Article ID: 620202

Released: 9-Jul-2014 10:45 AM EDT

Source Newsroom: University of Chicago

  • Credit: Robert Kozloff/University of Chicago

    Sora Kim is the T.C. Chamberlin Postdoctoral Fellow in Geophysical Sciences at the University of Chicago. In her latest study, she examined environmental conditions of the Arctic Ocean during the Eocene epoch (53 to 38 million years ago), as a prehistoric proxy for what could happen to the region if modern Earth experiences a runaway greenhouse effect.

  • Credit: Robert Kozloff?University of Chicago

    Sora Kim has analyzed oxygen 18 and oxygen 16—two isotopes of the oxygen atom that contain only slight subatomic variations—of fossil and modern sharks teeth for insights into the potential adaptability of sand tigers to global warming. These teeth come from the Eocene epoch, which ended approximately 38 million years ago.

  • Credit: Erik Rebeck

    Both graceful and docile, this modern sandtiger shark swims through a school of round scad fish in the coastal waters off North Carolina. Modern sand tigers prefer waters of high salinity, though they tolerate brackish waters for short periods as well.

  • Credit: Jaelyn Eberle

    A group of orange tents in the middle of this photograph marks the destination of one of Jaelyn Eberle’s expeditions to Aulavik National Park on Banks Island in the western Arctic Ocean. Here is where the Eames Rivers weaves its way into the Arctic Ocean. This area marks one of only two known sites that yield vertebrate marine fossils from the Eocene epoch, dating from 53 to 38 million years ago.

  • Credit: Jaelyn Eberle

    This fossil sand shark tooth was among those recovered from Aulavik National Park by Jaelyn Eberle. Sharks teeth from the Eocene epoch, dating from 38 to 53 million years ago, are the only vertebrate marine fossils avaiable from the Arctic region during this period of Earth history.

  • Credit: Jaelyn Eberle

    This map and the inset show the location of Banks Island and Aulavik National Park, which ajoins the western Artic Ocean. Jaelyn Eberle of the University of Colorado, Boulder, has led multiple expeditions into the region, where she has recovered abundant fossil sharks teeth from a coastal site in the park.

Newswise — A new study shows that some shark species may be able to cope with the decreasing salinity of Arctic waters that may come with rising temperatures.

The Arctic today is best known for its tundra and polar bear population, but it wasn’t always like that. Roughly 53 to 38 million years ago during what is known as the Eocene epoch, the Arctic was more similar to a huge temperate forest with brackish water, home to a variety of animal life, including ancestors of tapirs, hippo-like creatures, crocodiles and giant tortoises. Much of what is known about the region during this period comes from well-documented terrestrial deposits. Marine records have been harder to come by.

A new study of shark teeth taken from a coastal Arctic Ocean site has expanded the understanding of Eocene marine life. Leading the study was Sora Kim, the T.C. Chamberlin Postdoctoral Fellow in Geophysical Sciences at the University of Chicago, in coordination with Jaelyn Eberle at the University of Colorado, Boulder, and their three co-authors. Their findings were published online June 30 by the journal Geology.

The Arctic is of special interest today because it is increasing in temperature at twice the global rate. According to Kim, past climate change in the Arctic can serve as a proxy to better understand our current climate change and aid future predictions. The Eocene epoch, she said, is like a “deep-time analogue for what’s going to happen if we don’t curb CO2 emissions today, and potentially what a runaway greenhouse effect looks like.”

Before this study, marine records primarily came from deep-sea cores pulled from
a central Arctic Ocean site, the Lomonosov Ridge. Kim and Eberle studied shark teeth from a new coastal site on Banks Island. This allowed them to better understand the changes in ocean water salinity across a broader geographic area during a time of elevated global temperatures. Shark teeth are one of the few available vertebrate marine fossils for this time period. They preserve well and are incredibly abundant.

To arrive at their results, Kim isolated and measured the mass ratio of oxygen isotopes 18 to 16 found in the prepared enameloid (somewhat different from human tooth enamel) of the shark teeth. Sharks constantly exchange water with their environment, so the isotopic oxygen ratio found in the teeth is directly regulated by water temperature and salinity. With assumptions made about temperatures, the group was able to focus on extrapolating salinity levels of the water.

The results were surprising. “The numbers I got back were really weird,” Kim said. “They looked like fresh water.” The sand tiger sharks she was studying are part of a group called lamniform sharks, which prefer to stay in areas of high salinity.

“As more freshwater flows into the Arctic Ocean due to global warming, I think we are going to see it become more brackish,” said Eberle, associate professor of geological sciences at CU-Boulder. “Maybe the fossil record can shed some light on how the groups of sharks that are with us today may fare in a warming world.”

Because the teeth are 40 to 50 million years old, many tests were run to eliminate any possible contaminates, but the results were still the same. These findings suggest that sharks may be able to cope with rises in temperature and the subsequent decrease of water salinity. It has long been known that sharks are hardy creatures. They have fossil records dating back some 400 million years, surviving multiple mass extinctions, and have shown great ecological plasticity thus far.

Additionally, these results provide supporting evidence for the idea that the Arctic Ocean was most likely isolated from global waters.

“Through an analysis of fossil sand tiger shark teeth from the western Arctic Ocean, this study offers new evidence for a less salty Arctic Ocean during an ancient ‘greenhouse period,’” said Yusheng (Chris) Liu, program director in the National Science Foundation (NSF)’s Division of Earth Sciences, which co-funded the research with NSF’s Division of Polar Programs. “The results also confirm that the Arctic Ocean was isolated during that long-ago time.”

While Kim has hopes to expand her research both geographically and in geologic time in an effort to better understand the ecology and evolution of sharks, she remarked that “working with fossils is tricky because you have to work within the localities that are preserved. “You can’t always design the perfect experiment.”
— Isabella Penido


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