Unit 3: Geologic Record of Past Climate

1. Students will be able to define a paleoclimate proxy and give an example.
2. Students will be able to evaluate paleoclimate proxy data and use these data to interpret ancient temperature variations.
3. Students will be able to predict in general terms how stable isotope ratios should change in Earth reservoirs in response to changes in climate.
4. (Optional) Students will be able to compare and contrast the "Snowball Earth" and "PETM" episodes as exemplars of the "end members" of climate change induced by perturbations to the carbon cycle.

This unit is designed to be applicable in an introductory level college Earth science course or environmental science course. This unit consists of classroom activities, discussions, and an out-of-class assessment project. It would be appropriate for classes ranging in size from 5–50, though an instructor of a larger course with teaching assistants could utilize the TAs to help manage the activities in a larger classroom. In the most student skills-focused version of the unit, it requires access to Microsoft Excel. The Teaching Tips section below describes alternative teaching strategies for those who are unable to use the Excel versions of the activities. The activities are designed to be flexible and adaptable for use in other circumstances. Several of the activities deal with isotopic paleotemperature proxies, and a basic understanding of isotopes is assumed among the students.

The overall flow of this unit is as follows:

1. Engagement: Students hypothesize connections between giant dragonflies and ancient climate. Instructors tie atmospheric oxygen and carbon dioxide levels to fossil fuels, plants, and weathering together into a complex system, and examine oxygen fluctuations in the context of late Paleozoic carbon burial. (5 min)
2. Paleoclimate proxies: Parts I and II. Students manipulate data from foram coiling direction and hydrogen isotopes from Antarctic glacial ice to arrive at paleotemperature interpretations.(10 min + 15 min, for 25 min total)
3. (Optional) Case studies: PETM activity and Snowball Earth discussion. Students compare two graphs from the scientific literature with a photograph of a sediment core, then interpret the physical evidence of the PETM. This leads into a discussion of the Snowball Earth hypothesis, and a comparison of the two extreme climatic episodes. (15 min)
4. Assessment (homework)
5. Assessment (quiz)

All of the following activities can take place during class time, with the exception of the homework assessment assignment on oxygen isotopes.

1) Engagement:

Background presentation by instructor: The late Paleozoic story of CO₂, O₂, climate, and sea level change. The students begin with a prompt: "How could a dragonfly tell us about the climate of the ancient past?" After the brief presentation, students will see biogeochemical connections writ large (very large) by connecting Carboniferous burial of vegetation with a buildup of oxygen in the atmosphere, and this being a necessary precondition for the evolution of lungless giant arthropods. (Time estimate: 5 min)

PowerPoint slideshow (8 slides, includes presenter notes) : Introduction to paleoclimatology (PowerPoint 1.7MB Aug16 16)

This is an excellent opportunity for the instructor to emphasize how geoscientists think: the development of logical conclusions based on converging lines of evidence; the "lessons" of the past as a source of guidance for the present day, and the interconnectedness of the Earth system, as well as the interdisciplinary nature of the study of the geologic past.

2a) Paleoclimate proxies, Part I: Foram coiling direction activity*

This activity uses foraminiferid coiling direction data from the North Atlantic (our first proxy data source) to examine a recent period of paleoclimate (late Pleistocene and Holocene, the most recent "Ice Age" glaciation). The quickest version of this activity focuses on data interpretation, but not manipulation: Assuming time or computer access are limited, the instructor distributes the graph to the students and then asks them to complete the worksheet questions. If the instructor has the time and inclination for students to practice basic manipulation of an Excel spreadsheet, that is an option, too: the spreadsheets below will automatically draw a graph of their results for them. (Time estimate: 10 min)

Student spreadsheet: Foram coiling direction activity - student version (Excel 2007 (.xlsx) 15kB Mar13 16)
(with blank cells to be filled in by the students so that the graph will be automatically rendered)

Instructor's copy of the spreadsheet:

Foram coiling direction activity - instructor version -- private instructor-only file

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(with all cells to be filled in and graph pre-drawn)

Student worksheet as a Word document: Foram coiling direction activity worksheet - student worksheet (Microsoft Word 2007 (.docx) 33kB Aug16 16)

Student worksheet as a PDF: Foram coiling direction activity worksheet - student worksheet (Acrobat (PDF) 66kB Aug16 16)

Instructor's directions and "right" answers for post-worksheet discussion as a Word document:

Foram coiling direction activity worksheet - instructor's copy -- private instructor-only file

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Instructor's directions and "right" answers for post-worksheet discussion as a PDF:

Foram coiling direction activity worksheet - instructor's copy -- private instructor-only file

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If the instructor wants to show the students images of the foram in question, Neogloboquadrina pachyderma, I suggest a Google Image search.

