Dating Hagerman’s Fossils With Volcanic Ash: 13 Ash Layers Split a Million Years

Hagerman Fossil Beds National Monument in Idaho is often treated as a single snapshot of the Pliocene, but its fossils span more than a million years. A Park Paleontology News article describes how volcanic ash is helping to separate that single snapshot into 14 distinct communities.

Image from the National Park Service's page: a view of the monument. The steep slopes preserve one of the best records of Pliocene fossils in North America, the article says.

One site, a million years

The fossil sites are spread across steep bluffs with a vertical range of 600 feet and a horizontal spread of approximately 4,350 acres. They preserve a rich ecosystem of some 200 species, including rodents, birds, frogs and carnivorans such as otters, canids and saber-toothed cats, with larger animals such as mastodons and giant ground sloths less common. The best-known site is the Smithsonian Horse Quarry. The fossils range from approximately 4.2 to 3.07 million years old, so a single "window" into the Pliocene is very time-averaged. That would be like lumping today's humans and animals together with those of the Holocene and much of the Pleistocene ice ages.

Why ash

The fossils are too old to keep the carbon needed for carbon-14 dating, but the volcanic ashes, called tephra, are the right age to date. The dating, tracing and mapping of ashes is tephrochronology. A fossil layer is younger than the ash below it and older than the ash that caps it. Earlier studies dated some ash outcrops and projected the dates across the monument, assuming a flat floodplain. But faulting and the scouring and deposition of a river and lake made the landscape anything but flat, so a few isolated ashes were not enough for the resolution paleontologists want.

The project

The Hagerman Paleontology, Environments, and Tephrochronology (PET) Project formed to fill that gap. Starting in 2016, a U.S. Geological Survey geologist from the USGS Tephrochronology Project spent part of each summer finding ashes and tracing them across the bluffs, digging them out from under sediment and vegetation on steep slopes, and taking samples to the lab to identify each one's chemical signature. The work identified 13 volcanic ashes and traced them across the monument. Most had been known only from single outcrops, and several were not known at all. The monument's fossil sites now fall into 14 time slices, each a distinct animal community and environment, which should improve understanding of animal behavior, community structure and climate change there.

Image from the National Park Service's page: steep slopes like these had to be crossed to trace the ashes. A geologist is somewhere high on the slope.

How the matching works

Each magma chamber has its own geochemical composition. In an explosive eruption, bits of magma cool quickly into volcanic glass, the main component of ash, and the glass shards keep that composition. Matching the glass chemistry, together with the sediments above and below, shows that ash in one gully came from the same eruption as ash in the next.

Image from the National Park Service's page: a scanning electron microscope photograph of volcanic glass shards. Well-preserved glass gives a better chance of dating the ash.

Dating needs minerals formed in the magma chamber, such as feldspars (argon/argon dating) or zircons (uranium/lead). Because those minerals are heavier than glass and tend not to travel far, ash far from its volcano often lacks them, so not every deposit can be dated. If its chemistry matches a deposit near the source, an undatable ash can still be given an age.

What may be missing

More ash layers probably exist. Ash is laid down fairly evenly but is preserved unevenly: rivers wash it downstream, higher ground erodes, and marshes and deep lakes tend to keep it as it fell. Some layers found are thin and preserved only in the northern half of the monument. Younger deposits were searched less because they have produced fewer fossils.

What comes next

The geologist is finishing sample analysis and plans to publish the data. The team would like a sedimentologist or stratigrapher to join and tie the environments of the sites together through time, to tell which animals lived together, and to link Hagerman to other fossil sites in and around Idaho using the same ash analysis.

Sources

• National Park Service, Hagerman Fossil Beds National Monument, Park Paleontology News, From the Ashes: how volcanologists can help paleontologists reconstruct the ancient past at Hagerman Fossil Beds National Monument, written by a Geoscientists-in-the-Parks guest scientist and the monument's lead paleontologist, drawing on an interview with the USGS geologist; the interviewee's name and the photographs of people left out; rewritten in hubnx's own words.