Assessing vegetation and fluvial responses to the Paleocene-Eocene Thermal Maximum in the Hanna Basin (Wyoming, U.S.A.)
Overview: This project focuses on developing the basin evolution and paleobotanical history of the Hanna Basin spanning the transition between the Paleocene and Eocene epochs. Globally a greenhouse climate state dominated the early Paleogene, and in the Western Interior of the United States the Laramide Orogeny produced a series of uplifts and sedimentary basins. Overall the paleoclimate appears to become drier and potentially more seasonal between the Paleocene and Eocene, likely related to global warming trends and changing topography due to uplift of the Rocky Mountains. In several basins there are attendant shifts in fluvial and floodplain deposition indicating drier and more seasonal conditions as well as moderate shifts in floral communities. However, the Hanna Basin of south-central Wyoming appears anomalous. Initial studies suggest this area remained largely wet and humid, defying regional trends. This uniqueness is particularly important because the basin likely contains one of the most stratigraphically-expanded records of the Paleocene-Eocene Thermal Maximum (PETM) of anywhere in the world.
Searching for fossil ferns in the Hanna Basin, Wyoming
The PETM is an abrupt global warming event linked to the massive release of carbon into Earth’s atmosphere and oceans (McInerney & Wing, 2011). In other basins in the Rocky Mountain region it is correlated to substantial changes in the hydrologic cycle exacerbating seasonality in rainfall and vegetation overturn (Wing et al., 2005; Foreman et al., 2012; Kraus et al., 2015). The effects in the Hanna Basin are largely unknown and may be unique in their character. Student projects will focus on one of three topics: (1) reconstruction of paleoenvironment from fluvial and floodplain deposits and recovering information on ancient landscape dynamics; (2) understanding the interacting components of the local to global carbon cycle through stable carbon isotope records; and (3) characterizing vegetation cover spanning the short- and long-term climate changes using a new plant cuticle-based proxy method.
When: July 6 – August 5, 2018
Where: Laramie, Wyoming (introduction and lab work) and Hanna, Wyoming (field work)
Who: Four to six students and project leaders Dr. Brady Foreman (Western Washington University, email@example.com) and Dr. Ellen Currano (University of Wyoming, firstname.lastname@example.org). USGS stratigrapher Marieke Deschesne and Field Museum paleobotanist Dr. Regan Dunn will also take part in field and lab work and provide additional mentorship for student projects.
Prerequisites and Recommended Courses: Suggested (but not required) are core courses in the Geology major: Historical Geology, Structure/Tectonics, Stratigraphy, Mineralogy, Paleontology and Geochemistry. Students should have completed key cognate courses in Chemistry and Math. Experience at a field camp or in a field geology course is recommended but not required. We are particularly interested in applicants with an interest in Paleoclimates, who have a high degree of comfort in rugged outdoor settings, are able to hike several miles in warm temperatures (on occasion greater than 95° F in the high desert climate), are flexible eaters, and who want to use this work to complete a senior thesis (or equivalent) in geology. Helpful, but not required in the letter of recommendation from the on-campus sponsor is an indication of how well the applicant will function in a remote field setting with primitive camping (i.e., no running water, no bathrooms, limited cell phone service).
Expectations and Obligations:
- Participation in all project-related work during the summer (July 6-August 5, 2018).
- Follow up data analysis at home institution and regular conference calls with research team throughout academic year.
- Write an abstract and present a paper (poster or talk) for the Geological Society of America Rocky Mountain Section meeting in Manhattan, Kansas (conference is March 25-27, 2019).
- Write a short contribution (4-6 pages text + figures) to be published in the Proceedings of the Keck Geology Consortium 2019 Volume (first draft due Mid-February).
- Expected but not required: Use this work for the completion of a senior thesis (or equivalent) at your home institution.
The Paleocene-Eocene Thermal Maximum is viewed as one of the premier geologic analogs for modern, anthropogenic global warming. However, our understanding of the major terrestrial impacts is limited to one well-documented example (Bighorn Basin of northwest Wyoming) and a few other locations globally where only a few key hydrologic or temperature constraints exist. Broadly speaking, the terrestrial response to elevated carbon dioxide levels is expected to be highly variable due to topographic variability and shifting atmospheric circulation patterns under different latitudinal thermal gradients. As such it is critical to develop additional continental locations where the temperature, hydrologic, and biologic changes are constrained. The Hanna Basin is a logical extension of the work in the Bighorn Basin, and, more importantly, a key example for changes that may mimic those predicted for modern river and vegetation systems during future anthropogenic climate change.
