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Unit 2.2 - Basic Critical Zone Concepts

Susan Gill (Stroud Water Research Center) and Ashlee Dere (University of Nebraska - Omaha)

Summary

Students will learn about geoscience-specific methods used to analyze data in the Critical Zone from data-driven activities and short presentations by their peers. The topics include the use of carbon isotopes, rock and soil profile weathering rates, stream discharge, demographics, and soil carbon. Activities will build data analysis and communication skills while using real data to interpret Critical Zone processes and begin to think about human interactions in the Critical Zone. Students will use geoscience-specific methods when developing their research proposal for the summative assessment activity.

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Learning Goals

The goal for this unit is to introduce students to geoscience methods used in Critical Zone science and to prepare them for future units in this course. Content learned is somewhat dependent upon peer-presenters and what topics are selected but the broader learning goals include:

  • Reading short reports and analyzing data to extract, organize and present a summary of that information.
  • Critically examining geoscience methods and their potential application to Critical Zone science.

Context for Use

This week-long unit for advanced undergraduates/graduates in geoscience or environmental science reviews several scientific skills as they apply to Critical Zone science during two 75 minute class periods. This module, "Methods in Critical Zone Science," the InTeGrate interdisciplinary course called "Introduction to Critical Zone Science." These exercises assume that students have had introductory course in geoscience or environmental science.

Description and Teaching Materials

Background

Although each Critical Zone Observatory (CZO) pursues unique research questions, hypotheses and experimental designs, the CZO network aims to collect common measurements that can be used to compare CZ processes and function across all sites. In effect, common measurements mean the sum of the network is greater than its parts (CZO).

All U.S. CZOs seek to quantify, through a common set of measurements:

1. CZ structure and evolution

2. Event-based and continuous fluxes across CZ interfaces

3. Changes in budgets, including energy, water, solutes and sediment

To accomplish this, CZOs are working to collect the following data at as many sites as possible:

1. Land-Atmosphere

  • LiDAR datasets
  • Eddy flux for momentum, heat, water vapor, CO2 exchanges
  • Wind speed and direction (sensors)
  • Solar radiation and temperature (sensors)
  • Precipitation and through-fall (samplers)
  • Wet and dry deposition (samplers)

2. Vegetation and associated microbiota

  • Above- and below-ground vegetative and microbial biomass
  • Relations between ET and species composition and structure
  • Soil/plant respiration, net ecosystem exchange

3. Soil (vadose zone)

  • Solid phase (campaign sampling for spatial characterization)
    • Texture and physical characterization
    • Organic matter content
    • Elemental composition and mineralogy
    • Stable and radiogenic isotope composition
  • Fluid phase (sensors and samplers for time series)
    • Soil moisture (sensors)
    • Soil temperature (sensors)
    • Soil solution chemistry (samplers)
    • Soil gas chemistry (samplers/sensors)
    • Rates of infiltration and groundwater flow

4. Saprolite and bedrock (saturated zone)

  • Solid phase (campaign sampling for spatial characterization)
    • Texture and other physical and architectural traits
    • Petrology and mineralogy
    • Elemental composition and organic matter content
  • Fluid phase (sensors and samplers for time series)
    • Potentiometric head and temperature (sensors)
    • Groundwater chemistry (samplers/sensors)
    • Gas chemistry (samplers/sensors)

5. Surface water

  • Discrete and instantaneous discharge (flumes, weirs, stage sensors)
  • Channel morphology
  • Stream water chemistry, dissolved and suspended (samplers/sensors)
  • Sediment and biota (samplers/sensors)

More information about common measurements, including a matrix showing which observatories currently collect various measurements, can be found in the 2015 CZO common measurements white paper CZO common measurements white paper (Acrobat (PDF) 239kB Jan19 17). Measurements such as those collected at CZOs form the foundation for some of the CZ methods employed in the following exercises.

