Microscopic sea animals can give us big clues about climate change and the causes of global warming. 
 

The gear you need to study global warming can include long underwear and parkas—at least some of the time.

Meteorologists, oceanographers, biologists, and other scientists are traveling everywhere from the poles to the tropics to get a better handle on climate change and the extent to which the warming we see today is due to human production of greenhouse gases, such as carbon dioxide, which trap heat in the atmosphere.

Scott Ishman, a paleobiologist and assistant professor of geology at SIUC, studies some tiny benchmarks of climate: single-celled marine creatures called foraminifera. Thousands of species of foraminifera—"forams," for short—live around the globe, from cold, deep-sea troughs to tropical lagoons. Some inhabit surface waters, but most are adapted to life on the sea floor. 

These animals are much like amoebas, except that they have shells (typically made of calcium carbonate). Beachgoers sunbathe upon millions of speck-sized foram shells among the sand grains. It takes a microscope to identify forams and an electron microscope to tell some closely related species apart, by subtle shell variations. 

What groups, or assemblages, of foram species occur in a given area depends upon the water temperature, salinity, and oxygen content; the nutrient density; even the type of sea-floor sediment. By learning what environmental conditions living foram assemblages require, scientists finding the same assemblages in the fossil record can tell what a region’s environment was like in the past and can better interpret what’s happening with climate today.

In recent years Ishman has focused much of his research on the Antarctic Peninsula, the continent’s long, skinny arm. Average annual air temperature here has increased about 5 degrees Fahrenheit in the past half-century, a huge rise. As a result, many of the peninsula’s ice shelves—the floating platforms of ice hundreds or thousands of feet thick that extend many miles from the continent—are shrinking.

Almost half of the huge Larsen Ice Shelf, third-largest in Antarctica, has disappeared in a few short years. Larsen A, the northernmost segment, has completely melted since the mid-1990s. Larsen B, the much larger middle segment, shrank rapidly through the late 1990s, then lost most of its remaining mass in February 2002, when a chunk the size of Rhode Island—some 1,250 square miles of ice—broke off and disintegrated. 

In May 2000, Ishman joined several American, Canadian, and Italian colleagues for four weeks aboard the wave-tossed Nathaniel B. Palmer, a U.S. research ship, collecting water samples, sea-floor samples, and sediment cores from areas that had recently been covered by Larsen A. 

"We’re looking at the processes as this ice shelf disappeared—what types of sediments were deposited as it was retreating and how the foram assemblages were changing," Ishman says. 

"What were the first forams to occupy the sediments, inhabiting what was sort of a desert environment? Can we recognize a progression to communities of forams forming different assemblages? If the same thing happened in the geologic record, we should see the same pattern" in sediment cores. 

Is today’s melting an unprecedented event in recent geologic time? If so, then it can’t be chalked up to natural climate fluctuation, and policy makers will have more ammunition to argue for reducing greenhouse gas emissions. The research team, led by sedimentologist Eugene Domack of Hamilton College, is trying to answer the question by determining the history of the Larsen Ice Shelf during the 12,000 years that have passed since the end of the last great ice age. 

The National Science Foundation’s Division of Polar Programs is supporting Ishman’s portion of this research. Among other things, his NSF grant funded a January 2002 trip made by Scott McCallum, a master's student working with Ishman, to help collect water and sediment samples from sites recently covered by Larsen B. 

Geology undergraduate Phillip Szymcek is working with them to identify foram species in the samples. Ishman and McCallum then run statistical analyses to determine the proportions of species present and to see how those assemblages correlate with environmental conditions.

Foram samples from the May 2000 sampling trip hint that Larsen A may have melted once before, between 9,000 and 6,000 years ago, when the earth in general was warmer. The research team also found evidence of ancient algae blooms—a surface phenomenon that indicates the presence of open water in the past. 

Larsen B is a different story. So far, the foram species recovered by McCallum last January support the conclusion that Larsen B had been stable since the ice ages—until now. And no fossil remnants of algae blooms were found in cores taken from the sea floor in this area.

