Dusty times provide iron to grow rich ocean plankton pastures
Like humans, phytoplankton (tiny plants that drift on ocean currents) need iron to survive. The difference is that while we humans and every other red blooded life form on earth need 100 parts per million iron, ocean plankton need less than 100 parts per trillion to thrive. Normally iron is in very short supply in the ocean, just 3 parts per trillion. Only when the wind blows dust to ocean pastures does the iron level rise sufficiently for the pasture to bloom in abundance.
A recent study funded by the National Science Foundation suggests that over the last 1 million years, periodic increases in iron — which mainly reaches the open ocean in the form of windblown dust — have caused dramatic spikes in phytoplankton numbers. When ever the plankton was growing it consumed massive amounts of CO2, in the past delicately balanced in nature, when the CO2 levels dropped the planet cooled.
Here’s how: Phytoplankton consume carbon near the surface of the ocean through photosynthesis. Because the upper ocean and the atmosphere are closely connected through chemical exchanges, more carbon consumed in the ocean may mean less carbon dioxide in the atmosphere.
While some argue over the minutia about just how direct of an effect on climate phytoplankton, the new study supports the Iron Hypothesis, an idea developed over 20 years ago by marine scientist John Martin. Martin argued that when the Earth goes through dry, dusty climate periods, more iron reaches the ocean in the form of windblown dust, which acts like a vitamin for phytoplankton. As the phytoplankton become more numerous, they draw down carbon from the atmosphere, thereby helping to cool the planet.
Other research such as that of Russ George has supported a link between iron and phytoplankton in the present-day ocean, but this latest study provides a unique look back in time.
To investigate the long-term influence of iron on phytoplankton, Richard Murray of Boston University and a multi-institution team of scientists studied prehistoric sediments buried in the seafloor. They found that as iron levels periodically increased and decreased over time, so too did levels of opal — a material many people associate with jewelry, but which marine scientists use as an indicator of phytoplankton abundance.
Opal indicates phytoplankton abundance because it is secreted by diatoms as material to form diatom shells. Diatoms are among the most common and important kinds of phytoplankton. As the abundance of diatoms changes over time, these changes are reflected in the amounts of opal (dead diatom shell) that settles on the seafloor and eventually is buried by marine sediment. By tracking opal and iron in the sedimentary record, Murray and his team were able to show that the relationship between phytoplankton and iron is long-standing, even ancient.