Unit 2.2 (Day 1)

Activity 2.3 - CZ Methods and Data Activities (75 min)

  • Assign different groups of students to complete ONE of the following activities, described below, and then to report back to the class on what they found in the second class period in a 10-minute group presentation. Recommended group size is 2-3 students/group. Students should include the following elements in their presentation:
    • What is the main idea?
    • How does this method work?
    • What kind of results were obtained using this method?
    • How is the data analyzed? Where should it be used? Over what timescales is it useful?
    • What are its advantages and disadvantages?
    • What are the limitations to this method of analysis?
    • How is this relevant to the Critical Zone?

Unit 2.2 (Day 2)

Activity 2.4 - Group Method Reports (75 min)

  • Each group will provide a 5-10 minute presentation for the class and will be assessed by the instructor and their peers on how effectively the science and methods are communicated.

Unit 2.2 Group Activities

The following activities are from projects hosted on the SERC website. All of the material necessary to complete these activities can be accessed through the web links and additional activity summaries and modified worksheets are provided below each activity. Note that these activities are designed for instructor use, so you are encouraged to download the necessary worksheets and materials rather than directing students directly to these sites. Instructors should pick the activities that best suit the size and interests of the class - not all activities need to be completed.

  1. CARBON ISOTOPES: This is a series of exercises designed to introduce undergraduate students to the role of sediments and sedimentary rocks in the global carbon cycle and the use of stable carbon isotopes to reconstruct ancient sedimentary environments. Students will make some simple calculations and think about the implications of their results.
    NOTES: Nice student worksheet with progressively more challenging calculations. See Leithold link below for full activity. Will likely need to abbreviate the worksheet to make the time requirement for this activity similar to others listed here or assign to more advanced students. Activity is broken out below into three parts with associated answer keys; instructors can select one or all of the activities, depending on time and student level.
  2. SOIL CARBON: How can the carbon in a dead, rotting rabbit or rotting leaves end up in the atmosphere? To understand this important carbon cycle question, you will need to understand the following about soil—what lives in soil, what soil is made of, and how soil behaves under different environmental conditions.
    NOTES: Actual lab takes a week of observations but activity includes lots of background info. Instructor could provide a set of data that has already been collected to avoid this delay, such as the paper by Bekku et al. (2003) that discusses the temperature dependence of soil respiration.
    • Original Activity: EarthLabs, Lab5a: Soil and the Carbon Cycle
    • Soil Carbon Activity Summary Soil carbon summary (Microsoft Word 2007 (.docx) 15kB Feb22 17)
    • Bekku, Y.S., Nakatsubo T., Kume A., Adachi M. and Koizumi H. 2003. Effect of warming on the temperature dependence of soil respiration rate in arctic, temperate and tropical soils. Applied Soil Ecology. 22:205-210.
  3. WEATHERING RATES: Students are asked to calculate weathering rates from tombstone weathering data. Atmospheric (and precipitation) chemistry determines the rate of weathering for marble tombstones. Show the students data from a rural and an urban cemetery, ask them to estimate rates, and then have them speculate as to why the rates are so different. 
    NOTES: Pretty basic but straight-forward. Works well to provide students will the original Dragovich (1986) paper that includes the context for the data.
  4. BIOGEOCHEMISTRY: Students use elemental chemistry data in a soil profile to explore major biogeochemical processes that dominate in critical zone. Data will be provided, and students calculate and graph the mass transfer coefficients as a function of depth using Excel. Based on these plots, student make generalized statements about how different elements behave in this soil profile and what processes dominate, e.g., depletion by rock-water interaction, addition by dust inputs or elemental loading by human activities etc. 
    NOTES: Missing short explanation of theory to make simple calculation, so need to provide students with tau equation from Jin et al. (2010) (data source for this activity). Data set is presented in solution key but not stand-alone. Also helpful to refer students back to the Brantley et al. 2007 paper for how to interpret geochemical profiles.
    • Original activity: Jin, L. (UTEP) How to read elemental soil profiles to investigate biogeochemical processes in Critical Zone *includes answer key
    • Biogeochemistry Activity Summary Biogeochemistry summary (Microsoft Word 2007 (.docx) 15kB Feb22 17)
    • Biogeochemistry data Biogeochemistry Data (Excel 2007 (.xlsx) 16kB Dec23 16)
    • Biogeochemistry Solution Set
    • Jin, L., Ravella R., Ketchum B., Bierman P.R., Heaney P., White T., and Brantley S.L. 2010. Mineral weathering and elemental transport during hillslope evolution at the Susquehanna/Shale Hills Critical Zone Observatory. Geochimica et Cosmochimica Acta 74:3669-3691.
    • Brantley, S.L., Goldhaber M.B. and Ragnarsdottir K.V. 2007. Crossing disciplines and scales to understand the critical zone. Elements 3:307-314.
  5. DEMOGRAPHICS: Hosted by the Council on Environmental Quality (CEQ), this site contains monthly tables with statistics about United States environmental quality. The major topics covered in these tables are population, economy and the environment, public lands, ecosystems, air quality, aquatic resources, terrestrial resources, pollution prevention, energy, transportation, and the global environment. The tables indicate data sources and an archive of statistics for earlier years is provided. 
    NOTES: Just tables of data - it is up to the user to devise some interesting way to visualize this data. Suggest students contrast some Critical Zone factor at multiple sites or look at dust bowl demographics, which will link to concepts covered in Unit 7. Because of a change in policy, the CEQ reports produced as a result of NEPA were halted after 1997. Students should be aware that this is historical data. 
  6. STREAM DISCHARGE: In this Spreadsheets Across the Curriculum activity, students use USGS data from 2020-2024 to compare the discharge per unit area in July for a pair of nested watersheds in the high country of Glacier National Park. The calculation illustrates how discharge per unit area varies with elevation and demonstrates the distinction between extensive quantities (discharge) and intensive quantities (discharge/area). The module also introduces the hydrologic concepts of watershed, stream discharge, and orographic precipitation.
    NOTES: Original datasets linked in activity powerpoint are out of date. Use activity as a reference but look at USGS stations for up-to-date info. Two datasets for July 2020-July 2024 are included in the attached .csv files.

