A slow day on the Palmer as we steam from the mooring sites to the next stations. This allows me to somewhat catch up with recent events. Here is some of what happened this past Saturday.
Science continued on the Palmer on a stormy Saturday when Dr. Chris Measures from the University of Hawaii gave a fascinating talk titled 'The CLIVAR trace metal program: why they fund us and what have we learned'. To begin with, I learned a lot during a very interesting hour or so.
What are trace metals? I asked Hugo, who is part of Chris' group, and was sitting at his computer next to me. Trace means 'a very small amount'. It is a relative term since each scientific discipline defines small differently. Trace metals in the oceans are metals that are found dissolved in water at very, very low concentrations. How low? Chris said during his talk that it is like having a billionth of a gram of iron in one liter of water. ¡A billionth! A billion in the US is a thousand millions (most other countries use a billionth for a million millions), so that is a very small amount (take a pound of iron and cut it into 454 000 000 000 equal pieces). Other scientific disciplines use the term trace for things with higher or even lower concentrations.
Why would we want to measure something that is at such a low concentration in the sea? It turns out that some trace metals, like iron, are extremely important for life in the oceans.
Let us review a bit about life at sea. The first link on the marine food chain is formed by the phytoplankton: tiny plant-like organisms that float on the surface of the oceans that, like land-based plants, take the energy from the sun through a process called photosynthesis. The most common pigment used by phytoplankton to capture the solar energy is called chlorophyll. Besides solar energy, phytoplankton require carbon dioxide, water and nutrients (nitrates, phosphates and silicates). The fertilizers that we add to our land-based plants are their nutrients.
Chris explained that there are large areas in the oceans where there are plenty of nutrients, CO2, water and sunlight for phytoplankton to grow, that are like deserts in terms of phytoplankton. We know this because we measure small concentrations of chlorophyll amounts of chlorophyll, which indicates there is not many phytoplankton. Scientists call these areas 'High nutrient - Low chlorophyll areas'. You can see on the map that the Polar oceans represent a large percentage of these areas.
The map shows surface areas of the world's oceans in which we find high concentrations of nutrients but small amounts of phytoplankton
Why do we care if phytoplankton do not grow in these areas? Because phytoplankton are the basis of a process by which the oceans remove CO2 from the atmosphere, and we are very interested on reducing atmospheric CO2. Some people think it would be great if phytoplankton were to colonize these vast deserts of the ocean.
You might wonder how can tiny floating organisms help reduce the atmospheric CO2. Phytoplankton use CO2 for growth and reproduction, so they take CO2 that came from the atmosphere and incorporate it into their bodies. We call this organic matter. The carbon is passed to other organisms as the phytoplankton get eaten. All organisms release back to the water some of the carbon in solid form as waste and when they die, while some other carbon is released back as CO2 through respiration. Some of the carbon in solid form is recycled in the upper layers of the oceans by bacteria. They turn the organic matter back into mineral forms like the worms do in our soil. A small fraction of the organic matter sinks to the bottom where it cannot be reached by the organisms at the surface. At the end, this process helps reduce the concentration of CO2 from the atmosphere by moving it to the great depths.
Now that we know why some people are interested on studying the high nutrient-low chlorophyll areas let us go back to the question, what is preventing phytoplankton from growing there? It turns out that some phytoplankton need very small amounts of iron to survive (mostly to form chlorophyll).
Chris pointed out that phytoplankton need iron like you and I need vitamin C. We all need carbohydrates, proteins and fats to survive; they are our nutrients. But we also need trace amounts of vitamins and minerals. We do not even realize how important vitamin C is for us since we get at least the minimum we need of it in our diet. Travelers of past times did not know they needed to pack citrus foods while traveling for months on their ships, and they suffered a disease called scurvy that can be fatal. Phytoplankton need other trace metals besides iron, and that is why we need to measure more trace metals than just iron.
People have speculated that the high nutrient - low chlorophyll areas do not have enough iron for phytoplankton to grow (iron would be considered the limiting factor). There have been few measurements of iron dissolved in the oceans that confirm tis idea in some parts of the oceans, including some areas of the Southern Seas, but the majority of the oceans remain unexplored in terms of trace metals. Most studies have had to make big assumptions of what the values could be in large areas where we do not have measurements (we call this extrapolations).
Chris during his great talk about trace metals
Some scientists recently began an extensive global survey to measure the amount of trace metals in the oceans, and this cruise is part of that effort. Chris calls it just a taste, since this program is measuring only three metals (iron, aluminum, manganese) up to 1000 meters of depth. A more ambitious program, built on the preliminary results of this one, will begin soon.
We could now ask, what are the sources of iron in the oceans? Why are the Polar regions low in iron? In coastal waters, were iron is abundant, the main source is river outflow. The source for open waters is the wind. Satellite images have shown the spread of very large plumes of dust over the Atlantic ocean from the Sahara desert, for example. There are other big dust producing areas in the world that feed the oceans.
It is very hard to measure dissolved iron in the oceans because phytoplankton use it very fast. Chris is collaborating on this cruise with Dr. Bill Landing from Florida State University to estimate the amount of dust that arrives to the oceans. They are measuring the amount of dissolved aluminum to estimate the amount of dust instead of using iron. The benefit of aluminum is that it stays longer in the water (longer residence time) after it has been deposited than iron.
Bill is also collecting aerosols (fine particles floating in the air) in hopes of measuring dust in the air. I hope he will give a Saturday talk. I will describe better what he does in a later entry. Meanwhile I can write that Bill says that there are barely any aerosols over these waters.
Chris shared the following graph that shows a vertical slice of the ocean a long a cruise track in the Atlantic ocean that went from Iceland in the north to Brazil in the south (see the map on the left side of the figure). The upper graph is for aluminum and the lower for iron. The vertical axis is depth in meters and the horizontal axis is latitude with south on the left and north on the right. The units for the concentrations are nanomoles ( about a billionth of a gram in a in liter, remember?)
Vertical sections along the Atlantic ocean for ALuminum concentrations (top) and iron (bottom). Vertical axis is depths while horizontal axis is distance. Units are nanomoles.
We can observe in the surface of the aluminum graph a red area that corresponds to the Saharan dust plume. This is not present at the surface on the iron graph because phytoplankton used it very fast. We do observe a large value for iron around 300 m at 15 N. This is iron that is being released by the bacteria decomposing the waste and remnants of plankton and other organisms that have sunk from upper layers. As we said above, this also happens to some of the carbon in organic matter as well as the nutrients. Scientists say the elements have been remineralized. You could remineralize some organic matter in your house by leaving a banana or an apple on your kitchen counter for a very long time. Bacteria will turn it back to CO2 and water; your mom will probably not be amused.
I invite you to come with me and visit Chris' and Bill's lab to see how they measure the trace metals. We have already seen the trace metals rosette, but there is a lot more to learn about how they analyze the samples. We will do that on another day. Thanks for following our journey.