Back to the future: ancient sediment samples shed light on climate change

Sediment samples, collected 700m below the floor of the Bering Sea, helps to predict the impacts of climate change.

The research vessel JOIDES Resolution on an earlier expedition.

A team of researchers, led by Professor Christiana Ravelo of the University of California, embarked on a nine-week expedition last summer to collect samples from the Bering Sea. The research vessel JOIDES Resolution drilled 700 meters below the sea floor to reach sediment samples deposited during the Pliocene Warm Period, 3.5 to 4.5 million years ago.

“Evidence from the Pliocene Warm Period is relevant to studies of current climate change because it was the last time in our Earth’s history when global temperatures were higher than today,” Ravelo said.

Climate scientists believe studying the Pliocene Warm Period can help to predict the impacts of global warming, particularly in polar regions, which are thought to be experiencing greater warming than elsewhere on Earth.

Analysis of the soil samples demonstrated exaggerated warming also occurred at the poles during the Pliocene Warm Period. The evidence indicates temperatures in the Bering Sea were five degrees Celsius warmer that today, whereas average global differences were closer to three degrees.

Large swathes of the Bering Sea, which borders Alaska in the East and Siberia in the West, are covered with sea-ice from November to June every year. Sea-ice forms an important part of polar ecosystems, while the annual melting of the sea-ice alters regional water conditions and influences biological productivity.

Microorganisms typically associated with sea-ice and debris carried from land by ice floes were absent from sediment samples, suggesting the Bering Sea was ice-free all year round during the Pliocene Period.

Fossils of phytoplankton and other microorganisms found in the sediment indicate a strong community of organisms existed.

Deep sea organisms where also discovered in the sediment, which require greater oxygen concentrations than are present in the Bering Sea today. Ravelo believes this suggests mixing of water layers was greater during the Pliocene Warm Period than it is today, contradicting current theories of ocean circulation.

“We usually think of the ocean as being more stratified during warm periods, with less vertical movement in the water column,” she said. “If the ocean was actually overturning more during a period when it was warmer than today, then we may need to change our thinking about ocean circulation.”

Professor Ravelo presented the results of the expedition at an American Geophysical Union (AGU) conference in San Francisco earlier this week. She believes this research will be beneficial to the wider scientific community.

“The information we found tells us quite a bit about what things were like during the last period of global warming. It should benefit the scientists today who are sorting out how ocean circulation and conditions at the poles change as the Earth warms,”

Planning is already underway for the next ocean drilling expedition aboard the JOIDES Resolution.


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