(Introducing isotope fractionation)

This video should be assigned as pre-class viewing, or viewed in class if the instructor has the luxury of 17 extra minutes of class time. The video introduces natural examples of physical fractionation of isotopes of H, C, and O, and it explains what isotopes are and how to calculate "delta" values, and finally, how to interpret higher and lower values of δD, δ¹³C, and δ¹⁸O. (17 min)

Video introduction to isotope fractionation Isotope fractionation (Quicktime Video 20.1MB Aug4 14)

Isotope Fractionation video on YouTube

2b) Paleoclimate proxies, Part II: Vostok ice core activity*

This activity uses δD data from the Vostok ice core to examine the same period of paleoclimate (late Pleistocene and Holocene) as the previous activity on foram coiling direction. In explaining the activity, the instructor (and the student worksheet background reading) detail δ notation (useful for isotopes of H, C, and O) and emphasize the notion that these isotopic ratios are another kind of proxy data. [For example, if a sample is said to have a delta value of +6 ‰ δ¹³C then it is 6 parts in 1000 enriched in ¹³C compared with a standard of reference.] Students complete slightly more advanced manipulation of an Excel spreadsheet, and as with the previous activity, the spreadsheet automatically draws a graph of their results for them. Several of the interpretive questions refer back to the foram coiling direction activity, so if the instructor opts to do only this activity, he or she will need to modify those interpretive questions (# 7 and 8). As with the foram coiling activity, if time or computer access is limited, or if the instructor deems the students' spreadsheet skills too undeveloped to bother with, the instructor could just distribute the final graph to the students and then ask them to use it as a basis for completing the worksheet questions. (Time estimate: 15 min)

Student spreadsheet: Vostok ice core data (student spreadsheet) (Excel 2007 (.xlsx) 126kB Jun21 14)

Instructor's copy of the spreadsheet:

Vostok ice core data (instructor's spreadsheet) -- private instructor-only file

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Vostok ice core data (instructor's spreadsheet)

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Student worksheet as a Word document: Vostok ice core activity (Microsoft Word 2007 (.docx) 158kB Aug16 16)

Student worksheet as a PDF: Vostok ice core activity (Acrobat (PDF) 117kB Aug16 16)

Instructor's directions and "right" answers for post-worksheet discussion as a Word document:

Vostok ice core activity - Instructor's copy -- private instructor-only file

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Instructor's directions and "right" answers for post-worksheet discussion as a PDF:

Vostok ice core activity - Instructor's copy -- private instructor-only file

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A brief discussion of mass spectrometry, available from the Niels Bohr Institute, could be used if the instructor wants to emphasize the instrumentation that allows these measurements.

Discussion concludes with a reminder that proxies are physical evidence that, when viewed through the lens of our theoretical understanding of climate, can be interpreted as evidence of past (pre-human observation) climates. Ask students to list other examples of paleoclimate proxy data. Hopefully they will come up with some of the following: glacial geomorphology, sedimentary rocks that indicate warm/cold or arid/wet conditions, O & C isotopes (in carbonate rocks, ice, tree-rings, speleothems, corals, animal bones, shells, and otoliths), sea level transgression/regressions, paleoshore lines, marine fossils found in inland locations, terrestrial fossils found in offshore locations, tree ring thicknesses, and leaf fossil shapes. There is a lot of potential evidence about the climates of the ancient past!

* If time is limited, and it seems unrealistic to complete these activities in the available class time, then instructors can modify the activity by skipping one of them (2a: foram coiling, or 2b: Vostok deuterium interpretation). Alternatively, the class could be split in half, with half doing 2a, and the other half 2b.

3) (Optional) Case studies: PETM activity and Snowball Earth discussion:

Note that the discussion of δ¹³C in the introduction for this activity mimics the discussion of δD in the Vostok activity. Depending on the instructor's tolerance of repetition, he or she may opt to de-emphasize or remove this. (Time estimate: 15 min; Optional depending on class progress and time available)

Instructor's copy (including implementation instructions and "right" answers"):

Microsoft Word document version:

PETM data comparison activity (instructor's copy) -- private instructor-only file

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PDF version:

PETM data comparison activity (instructor's copy) -- private instructor-only file

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Student copy (only includes the questions and data documents)

Microsoft Word document version: PETM data comparison activity (students' copy) (Microsoft Word 2007 (.docx) 2.2MB Aug16 16)

