Geology Professor and Students Look for Clues to Understanding Environment Following Mass Extinction

Posted July 24th, 2012 at 2:57 pm.


Stromatolites in Shark Bay Australia

New research findings by Assistant Professor of Geology Pedro Marenco–aimed at understanding the environment following Earth’s most dramatic mass extinction–will appear in the August print edition of Geology and are now available online.

Results presented in the paper were partly generated in a class Marenco taught during the spring of 2011 called Carbonate Petrography and Geochemistry, and Julie Griffin ’11 is credited as a co-author.

Marenco and his students examined samples from reef-like mounds at Lost Cabin Spring, Nevada, to try to better understand the environment that existed during the aftermath of the End Permian Mass Extinction.

Conventional wisdom is that it took about five million years for life to recover from the events that led to the extinction. However, it remains unclear as to what the environmental factors were that made it so difficult for life to reestablish itself, explains Marenco.

While there was little diversity to life during this period, one thing that did thrive were stromatolite-building microorganisms.

“Before the evolution of animals these stromatolite structures were all over the place and grew to be really spectacular reefs. Once animals evolved, the stromatolites declined both in terms of abundance and diversity,” says Marenco. “At first blush, it seems like simple cause-and-effect animals became more populous and ate the microorganisms that created the stromatolites.”

This thinking seems to be validated by the fact that today stromatolite mostly survive in places like Shark Bay Australia, where the hyper-saline environment greatly limits animal life.

Previous research pointed toward a lack of oxygen (anoxia) as being an environmental factor that would limit animal life and may have helped the stromatolites to flourish.

“But the thing is, when you look at just how well these organisms thrived during this period, the absence of animals might not have been the only factor,” Marenco says. “That’s what we were interested in finding out. Was there something else about the environment at that time that was advantageous to the growth of these microorganisms and structures?”

To do their research, Marenco, Griffin, and other students in the Carbonate Petrography and Geochemistry examined samples of mounds of stromatolite from Lost Cabin Springs using the ELTRA CS2000 carbon/sulfur determinator in the new geochemistry lab.

The ELTRA carbon/sulfur determinator measures the amount of sulfur and organic carbon in a sample, key indicators of the amount of oxygen present when a rock was deposited.

“Although other people have interpreted the presence of these stromatolite to indicate the mounds were formed under anoxic conditions, our analysis–which was the first of this kind done on samples from this area–suggests otherwise,” says Marenco. “After we ran the tests and started to interpret the data we found that levels of sulfur and carbon were super low, which is not at all what you’d expect from an anoxic environment.”

While collecting samples the researchers came across something else that leads them to believe the mounds were not the result of anoxic conditions: evidence that sponges (which require oxygen) were also present at the time the mounds were formed.

“This is one of the earliest examples of sponges building reefs after the mass extinction and it also suggests that oxygen had to be present,” says Marenco.

The next step for Marenco and his students is to study samples they’ve gathered this summer in Utah.

“There are all of these assumptions that we’ve made about these things because their heyday was when there were no animals but the truth is that the environmental controls on these structures are very poorly understood. The opportunities for new research are tremendous,” says Marenco. “Our research has led to a whole new set of questions.”

Marenco points to the small class sizes at Bryn Mawr as an important factor in allowing undergraduates to do this sort of research.

“You’ve got to be able to spend time in the labs on the machines. We had seven students in the Carbonate Petrography and Geochemistry class. Probably the most we could have had is about 10. There just aren’t that many schools that have the combination of equipment and small class size necessary to make this sort of class possible,” says Marenco.

Marenco, who is on leave for the 2012-13 academic year, plans to teach the course again using the Utah samples in the fall of 2013.

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