Sambrotto, R.

Lamont-Doherty Earth Observatory of Columbia University, 61 Rt. 9W, Palisades, NY 10913, E-mail: sambrott@ldeo.columbia.edu

The role of surface ocean biological flux in the regulation of atmospheric CO₂: Have we made an honest woman of her?

Extensive sampling in the Southern Ocean in recent years revealed that, on an areal basis, it does not support exceptionally high carbon productivity. Factors such as light and iron limitation combine to restrict the amount of carbon reaching deeper waters. However, in contrast to carbon, exceptional amounts of silica and to a lesser extent, phosphorus are exported to deeper waters. This departure from balanced growth in surface waters appears to be associated with the growth of penate diatoms such as Fragilariopsis sp. that characterize surface waters from the coast to the Antarctic Polar Front. In low light conditions, luxury consumption of phosphorus leads to N/P export ratios less than 10. Conversely, the frequent iron limited conditions in Antarctic surface waters result in very high Si/C export ratios. Much of this export is regenerated before reaching the bottom in Upper Circumpolar Deep Waters. Thus, the elevated levels of these nutrients in Antarctic waters may be largely due to the nature of biological export. Such nutrient trapping may impact the biological carbon pump in lower latitude waters because it would restrict silica for diatom growth and phosphorus for the growth of all phytoplankton. The trap appears to be leaky off the East Antarctic Coast and this leads to enhanced nutrient supply to deep waters in the western south Pacific.