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.


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Posted by: jurys
Posted on: April 21, 2006 07:52 AM

Very good site, congratulations!


Posted by: bob smith
Posted on: August 29, 2007 06:54 PM

Hi there

A great read and certainly makes ya sit up and take interest

looking forward to your next great post

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