ALUMINUM AND SILICIC-ACID IN WATER AND SEDIMENTS OF THE ENDERBY AND CROZET BASINS

The distributions of dissolved aluminium (Al) and silicic acid (H4SiO4 ) were investigated in the Indian sector of the Southern Ocean along a hydrographic section from 52 degrees S in the eastern Enderby Basin t o 39 degrees S in the western Crozet Basin. Going northward, there is a clear increase of Al concentrations at all depths, starting with a n early featureless profile (Al generally below 1 nM) at the southernmos t station, to profiles with maxima of 4.09 nM at 500 m and 3.57 nM in bottom waters at the northern stations. The general trend is interrupt ed by intrusions of Antarctic Intermediate Water with low Al, coincidi ng with oxygen maxima > 260 mu M. In the surface layer, H(4)Si0(4) ran ged from 1.5 to 5 mu M and increased continuously with depth to 158.7 mu M in the southernmost bottom water and to 141 mu M in the bottom wa ter of the Crozet Basin. There is a linear relation between H(4)Si0(4) and salinity, except in the eastern Enderby Basin, where the bottom w ater is enriched in H(4)Si0(4) At the eastern flank of the Crozet Plat eau, a western boundary current was observed carrying deep water with about 2.3 nM Al and 155-156 mu M H(4)Si0(4) This indicates a net input of H(4)Si0(4) and no net input of Al during water transport from the western Weddell Basin to the Crozet-Kerguelen Passage. At the southern most station all Al concentrations, ranging from 0.84 to 1.2 nM., with minima of 0.3-0.5 nM between 500 and 1000 m, are below the values rep orted for the western Weddell Sea, where the water originates. Also th e Al Values in the salinity maximum of the Circumpolar Deep Water are lower than expected from the intrusion of saline, Al-rich North Atlant ic Deep Water. Both observations suggest that scavenging of Al, possib ly by diatoms, occurs in parts of the Southern Ocean. In the Crozet Ba sin deep waters, H(4)Si0(4) behaves conservatively, while increased co ncentrations of dissolved Al in bottom waters are due to input from th e sediments. In the eastern Enderby Basin, excess H(4)Si0(4) in bottom waters indicates input from the sediment, without such an input of Al . This is explained by the higher concentrations of H(4)Si0(4) and low er dissolved Al in pore waters of the Enderby Basin, compared to the C rozet Basin. The Al/Si ratios of biogenic silica in the sediment were 0.0027 in the Enderby Basin and 0.011 in the Crozet Basin, both higher than in diatoms from surface waters. It is suggested that the biogeni c silica-rich surface sediment in the Enderby Basin acts as a filter f or Al, diffusing upward from pore water, whereas the low Al/Si ratios in biogenic silica support a marked H(4)Si0(4) flux into the bottom wa ters. In the Crozet Basin sediments, with less biogenic silica and mor e Al-releasing terrigenous material, some dissolved Al can escape into bottom water, but the higher Al/Si ratios in biogenic silica suppress release of H(4)Si0(4) (C) 1997 Elsevier Science Ltd.