Southern ocean iron may have come from the depths, not the atmosphere, researchers conclude

December 19, 2001

WASHINGTON - Scientists believe that increases in plant life in the Southern Ocean are associated with increases in iron, which acts as a fertilizer, in the ocean water. This "Iron Hypothesis" was put forward a decade ago by the late John Martin. Iron is usually in short supply but, according to Martin, could have been delivered in greater amounts via dust falling into the ocean during intervals between glacial periods. Two researchers from Indiana University - Purdue University Indianapolis (IUPUI) now cast doubt on dust as the principal source of iron and propose an alternative source of iron in the Southern Ocean.

Jennifer Latimer, a doctoral student, and Professor Gabriel Filippelli suggest that increased amounts of iron may have been delivered predominantly from deep ocean waters that rose, or upwelled, to the surface. They presented their results at the American Geophysical Union's Fall Meeting in San Francisco and in a peer reviewed paper to be published in Paleoceanography, an AGU journal, both in December.

The researchers found that increases in biological productivity during intervals between glacial periods in portions of the Southern Ocean, which surrounds Antarctica, coincided with increases in biologically available iron and the input of material from continents. This input may have been closely linked with increased weathering and delivery of material from continental shelves, which are exposed during glacial periods, when sea level is lower. Material from the continents runs off into the ocean, and the intensified ocean circulation associated with glacial periods helps to mix the material, bringing nutrients from deep ocean waters to the surface through upwelling. Latimer and Filippelli conclude that the major source of iron in the Southern Ocean was not from wind-blown dust falling from the atmosphere, but from deep ocean waters below, which they call the "Upwelled Iron Hypothesis."

Latimer and Filippelli performed an extensive array of geochemical tests on sediments from cores collected across the Polar Front Zone in the South Atlantic and Indian Oceans. They sought to identify the potential sources for minerals from land masses found in these core samples, the availability of iron for organisms from this matter, and its biological effect. They are currently analyzing sites spanning a wider range of latitudes in the Southern Ocean. Together with other scientists, they hope to examine the duration of this phenomenon by using several cores recently extracted from the South Atlantic, containing continuous sediment records spanning the last several million years.

The implication for this increased iron-fertilized plant growth is far-reaching. During these periods of increased phytoplankton growth, the larger number of organisms engaged in photosynthesis in the ocean might have tipped the carbon balance such that atmospheric carbon dioxides decreased. (In photosynthesis, carbon dioxide and water combine in green plants to form simple sugar and oxygen.) This may in turn have provided a positive feedback leading to cooler global conditions.

American Geophysical Union

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