August 2013 Archways

Why Did Life Flourish?

Faculty-student collaboration yields a paper on the origins of biodiversity.


Rachael Lubitz ’11 conducted field research at Fossil Mountain, Utah in 2010. Photo by Sophia Wolfenden ’10.

Rachael Lubitz ’11 conducted field research at Fossil Mountain, Utah in 2010. Photo by Sophia Wolfenden ’10.

Some 480 million years after what is arguably the most important increase of marine biodiversity in the history of the Earth, a Bryn Mawr professor and his students are asking what happened. During a 25 million-year period, called the Great Ordovician Biodiversification Event (GOBE), “animal life really got a foothold and started to take off,” explains Assistant Professor of Geology Pedro Marenco. Early fish swam the oceans, the first moss-like plants grew in swamps, and invertebrate animals (specifically arthropods) walked on land.

But researchers remain baffled about what sparked this explosion of life, and Marenco’s Carbonate Petrography and Geochemistry class set out to look for causes. Marenco, his wife and Geology Lecturer Katherine Marenco, Rachael Lubitz ’11, and Danielle Niu (Hfd ’12), turned to the fossil record at Utah’s aptly named Fossil Mountain. Their efforts were published recently in the scientific journal Palaeogeography, Palaeoclimatology, Palaeoecology.

Fossil Mountain offers up conflicting clues: On the one hand, shale deposits at the site show few fossils, but limestone from the same period is rich in evidence of life. Based on that evidence, some have posited that the area was an oxygen-poor basin cut off from the rest of the ocean. In this picture, storms washed fossils into the basin from shallow, oxygen-rich waters. Other researchers argue that, shale evidence aside, the water in the Fossil Mountain area did indeed have enough oxygen to support the creatures captured in the limestone record. 

“So we have these different hypotheses, but there’s been very little testing done to support either,” Marenco says. “That’s where our students come in.”

Summer 2010 found Lubitz conducting field research at Fossil Mountain, and that fall, she offered up her rock samples to the Carbonate class for further study. Using equipment in Bryn Mawr’s Geochemistry Lab, the class analyzed the samples and found evidence of abundant oxygen.

Says Marenco, “So we decided to try an additional approach.”

In 2012, Niu undertook the laborious process of extracting sulfate minerals from the samples and sent them to the University of California-Riverside to measure stable isotopes of sulfur—a technique that can provide insight into the oxygenation of the whole ocean. Niu’s findings confirmed those of researchers looking at rocks of the same age in Newfoundland, Canada. Both results, explains Marenco, “suggest that the deep ocean was anoxic even though the data generated by the Carbonates class suggests that the shallow ocean had plenty of oxygen.  So again we have this question of ‘Why did life flourish during this time?’” The answer, Marenco and his fellow researchers hypothesize, might lie not in the deep oceans but precisely in the kind of shallow environments found at Fossil Mountain.

“One of the dominant hypotheses to explain why the GOBE occurred is that oxygen levels increased in the deep ocean,” says Marenco. “But our data suggests that this is not the case and that life can thrive in shallow settings regardless of what is happening in the deep.”

In fact, between the Fossil Mountain and Newfoundland findings, another hypothesis is supported. “This is a time during which there were these massive continental shelves covered in shallow water,” Marenco explains. “And even though the deep oceans were pretty nasty, the shallows had enough exposure to atmospheric oxygen to teem with life.”

“This is exciting stuff—and it’s the sort of science that shows the value of a liberal arts approach to science,” Marenco says. “We didn’t just look at our results in isolation. By also looking at the work of these other researchers, we were able to get a much broader understanding of the environment we were studying and to make a significant scientific contribution.”