July 10, 2014
Better Understanding Of Arctic Climate Change Based On Shark Teeth
April Flowers for redOrbit.com - Your Universe Online
Today, polar bears and other animals adapted to extremely cold environments inhabit the Arctic tundra. In the past, around 53 to 38 million years ago (the Eocene epoch), the Arctic was not a frozen tundra -- rather, it was more like a huge temperate forest with brackish water. This forest was home to a wide variety of wildlife, including the ancestors of modern-day tapirs, hippo-like creatures, crocodiles and giant tortoises. Well-documented terrestrial deposits have given us much of our current knowledge about this region as marine records have proven more difficult to come by.
Last month, redOrbit reported that a University of Chicago-led study had shown that Arctic sharks, specifically Arctic sand tiger sharks, lived in brackish water rather than the salt water modern sharks inhabit. A new study from the same team, published in Geology, demonstrates an expanded understanding of Eocene marine life.
Scientists consider the Arctic an important region because the temperature there is increasing at twice the rate of the rest of the globe.
Kim said that understanding past climate change in the Arctic can provide information about the current situation, and help improve the accuracy of future predictions. According to Kim, the Eocene epoch 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."
Previously, marine records came from deep-sea cores pulled from a central Arctic Ocean site called the Lomonosov Ridge, for the most part. The current study used shark teeth collected from a new coastal site on Banks Island, which allowed them to gain a more complete understanding of the changes in ocean water salinity across a broader geographic area. Because they preserve well and are incredibly abundant, shark teeth are one of the few available vertebrate marine fossils for the Eocene.
The mass ratio of oxygen isotopes 18 to 16 were isolated and measured from the prepared enameloid of the shark teeth, a substance which is somewhat different from human tooth enamel. The isotopic oxygen ratio found in the teeth is directly regulated by the water temperature and salinity of the water that sharks constantly exchange with the ocean. The team made assumptions about the water temperature that allowed them to focus on extrapolating salinity levels.
"The numbers I got back were really weird," Kim said. "They looked like fresh water." Kim was surprised by these results because 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."
The researchers ran many tests to eliminate any possible contaminants, due to the age of the teeth being sampled. The results remained constant, however. They believe that the findings could mean sharks will be able to cope with rises in temperature and decreases in water salinity. Shark fossil records date back over 400 million years, showing that they have survived many mass extinctions and have great ecological plasticity.
The theory that the Arctic Ocean was isolated from global waters is also supported by this new evidence.
"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."
Kim hopes to continue her research to understand the ecology and evolution of sharks by expanding it both geographically and in geologic time. However, she said 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."