Hagerman Fossil Beds National MonumentSida 16 av 21
Dating Hagerman’s Pliocene Fossil Beds With Ash, Magnetism and Shells: The PET Project
Hagerman Fossil Beds National Monument, a stretch of brown bluffs by the Snake River in southern Idaho, holds nearly 200 species from the Pliocene, but the species list alone cannot say how the place changed over about 1.7 million years. A multi-disciplinary team, the Hagerman Paleontology, Environments, and Tephrochronology (PET) Project, is building that picture. This summary follows an article by Kari Prassack, a former senior paleontologist at the monument and co-lead of the project, in the National Park Service's Park Paleontology News.
The place and the problem
Horses, beavers, ducks and peccaries, whose descendants are familiar, lived alongside saber-toothed cats, giant ground sloths and mastodons. Hagerman is most famous for a large accumulation of early single-toed horses, and several species were first described from here. The species range in age from 3.07 million years (Ma) to as old as 4.8 Ma and did not all live at once, so to read how the environment and animal community changed, the team first needs the geologic context. The project was inspired by Prassack's earlier work at Olduvai Gorge in Tanzania, where volcanic ash dating let teams study short slices of time, a few thousand years, instead of long time-averaged deposits. Hagerman offered diverse species, numerous ash layers and an ancient lake and river that likely made a mosaic of settings. At the time its span was thought to be only 500,000 years.
What each method adds
| Method | What it gives |
|---|---|
| Volcanic ash (tephrochronology) | Ash is chemically fingerprinted, so deposits from one eruption can be matched across the landscape. Mapping it showed that Hagerman's paleo-topography was far from flat, split the beds into manageable time slices, and gives indirect ages for fossils whose carbon was long since replaced by minerals |
| Argon-argon dating | Dates ash by the decay of argon 40 to argon 39 in feldspar. Before this project only two ashes were dated: Bed G at 3.8 Ma and the Horse Quarry Ash at about 3.07 Ma. New samples were with the lab for results |
| Paleomagnetism | Basalt records the direction of Earth's magnetic field when it cooled. Hagerman's deposits formed during the Gauss normal and Gilbert reverse polarity chrons, which helps identify and match basalt flows |
| Cyclostratigraphy | Looks for Milankovitch cycles of 40,000 or 100,000 years in the magnetic susceptibility of sediments, hinting at a general time sequence |
| Chemistry | Detrital zircons and stable isotopes trace where sands came from and how drainage shifted; isotopes in clam and snail shells give paleotemperature |
| Diatoms | These single-celled algae with silica shells, in many shapes tied to habitat, indicate lake depth, wetland distribution and river flow |
One of the project's sampling sites, showing a newly exposed layer of volcanic ash (National Park Service).
A fence diagram of volcanic ashes from distinct eruptions traced and mapped across the landscape by a USGS team member. Differences in how the ashes map reflect differences in deposition, erosion and topography, including tectonic faulting (USGS).
Diatoms come in shapes and sizes related partly to their environment, so they make good environmental indicators (USGS photo).
Prassack notes the older picture of Hagerman's lake, a lower lacustrine plain, then a warm wet marshy floodplain, then drier and cooler with a lake retreat, is probably too simple, and the oldest deposits may be deep lake deposits. The orientation of the ancestral Snake River is not worked out.
What the dates have already changed
The project mapped six sites, named Horse, Bear, Otter, Dog, Badger and Peccary. Horse and Peccary, long thought contemporaneous, are not, the peccary site being clearly older. The Bear site is at least as old as thought.
- Otter. In 2016 the date of a new fossil otter, Lontra weiri, was inferred by projecting an isolated outcrop of Bed G across bluffs, a flawed method. Mapping showed Bed G above the otter site and even further above than projected. The otter was being touted as the origin of the genus Lontra in the Americas, so the date mattered.
- Dog. A robust canid jaw looked like Canis lepophagus, a larger canid not otherwise recorded at Hagerman and found elsewhere mostly in younger deposits. After the bluff was mapped it was dated to around 3.9 Ma, one of the oldest and most robust examples of the species.
- Badger. A new lower jaw of the giant badger Ferinestrix lay on ash bed F, dating it between 4.2 and 3.9 Ma, considerably older than F. vorax (3.4 to 3.0 Ma). The team calls it by its genus name only, because it may be a new species.
Next
Prassack moved to be an archaeologist at Glen Canyon National Recreation Area while still co-leading the project. Two colleagues were preparing work in the similarly aged Ringold Formation of Washington. The team invites qualified researchers in unrepresented fields, such as sedimentology, ostracods, plants and rodents, to join.
Sources
- National Park Service, Kari Prassack, The Hagerman Paleontology, Environments, and Tephrochronology (PET) Project Expands its Scope, Park Paleontology News; photos of team members are left out; rewritten in hubnx's own words.