Weathering Trends

I was tasked to characterize how bedrock was weathering in the Laurels Preserve in Chester County, PA. I collected samples for this weathering study by coordinating a drilling campaign that made a huge effort to keep the preserve as clean and undisturbed as possible. After being told that no drill rig would make it through the woods, I tracked down a Geoprobe Rotosonic (model 8140LC) mid-sized track-mounted drilling rig. 

 

The tradition in geology to characterize weathering is to calculate a "mass balance" of the elements or minerals in a rock formation. This method assumes that prior to weathering the rock had a somewhat consistent mix of elements prior to weathering and some of them stay in the material, even after the rock has weathered into soil. The method requires you to define what unweathered bedrock looks like so you can have a starting point for the mass balance. The Laurels Schist Formation proved to be extremely variable. 

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I asked several scientists in the weathering community how they would determine the depth to fresh, unweathered bedrock. I showed them showed my "mass balance" plots. Some said, "That's not a good place for a weathering study." I also heard, "Cut your losses and study something different." Since most of the earth is not made of homogeneous rock, I had stumbled on the need for a method to assess weathering in heterogeneous rock. 

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One evening at a gathering of friends, my friend Aaron, a statistician and academic, asked how my research was going. I told him I was pondering whether statistics could help me with my weathering problem. We forged a collaboration and produced an R package (available on GitHub) and a publication together.

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Fisher, B. A., Rendahl, A. K., Aufdenkampe, A. K., and Yoo, K. (2017) Quantifying weathering on variable rocks, an extension of geochemical mass balance: Critical zone and landscape evolution. Earth Surf. Process. Landforms, 42: 2457–2468. doi: 10.1002/esp.4212.