imageThe depth to which carbon sinks is critical to our understanding of how much carbon may actually be removed from the atmosphere.

This curve represents the age of seawater as a function of depth. This means that water in the surface equilibrates rapidly with the atmosphere, whereas water as deep as a 1000 meters may take 500 years to do so.

This is important when one considers the fate of CO2 taken up from the surface waters by phytoplankton and the tiny amount of organic carbon that actually gets buried in the deep ocean. The depth to which carbon is removed from the surface waters via the biological pump will have a dramatic impact on oceans' ability to sequester carbon. The depth to which carbon is transported is a function of the biological community and the biological pump.

imageOceanographers have repeated asked a question: Is there any evidence to suggest that iron was linked with CO2 in the past? One can go to Antarctica and take an ice core and trapped in between the snowflakes in the ice lattice, there are small pockets of atmospheric gases. The gas in these pockets can be analyzed for carbon dioxide. If you look back in time, you'll see that during the last Ice Age, 18,000 years ago, there was very little CO2 the atmosphere, and a huge amount of iron reaching the Earth's surface.

imageConversely, at the last climate maximum when atmospheric carbon dioxide levels was high, about 6000 years ago, there was very little iron entering the ocean. One might ask: Does this have anything to do with marine primary production? The white line represents iron flux, over time, and these blue bars represent a proxy for marine production. So what we see is that over the last few interglacial cycles, in fact marine production was closely linked with iron supplied to the oceans.


imagePresent-day evidence that iron limits ocean primary production is exemplified here in the Galapagos bloom, where this entire region of Equatorial waters is bathed in about six micromolar (mM or10-6 Molar) of nitrate, both upstream and downstream of the Galapagos Islands. This is a high level of nitrate for the open ocean. It was this question that led the late John Martin to suggest that downstream waters are rich in iron and upstream waters do not have enough iron to support plant growth. Our measurements during FeEx I cruise in 1993 substantiated this hypothesis, indicating that water impinging this Galapagos plateau has high levels of iron leading to increases in primary productivity downstream.

This conclusion required several experiments. The first were bottle enrichment experiments in which water was retrieved from high-nutrient, low-chlorophyll regions, amended with minute, quantities of iron, and then allowed to incubate on deck.

imageThis slide shows the difference of water to which no iron amendments are added, and this is an example of water to which subnanomolar (less than 10-9 Molar) levels of iron were added.

Subnanomolar levels of iron added to the incubation bottles represents an amount that you will see during glacial times in the surface waters

imageIn terms of primary productivity, you can see that the addition of iron to these samples results in dramatic increases in production. These bottle experiments are small--they're on the order of 20 liters--and you can tell from the biological pump diagrams that have been presented that carbon flux takes place over a much larger scale, involving many aspects of the marine community. So the testing of the iron hypothesis required, essentially, a much larger experiment.

These larger experiments were conducted in the two FeEx experiments, FeEx I and FeEx II. Both were conducted in the Equatorial Pacific in a region characterized by high concentrations of nitrate but otherwise low concentrations of iron and plant biomass.

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