Southern Ocean Iron Experiment (SOFeX) Cruise
January 5 - February 26, 2002
Skip to Log Entry from the USCGC Polar Star
February 20, 2002: Day 47
Position: 52 degrees, 23 minutes South, 166 degrees, 57 minutes West
R/V Melville Log Entry: There are some things that will not be measured on this trip. I have mentioned phytoplankton pigments as an example. The stable isotopic composition of seawater and plankton are others. Why dont we measure them out here? Because it takes elaborate extraction equipment, clean labs and a mass spectrometer that is too big and delicate to bring to sea. Even if we could, we are full to the gills and there is no space for this equipment. The distance between each work table in the main lab is just over half a meter and we have become quite skilled and tolerant of passing one another in the lab. In spite of our courtesies, we have neither the space nor the time to measure this stuff out here. Unlike thorium-234 (half life of 24.1 days), these isotopes can wait and dont decay or go stale. They are perfectly happy to sit on a filter or in a bottle for a long time. You can bet your extra neutron, however, that they will be measured back in the lab at a couple of different institutions.
David Timothy (with Mark Altabet, University of Massachusetts) and Eva Bailey (with Jim Bauer of the Virginia Institute of Marine Science) are looking at the stable isotopes of carbon and nitrogen (carbon-12, carbon-13, nitrogen-14 and nitrogen-15). Both carbon and nitrogen are the building blocks of biological tissue and comprise, in part, the currency of metabolic transformations within an ecosystem. The thing about the isotopes is that they are partitioned or fractionated when they are taken up by phytoplankton, or respired by the zooplankton that eat the phytoplankton.
Working on the open ocean, out here in "blue water", is something quite new to David. He has been studying various aspects of the oceanography of fjords in British Columbia, Canada, and recently received his PhD from the University of British Columbia. Working with naturally occurring stable isotopes is also a bit new to him, although he tells us there were a lot of people doing this sort of work at UBC, and he was able to osmotically assimilate the fundamentals of isotope fractionation. "This has been a very exciting trip", says David, "but working with isotopes can be a bit demanding. One is forever concerned about contaminating samples! Keeping things separated and clean is tricky business on a busy ship at sea."
For Eva, this is her first time working in the Southern Ocean and it has been very exciting. In planning for the trip she thought her work would be fairly simple: just filter some water into clean glass bottles, freeze them and transport them back to Virginia. In reality she has had to be constantly on her toes keeping her samples clean. With the isotope vans just steps away from where she collects her samples, she has to be extra careful and aware of how to avoid contamination. Whereas she is looking at naturally occurring isotopes, some on board use large quantities of these same isotopes for rate studies (more on this later). A tiny amount of tracer material from these studies could swamp the natural signal and she would have no idea until she had processed the samples back in the lab. Carbon is everywhere; grease, skin cells, powder on gloves, etc. and all these substances can cause contamination in her samples (she is bringing back between 700-800 bottles of frozen water samples).
So, for Eva and David, we wont know what they find until later, and they wont sleep easily until they get their samples back safely to the lab.
How does this isotopic fractionation stuff work? Well it is like thisremember from physics (stay with me here) the equation for kinetic energy: E=1/2 mv2? OK! At a given temperature all molecules have the same energyBUT they dont all have the same speed. Why? Because some have different massyep, its the isotope thing. Look at the equation (this is really simplified). At the same temperature (E), heavier isotopes go slower (have a smaller v) than lighter isotopes. In a molecule like nitrate (15NO3), a heavy isotope of nitrogen would go slower than its lighter counterpart (14NO3)and statistically, make fewer collisions with the active sites of enzymes (like nitrate reductase). This would make it less likely to be taken up by a phytoplankton. Consequently, phytoplankton nitrogen would be lighter than the nitrogen in the water and phytoplankton growth under these conditions would make the water nitrogen, isotopically heavier. This process is called fractionation. Well, it gets better. When nitrate is scarce, phytoplankton arent so picky and will scoop up anything they can get. When nitrate is abundant, they seem to be very particular and fractionate (discriminate) quite a bit. So in some measure, the isotopic composition of the phytoplankton community is related to nitrogen availability. Isotopically light phytoplankton have grown where nitrate is abundantly available; heavy phytoplankton result where nitrate is scarce.
It is similar with carbon. When plants grow they would normally take up the lighter stuff. But when they are really growing fast, they can become carbon limited and then they arent so picky about it and will take up anything they can get. Theyll eat brussel sprouts just as fast as they will eat pizza. So, the isotopic ratio of carbon 12:13 in a cell can be a measure of growth rate.
