Trull, T., Rintoul, S. R., Hadfield, M. and Abraham, E. R.
Circulation and seasonal evolution of polar waters south of Australia: Implications
for iron fertilization of the Southern Ocean. Deep-Sea Res II
48 2439 - 2466 (2001)
The physical and biogeochemical setting of the Southern Ocean Iron Release Experiment (SOIREE) is described. SOIREE was carried out in the southern Antarctic Zone (61o S, 140o E), between the southern branch of the Polar Front and the southern front of the Antarctic Circumpolar Current. The region is characterized by weak currents, high nutrients, low chlorophyll, moderate summer mixed layer depth, and is remote from the influence of sea-ice and coastal waters. In other words, the SOIREE site is typical of a broad area of the "open ocean" waters of the Southern Ocean. SOIREE was carried out in late summer (February), close to the time of maximum seasonal nutrient depletion and well past the usual seasonal peak in production and export. While the seasonal nutrient draw-down is substantial, levels of all three macro-nutrients remain high enough to support phytoplankton growth. In contrast, north of the southern branch of the Polar Front, silica is drawn down to levels which are potentially limiting for growth of large diatoms. The response to iron addition may therefore vary with season, at least to the north of the southern branch of the PF. The extent and consequences of silica limitation in Southern Ocean waters appear to have been under-appreciated. Only waters south of the southern branch of the Polar Front are high in silica year-round. In particular, water masses formed in the Southern Ocean sink with little pre-formed silicate (Antarctic Bottom Water (AABW) is an exception, but sinking of AABW is limited to a few small regions along the Antarctic margin). Therefore, in the absence of physiological changes, adding iron is likely to have little effect on carbon export by the silica pump, which plays the dominant role in carbon export in both the modern and glacial Southern Ocean. As a consequence, it may not be as easy to draw down all the macro-nutrients in the Southern Ocean as previously assumed in assessing the response of the global carbon cycle to iron fertilization. The iron hypothesis for glacial-interglacial changes in carbon dioxide requires that the silica limitation be alleviated in waters north of the present-day Polar Front (by changes in ocean circulation), as well as an increase in aeolian iron supply.