Exploring Soil Development & Conservation of Mass

Clifford Riebe
University of Wyoming
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This activity aims to develop understanding of soil formation using the principles of conservation of mass. This is best applied to a chronosequence of ≥ 2 soils/deposits, such as a set of terraces or glacial moraines. The deposits should differ as much as possible in age and appearance. The "type" deposits for this exercise are Wind River moraines (Taylor and Blum, 1995) that range in age by a factor of roughly 1000. Hypotheses for how soil composition should behave in the two scenarios (i.e., eroding and non eroding) are developed in class prior to the activity using conservation of mass. Field observations and geochemical data from the sites are used to test the validity of assumptions and assess whether data are more consistent with one hypothesis or the other. Results are delivered to the instructor in a final report, which has been revised after peer review by a classmate.

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upper division course in geomorphology
in a slightly extended form this would also work as a segment of a summer field camp

Skills and concepts that students must have mastered

basic math, general geology, mineralogy (optional). Pre-exposure to surveying basics is a must. Students will also need pre-exposure to lectures in conservation of mass, erosion, and chemical weathering.

How the activity is situated in the course

A weekend field trip, or two one-day trips (depending on logistics) plus laboratory exercises, building up to a written report. This might work well as a final project, at the end of the term.


Content/concepts goals for this activity

conservation of mass principles
residence time of soils and individual minerals within it
focused discussion of processes (which will be
region-specific, depending on site selection)

Higher order thinking skills goals for this activity

the main goal of the exercise is to give students context for critical thinking about conservation of mass and residence time in surface processes

a complimentary goal is to develop skills in hypothesis development and testing

Other skills goals for this activity

hands-on familiarity with surveying methods
taking field notes
introduction to geochemical analysis methods
reports: writing, reviewing and revising

Description of the activity/assignment

How do soils develop over time? Perhaps the best place to learn is a across a chrosequence of deposits that span a wide range in age (and appearance). If we can identify parent material and measure its chemical composition, it can be used as a benchmark for comparison with the chemical composition of soils that were formed from it. This enables us to quantify the degree of chemical depletion. In general we expect older soils to be more depleted, all else equal. But older soils might also be subject to greater physical erosion, in addition to chemical weathering. This complicates the assessment of soil development because the eroded material is no longer present.

Students are presented with two alternate hypotheses about the soils/deposits they visit:
a) the material has been weathering w/ little physical erosion since it was deposited
b) the material has been weathering and eroding since it was deposited
These hypotheses are developed in lectures before the activity and are based on principles of conservation of mass.

During their site visit, students coarsely characterize topography (@2–5 m scale) for several "representative" cross sections. If time is limited this step could be done remotely (e.g., with topo maps and Google earth).

Students assess and discuss evidence for erosional (and depositional) processes since the deposits were created. They look for broad topographic signatures and measure (for example) the spatial density and material volume of tree throw and animal burrowing mounds, if present.

Students also assess and discuss evidence for in-situ weathering (e.g., development of rinds, soil texture, and mineral alteration). The idea is to train their eyes to observe and key in on any site-to-site differences.

Students dig (and discover!) at select sites. They sample soils at regular intervals from pits (with discussion of merits of different sampling approaches e.g., random vs. stratified random). Students discuss relationships in excavated pits.

A jigsaw approach would be an effective way to tackle the large number of field tasks outlined here.

Back in the lab, using literature values, students estimate weathering rates for each deposit. They compare their estimates with back-of-the-envelop estimates for physical erosion rates (based on tree throw/animal burrowing density) and literature values of diffusivity (which can be coupled with curvature measurements).

The instructor promotes discussion of the implications of differences in residence time on weathering rate estimates.

Students analyze samples by XRF; depending on the course's time constraints students are provided with geochemical data from previous year's field effort or other existing data (in this case Taylor and Blum, 1995).

Students are asked to prepare a final report focusing on the following questions: Are soils products of erosion and weathering, or are they being formed in place by weathering alone? Under what circumstances can we expect erosion to dominate over weathering and visa versa? Students first prepare figures and then use them to develop an an outline (reviewed by the instructor) for their report. Students prepare a draft and engage in peer review (one review each). Students revise their reports, based on the peer review comments, and submit their final report.
Designed for a geomorphology course

Determining whether students have met the goals

Student performance is judged based on how well they analyze assumptions and synthesize evidence for/against the two hypothesis, as demonstrated in their written reports. Another crucial indicator is how well they are able to express the implications of their analyses and results. The instructor also judges students' accomplishments based on the quality of both their reports and the comments they provided in peer review.

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