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The Australian Coastal Zone and Global Change: Research Needs

Australian Academy of Science, Becker House, Canberra. Friday 16 December 1994

LOICZ-JGOFS interactions: carbon fluxes in the continental margins

Dr John Parslow

Thanks. I would like to talk about the interaction between JGOFS and LOICZ. I have subtitled the talk 'Carbon Fluxes at the Continental Margins'. There are two aspects I would like to cover briefly. One is the interaction between JGOFS and LOICZ at the international level and in terms of the underlying science. Then I will come back to Australia and look in a little more detail at the regional aspects in Australia and the research opportunities.

To remind you why JGOFS and LOICZ have a lot in common, the science plan goal for JGOFS was: to determine and understand on a global scale processes controlling fluxes of carbon and associated biogenic elements in the ocean, and to evaluate the related exchanges with the atmosphere, sea floor and continental boundaries. It was certainly recognised from the beginning that exchanges with continental boundaries would be a key aspect of JGOFS. When JGOFS developed an implementation plan, the implementation plan goal (Overhead 1) focused specifically on carbon. (There is still a strong recognition that in tackling carbon we have to tackle other associated elements in the ocean, particularly nitrogen.)

OH 1: JGOFS implementation plan goal

Overhead 1

Although explicit mention of the continental margin has disappeared from the JGOFS Implementation Plan goal, it has certainly not disappeared from the Implementation Plan. One of the objectives of the Implementation Plan (Overhead 1) involves knowledge and understanding of fluxes across the continental margins. These were seen as critical boundary conditions for global and basin-scale carbon models.

The JGOFS Implementation Plan contains a set of sub-objectives for JGOFS continental margin studies. These include quantifying the exchange of carbon between the margins in the open ocean; understanding the seasonal and inter-annual variations in carbon and nutrient fluxes and cycles on the margins; understanding the physical processes that govern exchanges between the continental shelves and the slope and the open ocean; looking at carbon deposition in the margins and, in particular, on the continental slope; looking at air-sea CO2 exchanges in upwelling areas in particular, where exchanges are thought to be high; and, of course, the problem that has been talked about a lot by the previous speakers, of extrapolation to global scales.

I will not go into these in any detail here but you can see very clearly the overlap between JGOFS' interests in the continental margins, as a boundary condition, if you like, for the ocean carbon cycle and as a critical component of the oceans in themselves, and LOICZ's interest, especially in the Focus one and three activities that we have just heard about (Overhead 2).

OH 2: LOICZ focus 1,3 activities

Overhead 2

As LOICZ became established, the two programs saw this overlap in interests as being so important that a joint task team was formed in 1992, under the chairmanship of Arthur Chen, specifically to look at the carbon cycle in the continental margins. That Task Team published a report earlier this year as JGOFS report No. 15. I certainly recommend it to anyone who is interested in the interaction between JGOFS and LOICZ and in carbon cycle studies in the continental margins. You can obtain copies of that report either from the JGOFS project office or from the SCOR office. The task team will continue, but will probably be reformulated now, as LOICZ enters its implementation phase and starts to look at joint field activities.

OH 3: importance of continental margins

Overhead 3

Overhead 3 summarizes the findings of the Task Team on the importance of the continental margins to JGOFS. As defined by the Task Team, the continental margins consist of the shelf and slope, and constitute about 5 per cent of the ocean area. This area sits between the terrestrial input of nitrogen, other nutrients and organic carbon, and the oceans. Processes there determine the fate of terrestrial input and how much of it reaches the oceans. The continental margins are an area of much higher average biological production per unit area than the open ocean.

About 50 per cent of the marine calcium carbonate production occurs in the margins. Calcium carbonate production actually means a net influx of CO2 from the oceans to the atmosphere. The margins generally are thought to be a site of intense air-sea exchange of CO2 because of the processes that drive the carbon cycle there, especially upwelling of deep water with high dissolved inorganic carbon, nutrient input, organic matter deposition and calcium carbonate production.

The margins differ from the deeper ocean in that much more of the organic matter that is produced there is exported to the sediments. The processes of remineralisation in sediments are much more important to the production processes in the water column on a short time scale. High sediment accumulation rates occur in the margins. It is estimated that about 90 per cent of the ocean carbon burial occurs on the margins, mostly on the upper slope.

The task team identified some key topics for process studies in the margins (Overhead 4). These include processes, both from the ocean side and the catchment side, which supply nutrients to margins. They include physical processes responsible for transport and deposition of particulate matter, and exchange with the open ocean; the coupling between primary production and organic matter supplied to the sediments, and processes controlling remineralisation of organic matter in the water column and particularly in the sediments.

OH 4: key process studies

Overhead 4