The Hanna Basin record is a particularly important complement to the record already collected in the Bighorn Basin because stratigraphic data suggest a difference in water availability between the two basins. Both the PETM interval and the entire early Eocene sequence preserved in the Willwood Formation in the Bighorn Basin contain abundant red beds, indicative of well-drained soils and seasonal precipitation (Kraus and Riggins, 2007; Kraus et al., 2015). The Bighorn Basin records the consequences of a semi-arid basin experiencing an abrupt global warming event. In contrast, the Hanna Basin sedimentary sequence remains drab-colored and coal-rich from bottom to top, suggesting that wet, swampy conditions prevailed through the PETM and early Eocene (Dechesne et al., in prep). It records the consequences of a more humid basin experiencing an abrupt global warming event. Thus, comparisons of the Hanna and Bighorn basins will allow us to disentangle the roles of temperature and water availability in driving vegetation change and the fluvial response in a dominantly humid setting. This water availability difference is likely due to the location of the Hanna Basin, farther to the east and potentially more proximal to moisture sources as compared to the Bighorn Basin (Sewall & Sloan, 2006). The Laramide Orogeny created a complex topography within the Western Interior that strongly influenced water vapor transport paths (Sewall & Sloan, 2006). In general, however, circulation models suggest the easternmost Laramide basins were wetter as moisture from the paleo-Gulf of Mexico moved north and westward. This resulted in semi-arid and dry conditions in the western and northernmost Laramide basins (Sewall & Sloan, 2006).
Figure 1: Cretaceous to Eocene sedimentary strata of the Hanna Basin, Wyoming.
The targeted field area for this study is the Hanna Basin of south-central Wyoming near the small town of Hanna. The Hanna Basin is a Laramide style basin bounded by the Rawlins uplift to the west, Sweetwater Arch in the north, the Simpson Ridge anticline in the east, and the Medicine Bow and Sierra Madre Mountains in the south. It is exceptional among the Laramide basins because of its high subsidence rate, extremely thick Cretaceous to Eocene sedimentary strata, and extensive coal deposits (Dobbin et al., 1929; Roberts and Kirschbaum, 1995; Wroblewski, 2003). The Paleocene-Eocene boundary is preserved in the Hanna Formation, which is over 2000 meters thick at the center of the basin (WOGCC, 2016; Gill et al., 1970; Wrobleski, 2003) and consists of conglomerates, sandstones, siltstones, carbonaceous shales, and coals. These deposits are interpreted as low gradient fluvial to paludal and lacustrine (Dobbin et al, 1929; Wrobelski, 2003; Lillegraven et al., 2004). The student projects will target a specific set of extensive outcrops along the Hanna Draw Road and in “The Breaks,” which have been the focus of new geologic and paleontologic work by Currano, Dechesne, and Dunn. The majority of research will focus on a 250-meter thick stratigraphic interval with pollen biostratigraphic constraints and initial bulk d13C values that indicate the PETM is preserved.
Goals and Significance of the Project
Overall the goal of the project is to characterize fluvial and vegetation changes spanning the transition between the Paleocene and Eocene epochs in the Hanna Basin of Wyoming. This is a critical time period in the history of life and the evolution of the Rocky Mountain region. Specifically, there were transitory changes in paleo-plant communities, persistent shift in the mammalian biogeography, and a general warming and drying trend in the Western Interior of the United States. The Hanna Basin potentially records the most complete history of this transition because it witnessed the fastest sedimentation rates of any Laramide basin in the region. This broad goal requires (1) refining the chronstratigraphy of the basin using pollen fossils and carbon isotope records; (2) detailed lithofacies analysis and outcrop mapping; and (3) application of new cuticle-based proxies for vegetation cover.
