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Linking the Biogeochemical Cycles of Nitrogen and Sulphur as Part of the Atlantic Meridional Transect (AMT)
by Tom Bell email

It has long been established that the cycles of reduced nitrogen and sulphur in the surface of the open ocean are driven by biological processes. Reduced sulphur (DMS) and nitrogen (NH₃) gases emitted by the ocean can undergo chemical reactions to form particles in the atmosphere; these particles are composed of approximately equal amounts of nitrogen and sulphur species ¹, indicating approximately equal rates of emittance from the ocean. This suggests that the elemental cycles between the surface ocean and lower atmosphere are coupled. A greater understanding of this is particularly important as aspects of these cycles can potentially affect cloud water acidity (and hence processing within the cloud) and the flux of solar radiation reaching the ocean surface. My PhD involves research cruises as part of the Atlantic Meridional Transect (AMT) project; these cruises provide a fantastic opportunity to study, test and develop ideas concerning the nitrogen and sulphur cycles.

Sulphur (see figure 1)

The volatile compound dimethylsulphide (DMS) is the major source of biogenic sulphur to the marine atmosphere. Dimethylsulphoniopropionate (DMSP) is directly synthesised by phytoplankton and this is readily broken down into DMS and acrylate in the surrounding seawater. DMSP production varies significantly between phytoplankton species. At the same time, numerous trophic interactions control DMS production and destruction, complicating the reduced sulphur cycle.

The dominant oxidation product of gaseous DMS - sulphur dioxide (SO₂), has been shown to influence the solar radiation flux by reflecting and refracting the sun's radiation. This can occur either directly, as gaseous SO₂, or upon further oxidation to a sulphate (SO₄^2-) aerosol, which attracts moisture from the air, creating Cloud Condensation Nuclei (CCN). In 1987, Charlson et al. put forward the hypothesis that oceanic phytoplankton might be regulating the climate through DMSP, and hence DMS, production (figure 1). Since Charlson and co-authors published their paper, extensive research has been carried out asking critical questions. Of particular importance were: 'Why and how is DMSP produced?' and 'What are the pathways by which DMSP is broken down into DMS?'. It is important to understand the answers to these questions as they will eventually lead us to understand how and why DMS emissions from the ocean can vary so much over spatial and temporal scales and if there really is a climate-regulating feedback loop. I will not go into our current understanding regarding these questions but recent, comprehensive reviews are available in the literature^3-5.

Nitrogen (see figure 2)

In the surface ocean, an equilibrium exists between gaseous ammonia (NH_3(g)) and its protonated form, ammonium (NH₄⁺_(aq)). This equilibrium is pH dependent and, as the surface oceans are ~pH 8, ammonium is the dominant form (NH₃:NH₄⁺ ~ 1:9). NH₄⁺ (along with other forms of nitrogen) is a key nutrient for primary production and is intensely cycled within the euphotic zone.

The rate of air-sea exchange of gaseous ammonia is at least partially controlled by the concentration gradient across the air-sea interface. Whether the concentration in the air exceeds the concentration in the surface ocean will directly affect the direction of gaseous exchange. The nitrogen cycle is severely perturbed due anthropogenic activities; modern agricultural practice and industry have significantly increased concentrations of NH₃ in many environments, including the atmosphere. It is generally accepted that the further a site is from a source of such pollution, the lower the atmospheric concentration is likely to be and the more likely the flux will be out of the ocean rather than into it. Until recently, quantifying the direction and intensity of the air-sea flux of nitrogen has been difficult as simultaneous sampling in the atmosphere and ocean rarely takes place. In the atmosphere, NH₃ reacts with acids to form ammonium salts, which then attract water and, in the same fashion as SO₄^2-, produce Cloud Condensation Nuclei (CCN). This not only affects the radiative flux reaching the ocean surface, but also influences the amount of nitrogen deposition back into the oceans (figure 2).

Interaction of the Nitrogen and Sulphur Cycles (see figure 3)

The amount of nitrogen reaching the ocean on a global scale is not well known,

but in some remote environments will almost certainly be a major limiting factor for phytoplankton growth. At the same time, the flux of radiation is important to algae in the surface ocean because they use it as a light source. Understanding the interaction of the nitrogen and sulphur cycles and their potential influence on climate and surface ocean biology is of important. In the atmosphere, the aerosols of ammonia and sulphate readily combine to form potent CCN.

The various factors affected by the interaction of these cycles (as mentioned previously and shown in figure 3) either comprise or add weight to the CLAW hypothesis¹ (i.e. that there are numerous feedback mechanisms in place which attempt to maintain a constant radiative flux). It should be noted that figure 3 not only simplifies the reduced nitrogen and sulphur cycles, but also does not include the numerous interactions with many other nutrient cycles (e.g. iron, silicate, phosphorous).

Aim

To quantify the air-sea exchange of NH₃ and DMS through simultaneous lower atmosphere and surface ocean measurements, and hence gain insight into the complex set of interactions outlined in figure 3.

How?

My PhD involves me taking part in the Atlantic Meridional Transect (AMT) project. Every year, the Royal Research ship, the James Clark Ross (JCR), sails from the UK to Antarctica and back in order to resupply the various British Antarctic Survey stations; it travels southward in the N. hemisphere's Spring, and returns in the Autumn (figure 4).

AMT makes use of this passage by sending researchers to take measurements between the UK and the Falkland Islands (the JCR's last/first port of call before/after Antarctica). The project involves a coherent set of coupled atmospheric and ocean measurements which will define the surface water biogeochemistry, phytoplankton, zooplankton and bacterial production, atmospheric aerosol and gas concentrations. This provides an excellent framework within which to interpret the DMS and NH₃ data I will collect (with particular assistance from Dr. Alex Baker and Malcolm Woodward) during 3 of these research cruises (each lasting approximately a month). For further information, see the AMT website at http://www.pml.ac.uk/amt/.

Application?

My research will have many uses:

An increased understanding of how our climate may react to the perturbations we are causing in natural elemental cycles (including other cycles such as the carbon cycle)
An extensive data set which will be of great use to modellers, helping them to build more comprehensive models predicting future climate change scenarios.
An improved understanding of any potential relationships between DMS concentrations and other parameters measure by satellite (all attempts have thus far proved unsatisfactory). Should such a relationship be found, it would enable an accurate method of scaling up fluxes.
Increased understanding of any relationships between DMS concentrations and other water column parameters (e.g. pigments, phytoplankton speciation, etc.)
Increased understanding of the global NH₃ cycle.

Thanks go to my supervisors - without which there would be no project!

Dr. Gill Malin
Professor Tim Jickells (including his assistance in creating figs. 1-3)
Professor Peter Liss

Sulphur (see figure 1)

Nitrogen (see figure 2)

Interaction of the Nitrogen and Sulphur Cycles (see figure 3)

Aim

How?

Application?

Further Reading