November 2014 Uncategorized

Life in Ordovician Seas


Assistant Professor of Geology Pedro Marenco

Assistant Professor of Geology Pedro Marenco began his career studying the Permo-Triassic mass extinction—the greatest of all time—but recently switched to a more optimistic moment in evolutionary history: the Great Ordovician Biodiversification Event, aka the GOBE.

The fossil record tells the history of life, and in his KIM Talk with Nancy Toure ’15, Marenco traced that story from the Cambrian to the modern era. Although they can still be found today, the Cambrian evolutionary fauna were most dominant—the most diverse and the most abundant—about 500 million years ago during the Cambrian period. Then came the Paleozoic evolutionary fauna. For Marenco’s money, these specimens (brachiopods and crinoids and nautiloids) are “the coolest fossils.” These are also visible in the very architecture of Bryn Mawr: Step outside, Marenco advised the audience, to study Goodhart and Thomas Great Hall. “The archways and doorways are made of Indiana limestone that is Paleozoic in age. It’s chock-full of crinoids and brachiopods and occasional nautiloids.”

But “during the GOBE,” Marenco explained, “the Cambrian fauna went by the wayside, and the Paleozoic fauna took over. This event set the stage for about 200 million years of evolution”—a process that culminated in the emergence of Modern fauna. But despite its importance in the evolutionary record, he continued, “we don’t have a full understanding of how the GOBE happened or even why it happened. But we have some clues.”

To investigate, Marenco and his team traveled to the Utah desert. During the Ordovician period, the configuration of the continents was very different from what it is today. Laurentia, an ancient continent that straddled the equator, formed the land mass that would become North America. Along its continental shelf lay what is now the state of Utah, back then a tropical, underwater breeding ground, “prime real estate for life,” as Marenco put it.

As on today’s Earth, the majority of the planet then was deep ocean—waters that were very nasty. “Specifically, I mean that it might have had little to no oxygen in it,” Marenco said, “and we have a term for that. We call it anoxia.”

One mystery is particularly intriguing: Did those nasty, anoxic deep waters ever infiltrate the shallow environment? Did the anoxia slow down the burgeoning of life? Or, as Marenco asked, “Could the Great Ordovician Biodiversification Event have been the Greater Ordovician Biodiversification Event?”

The clues that might answer that question were locked in the shallow Ordovician marine rocks that the team collected in Utah. And back in the lab, they began looking for evidence of anoxia.


Nancy Toure ’15

As Nancy Toure explained, “With less oxygen, more organic carbon is preserved, and more sulfide is produced.” To determine the oxygenation levels of these shallow marine environments, she measured the total sulfide abundance and total organic carbon abundance in the rock samples. What she found—low sulfide abundance and low organic carbon—suggested that the anoxic conditions of the Ordovician deep oceans did not affect life within the shallow marine.

But Marenco remained intrigued by the question: Could the GOBE have been greater?

Being a scientist, he dug deeper into the data. “It turns out,” he explained, “that even rocks formed in shallow environments have geochemical indicators in them that tell us a lot about the health of the entire ocean, about the health of the deep ocean, specifically the oxygenation of the ocean.”

Anoxic environments, like the deep Ordovician ocean, teem with bacteria that cannot tolerate oxygen and, by contrast, consume sulfate and turn it into sulfide. But these organisms turn out to be picky eaters: they display a decided preference for lighter sulfur and leave the heavy stuff behind.

As luck would have it for Marenco’s purposes, that heavy sulfate gets preserved, leaving behind “a signature, like a footprint, recorded in the rocks.” Looking at that record over the span of the Ordovician, Marenco’s team found indications of high anoxia for much of the Ordovician period. But their analysis uncovered an anomaly: a burst of oxygenation during the most intense period of the GOBE. “When the planet got even more oxygen, [the organisms] went crazy and diversified even more,” he said. And this finding, Marenco said, is something that no one else has seen. “The research is not complete yet, but we’re working on it. And it’s a real contribution that we’ve made as a lab, and I don’t think I could have done it without the help of wonderful students like Nancy.”

Curious to hear more? Listen to the KIM Talks.