Thousands of miles from his Southern university, Tim Shaw works to discover how glaciers' melting affects the pace of global warming.
Shaw, professor of chemistry at the University of South Carolina, analyzes the composition of icebergs that have calved from the main ice mass of Antarctica. On expeditions in 2005, 2008 and 2009 aboard the Nathaniel B. Palmer icebreaker ship, and in future trips, Shaw seeks icebergs' footprints - their unique trail left behind as they float through the ocean, melting as they drift. It turns out those icebergs carry a lot more than ice, and their trails include significant amounts of iron.
"Ultimately, we want to know how the presence of icebergs impacts carbon export from the surface ocean in the region," said Shaw, who first became intrigued by what icebergs leave behind when hiking a glacier in Greenland in the early 1990s. While climbing, he was struck by the amount of debris in the water dripping and flowing off glaciers and in the fine material produced when glaciers grind against rock. He has studied this earthen material lodged in ice ever since.
So far, Shaw, colleagues and student researchers have determined that icebergs significantly add iron to surrounding waters and affect the biological community. Now the team is working to compile data on iron's effect on carbon export.
Right now, Shaw says, icebergs are breaking off Antarctica at an unmatched rate that many scientists attribute to global warming. Chunks of land material trapped in these icebergs release elements such as iron when the icebergs melt, which may fertilize the Southern Ocean. When fertilized with iron, the ocean loses carbon from its surface waters and steals carbon dioxide from the atmosphere to compensate; in this way, iron fertilization can contribute to carbon cycling.
"This is a really interesting way to look at how global warming may change the ecology of the surface ocean. This clearly has important implications for humans as well," said Marsha Bollinger, chair of the department of environmental sciences and studies at Winthrop University, which isn't involved in the study.
The study differs from other work. Most researchers study how global warming affects glaciers, but Shaw studies what glaciers say about global warming.
He also studies how this added iron will affect surrounding oceanic communities. The team gauges phytoplankton - bottom rung microorganisms in the food chain - productivity and success as icebergs deposit nutrients into the ocean. Scientists aren't sure how the added iron will affect phytoplankton, tiny oceanic food sources that could skew how well other larger marine organisms survive and remain successful in the environment.
In the past, carbon dioxide levels have dropped dramatically in the atmosphere for short periods, and scientists say they think it could be because of iron's addition, Shaw said. But researchers don't know where that iron originates. Shaw and his collaborators say they think it may come from glacial melt, a hypothesis that originated with oceanographer John Martin in the 80s.
To find out, Shaw and colleagues slosh around the Southern Ocean, collecting pieces of icebergs and water samples to find just where the iron comes from. When glaciers melt, they release more than just iron; they let go of a slew of other elements and compounds. Since iron is found in both the ocean and in glaciers, it's hard for scientists to trace its travel path. So the researchers look for a proxy of terrestrial material called a tracer, also found in the glaciers but not found in surrounding water. They track this tracer element from glacial melt into the water because it parallels how iron travels.
"We cannot always monitor what we want to monitor, so we find things that are reasonable fill-ins for what we're looking to measure," Bollinger said.
In this case, the tracer element is a short-lived radium isotope found naturally in the ground. This isotope's parents, thorium and uranium, are found in the land material stuck inside icebergs, and they release their radium daughter when the glaciers melt and the land material hits seawater. When researchers find this type of radium, they know that bits of terrestrial material linger nearby.
Though radium acts as a great tracer element for scientists, it's still hard to measure. Shaw uses an analytical technique developed by a USC colleague that can detect the radium isotope down to a level of seven atoms per liter of seawater. And since these radium isotopes degrade entirely after a couple of weeks in seawater, much of his precise analytical work happens immediately at sea rather than back at the lab.
The team pumps and collects seawater, at about 1000 liters per sample for a good measurement, then extracts radium from the water. First, though, the team uses remotely operated vehicles, surface-deployed pumps and the ship's seawater system to collect water and use rescue boats to gather ice samples.
Shaw's lab - operating under a $500,000 National Science Foundation grant - collaborates with researchers such as Ken Smith at the Monterey Bay Aquarium Research Institute to study various topics, including the biology of communities of organisms living around the icebergs. But they focus on one goal:
"As the climate warms, there is an increasing number of icebergs in the Southern Ocean. Through our studies, we are hoping to define the importance of these icebergs and their associated communities on the global carbon cycle," Smith said.