This experiment is unique because through the addition of iron, we are changing the growth rates of cells. If we can measure the isotopic composition of the plankton and the water, over a range of growth rates, we may be able to develop a robust relationship between them. So what? The isotopic composition of phytoplankton is preserved in marine sediments, albeit with some caveats. Therefore, we can measure these isotopes at stages in the sediments that represent glacial periods, for instance, to see how much of nutrient pool was utilized by the phytoplankton at that time and whether iron really did influence the rate of growth during the last glacial maximum. In addition, these isotopic proxies can also be used to trace the fate of surface derived material. It can help to tell us who eats whom and may be able to be used to establish trophic relationships in the water column community.
Anyone for a "light" lunch?
Kenneth Coale, Chief Scientist
Ships Position: 52 degrees, 37 minutes South, 167 degrees, 1 minute West
R/V Melville Log Entry: We finally bushwhacked our way back to the Northern Patch and it is only a shadow of its former self. We have found, and extensively mapped, portions (remnants) of the initial infusion area, scattered in pieces and drawn into shreds. Values of SF6 in these locations look old with maxima only a few times background readings. These maxima, however, were coincident with high values of chlorophyll and degrading Fv/Fm ratios reminiscent of a bloom that once raged in green fury.
These pieces are run up against a temperature discontinuity (front) that trends NE/SW and is very narrow, much like the way Ken Johnson described it to us about two weeks ago, but it is now shriveled and pinching off into clumps. This is not necessarily what we were hoping for but it is definitely the Old North Patch.
We conducted an extensive survey with the hope that this filament with its signature of fading SF6 was contiguous with the more recently infused area. We chose as our station today the "Patch Central" as defined by the most recent iron addition and the drifting SOLO Float about 30 kilometers away. Yet things are different, the water is warmer and blue, not the green that Ken had related to us.
As we hove to on station this morning, we had to maintain 1.3 knots headway, just to stay in one place!!! The Acoustic Doppler Current Profiler (ADCP) shows currents on the order of 75 cm/sec to the South and about 20 cm/sec to the East!!! The wire angle during the CTD cast was 45 degrees!!! This has the appearance of a station that has been obliterated by a convergent front and swept away, scattered hither and yon. Our survey has been like kicking through the rubble of your house after it has been hit by a tornado, rummaging through what was once the living room, looking for precious belongings. This is the nature of frontal systems and it seems the Northern Patch ran right into one. We will survey a bit more this evening, let's hope we can find the fireplace, if not, it seems there are portions of the basement that are still intact for our station tomorrow.
One of the hints that there had been a large bloom is that there were considerable drawdowns in carbon dioxide, coincident with almost complete depletion of silicate, an essential nutrient for diatoms. What is so remarkable about the bloom in the north is that diatoms came up at all. We want you to understand why silicon is important.
Life on earth is generally characterized as carbon-based. In consideration of extraterrestrial life forms some scientists (and fiction writers) have speculated that they may be based on other elements. Silicon is an element that is right under carbon in the periodic table and is a great candidate for such consideration. It is multivalent just like carbon and can hold hands with lots of different atoms. As such, it is great for a structural material, rocks are made out of it. Wouldnt you know it? Some phytoplankton have been reading the same outer-space fiction as us and have already figured this out. These organisms are the diatoms. Diatoms are phytoplankton that deposit silicon in their cell walls forming what is essentially a shell made out of silicon and oxygen. They have evolved to absolutely require silicon for growth. Without silicon, diatoms just cannot grow. Diatoms get their silicon from the minute amounts of dissolved silicon present in seawater. They use it in sculpting structures whose intricacy and complexity far surpasses what the engineers of silicon valley can accomplish through micro-engineering and they do this at low temperature, low pressure and near neutral pH. We humans have to use extremes of temperature, pressure and pH to manufacture the silicon microcircuits of computer chips. Diatoms also process more silicon than we do. Each year diatoms extract over 5 billion metric tons of silicon from the sea to produce their shells!
The biogenic silica that diatoms produce has the same chemical form as the gem stone opal. It is basically clear glass which confers not only strength to the cell, but a clear cell wall that light can penetrate allowing the cell to photosynthesize, and sharp and spiky siliceous spines that help suspend the cell in the water column and make them difficult for zooplankton to eat. The siliceous shells of diatoms can accumulate on the sea floor forming sediments that consist almost exclusively of diatom remains. These deposits provide evidence of prehistoric ocean productivity and are mined to produce filtering agents. The organic matter in diatom oozes sometimes turns into oil. Thus, diatoms, as diatomaceous earth, help filter our swimming pools and, as hydrocarbon, power our automobiles. Our link to the diatoms goes beyond filters and fuel. Through photosynthesis, diatoms produce about 20% of the oxygen that is in our atmosphere. So for every fifth breath of air that you take you can thank the diatoms.
