Dive In: Iron from the seafloor to the surface

This is the first post in a new series called Dive Into Science.  Here I'll be explaining results from recent scientific papers in oceanography.  Dive Into Science gives you a glimpse of current research in an easy to read format that everyone can understand.  To read more about oceanography, just use the Dive Into Science tag.

Today's article is "Estimating the benthic efflux of dissolved iron on the Ross Sea continental shelf" by C.M. Marsay et al, published in Geophysical Research Letters, 2014.

This paper is all about iron supply from the seafloor to the surface ocean.  Specifically, in the Ross Sea, Antarctica, a place of extreme weather that's difficult to travel to and observe.

I've mentioned the importance of iron in the ocean before, and even talked about how it is measured, but let's do a quick recap.  Iron is known as a "micronutrient".  Phytoplankton need iron to carry out their biological processes; to grow and reproduce.  However, unlike other nutrients such as nitrogen, phosphorus, and carbon, iron is only needed in extremely small quantities.  Such small quantities, in fact, that it took oceanographers awhile to discover that iron was even used at all!

In most oceanic habitats, there is enough background iron that it doesn't really make a difference to the phytoplankton - they are more concerned about their main nutrients.  However, in certain locations, like the Southern Ocean, there are areas that are iron-limited, such as the Ross Sea.  Here, we see tons of nutrients in the surface waters, but no phytoplankton, because they are missing the iron they need.

The Ross Sea, Antarctica.  Shown with important landmarks, and the ship track for the cruise this data was collected on.

During spring time in the Ross Sea, which occurs in December in the Southern Hemisphere, the sea ice melts and the phytoplankton get enough sunlight to grow.  They multiply quickly during this "spring bloom" period, then die off as the iron runs out.  The iron in the surface ocean during springtime has either been mixed up from the bottom waters over the winter, or has been added from sea ice melting.

The authors of this paper are specifically investigating the iron that starts at the seafloor, often from the sediments on the bottom, and mixes upward to eventually reach the surface.  To do this, they took a series of measurements during a cruise of concentrations of dissolved iron in the water at various locations and depths in the Ross Sea.  Using these measurements, they can see how much iron is near the seafloor, and how quickly it depletes towards the surface.

Graph of how iron concentration changes with height above the seafloor in the Ross Sea.  Adapted from Marsay et al 2014.

Then, they combined these measurements with results from a regional ocean circulation model.  This model shows how the water moves and can give researchers a good idea of how long it takes to mix water upwards, and how much is mixed upwards.

So, if water at the bottom has a certain amount of iron in it, and is mixed upward at a certain rate, you can calculate how much iron from the bottom gets to the surface.  This calculation was the main purpose of this paper; to put a number on the flux of iron from near the seafloor.

Comparing this number with how much iron is used up in the surface showed that only about 1/10th actually made it from seafloor to surface.  The rest, even though it was mixed upwards as calculated, was probably exported from this area to other regions offshore by currents.

As a scientist, this paper holds two important results for the community.  The data that was collected was the first time detailed near-bottom sampling of dissolved iron was carried out in this area.  Then, the main result of the paper is the amount of iron that traveled from the seafloor to the surface.  In order to understand the iron-limited system in the Ross Sea, we need to know all the sources and sinks of iron: where does it come from, and where does it leave, and by how much?  This paper put a number on one of those sources, and estimated that a large portion of that source is exported before it reaches the surface.

This is a good example of how science progresses.  A cruise in early 2012 took water samples that were analyzed in a laboratory to produce dissolved iron data.  That data was then examined for patterns and explanations, combined with other available data (in this case model results), and published 2.5 years after the cruise.  The time it took from beginning to end is very reasonable for science, especially when you consider that the scientist is working on other projects and publications at the same time, perhaps even teaching classes, and the data is being used for other research.

Questions or thoughts about this new research?  I'll also take suggested topics for future article reviews.