Location, location, location – UKGEOTRACES 40°S

I promised a while ago that I would give some background to the research expedition that I was preparing for – UKGEOTRACES 40°S. I have been at sea, on the RRS James Cook, for the past week, setting of from Port Elizabeth, South Africa on the 23rd of December. Once we had a quick port call in Cape Town we set sail in earnest. Having reached 40°S yesterday, I thought it was time to come good on my promise.

Summer surface chlorophyll in south Atlantic

Figure 1: Surface chlorophyll at 40S extending from South America to the Cape of Good Hope in the south Atlantic. A productive belt (green) of high chlorophyll - high phytoplankton productivity - is visable extending across the south Atlantic along 40S. The areas to the north and south of this band (dark blue and purple) represent areas of low chlorophyll or low or very low phytoplankton concentration. In these areas micronutrients, such as iron, are at very low concentrations and this restricts phytoplankton growth and productivity.

There is life in this part of the ocean (Figure 1) stretching across from the Cape Horn to the Cape of Good Hope. We have some to 40°S to study the life and the controls on life in this region and to better understand why there is life here when either side of this narrow band the ocean is much like a desert? This a very interesting question, interesting enough to get a large bunch of oceanographers from a variety of different disciplines together to try and figure it out.

GEOTRACES is a collection of scientists that are looking at TRACE metals/elements/nutrients in the ocean. For UKGEOTRACES 40°S we are interested in the sources (where they are origination from) and sinks (where they reside after moving through the system) of these trace elements. We are interested in trace nutrients such as iron (Fe), manganese (Mn), aluminum (Al) and zinc (Zn) among others. These are all important nutrients for phytoplankton in the oceans as they are required by physiological processes inside the cells of these tiny algae. If there is not very much of one or more of these trace nutrients in the seawater phytoplankton growth will be limited – they will not be able to grow.

Some of these elements are more important than others. For example iron is known to be the limiting factor of phytoplankton growth to both the north and south of our current location. The Southern Ocean and the South Atlantic sub-tropical gyre are both known as high-nutrient, low-chlorophyll (HNLC). The major nutrients required for phytoplankton cell growth, e.g. nitrate, phosphate, silicate and ammonium are not limiting in these areas yet there is no phytoplankton (phytoplankton contain chlorophyll which allows them to make energy from sunlight) thus no chlorophyll. Large scale experiments where iron has been added to these small areas of HNLC regions has resulted in the growth of phytoplankton – phytoplankton blooms (Figure 2).

Until recently methods used by chemical oceanographers to detect certain trace nutrients were not accurate enough to detect the minute quantities of trace nutrients e.g. iron, in parts of the world ocean. It is now possible to detect tiny (nano molar) concentrations of iron, manganese, aluminum, zinc and a whole suite of other elements.

Now that we can measure trace nutrients in the ocean it is possible to start to unravel how trace nutrients or micronutrients affect the growth of phytoplankton, also referred to a primary productivity and therefore all productivity in the ocean.

Figure 2: Phytoplankton bloom

Figure 2: Natural phytoplankton boom. Iron fertilizarion experiments in HNLC areas produce booms as iron is the limiting micro nutrient, once it is present phytoplankton growht is no longer restricted.

We have come to 40°S to investigate the controls of productivity and to figure out where the micronutrients that make it a productive region of the world ocean come from. Of course we have some ideas where these micronutrients come from. The wind carries dust to the ocean (atmospheric sources), rivers bring micronutrients into the ocean (riverine sources), interactions between the ocean floor and the water that lies above it inject micronutrients from the sea floor into the overlying water column (benthic sources) and in areas of seafloor spreading (hydrothermal sources) micronutrients are also injected into the ocean. We hope to document concentrations of micronutrients in the ocean and unravel which micronutrients come from which sources and their role in controlling productivity in this region of the ocean.

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About Roisin Moriarty

Postdoctoral Research Fellow with the Isotope Geochemistry and Cosmochemistry group at the School of Earth, Atmospheric and Environmental Science at the University of Manchester, UK. I work with noble gases and tritium in seawater samples from the south Atlantic and the Southern Ocean. I participate in research cruises in order to collect the seawater samples that I analyse. I am now working as a chemical oceanographer/noble gas geochemistry but I have a background in ocean biogeochemical modelling and zoology.
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