Teaching Notes and Tips

In this unit there are several examples of methods that can be applied to Critical Zone research. There are more examples than you can cover in the time allowed if each student completed each exercise. Instead, you should have student pairs or, at most, three students undertake one of the exercises and present them to the class. It would be advisable to select students who have some familiarity with the content, if possible, so that they can present the method clearly to their peers. The instructor should encourage the students to ask questions and discuss the possible role of these techniques in Critical Zone science.

Brief discussion can follow each presentation to remind students how methods compare across activities or where multiple methods could be used to gain even more insight into critical zone processes. For example, the weathering activity presents a very basic, inexpensive method to determine weathering rates of known duration, but results may not be comparable across sites or precise enough to determine process. The biogeochemistry activity method could be used to investigate weathering over longer timescales and in more detail (element-specific), but this method is more expensive and weathering duration is often unknown. Or, demographic data are useful to identify environmental changes over time, but more information is needed to determine why those changes occurred. Carbon isotope data from estuaries can also be compared to soil carbon respiration rates measured in a lab - carbon isotopes measure changes over thousands of years and lab experiments less than one month; lab experiments, where variables are controlled, can provide insight into process but a system may not behave the same way when exposed to many variables in a field setting.


Assessment

Students should evaluate the effectiveness of their peers' presentation, which, with your assessment, should be the grade for this activity.

As a metacognative exercise, students should be asked to reflect on what they learned through these assignments and how they expect this unit will affect both later course requirements, their other courses and their view of Earth's Critical Zone.

Grading Rubrics

References and Resources

See links above

Teaching Themes

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These materials are part of a collection of classroom-tested modules and courses developed by InTeGrate. The materials engage students in understanding the earth system as it intertwines with key societal issues. The collection is freely available and ready to be adapted by undergraduate educators across a range of courses including: general education or majors courses in Earth-focused disciplines such as geoscience or environmental science, social science, engineering, and other sciences, as well as courses for interdisciplinary programs.
Explore the Collection »