Southern Ocean Iron Experiment (SOFeX) Cruise
January 5 - February 26, 2002
Skip to Log Entry 2 from the R/V Melville
February 21, 2002: Day 48
Position: 52 degrees, 53 minutes South, 167 degrees, 27 minutes West
R/V Melville Log Entry: We have spent the evening running transects and looking for the larger patch portions and remnants of the most recent infusion. Our efforts led us to concentrate on this site showing significantly elevated SF6 and increased fluorescence, but values are still low and reflect an area where significant consumption of and phytoplankton nutrients has occurred. Yet the drawdown in carbon dioxide is only about 20 microatmospheres here. Our CTD cast showed a very shallow 10 m mixed layer in which fluorescence was uniformly high and then dropped off rapidly.a pancake patch! In such a shallow skin, we could have used some of our gas signal to the atmosphere. Both SF6 escape and a depletion in carbon dioxide could be enhanced in a thin layer. So neither carbon nor SF6 may be telling us the whole story. Perhaps there is more below the surface.
From the likes of this study, it may seem like iron is the key element that controls the cycling of carbon. Certainly it does play an important role and that is what this experiment has been about, in part. But bricks without mortar are just a loose assemblage of building blocks. Many other elements besides carbon (and iron) are certainly required to assemble cells and make them grow and thrive. Among the most obvious are nitrogen and silicon. At this station in particular the drawdown of major phytoplankton nutrients will be an important part of the iron story.
Nitrogen is a key element in amino acids and thus is found in proteins and enzymes. In addition it is required in chlorophyll, nucleic acids, ATP and almost everywhere else you look. On average it is found in phytoplankton cells at a ratio of 106 carbon atoms per 16 nitrogen atoms - gotta have it, there is no way a plant, or anything else, can live without it. Lucky for them, the world is full of it. 80% of our atmosphere (and most of the nitrogen on the planet) is nitrogen in the form of dinitrogen gas and the second most abundant form is oxidized nitrogen in the form of nitrate. Unfortunately few organisms can use the dinitrogen because the atoms are tightly bound by triple bonds and hard to break apart (it takes bolts of lightening and special bacteria to do this). Neither is nitrate such a great form of nitrogen. It is highly oxidized and is 5 oxidation steps away from the form they need - they need reduced nitrogen like ammonium to produce amino acids. Phytoplankton cells have to use a lot of energy to reduce one nitrate molecule to a form they can assimilate. So, in spite of its abundance it can be limiting. There is an old sea saying about nitrogen: "Nitrogen, nitrogen everywhere but not an energetically bioavailable form to drink." Sound familiar? As a result, many ocean systems rely on the cycling of reduced nitrogen species: urea and ammonium. The rate at which they can use nitrate will be directly proportional to the rate at which they can utilize carbon and other elements. So, nitrogen cycling is very important. You cant live, or die, without it.
Dr. Bill Cochlan from RTC/SFSU has been studying nitrogen cycling on this cruise. His general hypothesis is that, when given sufficient iron, cells will increase their ability to reduce nitrate and take up nitrogen. But other hypotheses of the nitrogen cycle are also being tested during his experiments. For instance: The more ammonium there is in seawater, the less nitrate will be consumed because ammonium is generally used preferentially by phytoplankton, and ambient ammonium concentrations of less than half a micromolar can severely inhibit nitrate uptake by phytoplankton "so alleviating iron deficiency should reduce ammonium uptake and permit phytoplankton to use the abundance of nitrate ions found in the Southern Ocean" Are these things true? Well, Bill Cochlan and his crew (Atma Roberts from UCSC and Julian Herndon from RTC/SFSU) intend to find out.
Bill spikes his samples with stable isotopes of nitrogen in different forms and then, using a mass spectrometer ashore, can detect where these compounds end up. By conducting time course experiments (incubations) he can determine rates for the transformation of nitrate, ammonium and urea into phytoplankton cells and can also measure the activities of the actual enzymes used in these transformations. By adding small amounts of ammonium to these incubations, he can also see how these rates may be inhibited by ammonium.
Since this is our last vertical profile station, we hope to be able to analyze some of these samples on the transit to Lyttelton. We will let you know what we find out.
