06 April 2004
Tuesday, 6 April 2004
A sign of things to come?
It’s a clear, calm, low-flux day here in the patch, though we are hardly aware of that in our lab down in the bowels of the ship. The CO2 lab is below the water line – a nice stable place in the ship, though we can’t tell if it’s day or night, sunny or cloudy. Twice a day we emerge to take water samples from the CTD casts (in-patch and out-patch stations) then it’s back below to analyze the water for pH and alkalinity, from which we calculate total inorganic carbon concentration. In the lab we also have two systems carrying out measurements while underway – taking near-surface water from the ship’s seawater supply to analyze for pCO2 and pH. The data are used to determine the carbonate chemistry of the seawater. Phytoplankton use carbon from the water for photosynthesis, so as a bloom develops there is a concurrent decrease in the carbon concentration, resulting in a decrease in pCO2 and an increase in pH. While tantalizing changes have been seen in the biology, a dramatic bloom has not yet developed (which is of great interest to the biologists) and so we cannot yet see any change in these seawater chemical properties.
Team CO2 consists of two people: Kim Currie (NIWA, Dunedin) and Burns Macaskill (NIWA, Hamilton). Malcolm Reid (University of Otago) gives shore-based assistance. A decrease in pCO2 in the surface water will lead to an enhanced flux of atmospheric carbon dioxide into the ocean. Our data will be combined with that from the atmospheric CO2 measurements (Rona Thompson), and the piston velocity determinations (David Ho and Cliff Law).
There’s a smidgen of a hint of a rise in the concentration of dimethyl sulphide in the water today. Dimethyl sulphide (or DMS) is one of the ‘climate-relevant’ gases that we are interested in (see Dawn Devries’ explanation earlier). It is derived from a compound produced by microalgae and a complex network of processes governs how much DMS enters the atmosphere, where it may have a cooling effect on the planet. For possibly the first time (you don’t always know what other research is going on in parallel around the world), we have the capability of looking at both the processes in the water column that produce DMS (Graham Jones, Karl Safi and myself) and the really novel capability to measure its transfer rate and fate in the atmophere at sea (Mike Harvey, Murray Smith, Jill Cainey and Dawn Devries).
The figure illustrates the chromatograms from 6 and 4 April for the surface waters in the iron enriched patch. The two large peaks correspond to the quantity of DMS. Due to the detection method, the difference is exaggerated in the figure but the increase is real enough to get excited about. Is it going to continue to rise and is this different from the surrounding waters without the added iron? There have been a number of tantalising indicators that the microalgae are starting to escape the constraints that have held their population growth in check. If they do break free, we could be in for an interesting few days. If they don’t, one of the fascinating questions to come out of our study will be what’s holding them back?
During the day the small RHIB workboat of the Tangaroa was deployed several times, the met balloon was released and Murray and Mike were doing some vertical profiling of DMS concentration in the atmosphere. The sunny weather gave an opportunity to measure some of the gases generated by sunlight interactions in the near-surface waters. Deck incubation experiments are investigating the effects of micronutrients, light levels and grazing on phytoplankton growth. This data may help explain why a bloom has not yet developed.
A further infusion of iron and tracer gas (SF6) will be released this evening.
Kim Currie (NIWA)
Burns Macaskill (NIWA)
Steve Archer (Plymouth Marine Laboratory, UK)