Temperatures climbed into the triple digits last summer as the researchers hiked Sawtooth National Forest in southern Idaho. They passed trees charred by a wildfire that consumed more than 90,000 acres in the fall of 2020. Haze hung in the air from another fire that burned out of control hundreds of miles away in Oregon.
Defying the stifling heat, the scientists pressed on, searching for the right place to dig. Under their feet, and below the baking Idaho soil, lies evidence of a time now unrecognizable.
Nearly 300 million years ago, during the Permian period, this place sat at the bottom of a shallow sea. Phytoplankton bloomed on the surface and ancient fish, brachiopods, and sponges thrived.
As these creatures lived, died, and settled on the sea floor over millions of years, they formed deposits captured in the rock record. Kimberly Lau and a team of scientists arrived in Idaho to collect samples and bring them back to Penn State for new analysis that could improve our understanding of what was happening in the oceans of ancient Earth.
“Huge amounts of phosphate were deposited here millions of years ago in an environment we really don’t see today—this vast shallow inland sea,” said Lau, assistant professor of geosciences. “We hope these samples can shed new light on how environments changed in the deep past and how nutrients and by extension, life, responded.”
In the present, increasingly frequent and severe forest fires and rising temperatures—Idaho sweated through its hottest June and July on record in 2021—tell an important story of a changing climate. But understanding environmental changes in Earth’s ancient past is also important for predicting how the planet will respond far in the future.
“We can’t go back in time and directly measure things like temperature or carbon dioxide concentrations,” Lau said. “There are no thermometers to read or ice cores to sample when you go back a million years. Even when we look at rocks, we are taking some incomplete measure of what the environment was like.”
Scientists like Lau turn to geochemical clues in the marine sedimentary record to reconstruct environmental changes in the planet’s history.
Understanding the availability of nutrients, for example, has critical implications for the health of the oceans—levels that are too low can be insufficient to support a healthy ecosystem, and levels that are too high can result in eutrophication, an excessive richness of nutrients that can lead to oxygen depletion and animal die-offs.
“All we have are these tools called proxies,” she said. “And it’s our job as geologists to develop them, evaluate them, and make the most accurate and precise interpretations we can.”
The Great Dying
Some 250 million years ago, life on Earth nearly came to an end.
Massive volcanic eruptions spurred a chain of events that led to the largest mass dying recorded in history—the end-Permian extinction event. Entire ecosystems collapsed as a catastrophic number of species perished in the oceans and on land.
It took more than five million years, into the Middle Triassic period—which eventually gave rise to the dinosaurs—before life approached its former levels of diversity, the longest recovery time from any mass extinction.
When a doctoral student at Stanford in 2016, Lau was lead author on a paper published in the journal Proceedings of the National Academy of Sciences that identified ocean anoxia, or low oxygen conditions, that corresponded with the delayed recovery on a global level.
Lau found ocean oxygen levels plunged globally during the mass extinction and recovered slowly, only approaching their previous conditions after five million years. It was the first evidence showing this beyond a regional scale.
“This was a big extinction event, so you might expect a slow recovery,” she said. “But five million years is extremely long. It’s still a debate why the Earth system collapsed but then came back and recovered over that time scale. My research showed a really clear pattern that anoxia had a big role in the delay in recovery from the extinction.”
Lau was among the first to conduct this type of research using uranium isotopes, an emerging proxy for oxygen levels in past oceans. She measured uranium preserved in marine limestone rocks and found isotope values, expected to be higher in oxygen-rich waters, also dropped during the extinction event and didn’t return until biodiversity had risen near its initial levels.
“Dr. Lau has been developing novel proxies of ancient environmental conditions to investigate the relationship between environmental change and the health of the biosphere,” said Lee Kump, John Leone Dean in the College of Earth and Mineral Sciences.
Kump, who is also a professor of geosciences at Penn State, collaborated with Lau on the paper.
“As our climate changes and we worry about the effects on life and environments, Dr. Lau’s work allows us to see how the biosphere has responded to similar changes in deep time, providing an analog for the future,” he said.
At Penn State, Lau has continued her work developing isotopes and geochemical proxies to investigate oxygen levels in ancient oceans, including continued research on the end-Permian, the end-Triassic extinction, and the transition between the Ediacaran period and the Cambrian, when multicellular life exploded.
“To this day my research seeks to understand how the Earth has changed in terms of its environments, what causes those changes, and how those changes are linked to the evolution and extinction of life on our planet,” she said. “I’m particularly interested in understanding what causes anoxia and how those low-oxygen conditions can be sustained for long periods of time.”
A geochemist’s life
Last summer, Chelsie Bowman got to be a different kind of scientist. She climbed the mountainous Idaho terrain, dug into hillsides with her rock hammer, and exposed sections of soft rock previously hidden away for millions of years.
“At this point, even though I’ve been doing this for a decade, my parents still think what I do must be similar to Indiana Jones or Alan Grant from Jurassic Park,” said Bowman, a postdoctoral scholar who works with Lau at Penn State. “To start with the basics, we’re not looking for dinosaurs.”
In reality, geochemists like Bowman and Lau spend much of their time in the laboratory, running complex chemical analysis on rocks that collaborators collected. But last summer, the pair, and researchers from Albion College in Michigan, embarked on the two-week journey to Idaho.
“Because these samples were formed in an ancient ocean basin, we can learn things about the chemistry of those ocean waters based on the chemistry of the rocks we’ve collected,” Bowman said. “We’re trying to learn things like how oxygen concentrations might have changed through time and space in the basin we’re looking in.”
Low oxygen concentrations in the sea are a clue to why large deposits of the nutrient phosphate can form, sometimes over millions of years. Idaho has one of the country’s largest reserves—the ore has been mined there for more than a hundred years, mainly for use in fertilizer products.
“Our end goal is to be able to tell a story with our data about random events that happened hundreds of millions of years ago,” Bowman said. “Part of that is using as many different lines of proxy evidence as possible to narrow in on what was going on and to make our results as irrefutable as they can be. That’s one of the best things about what we do—it’s very much like putting a puzzle together.”
Today, oxygen-depleted dead zones occur in oceans when nutrient-rich runoff feeds massive blooms of phytoplankton. As these organisms die and sink to the ocean floor, large amounts of microbes break them down and consume most of the available oxygen in the process, starving ocean creatures of what they need to survive.
This same process may have played out repeatedly in the deep past, fueled by ocean currents bringing deep, nutrient-rich water to the surface, or from continental weathering, the processes that break down rocks and allow water to transport ions dissolved from minerals once stored inside. More weathering occurs when the planet is warmer and wetter, like in times of high greenhouse carbon dioxide concentrations after a major volcanic eruption.
“Weathering is often called Earth’s thermostat,” Lau said. “It’s how we have maintained habitable conditions for life for almost two billion years. It doesn’t happen next year or in ten years. This is a really slow process. That’s why understanding how sensitive Earth is to this thermostat activity helps us understand how quickly it can bounce back if something is perturbing it.
“Understanding these ancient events gives us some sense of how the Earth can respond in the future.”