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Fossils Help Determine Ocean’s Role in Last Ice Age

Scientists from The University of Tasmania’s Institute for Marine and Antarctic Studies and Simon Fraser University have used the fossil record to pull together the first global database of ocean temperatures over the past 125,000 years in order to explain why carbon dioxide levels were low at the time.

The study, published in Earth and Planetary Science Letters, combined ocean temperature records with other studies to show how carbon dioxide took different paths into the deep sea during different phases of the ice age.

“This study shows for the first time how temperatures changed across the whole ocean as the earth entered the last ice age,” said lead author A/Prof Karen Kohfeld. “This new understanding of ocean temperature changes hints at some important thresholds in the climate system.

“It’s clear that some parts of the system, such as sea-ice around Antarctica, responded rapidly when the ocean cooled. Other parts, like deep ocean circulation, changed very slowly and needed a nudge of extra cooling to push the system into a new state, a whole 30,000 years after the sea-ice changed.”

With direct measurements only providing ocean temperatures for approximately the past 100 years, the researchers used chemical and biological clues left by tiny fossils in mud from the sea floor to understand past temperatures. Past ocean temperatures can be estimated by counting the number of cold versus warm fossils of species with known distinct temperature preferences.

The team trawled the scientific literature for studies of past sea surface temperatures, with data from 136 locations around the globe providing more than 40,000 estimates of temperature. “We’ve combined the individual efforts of hundreds of scientists,” said A/Prof Zanna Chase. “What emerges is a remarkably clear picture of how the ocean changed during the last ice age.”

The data revealed that atmospheric carbon dioxide dropped in several steps over 100,000 years as the Earth entered the last ice age 20,000 years ago. The first drop in carbon dioxide, 115,000 years ago, occurred because of early cooling of the poles and expansion of sea-ice around Antarctica. The second carbon dioxide drop 70,000 years ago was accompanied by a reorganisation of the deep ocean and heightened ocean productivity. The lowest ice age carbon dioxide levels occurred 20,000 years ago when ocean temperatures, productivity, deep circulation and sea-ice had changed the most.

The team’s next step is to combine the new temperature database with palaeoclimate models to test their theories.