PDF version: PETM data comparison activity (students' copy) (Acrobat (PDF) 2.1MB Aug16 16)

Instructors then review ("debrief") the PETM activity, discussing any issues revealed through student answers. [Optional discussion afterward: Compare the "direct" paleotemperature proxies (δD and δ¹⁸O) with the less-direct δ¹³C, and a metacognition-focused discussion of confidence in conclusions when the chain of evidence grows longer but the logic is less direct. The importance of having multiple independent lines of evidence for increasing confidence in "indirect" conclusions is a key part of the scientific process (example: the PETM carbon isotope excursion being caused by a sudden release of extra organic carbon is reinforced when the evidence of attendant ocean acidification is considered).] This leads to a brief discussion of the Snowball Earth hypothesis (Neoproterozoic glaciation), another example of ancient climate change caused by perturbations to the carbon cycle, though this time the effect is to drive the climate into a colder state. In a nutshell: excessive weathering of silicate minerals in the continental crust consumed greenhouse gas CO₂, and made Earth much colder. Later, Earth was "rescued" from a frozen fate due to natural emissions of volcanic greenhouse gases, and a concomitant lack of "scrubbing" this new carbon from the atmosphere, causing it to build up. The attached PowerPoint slideshow covers the basics of this idea. (5 min more)

"Snowball Earth" PowerPoint slideshow for instructors (7 slides, includes presenter notes and a concluding slide comparing the PETM and the Snowball Earth): Snowball Earth (PowerPoint 1.6MB Aug16 16)

Additional slides and images may be downloaded from the SnowballEarth.org website's teaching slides collection.

If time permits, the instructor can achieve a quick, informal formative assessment with the discussion prompt: "What can the PETM and the Snowball teach us about the modern unbalancing of the carbon cycle and ensuing climate change?" Student responses can be fodder for metacognitive insight with further reflection: "Sally, why do you think James gave that answer?" or "James, is there a reason you are emphasizing natural variability over the role of the carbon cycle?" (In other words, harken back to the lessons of Unit 1 of this module in a quest to root out bias and logical fallacies.)

*If time is limited, and it seems unrealistic to complete these activities in the available class time, then instructors can modify the activity by skipping any one of them (2a: foram coiling, 2b: Vostok deuterium interpretation, or 3: PETM/Snowball Earth).

• Students will likely find the concept of δ (delta) notation to be the most challenging concept, because it is (a) a foreign letter, and not even the capital delta they may be more familiar with; (b) it is math, and math repels many intro-level students in geoscience courses; and (c) the inclusion of the standard may be nonsensical at first. An analogy the instructor could offer for the latter is comparing exotic currencies by reference to a dollar. A student can get a better sense of the purchasing power of a Japanese yen relative to a Turkish lira by comparing both to the common standard of a U.S. dollar.
• *One option with Activity 2 is to have the entire class do 2a (forams), followed by the entire class doing 2b (ice and δ¹³C). Another option is to split the class in half, and have half the students work on 2a, while the other half works on 2b.
• If the instructional space does not include access to Microsoft Excel, or if the logistics of plotting in Excel are deemed too labor-intensive, the instructor could modify the "Foram coiling direction" activity as well as the δ¹⁸O assessment with benthic foram data by providing the completed graph(s), and having the students simply answer the worksheet questions based on their observations of the graph.
• Going further, here is a PowerPoint slideshow with the three graphs (one of the foram coiling direction data, a second of the oxygen isotope data, and a third of the Vostok deuterium data). For instructors who are short on time or who do not want to engage in the use of Excel, these images can at least provide the basis for students' interpretation of paleoclimate data: Paleoclimate_proxies_3_graphs (PowerPoint 2007 (.pptx) 186kB Aug16 16)
• Four possible extensions include:
• National Geographic magazine ran an excellent overview article 'Hothouse Earth' by Robert Kunzig on the PETM in the October 2011 issue. The article includes some provocative photography by Ira Block. Students could read this article as background, or the instructor could assign it as pre-lesson preparatory reading.
• Kevin Theissen of the University of St. Thomas (MN) produced a more in-depth examination of the PETM data from the Zachos, et al (2005) paper, posted on the activity page: An abrupt global climate change event in Earth history—Evidence from the ocean. This is intended for a 200-level oceanography course, and he devoted 2.5 weeks to the assignment. In his version of the activity, students plot the raw data from the paper and interpret it independently.
• Ron Blakey (Northern Arizona University and Colorado Plateau Geosystems) has produced a lovely paleogeographic map for 50 Ma. This map could be used as an additional data source to supplement the PETM activity. Although it is not for exactly the right time (50 Ma instead of 55 Ma), it at least could provide a paleogeographic perspective on higher sea levels and lack of continental ice sheets. This would probably be most useful for a historical geology class utilizing the module.
• The GISP2 project has a succinct web page comparing the Greenland core and the Vostok core (Antarctica) in terms of atmospheric gas isotope ratios. Correlations are provided between the Vostok δD and the GISP2 δ¹⁸O records.