Kenneth Coale, Chief Scientist
R/V Melville Log Entry: Mike Gordon at the trace metals winch. Normal hydrographic winches carry thousands of meters of 0.82-cm diameter steel cable that is galvanized to reduce corrosion. This contaminates the whole water column, especially the area near the samplers, with iron and zinc. The winch depicted here has special Kevlar cable on it that is non-metallic and a stainless steel level-winder. The breaking strength of this cable is over 1100 kilograms.
At our last station today it occurred to me: "Wow, this is our last station! In fact, this is our last trace metal cast..and I dont have one picture of this operation." I was operating the crane so I couldnt leave just then, but as the messenger fell (a messenger is a weight that slides down the cable to trip our bottles closed), I ran inside (walked safely and swiftly) and took a couple of pictures of the trace metals aspect of our operation. They are bad but you can see them anyway. Why do we care about trace metals? Well I am glad you asked.
Before it involved the whole periodic table, this whole program (SOFeX and IronEx before it) was centered on one element: Iron. Like many other ecosystems, there is no single factor upon which the whole system depends. Although this is true for ocean systems as well, iron comes pretty close. For its overwhelming and dramatic impact on ocean ecosystems and potential role in carbon sequestration, some have called it the key to global warming. Overstated? Perhaps, but why is it so important and how is it that we figured this out? Here is the short story:
When Mike Gordon was a graduate student at Moss Landing Marine Laboratories, his advisor, Dr. John Martin, gave him a project: Figure out a method, then measure the trace metal concentrations in San Francisco Bay. Mike, a masters student from San Jose State University rose to the challenge, developed both an organic extraction and ion exchange method to preconcentrate metals from seawater, then perfected a method by which to detect them using graphite furnace atomic absorption spectrometry. I know - this sounds soooo simple; any masters student could do this.
Well its not. Think about the sources of trace metal contamination. They are everywhere: on the ground, in drinking water, in the dust we breathe, on our clothes, in our hair, in our sample bottles and ships especially ships, are made out of them. They are made out of iron, have sacrificial anodes made of zinc, they pump huge amounts of water through their engines to cool them off, there is water running all over their decks constantly It is like trying to measure cigarette smoke in the middle of a forest fire. Not only that, but seawater, my goodness! It has everything in it and except for a few elements, all the rest are at vanishingly small concentrations in a matrix of half molar sodium chloride!! So it is really like trying to measure cigarette smoke in a forest fire on Venus, or trying to find a postage stamp on the area of Texas (this is actually an accurate aerial comparison).
Mike did it! He produced some of the most beautiful data John had ever seen and some of the first data that really made sense. John was impressed and hired Mike as a technician. Over a number of years Mike involved in some landmark programs his data leading John Martin into new controversies with the most prominent of geochemists. Why? Because Mikes data was very good and led to new and more accurate paradigms for trace element geochemical cycling and the old data was for the most part, contaminated and told us nothing about how the world works (and John loved a good fight).
One day Mike brought John a vertical profile of dissolved iron from the North Pacific. John couldnt believe it at first, the values were too low, way below published reports. He asked Mike to recheck his values. He did and confirmed they were correct. The profile revealed the distribution of iron that was very low in the surface waters and increased with depth. They plotted it together with the phytoplankton nutrient nitrate from the same station. straight as an arrow! It looked like iron was distributed like a phytoplankton nutrient! Furthermore, the surface values were so low that John thought they could be limiting phytoplankton growth.
He was right. Over the next decade, John and Mike, Steve Fitzwater, Sara Tanner and others from MLML and UCSC performed trace metal enrichment experiments that confirmed this hypothesis. These experiments showed dramatic growth when nutrient rich waters were given tiny amounts of iron. Yet these experiments were also criticized because the bottles did not reflect the whole community response one might expect in the ocean. Fair enough, John thought and devised a larger experiment that would involve the whole ecosystem - an open ocean iron fertilization experiment. Tragically, John died of prostate cancer before it could be carried out. Mike has hung in there through Johns untimely death, the earthquake destruction of the laboratories, the transition in leadership at the Labs, and the reconstruction of new facilities and has participated on every MLML enrichment experiment to date.
This one is special. John always had his eyes on the Southern Ocean but I think John would have been surprised (and delighted) at the North and South Patch experimental results. But as Mike heads home with hundreds of liters of samples, we ask: What is the next limiting nutrient and how does it affect ecosystem structure? I think the answer may be in these bottles. So does Mike, devise a method and measure it? Or is this a task for the next graduate student?
Kenneth Coale, Chief Scientist