Iron in Antarctic Waters Is Focus of Climate Change Research
Oceanographers on the faculty of Old Dominion University have key roles in a new climate change-related research project that will focus on inputs of minute quantities of the micro-nutrient iron to surface waters of the Ross Sea off Antarctica.
John Klinck and Eileen Hofmann, who are physical oceanographers, and Peter Sedwick, a chemical oceanographer, are leading separate portions of a three-year project titled "Impact of Mesoscale Processes on Iron Supply and Phytoplankton Dynamics in the Ross Sea." Their awards from the National Science Foundation (NSF) total more than $700,000.
This work reflects how closely scientists are watching Antarctica and the surrounding Southern Ocean because of rising levels of carbon dioxide in the atmosphere, and anticipated climate warming and related environmental changes. Of special interest to the ODU researchers is the Ross Sea, a deep bay over the continental shelf of Antarctica, south of New Zealand.
The Ross Sea is rich in nutrients and among the most productive regions in the vast Southern Ocean. This means that, generally speaking, the growth of the microscopic plants known as phytoplankton supports a robust marine ecosystem in this region.
The growth of phytoplankton, and the formation of oceanic deep waters, both of which are important processes in the Ross Sea, act to transport carbon from the surface to the deep ocean, meaning that the Ross Sea is likely to be an important oceanic "sink" for carbon dioxide. Phytoplankton pull in carbon dioxide as they grow, and they also need nutrients and sunshine, both of which are abundant in the Ross Sea during summer. The ODU researchers are interested in the small quantities of iron that are necessary for phytoplankton growth.
Iron is generally scarce in the ocean waters that surround Antarctica, where its main sources are thought to be seafloor sediments, as well as the melt waters from sea ice and glacial ice, and possibly the upwelling of deep waters. In offshore Antarctic waters, dissolved iron concentrations are typically less than 5 parts per trillion, which is low enough to limit the growth of phytoplankton. This has led some to suggest that Southern Ocean waters might be purposefully 'fertilized' with iron to stimulate huge blooms of phytoplankton, thereby removing carbon dioxide from the atmosphere into the deep ocean.
The greater productivity of the Ross Sea, compared to offshore Antarctic waters, is thought to reflect an enhanced availability of iron in surface waters. This is supplied from the seafloor during winter water-column mixing and from sediments in melting sea ice during the spring. These inputs appear to decrease during the late spring and summer, when very low levels of iron have been observed in the Ross Sea. Despite this, field and satellite observations reveal that phytoplankton continue to grow in these waters throughout the summer season, which requires some source of iron to the Ross Sea over this period.
"Where does this iron come from?" Hofmann asks, framing one of the questions that the researchers are seeking to answer. "And how does it get into the upper water column where there is light" enough for phytoplankton to exist?
"We're looking at natural sources of iron," explained Klinck, "and how these sources may respond to future climate change".
Added Sedwick, "Our project seeks to understand the sources and pathways by which iron reaches the surface waters of the Ross Sea during the austral summer, the way in which the iron supply limits biological production, and, using numerical modeling simulations, the way in which these processes may change in response to a warming global climate."
Climate change affecting sea ice, glacial ice and ocean circulation would impact the ecosystem and associated biological carbon dioxide sink in the Ross Sea, Sedwick said. This is because the supply of iron to phytoplankton during the growing season could be affected.
"If iron supply increases during a warming climate, this might serve to ameliorate climate change by increasing the ocean's biological carbon sink, all other factors being equal," Sedwick said. "Conversely, a decreased supply of iron under a warming climate might exacerbate climate warming by lessening the oceanic carbon dioxide sink. Of course, we also need to consider what happens to the rest of the ocean, particularly the physical process of oceanic deep water formation, which transports carbon into the deep ocean, away from the atmosphere."
Sedwick also noted that the warming of surface waters could decrease the amount of cold, heavy water at the poles that drops down through the water column into the deep ocean. This process transports organic carbon into the deep waters, creating the so-called carbon dioxide sink.
Sedwick and Klinck will be among the project's scientists who will be aboard the U.S. research vessel and ice breaker Nathaniel B. Palmer from late December to mid-February for an expedition to the Ross Sea. With them will be Pierre St. Laurent, a post doctoral researcher at CCPO, oceanography graduate students Diego Narvaez and Candace Wall, research associate Bettina Sohst, and Stephanie Hathcock, a doctoral student in ODU's Darden College of Education, who is responsible for the project's public outreach effort.
That outreach will involve audio-visual communications between the scientists on the cruise and schoolchildren in Portsmouth and Norfolk. Hathcock is the student of Daniel Dickerson, professor of science education.
The "mesoscale" aspect of the project reflects a research scheme for the expedition that is different from most. The general definition for the word is "middle sized." In this case, data and samples will be collected - to measure salinity, water temperature, chemical composition, currents, biological activity, etc. - from closely-spaced stations across relatively small "mesoscale" oceanographic features that are 7-15 miles across. On most research vessel cruises, the stops, or stations, for comprehensive data/sample collecting are at least 30 miles apart.
Here the focus on the mesoscale is important because of the mesoscale-sized eddies or swirls of water in the Ross Sea may contain higher productivity, as a result of higher iron levels. "They are a lot like the Gulf Stream rings in the Atlantic Ocean" which have abundant phytoplankton, said Klinck. "They are like teacups of productivity." In other words, these phenomena exist on the mesoscale, and might be missed if sampling stations are 30 or more miles apart.
Because the dissolved iron that the researchers will be looking for is present in such minute quantities, samples must be collected very carefully, to avoid contamination. Large quantities of iron are present in the research vessels themselves, and in the equipment that is routinely used to collect and process seawater samples. Sedwick has earned a reputation for using the specialized "trace metal clean" techniques that are required to accurately sample and measure the very low concentrations of iron in ocean surface waters, and will be responsible for these measurements during the cruise.
Other researchers working on the project include Dennis McGillicuddy of Woods Hole Oceanographic Institute in Massachusetts and Blair Greenan of Bedford Institute of Oceanography in Nova Scotia, who will be dealing with physical circulation of waters and the phytoplankton distribution. Walker Smith of the Virginia Institute of Marine Science and Thomas Bibby of the National Oceanography Centre, Southampton, England, will be examining the relationship between iron levels and phytoplankton growth.
Hofmann and Klinck, along with Mike Dinniman, a research scientist at CCPO, will contribute their expertise in the development of numerical model simulations of these processes, which will be compared to the measurements from the expedition and will help to describe the processes that provide iron to the ocean surface, including projections of how such processes may be affected by future climate change in the Antarctic region.