ON SUSPENDED BARITE AND TIlE OXYGEN MINIMUM IN THE SOUTHERN OCEAN

Particulate Ba profiles were measured in the Indian sector of the southern ocean. The largest fraction (>80%) of this barium is present as barite microcrystals. The profiles of total barium are characterized by a subsurface maximum between 200 and 500 m depth in the vicinity of the oxygen minimum. Highest barium values are found just south of the Polar Front, while lowest values occur close to the Antarctic Divergence. Between the divergence and the Polar Front a tight inverse relationship is observed between oxygen in the oxygen minimum and barium in the barium maximum. This relationship disappears north of the Polar Front. Since suspended barite is known to be of biological origin, the correlation of barite with oxygen suggests that the observed decrease of oxygen in the oxygen minimum, between the divergence and the Polar Front is due to local consumption of oxygen. It is proposed that deep low oxygen water is advected towards the Divergence where upwelling occurs and where this water subsequently partly spreads out to the north, north-east, as entrained by the Antarctic Circumpolar Current. Oxidation of locally produced organic matter, with which barite crystals are associated, consumes oxygen and sets free individual discrete barites. As a result, oxygen decreases and barite increases away from the divergence, with barite integrating former biological processes.