From: Editor ENN from EurekAlert
Published July 19, 2013 05:47 AM

Oceanic Iron content more variable than thought

The supply of dissolved iron to oceans around continental shelves has been found to be more variable by region than previously believed — with implications for future climate prediction.

Iron is key to the removal of carbon dioxide from the Earth's atmosphere as it promotes the growth of microscopic marine plants (phytoplankton), which mop up the greenhouse gas and lock it away in the ocean.

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A new study, led by researchers based at the National Oceanography Centre Southampton, has found that the amount of dissolved iron released into the ocean from continental margins displays variability not currently captured by ocean-climate prediction models. This could alter predictions of future climate change because iron, a key micronutrient, plays an important role in the global carbon cycle.

Previously assumed to reflect rates of microbial activity, the study found that the amount of iron leaking from continental margins (the seafloor sediments close to continents) is actually far more varied between regions because of local differences in weathering and erosion on land. The results of the study are published this week in Nature Communications.

"Iron acts like a giant lever on marine life storing carbon," says Dr Will Homoky, lead author and postdoctoral research fellow at University of Southampton Ocean and Earth Science, which is based at the Centre. "It switches on growth of microscopic marine plants, which extract carbon dioxide from our atmosphere and lock it away in the ocean."

Continental margins are a major source of dissolved iron to the oceans and therefore an important factor for climate prediction models. But until now, measurements have only been taken in a limited number of regions across the globe, all of which have been characterised by low oxygen levels and high sedimentation rates. The present study focussed on a region with contrasting environmental conditions — in Atlantic waters off the coast of South Africa.

"We were keen to measure iron from this region because it is so different to areas studied before. The seawater here contains more oxygen, and sediments accumulate much more slowly on the seafloor because the region is drier and geologically less active," says Professor Rachel Mills, co-author at the University of Southampton.

The team found substantially smaller amounts of iron being supplied to seawater than measured anywhere before — challenging preconceptions of iron supply across the globe.

The electric blue glow in the pacific coast waves shown in the photo is caused by a dinoflagellate bloom commonly referred to as a red tide. Image credit Shutterstock.

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