Tracing elements in the ocean
The surface waters of the vast Southern Ocean are suffering from ‘marine anaemia’ – a serious deficiency in the micronutrient iron. Just as iron deficiency negatively affects the health and productivity of humans and other land-based creatures, so it affects the phytoplankton (microscopic marine plants) existing in the oceanic realm.
The environmental consequences of this condition restrict primary production (photosynthesis) in the surface waters of the ocean. This means that less carbon dioxide is taken up from the atmosphere to fuel phytoplankton growth. The smaller the transfer of this greenhouse gas is from air to sea, the greater are the consequences for Earth’s climate. It is the sequestering of carbon dioxide in the deeper ocean and deep-sea sediments that provides a natural contribution to solving the problem.
During the International Polar Year (IPY), teams of international scientists on several research ships are investigating iron and other trace element micronutrients (such as zinc, cobalt, aluminium, copper and manganese) in cold, high-latitude waters around both poles. The research aims to understand the distribution and cycling of these important micronutrients, how they are supplied to the ocean, and how they affect the different phytoplankton communities in polar waters of both Hemispheres.
Under IPY-GEOTRACES, research voyages are being run along key transects in the Southern Ocean – the ‘choke points’ for the Antarctic Circumpolar Current, where land masses impinge on this massive radial circulation. German and Dutch scientists on Polarstern recently surveyed along the ‘Zero Meridian’ (0º longitude), then back northward across the Strait of Magellan, through the Drake Passage and along the South American coast (‘Zero & Drake’). French and Belgian scientists on Marion Dufresne made a voyage (‘Bonus-Good Hope’) out of Cape Town, South Africa, which ran diagonally across to the Zero Meridian, then overlapped the track of Polarstern to the coast of Antarctica and returned to Durban. Concurrently, US scientists followed a transect line south of Africa ('I6S'), at approximately 30oE.
In March-April this year, an Australian-led voyage aboard Aurora Australis, organised by the Antarctic Climate and Ecosystems Cooperative Research Centre, followed the 'SR3' transect along approximately 140°E, between Tasmania and the coast of Antarctica. This transect has been well characterised oceanographically in the past two decades (Australian Antarctic Magazine 8: 4). Scientists gathered samples with great care, to prevent contamination from the water column, at each degree of latitude along this north-south transect. Some trace elements (such as iron and mercury) were measured directly using ship-board instrumentation. Others, in their various forms, will be analysed in laboratories later. A high volume aerosol sampler was also deployed to measure the concentration of dust in the atmosphere, since dust blown from continental land masses is likely to be the main source of iron in the remote ocean south of Australia.
The results of this work will yield the first synoptic, circumpolar measurements of micronutrient trace elements in the Southern Ocean. Dissolved iron data along the SR3 transect represent an order of magnitude improvement in sampling resolution and show widespread iron limitation in upper ocean waters along the transect. These and related observations promise to reveal important information concerning primary production in this region, phytoplankton community structure, and ultimately, a better understanding of the important regulators of climate change. Through this and similar projects, scientists are beginning to learn how the functioning of Southern Ocean ecosystems is constrained by the small ‘cogs and gears’ of iron and other trace element micronutrients.
EDWARD BUTLER1,2 and ANDREW BOWIE1
2Centre for Australian Weather and Climate Research