Ideas about what fertilizes the southern oceans may need rethinking
The rate at which dust carries iron to regions of the South Pacific and the sea around Antarctica may be much lower than previously indicated by models of the process, a new study concludes. The findings call into question the traditionally accepted notion that atmospheric dust is the primary shuttle of iron to the open ocean, and could have implications for theories about climate change in the ice ages.
Photosynthetic plankton that fuel the bulk of the oceans’ primary productivity need iron, and its scarcity linits their growth over large parts of the oceans. Since these organisms soak up carbon dioxide during photosynthesis, their abundance in turn affects how much CO2 is removed from the atmosphere and absorbed by the oceans. This means that the amount of iron deposited in the oceans by dust swirling off the continents is a significant factor in the carbon cycle and its impact on global climate. It has been suggested that an increase in atmospheric dust could have been part of the biogeochemical shift that led to low levels of carbon dioxide in the ice ages.
Atmospheric dust in the Southern Ocean is notoriously undersampled due to the great distances from continental landmasses and the difficulty of measuring trace levels of minerals in aerosols. Now a team of French researchers has collected and analyzed dust aerosols on cruises in the southeast Pacific and the Southern Ocean, and found that concentrations of iron deposited by dust over these regions are two orders of magnitude lower than models have formerly predicted. They report their findings this week in Global Biogeochemical Cycles1.
The dust settles
“We had some suspicions that our models weren’t doing a particularly good job in the Southern Ocean from the few sites we had, but we didn’t have any observations in this region,” says Natalie Mahowald, an atmospheric scientist at Cornell University at Ithaca in New York state and a coauthor on the study. “The new observations tell us a huge amount.”
By replacing previous estimates with actual data of iron in the atmosphere, the researchers concluded that atmospheric dust may account for only half the amount of iron that is brought in through upwelling plumes of ocean water. Based on the results, the researchers argue that upwelling – rather than wind-borne dust – may be the dominant factor for fertilizing southern hemisphere oceans with iron.
“It’s difficult to know where the overestimation was coming from,” says Thibaut Wagener, an oceanographer at the Oceanographic Laboratory of France’s National Centre for Scientific Research in Villefranche-sur-Mer and lead author of the study. Wagener points to difficulties in obtaining meteorological data in the region to plug into the models. Another reason for the paucity of measurements, he says, is that aerosol samples are easily contaminated by engine smoke, traces of iron from ship parts and sea salt from the turbulent seas. In addition, the regions where samples were collected in the study have very low levels of dust and iron, making it difficult to achieve accurate measurements.
But scientists caution that the data set is still limited and should not be used to extrapolate to the Southern Ocean as a whole or to northern hemisphere regions. “They haven’t demonstrated necessarily that it’s overestimated everywhere in the Southern Ocean,” says Scott Doney, an oceanographer at Woods Hole Oceanographic Institution in Massachusetts. “It just suggests that they were overestimating in some regions.”
Some areas of the ocean, such as those near continents, are likely to encounter more wind-borne dust, for instance. And factors like solubility of iron come into play. “Not all iron is equal,” says Mahowald. “In these remote regions, even though there might be very little iron, the iron might be much more soluble than it is, say, close to the Sahara.” Organisms that are dependent on iron uptake may be able to harness iron even when it is in short supply. “So I still think there’s good evidence that the dust inputs to the Southern Ocean could be very important,” Mahowald says.
Understanding the role of dust in transporting iron between the atmosphere and oceans is becoming increasingly critical as some desert regions become more arid under a changing climate. More iron fertilization of the oceans from dust plumes could lead to enhanced CO2 uptake. This study, says Andrew Watson, an oceanographer at the University of East Anglia, UK, who was not involved with the research, “is one brick in the wall towards getting a proper understanding of the earth system as a whole”.
Wagener et al. Global Biogeochem. Cycles 22 (2008) doi:10.1029/2007GB002984