Diatoms can dominate the phytoplankton (in terms of biomass) where concentrations of dissolved silicon in seawater are high. Diatoms have been the group of phytoplankton that have responded to iron enrichment most strongly in both of our iron patches. One surprising finding so far is how little dissolved silicon diatoms need to grow. Especially at the North Patch station where the concentration of dissolved silicon in the seawater was less than one micromolar, there now appears to be a big bloom of diatoms!!? This study may change how we think about silicon limitation in diatoms and how iron may lower the limit at which diatoms can take up silicon.
To understand how iron affects silicic acid uptake and the production of "biogenic silica" Dr. Mark Brzezinski and his team from UCSB (Janice Jones and Mark Demarest) have been conducting a series of experiments and sampling efforts to understand the cycling of this important element in this Southern Ocean system. Not only is he measuring the rate at which diatoms are utilizing dissolved silicon, he is also measuring the concentration of silica in the particulate phase (biogenic silica) and determining the rate at which silica dissolves. He is also assessing whether the diatom blooms in the iron patches will end because they run out of dissolved silicon. Iron plays strange tricks on diatom physiology and a lack of iron dramatically changes the ratio of silicon to organic matter in diatom cells. Dr. Brzezinski is testing the hypothesis that the addition of iron will diminish diatom demand for silicon relative to carbon and nitrogen. In this way the system gets more organic carbon bang for each silicon buck utilized by the diatoms. Such a response would allow an iron-induced diatom bloom to fix more carbon before it runs out of dissolved silicon.
Brzezinskis group is also collecting samples to examine the isotopic composition of diatom silica and the dissolved silicon in seawater. The isotopic composition of this material may shed light on how diatoms contribute to carbon burial on the sea floor. The occurrence of diatom tests in the sediments is thought to indicate times of high ocean productivity. But this sediment record is confounded by the extent to which their shells may dissolve on the sea floor, or the thickness of the tests themselves. The isotopic composition of the shells may be free of these confounding effects and supply information on the history of diatom growth. As diatoms grow under different dissolved silicon concentrations, the isotopic composition of their siliceous shell changes. Although some shells may dissolve in the sediments, their isotopic composition should be unaltered and can be used to infer the contribution of diatoms to productivity when the cells were alive and growing in the surface ocean. Perhaps silicon isotopes will be the best proxy for ocean production.
Says Mark at every opportunity "Diatoms Rule!" and I am beginning to believe him, even in these dynamic and silica-poor waters.
USCGC Polar Star Log Entry: Well its official- sampling ended for SOFeX at the southern patch yesterday at the "Last Call" CTD/Rosette station taken on Feb. 20th at 65.874 degrees South 171.958 degrees West. It was a moment of mixed emotions- relief that the hard work was over and that the weather and patch conditions had held out for so long, but also curiosity about what fate would bring to the patch in the days to weeks to come. Remote sensing via satellites of patch conditions and a couple of autonomous monitoring buoys in the area are all that remain after we head east out of the patch area.
Scientifically, this Polar Star leg of SOFeX achieved all its scientific objectives, and then some. We intend over the next couple of days to post a few specific scientific results for those interested. Ill discuss a few teasers today, but it will take many months of continued sample and data analyses to see the full picture emerge from this experiment.
Before I drift back to the science, the end of this leg also leaves me in great debt to a number of groups. I hope in my reports that Ive given you some inkling of the quality and dedication of the small team of 13 scientists that came on board with me on the Polar Star. They deserve high praise for pulling off this final patch occupation- many thanks to all of them! On the Polar Star, we were assisted by the skilled USCG Marine Science Tech group here and the officers and crew of the Polar Star. While the resources were limited for our science objectives, the Polar Star crew always found a way to meet our demanding sampling needs. It also goes without saying that our appreciation goes to Kenneth Coale, Ken Johnson and colleagues on the Revelle and Melville who left us with such a well behaved patch and early data that allowed us to optimize this final occupation.
Long before we went to sea, we had to gain the support of the agencies and sponsors who funded this work. The Division of Ocean Sciences at the US National Science Foundation is the primary sponsor of this experiment. In addition, the US Department of Energy provided direct support for some of the SOFeX Investigators and this cruise in particular. At the risk of leaving someone out, thanks Don Rice, Phil Taylor and Anna Palmisano for your work within these agencies to set us up with the resources needed to pull this off. With the wide range of US and International teams involved, Im sure there are many other agencies and organizations that contributed in small and large ways that I cannot properly acknowledge in this web page. This type of field work does not come cheaply, but the short and long term advances in our understanding of ocean processes that comes out of such process studies are staggering and will be felt for a long time to come.