There are several ungraded (formative) and two graded (summative) means of assessment for this unit. (Ungraded assessments include keeping track of correct and insightful observations and statements during discussions of unit material and activities, as well as incorrect or "off-base" comments. In particular the initial "engagement" activity should provide baseline formative assessment for the instructor. Instructors are encouraged to assess groups' work as they tackle either the foraminifera or oxygen isotope data.) If time and student achievement level permits, the optional graded homework assignment (#4, below) could be assigned as a summative assessment. It could also work as an in-class group activity, if time permits.

4) (Optional) Summative Assessment

Instructors who want to be sure students have mastered the interpretation of paleoclimatological isotope data can assign this activity as an in-class or take-home assessment. They should share δ¹⁸O data with students (for the sake of coming full circle in the activity, we collect these data from foraminferids, with their CaCO₃ tests) and ask them to plot and interpret it.

Introduce oxygen isotopes as another paleotemperature proxy. Point out that for every 10,000 atoms of oxygen, the vast majority (9,977) are ¹⁶O, about 20 are ¹⁸O, and 3 would be ¹⁷O. Because there is more than six times as much ¹⁸O than ¹⁷O, we use ¹⁸O/¹⁶O ratios as our proxy (easier to measure that way). We calculate δ¹⁸O the same as we would other stable isotopes, like δ¹³C or δD. Optional: Instructors who want to streamline the assignment (focusing on graph interpretation alone rather than data manipulation, graphing skills, and then graph interpretation) can share the completed graph in the instructor's copy of the assignment and focus on the interpretive questions.

Student copy of the assignment:

Word document: Student assessment activity w/ delta 18O data from benthic forams (Microsoft Word 2007 (.docx) 36kB Aug6 24)

PDF: Student assessment activity w/ delta 18O data from benthic forams (Acrobat (PDF) 195kB Aug6 24)

Spreadsheet to distribute to students: ODP core 677 data (Excel 2007 (.xlsx) 52kB Jul2 14)

Instructor's copy of the spreadsheet (with graph):

ODP core 677 data - with graph -- private instructor-only file

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Instructor's copy of the assignment:

Word document:

Student assessment activity w/ delta 18O data from benthic forams -- private instructor-only file

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Student assessment activity w/ delta 18O data from benthic forams

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PDF:

Student assessment activity w/ delta 18O data from benthic forams -- private instructor-only file

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This assessment may be assigned as homework. With the plotting exercise in Microsoft Excel, the time estimate is 1 hour. If the instructor opts to share the graph and merely have students interpret it (rather than plot it), then it should take only 20 minutes.

5) Summative Assessment

The summative assessment for this unit, which can also function as a "pre-test" or a part of a larger, whole-module exam, is a quiz covering key aspects of paleoclimatic data and interpretations. Some of these questions are basic recall, while others ask students to apply their understanding. (Time estimate: 10 min)

Quiz:

Unit 3 quiz -- private instructor-only file

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Instructor's key for scoring the quiz:

Unit 3 quiz instructor's key -- private instructor-only file

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The foram coiling direction activity is modified from Climate Analysis Using Planktonic Foraminifera: A Classroom Activity Integrating Science and Mathematics by Hilary Clement Olson, University of Texas.

The Vostok ice core activity is modified from the activity Vostok Ice Core: Excel (Mac or PC) by Stephanie Pfirman, Barnard College. Based on data of J. Chappellaz, Laboratoire de Glaciologie et Geophysique de l'Environnement, Grenoble, France.

The formative assessment is modified from the activity Marine Oxygen Isotopes and Changes in Global Ice Volume by Ben Laabs and Scott Giorgis, SUNY Geneseo.

The data used in the exercise were first published in: Shackleton, N.J., A. Berger, and W.R. Peltier. 1990. An alternative astronomical calibration of the lower Pleistocene timescale based on ODP Site 677. Transactions of the Royal Society of Edinburgh: Earth Sciences 81:251.

EPICA Dome C ice core data is available for download from the NOAA Ice Core Data website (space delimited text files; these can be imported into a spreadsheet program like Excel).