Bryn Mawr Geologists and Students Study the Causes of Key Biodiversity Event

Posted May 2nd, 2013 at 12:11 pm.

After looking to better understand Earth’s most devastating mass extinction, Assistant Professor of Geology Pedro Marenco and his students have now turned to a period in which life flourished dramatically.

“Contrasting long-term global and short-term local redox proxies during the Great Ordovician Biodiversification Event: A case study from Fossil Mountain, Utah, USA” is the second manuscript to result from Marenco’s Carbonate Petrography and Geochemistry class and many of the results were produced in the new Geochemistry Lab Suite. In addition to Marenco, the paper’s authors are Geology Lecturer Katherine Marenco, Rachael Lubitz ’11, and Danielle Niu (Hfd ’12).

The Great Ordovician Biodiversification Event or (GOBE) occurred during the Ordovician (about 480 million years ago) and is a time in which “animal life really got a foothold and started to take off,” explains Marenco.

While researchers are in agreement that this was a time of tremendous growth in terms of marine animal diversity, they’re still not in agreement as to why it happened, he says.

At places like Fossil Mountain, the geologic record of the time presents somewhat conflicting clues. Shale deposits from the time show few fossils but limestones of the same period are rich with the evidence of life.Marenco article

This has led some researchers to believe that in areas like Fossil Mountain there was little oxygen present when the sediments were deposited, and the fossil-rich limestones were formed by storm events that washed the fossils in from shallow, better-oxygenated environments.

“So in this interpretation, the Fossil Mountain area is sort of like a basin cut off from the rest of the ocean, and we would see little animal life except as a result of these storm events,” explains Marenco.

Other researchers noted that there is evidence that the fossils found at Fossil Mountain were from creatures that actually lived in the area and theorized that, despite evidence from the shale, the water in the area must not have been anoxic (lacking in oxygen). However, this model still left open for debate whether the non-anoxic conditions were local or global.

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

In the summer of 2010 Lubitz was at Fossil Mountain studying the paleontology of the fossils there with Katherine Marenco.

Lubitz was about to take Pedro’s carbonates class and offered up her samples to be used in that class and to undergo geochemical analysis.  Together, the class analyzed Lubitz’s samples using the ELTRA CS2000 carbon/sulfur determinator, which is housed in Bryn Mawr’s Geochemistry Lab Suite.

The results came back and “the pattern for anoxia was not there,” says Marenco. “So we decided to try another approach.”

The second type of analysis was done in 2012 by Niu.

Niu extracted sulfate minerals from the samples (a laborious process) and sent the extracts to U.C. Riverside to measure stable isotopes of sulfur, which can provide insight into the oxygenation of the whole ocean.

“What was interesting is that Danielle’s sulfur isotope results, and those of another group of researchers studying rocks of the same age in Newfoundland, suggested that the deep ocean was anoxic, even though the data generated by our carbonates class suggested that the shallow ocean had plenty of oxygen,” says Marenco. “In fact, this deep ocean environment doesn’t seem all that different from the environment we theorized existed during the End Permian mass extinction. So again we have this question of ‘Why did life flourish during this time?’”

The answer, Marenco and his fellow researchers theorize, may lie not in the deep oceans but in the shallow environments like that at Fossil Mountain.

“This is a time during which there were these massive continental shelves covered in shallow water,” he explains. “When you combine our findings with the findings from Newfoundland, you get pretty strong evidence that you’re not dealing with an isolated basin at Fossil Mountain. This was the global ocean that was harmful to life. But even though the deep oceans were pretty nasty, the shallows had enough exposure to atmospheric oxygen to teem with life. This is important because one of the dominant hypotheses to explain why the GOBE occurred is that oxygen levels increased in the deep ocean. 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.  Indeed, our work supports another hypothesis for the GOBE that argues that it is the availability of shallow water habitat during this interval in Earth’s history that allowed for dramatic biodiversification.”

The next step, Marenco says, is for students to continue to test this hypothesis by examining the evidence of life from the time in deep ocean rocks.

“This is exciting stuff. And it’s the sort of science that shows the value of a liberal arts approach to science,” says Marenco. “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.”

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