoms ...

The biggest problem with paleoclimate reconstruction (essentially, trying to predict the past) is that even if you find a beautiful, clearly-defined trend in your data, you can't be quite sure what it means. There are several factors that influence a certain proxy in a certain way, and so at first you can't tell whether it was Influence A or Influence B or a combination of the two. Or a completely new and never-before-thought-of Influence C.

Interpretation of isotopes is particularly tricky for me at the moment. Not just because the relative influences on this proxy are hard to understand, but also because the literature (meaning, scientific papers written by other people all over the world on this topic) actually contradicts itself. Whom to believe? Now I have to go deep -- actually delve into methods, data tables, and other snarly details -- and decide who I think is right! Or come up with my own ideas ... what a thought ...

I suppose I should be excited about this, because that is exactly what science is all about! It's a little frustrating, all the same.