It’s bottoms up for iron at sea’s surface

Biological activity at the base of the food chain in many regions of the ocean is limited by the availability of dissolved iron. The algae that convert sunlight into food in the sea’s top 100 meters or so can’t fully use the other nutrients present because there isn’t enough iron. Just where the crucial metal comes from is often not known.

An analysis of seafloor sediments obtained off Antarctica suggests that the dissolved iron in surface waters that fuels much of the region’s biological productivity comes from deeper waters via upwelling currents.

Many scientists have thought that much of the ocean’s dissolved iron comes from dust blowing off the continents, notes Gabriel M. Filippelli, a biogeochemist at Indiana University–Purdue University in Indianapolis.

However, his analysis of the seafloor sediments suggests that only 2 percent of the iron in the southern oceans comes from airborne dust. The rest is supplied by deep currents that hug the ocean bottom for hundreds of years before rising to the surface around Antarctica.

Filippelli and Jennifer C. Latimer, a geologist at Indiana University in Bloomington, analyzed sediments taken from two locations in the South Atlantic Ocean and one from the southern Indian Ocean. Evidence from these ancient muds, some up to 270,000 years old, show that 10 times as much iron had settled into the region’s ocean-floor ooze during recent ice ages than is being deposited there now. The researchers presented their findings at the American Geophysical Union meeting in San Francisco last month, and their analysis also appeared in the December 2001 Paleoceanography.

By comparing the changing concentration of life-derived phosphorous in the mud with the relatively stable concentration of titanium–a metallic element that erodes from rock as iron does but that organisms don’t typically use–Filippelli and Latimer could estimate variations in biological productivity in the southern oceans. At all three sites, the productivity went up during ice ages and dropped during interglacial periods, even though the high-latitude sites were much more biologically productive overall.

Scientists already knew that much of the dissolved iron in the South Atlantic Ocean today arrives there via a deep current that originates in the far reaches of the North Atlantic, says Filippelli. There, warm water from the Gulf Stream cools down after passing Greenland, sinks to the ocean floor, and begins a 500-year journey southward. Along the way, this submarine torrent, with six times the flow volume of the Amazon River, picks up iron and other elements dissolved from material washed off the continents.

If changes in biological activity are indeed linked to rises and falls in the ocean concentration of iron, Filippelli contends, then it’s much more likely that those variations originated in the deep water.

Although Antarctic ice cores show that increased amounts of dust settled on the continent during ice ages, there probably wasn’t enough dust to account for the entire increase in the sea’s biological productivity, says Filippelli.

The larger contributor to ice age boosts in iron is the material left high and dry by the lower sea levels, he notes. This silt would have been washed to the seafloor by rains, where it eventually released iron to the deep current.

Researchers previously confirmed that biological productivity in the equatorial Pacific Ocean is nourished by iron upwelling from a deep current that originates in the western Pacific, notes Richard W. Murray, a marine geochemist at Boston University. Filippelli’s study suggests that this mechanism is also at work in the other major region of biological productivity. Says Murray, “This research contributes to a fundamental shift in the way scientists think about where the iron in the ocean’s surface waters comes from.”