Student projects will be sub-divided into three discipline related groups. These groups entail Sedimentology-, Geochemistry-, and Paleobotany-themed projects. All students will receive training and gain experience in the three disciplines, and interact extensively with other students, faculty, and collaborators. However, the specific projects they pursue will fall into one of these three categories. Sedimentology projects will involve detailed lithofacies analysis and paleoenvironmental reconstructions of the river and floodplain deposits (n = 1 student project). These students will develop field datasets that address key questions regarding river channel morphology, avulsion behavior, floodplain drainage, and overbank flooding frequency. The majority of data can be derived from the detailed stratigraphy section we will measure at Hanna Draw area. A separate Sedimentology project (n = 1 student project) will focus on mapping out and describing the larger alluvial architecture of the fluvial sandbodies. This entails identifying and tracing out the amalgamation of distinct channel occupation events and the lateral migration of channel bodies. This information will yield insights into the mobility and paleo-dynamics of the ancient river systems.
The Geochemistry-themed projects will focus on developing a detailed δ13C bulk organic isotope stratigraphy through the 250 meters of section thought to contain the PETM (n = 2 student projects). This will entail a combination of stratigraphic section measuring (fieldwork), and sample preparation at the University of Wyoming. This includes powdering rock samples, acid digestion, and distilled water treatments, followed by drying and weighing steps. Time permitting students will run samples themselves on the Isotope Ratio Mass Spectrometers at the University of Wyoming Stable Isotope Facility. Two students will focus on this component of the project. One student will recover secular variation from densely sampled (every 0.5 m or finer) vertical samples distributed throughout the 250-meter target interval. A second student will focus on constraining lateral variability of the isotope signals. Since bulk organic δ13C records mix vegetative matter from several different types of plants it is important to constrain the total amount of variability across a landscape. This is rarely done in ancient strata (Magioncalda et al., 2004; Foreman et al., 2012), and this student will produce one of the most detailed evaluations of this uncertainty to date.
Paleobotany-themed projects will utilize a new proxy for vegetation cover that uses leaf epidermal cell size and shape to reconstruct leaf area index (LAI=foliage area/ground area; Dunn et al., 2015). This proxy has been effectively used to correlate changes in vegetation cover in South America to environmental changes, as well as to vertebrate evolution. The work in the Hanna Basin will be the first attempt to quantitatively document changes in vegetation structure across the Paleocene-Eocene boundary. The samples contain fragments of fossil leaf cuticle that preserve epidermal cell morphology. Students will photograph cuticle fragments using the microscope set-up in Currano’s lab, measure leaf epidermal cell size and shape using a Wacom Cintiq tablet and ImageJ, and reconstruct LAI using regression equations between size and shape parameters and LAI produced by Dunn from using a modern calibration datasets (Dunn et al., 2016). One student will analyze the vertical transect and a second will analyze a set of lateral samples by tracing out a stratigraphic marker bed over several 100’s of meters. This approach will allow both temporal and spatial changes in vegetation to be constrained.
Summary of Projects
Student 1: Lithofacies analysis of fluvial and overbank facies
Student 2: Mapping and description of sandbody geometries and floodplain relationship
Student 3: δ13C bulk organic isotope stratigraphy vertical variation
Student 4: δ13C bulk organic isotope stratigraphy lateral variation
Student 5: Cuticle variability and vegetation cover, vertical section
Student 6: Cuticle variability and vegetation cover, lateral section
Figure 2: Measuring section using a Jacob’s staff.