Supporting evidence of another kind may come from the foram shells themselves. Mike Prentice, a geochemist at the University of New Hampshire who’s working with Ishman on the NSF grant, is studying the shells’ isotope composition—the percentages of different varieties, or isotopes, of the oxygen and carbon making up the shells.

"Different water masses have different isotopic properties," Ishman says. "When forams secrete their shell, they’re using carbon and oxygen from the water. So the shell should have the same isotope ratios as the water mass it was derived from.

"We need to see how much variability there is in the modern samples to interpret the variability we see in the fossil record. If we see a big change in the isotopes [over time], that’s telling us there was probably a significant change in the properties of the water mass." Such information will shed light on ice shelf extent, climate conditions, and ocean circulation patterns in the past.

Ishman and his colleagues have concluded that the collapse of the Larsen Ice Shelf appears to be due entirely to atmospheric warming. Scientists had assumed that any rapid collapse would also have to involve warmer sea water eroding the ice from underneath. The fact that air temperature alone can do the trick in such a short time is worrisome. Why? Changes in Antarctica’s climate can affect ecosystems and weather systems thousands of miles away. 

Much depends on the ice shelves, which serve to buttress the massive freshwater ice sheets covering the continent. Where the shelves have disappeared, the ice sheets can flow like glaciers into the sea, adding water volume as they melt. 

If warming continues, this melting would raise sea level, and it also would change the salinity and temperature of the water masses around the continent.

"Warming things up can produce an oceanographic change, which can further disturb the climate system," Ishman says.

Global ocean circulation patterns are influenced by the interplay at the bottom of the world of Antarctic Deep Water (cold, salty bottom currents flowing north from the continent) and Circumpolar Deep Water (a warmer, globe-traversing current that upwells around Antarctica).

The latter, Ishman explains, is "very important to heat flux to the southern ocean, which influences the organisms living there, the climate, and the extent of seasonal sea ice. Sea ice behavior is critical to a major portion of the biota around Antarctica—seals, penguins, and krill, which is one of the primary food sources in the southern ocean." 

If the numbers of krill were to decline significantly, the world’s fisheries—not to mention its whale species—would suffer. Another example of the ocean’s interconnectedness: Shifts of ocean currents around Antarctica, as well as periods when there is less sea ice, appear to correlate with episodes of El Niño, the warming of tropical Pacific waters that changes and intensifies weather over the Americas.

Antarctica isn’t the only focus for Ishman’s work on climate change. He has analyzed fossil and live forams from Chesapeake Bay, for example. That research, in combination with the Antarctic research, is "giving us a better idea of how the global climate system is working," he says. 

"For example, we’re able to look at the El Niño record in the Chesapeake, plus we’re also able to look at human impact on that environment and compare that to an area like Antarctica, which has not directly been impacted by human activity.

"What we’ve seen so far in the Chesapeake Bay area is that with global warming there’s been an increase in the magnitude of weather events, such as storms and droughts. The extremes have become more extreme."

With doctoral student Christopher Williams, Ishman also is using foram analysis to link human activity to ecosystem changes in south Florida over the past century. And in northern Chile, he and master’s student Tim Reilly have collected ancient forams from coastal sediments once covered by the cold, upwelling equatorial currents that make South America’s fishing industry so good. Oceanographers know that climate change in Antarctica can disrupt those currents.

"Some good records from deep-ocean cores worldwide indicate changes in ice volume in Antarctica" over millions of years, Ishman explains. Dating sediments from old coastlines in northern Chile will allow the researchers to confirm that local changes in sea level match the deep-sea records. Studying the forams from those time periods will then allow them to track any associated changes in sea life productivity in the region. A new three-year grant from NSF is funding this work. 

"If we can say that decreased ice volume in Antarctica will or won’t affect productivity, based on the geologic record, that can allow countries like Chile and Peru to prepare for effects on their fishing industry from global warming," says Ishman.

By investigating some of the tiniest animals on the planet, Ishman and his students are adding key details to an emerging big picture of the earth’s climate, past and present. 

It’s a picture we need to help us understand what’s likely to happen tomorrow—and what we may need to do about it. 

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