A final word of thanks and appreciation goes out to our families, who put up with our at sea voyages and long days and weeks away from home.
Getting back to the science, one objective of this cruise, dubbed SOFeXport, was to look at the impact of iron fertilization on carbon flux to the deep ocean, i.e. how much of the green patch sinks out? And how much simply degrades in surface waters and thus has little net effect on atmospheric uptake of CO2, since the extra C is only stored temporarily. Past results in this regard were mixed. As of our last occupation of the southern patch, the carbon flux out of the surface ocean at this site in a relative sense is not significantly different than the surrounding waters.
This statement is based primarily upon my own research into the distribution of the natural particle reactive radionuclide thorium-234. We had groups on both the Melville and Polar Star from my "Caf Thorium" research team, and samples collected for us on the Revelle (which I have not yet analyzed). So while not complete, what I show below is a set of time series profiles of thorium-234 in the patch. You dont have to know the decay equations or particle chemistry of thorium for me to tell you that lower levels of 234Th are indicative of higher loss of this element (and carbon) on sinking particles. So surface waters are lower, and they decrease with time of the experiment to even lower values, or higher particle export.
So why am I telling you that iron had little impact if the particle export increased with time? Essentially this is where the "out" patch stations are so important. Not shown today (mostly because my favorite graphics software crashed in McMurdo) are "out" patch stations that look similar for 234Th. On this Polar Star leg, we have preliminary data that show that levels of 234Th were both higher and lower surrounding the study area. While this data is very preliminary (final samples processing for this tracer takes >6 months), it looks as if natural variability will be as large as the iron-induced effects on particle export during this 5 week occupation of the South patch.
While the details of this story will change, we did not witness the large-scale demise of the bloom and sinking out of carbon taken up by the plankton in response to iron. Why? Exactly how little was lost? And what will be the ultimate fate of this bloom, are questions we will try to assess with the larger data sets.
One clue to the answer to this export story is related to the types of phytoplankton species that grew up in the fertilized vs. non-iron fertilized areas. Shown below is a microscopic image (400x magnification) of plankton from the South patch caught on a filter (the round small holes are pores in the filter). In this image you can see long chains of diatoms and their spines as well as individual centric and pennate shaped species. Diatoms are a class of plankton with high iron requirements as well as high silica demands, since they have siliceous tests or shells. Southern Ocean waters are high in silicate and low in iron, so here the iron stimulated the growth of some of these diatom species over other plankton types. In patch vs. out patch and time-series comparisons of species composition will be important to the interpretation of the geochemical changes we see.
One of the quickest ways to assess community structure for a given parcel of water is to look at the relative size distribution of the chlorophyll bearing plankton. Shown below is a graph of the Chlorophyll content caught when water is passed sequentially through a 20 micron (= um = 10^-6 m), 5 um and 2 um filter. These data are from just one of our large scale transects, in this case running some 130 kilometers from South to North. The peak in the 5 um curve is at "patch central", where Fv/Fm and our patch tracer SF6 were highest (data not shown). The blue line represents the larger, >20um plankton, and these are found at higher abundances at the ends of this sampling line, both N and S outside the patch.
This is just one example of variability we are seeing in the region that will need to be explored in more detail. Im throwing it out here as a possible reason why my particle export values were not especially high in the patch relative to outside. It is thought that large cells are more efficient at sinking, thus even fewer large cells outside the patch might export as much carbon, as more abundant but smaller cells in the patch. Again, we need to compare this with the Revelle and Melville to see if they saw similar or different distributions.
None of the above science teasers are intended to be the final word or meant to suggest that iron had little effect. In fact the addition of iron induced the production of many tons of phytoplankton growth in our study patch, and these effects were obvious just looking at the color of this "green" water in our study site. What it does point to are some of the complications of making simple or quick extrapolations from this study to large issues of C uptake and climate change. Does iron induce phytoplankton communities that are more or less efficient at removing CO2 from the atmosphere? What are the secondary effects of Fe on community structure and surface ocean chemistry? How long would these effects be felt in terms of local biological or chemical change? What other secondary controls are there on C uptake, export and sequestration to the deep ocean?
Im just skimming the surface of this interesting story for our web site. Keep in mind that the interpretation and even the data themselves are subject to change as we recalibrate our instruments, check data and make final measurements. For now, we are pleased to have contributed to this experiment and hope you will keep an eye out for the real science stories that emerge.
At this moment, it is as if each of the 3 ships left with small pile of pieces to a complicated jigsaw puzzle. Now it's time to put our pieces on the table (hopefully we none have fallen off the table!) and see what image emerges from this amazing experiment.Over and out- Ken Buesseler