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Like Winter in July?

by Jessica Tanenbaum

In the past, when some scientists found odd clues pointing to ice sheet growth ninety million years ago, climatologists scratched their heads. Given the Earth’s sauna-like conditions at that time, how could ice sheets possibly be expanding?

Now, by analyzing goop from the ocean floor, a team of researchers has discovered new evidence that ice sheets were indeed growing for a 200,000 year-long interval during the Turonian period, which spanned from 93.5 to 89.3 million years ago.

During the Turonian period, the world was something of a sweat box, with warm conditions in the high Arctic, and even sultrier temperatures in the tropics. “Ninety, 100 million years ago, we had alligators, and tropical forest plants living up in the high arctic,” explains Richard Norris, a geoscientist with the Scripps Institution of Oceanography of the University of California at San Diego and a coauthor of the new study. “It was the kind of place that if you stepped outside of your nice air-conditioned office, you would instantly be covered with perspiration. It would be soaking your shirt very quickly,” says Norris. Also, back in the Turonian, which is part of the Cretaceous Epoch, tropical oceans had temperatures “equivalent to your hot tub,” Norris explains.

It may be hard to picture a world with both sweltering temperatures and expanding ice sheets. But to Norris, the counterintuitive result just underscores the complexity of climate science. He says that his team’s research overturns “simplistic notions that you make the planet warmer, and all the ice goes away.”

In fact, says Norris, “Very warm conditions are actually kind of nice for growing ice sheets, as long as you have a cold enough spot someplace on the planet.” Warm global temperatures mean that a lot of water is evaporating, and this water can return to the earth as rain or, in cold areas, as snow. If these snowy areas stay cold enough year-round, the snow will accumulate over time.

Taking the Temperature of the Ancient Ocean

The research team, which included Norris as well as scientists from the United Kingdom, the Netherlands, and Germany, deduced the growth of ice sheets by using two methods. Known as paleothermometers, these techniques allow today’s scientists to estimate ancient ocean temperatures.

In one test, the team examined samples gathered from the Demerara Rise, a plateau in the Atlantic Ocean floor off the coast of French Guyana. Using a well-known method, they analyzed the amount of a certain isotope of oxygen, O-18, in the shells of fossils called Foraminifera, or forams.

Fortunately for the research team, the fossil shells they discovered were in good shape for compositional analysis. “We’re talking about ninety million years ago. You wouldn’t think this stuff would survive very well,” says Norris, “but there is actually some beautiful preservation of these fossils.”

Forams construct their shell out of elements in their environment, so analysis of the shells provides an indirect way of measuring O-18 levels in the ancient ocean, data to which our oceanographers obviously have no direct access. Usually, O-18 is relatively uncommon in the ocean compared to the O-16 isotope. But when ice sheets are growing and sucking up the O-16, O-18 levels rise in comparison to O-16. The researchers’ data show high levels of O-18 in foram shells, and therefore, glaciation.

Besides the O-18 evidence, the researchers also looked at another paleothermometer, known in the meteorology business as TEX86. To use this recently developed method, researchers must extract compounds called tetraether lipids from their ocean floor samples.

These tetraether lipids contribute to the cell membrane of ocean-dwelling Crenarchaeota, ancient organisms that are found at a wide variety of temperatures. In 2004, a team of paleooceanographers showed that the type of tetraether lipids in these cell membranes varies directly with ocean temperature. That means chemical analysis of these compounds can serve as a makeshift thermometer. The TEX86 data confirm that the Turonian tropical ocean was just like a warm bath, with temperatures in the 90s Fahrenheit.

Sea levels, past and future

Previously, studies have shown that during the mid-Turonian, sea levels were decreasing. That sea level evidence matches up with the new geochemical clues of ice sheet growth. Norris explains, “When you grow big ice sheets, that’s fresh water that’s being stored on land, taken out of the ocean, and therefore the sea level should drop as ice sheets grow.” The sea level data come from Siberia, northwest Europe, and “of all places, in New Jersey,” says Norris.

According to these prior studies, sea levels dropped at least 80 feet, and possibly over 130. Although shifting among tectonic plates can affect sea levels, sliding lithosphere slabs probably could not have caused such steep and globally distributed changes in sea level.

Norris and his colleagues estimate that the Turonian ice sheet weighed in at about sixty percent of the Antarctic ice sheet of today.

But new findings about this ancient system have little bearing on today’s rising temperatures and melting ice caps, cautions Norris. Although it’s possible that ice masses in eastern Antarctica are growing now or could grow in the future, the melting of other ice sheets and the disappearance of sea ice are well documented. “Other big ice masses that we have to worry about, like Greenland–as far as I can tell, those are just doomed. Which is not good news for us because Greenland has something like 6 or 7 meters of sea level equivalent stored in the ice cap there,” he says, adding, “I think that is a profound worry for people who work on ice processes or anything associated with global warming.”

This research appeared in the journal Science on January 11, 2008. Research funded by: the German Research Foundation and the National Science Foundation, under the management of the Joint Oceanographic Institutions.

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1. Faster Sea Level Rise | ScienCentral | Science Videos | Science News says:
   

   September 30th, 2008 at 12:00 pm (#)

   [...] comparison video 01.22.01 Supergreenhouse Climate Mystery 02.26.08 Melting Glaciers [...]

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