What is the Critical Zone?

Critical Zone science is a new field of science that links together many existing fields.  It studies water, air, ecosystems and hard rock geology but it is not soil science. However, Critical Zone science is built upon many of the same ideas and principles of soil science. What makes Critical Zone science different is that it focuses on how rocks, soils, and plants help sustain life, especially humans, while soil science has a much broader focus on soils. In addition, Critical Zone science is different because it puts soils into a much deeper context, with respect to depth and time. So is it really a new field of science and do we need the Critical Zone concept?

Daniel Richter and Anna Wade examining a soil pit at the Calhoun CZO in South Carolina. Photo by Justin Richardson.

For those who are regular readers of Soils Matter, you know that soil scientists describe the soil-forming process based on the Climate, Organisms, Relief, Parent Material and Time (or, CLORPT for short! For more on CLORPT, read this Soils Matter blog).

Critical Zone scientists are focusing a bit differently on their research than general soil scientists. In 2001, the National Research Council added a definition of the Critical Zone. And, it’s this small change in focus that is leading to new concepts that are important to the role soils, and their parent materials, play in sustaining life.

To study soils, soil scientists dig pits, revealing a “soil profile.” Many pits go several feet deep – until we hit the parent material. Critical Zone scientists are digging and drilling much deeper than just the soil profile. This is because new research shows that what is happening to the parent material can later affect soil formation, and the soil itself. Think of it as looking at your genetic background through a genetic test!

In some cases, they are listening to deep rocks. Critical Zone scientists have implemented geophysics to listen to sound moving through rock to learn about how crumbly or intact it is. An important study of the Critical Zone¹ showed that stresses on bedrock on hillsides help control how fractures are created. In other words, the shape of a hill and the internal stresses within bedrock plays a role in producing material that will one day become soil. This challenges the general idea that soil production is dominated by what happens when rocks are exposed to the surface.

Critical Zone scientists discussing the Elder Creek next to a large river-incision at the Eel River critical zone in northern California. Photo by Justin Richardson

Critical Zone scientists recognize that deep weathering can be as important as weathering in soils. We like to think weathering is primarily occurring in soils because of organic acids and plant roots. While is generally true, bedrock weathering can also be very important for groundwater chemistry and the production of soils. The Shale Hills Critical Zone Observatory conducted a study with others that showed that bedrock weathering is happening as deep as 23 feet below the surface². This is 20,000 leagues deeper compared to most 3 or even 6 foot soil pits!

At this deep depth, pyrite, a mineral made of sulfur that can produce rotten egg smells, can create sulfuric acid during weathering. The sulfuric acid accelerates bedrock weathering. Pyrite isn’t stable up closer to the soil surface; these findings have important impacts in future research of soil formation.

Critical Zone scientists are even taking soil science to new dimensions of time. Many soil scientists are concerned with the loss of soils. They focus on erosion control and best practices to prevent further soil loss, which is an important factor in making sure that the soil we have can help us grow our food, clean our water, and all the other “jobs” soils do for the earth.

Critical Zone scientists are researching another side to this equation: how fast rocks can generate soils. Since we do not have time machines to speed up things like weathering, Critical Zone scientists have focused their efforts on formulating models to predict soil generation. Research at the Catalina-Jemez Critical Zone Observatory is tackling this question³. They are comparing research sites in southern California, New Mexico, and Arizona. They are monitoring several sites to see if they can find ways to predict whether carbon, water and other variables influence deep weathering.

Critical Zone science and soil science may have a few differences but they are very similar. Although we approach our research a bit differently, we are all working to protect soil as an important natural resource. Learning more about soil formation – and going back in the process – will lead to new discoveries that we may eventually apply to other planets!

Answered by Justin Richardson, University of Massachusetts, Amherst

Interested in more? Read about research here: Critical zone, critical research.

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1. Clair J, et al 2015) Geophysical imaging reveals topographic stress control of bedrock weathering. Science 350:534–538. doi:10.1126/science.aab2210
2. Brantley, S.L., et al 2013. Probing deep weathering in the Shale Hills Critical Zone Observatory, Pennsylvania (USA): the hypothesis of nested chemical reaction fronts in the subsurface. Earth Surface Processes and Landforms, 38(11), pp.1280-1298
3. Chorover, J. et al 2011. How water, carbon, and energy drive critical zone evolution: The Jemez–Santa Catalina Critical Zone Observatory. Vadose Zone Journal, 10(3), pp.884-899.