The planned projects will involve 4-6 undergraduate students at junior and senior levels in their academic career. Students and faculty will rendezvous at the University of Wyoming on July 6 where initial gear checks will occur, and safety procedures and project details will be presented by faculty members. After this orientation we will proceed to the field area near Hanna, Wyoming (~2 hour drive). Field transport will include a rented university vehicle and two private vehicles. The field area is remote, but camping will occur in close proximity to vehicles. Our camp will be set up on public (BLM) lands, and will be primitive (i.e., no running water, no toilets, limited cell phone service). The first field day will entail a tour of the study area, an overview of the depositional history of the basin, and an additional field safety orientation. Subsequently, the students will be subdivided into the sedimentology, geochemistry, and paleobotany research groups and begin their projects. The remainder of the time will be divided into four phases:
Phase 1 (07/07 thru 07/14): Fieldwork
- main stratigraphic section measuring (3 students)
- outcrop mapping (1 students)
- paleobotanical excavation (2 students)
Phase 2 (07/15 thru 07/23): Field and Lab work
- main stratigraphic section measuring & outcrop mapping (2 students)
- paleobotanical excavation (2 students)
- geochemistry sample preparation (2 students)
Phase 3 (07/24 thru 08/05): Field and Lab work
- main stratigraphic section measuring/outcrop interpretation (2 students)
- paleobotanical proxy training (2 students)
- geochemistry sample preparation (2 students)
Phase 4 (March, 2019): Professional Development Component
- poster presentations at Rocky Mountain Section GSA meeting
The expectations for students include (1) a positive, flexible attitude, (2) a responsiveness to the needs of the group, (3) a collaborative mentality, (4) strong work ethic, and (5) interest in developing quantitative and interpretative skills. These will be rough working conditions. Temperatures can exceed 95º F in the high desert climate of Wyoming. Strong winds and intense rainstorms are also likely. Students will be expected to have appropriate field gear (e.g., boots, hat, sunglasses, tent, sleeping bag, sleeping pad, backpack, water bottles). Students will need to be physically fit, and able to walk several miles per day on steep slopes in the heat, dig quarries that requires the use of shovels and pickaxes, and be capable of carrying and packing out over 30 lbs of rock per day over 3 miles. Days will likely be long, in excess of eight hours. Dietary restrictions will be accommodated if possible. Each research sub-group will be expected to produce a cohesive presentation for the annual Rocky Mountain Section meeting of GSA in May 2019 (location of meeting is unannounced). Faculty advisors will coordinate with the students towards this endeavor and aid in the construction of the poster presentation. All students will be expected to attend the meeting and participate in associated activities.
WYOMING SAFETY ISSUES
As with any field and lab work there will be safety concerns. Field risks include dehydration, heat stroke, exhaustion, sunburn, insect bites, poisonous snakes, and physical injuries such as sprained ankles and broken bones. Lab risks are relatively minor, but include the use of (weak) acids when preparing stable isotope samples and airborne dust respiratory concerns when handling and preparing plant fossils. Risks will be mitigated by following proper training protocols, outlining and identifying the risks, and holding several safety meetings. Faculty associated with the project have extensive experience in all pertinent field and lab methodologies, wilderness first aid training, and will coordinate with lab technicians to maintain safety standards.
The faculty members involved in this project and collaborators collectively have over 30 field seasons of experience working in the region. They have published over 15 scientific articles and reports on the field area and equivalent rocks in the Laramide basins of the western United States. All have led several field crews of undergraduate and graduate students under similar circumstances. Foreman and Dechesne are certified in wilderness first-aid, and we will be maintaining a list of nearby medical facilities in case of emergency. In terms of professional expertise Dr. Brady Foreman (WWU) and Marieke Dechesne (USGS) are sedimentary geologists who have worked extensively in the surrounding Laramide and Sevier foreland basin deposits. Dr. Ellen Currano (University of Wyoming) and Dr. Regan Dunn (Field Museum) are paleobotanists who have worked extensively on cuticle and plant macrofossil records in the region. Dechesne, Dunn, and Currano have collected the initial datasets upon which this proposal is based, and have identified specific areas in the basin to target for this study. This will maximize the chances of research success for the students. Foreman and Dechesne will advise students with sedimentologic projects, paleobotany projects will be advised by Currano and Dunn, and geochemistry projects will be advised by Foreman and Currano.
Figure 3: Cnemidaria fern fossil.
We plan to take all participants to the GSA Rocky Mountain section meeting in Manhattan, Kansas (25–27 March 2019). We hope most students will be first author on one paper, and probably secondary authors on others due to the collaborative nature of the project.
All students are required to complete a 4-6 page paper (short contribution) that will be published in the Proceedings of the Keck Geology Consortium 2019 Volume (see examples from previous years). The first draft of this paper will be reviewed by your research advisor at your home institution sometime in late February, 2019, with the revised version sent to the project directors by March 1. Final versions of your paper and figures will be submitted to the Keck Office in Mid-March.
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