Tankersley and a team of students recently spent a day on the karst
hilltop site of famed Serpent Mound, where they took core soil samples from several sinkholes, looking for signals of previous climate shifts and, possibly, their causes.
“What was the environment like? How did the environment and the climate change through time?” he asked.
To find out, Tankersley’s team is looking for interlayer differences in carbon.
The tales carbon can tell.
UC students who take organic chemistry know that there are 3 common, naturally-occurring carbon isotopes: “normal” carbon-12, slightly heavier carbon-13 and unstable, radiogenic carbon-14.
Biology students further know that different plants accomplish photosynthesis by different processes. In photosynthesis, sunlight is harnessed by green chloroplasts to catalyze a reaction in which carbon dioxide and water are metabolized to produce stored sugars and waste oxygen.
C3 plants, like potatoes and fescue grasses, accomplish photosynthesis by a relatively inefficient process, which becomes even less efficient in hotter, drier conditions. For this reason, C3 plants typically grow in temperate or cool, moist climates.
C4 plants, such as maize, introduce an extra carbon-fixing step in the photosynthetic pathway, which makes the overall process more efficient: less carbon dioxide is lost to photorespiration, and C4 plants thus better tolerate warm, dry conditions.
Because C3 plants are less efficient photosynthesizers, they’ve evolved to become “picky” about the isotopes of carbon they’ll transpire, Tankersley said. C3 plants will preferentially exclude heavier carbon isotopes.
Thus, C3-derived and C4-derived plant biomatter will exhibit differences in their isotopic carbon levels: C3 biomatter includes less carbon-13 and carbon-14 than C4 biomatter.
“What's all this mean in the cores? I can take a little piece of soil, extract the organics and put them into an elemental mass spectrometer,” Tankersley said, which reveals the sample’s isotopic carbon ratio.
Differences in layers’ isotope ratios, Tankersley said, can point to changes in the site’s vegetation over the centuries.
“Through the core, we can see the changing vegetation through time,” Tankersley explained.
Although several factors — including human habitation and agriculture — might explain a single, local environmental change, shifts correlated across cores, from widely-scattered sites, are best explained by historical changes in Earth’s climate, he said.
And, Tankersley asserted, there seem to have been many.
What caused ancient climate shifts?
It’s widely accepted that some of the most dramatic environmental shifts in Earth’s history had extraterrestrial origins: comet or asteroid impacts.
For example, sweeping climate change signals, abrupt cessations in the fossil record of many species (including the dinosaurs), and an abnormally high level of iridium — rare on Earth’s surface, but commonly found in meteorites and asteroids — at the worldwide Cretaceous-Tertiary (K-T) boundary have been cited as strong evidence for a cataclysmic impact event.
Asteroid and comet encounters aren’t the only potential suspects in Earth’s ancient climatic events. Supervolcanoes — like Krakatoa, Sumatra’s Mt. Toba, or the Yellowstone caldera — from time to time release enough gas, ash and dust into the atmosphere to affect the global climate.
Volcanic incidents are often identifiable in core samples because their eruptions bring rare metals — platinum, for example — to the surface. Like the unusual iridium layer at the K-T boundary, a worldwide spike of a rare metal at a given soil or glacial ice layer can indicate a past deposition of volcanic material.
And, Tankersley cautioned, anthropogenic (human-caused) changes to the landscape can also shift the climate.
Some scholars, for example, suspect that the deforestation of Europe may have contributed to a general warming trend during the late Classical and early Medieval period — a trend that suddenly reversed, worldwide, in the 14th Century CE.
That cooling, colloquially known as the “Little Ice Age,” lasted until the middle of the 19th Century CE.
It caused declines in civilizations around the world, Tankersley said, probably including Late Woodland societies — like the Algonquinian people he believes likely inhabited the Serpent Mound site — in the final centuries leading up to the European conquest of the Americas.
He suspects that a series of supervolcanic blasts could be the culprit for the Little Ice Age.
Why? It’s the best available explanation for both the abrupt, worldwide shift in temperatures, and a platinum spike he’s observed — at multiple sites throughout the Western Hemisphere — in a soil layer dating to around the start of the cooling period.
How did the Little Ice Age affect peoples in the Americas?
Tankersley believes that the Serpent Mound site was continuously or repeatedly inhabited over the 2,000 years prior to European contact, either by successive societies or — more likely, in his opinion — by a continuity of proto-Algonquin cultures.
“Behind that burial mound, there’s a village site,” Tankersley said, pointing toward one of the several mounded earthworks that are situated near the larger, snakelike effigy.
“There was a village down below,” too, he added. “You have winter and summer occupations.”
In summer, the Algonquinians likely lived on the hill because it was up out of the flood plain, he said, and thus safer to grow crops there.
The people who constructed the Serpent Mound effigy did so around a series of sinkholes, he hypothesized, to divert rainwater into them and create retention ponds: a prehistoric form of irrigation and municipal water management.
Another advantage of conducting their agriculture on Ohio’s hilltops was that the soil thereupon is loose, wind-deposited loess, whereas the soil in its river plains is highly clay-enriched,” Tankersley explained.
“They have bone, shell, wood [farm implements]. Go down in the floodplains and try to bust up that soil,” he chuckled, shaking his head.
Bottom-land cultivation was a European innovation, Tankersley said, made possible only by metal sodbuster plows and the domesticated animals that could pull them.
In the winter, the people probably moved back down to the valley below, where they foraged and hunted along rivers and creek beds.