Aluminium and silicic acid in water and sediments of the Enderby and Crozet Basins

Abstract

The distributions of dissolved aluminium (A1) and silicic acid (H 4SiO 4) were investigated in the Indian sector of the Southern Ocean along a hydrographic section from 52°S in the eastern Enderby Basin to 39°S in the western Crozet Basin. Going northward, there is a clear increase of A1 concentrations at all depths, starting with a nearly featureless profile (A1 generally below 1 nM) at the southernmost 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 interrupted by intrusions of Antarctic Intermediat Water with low A1, coinciding with oxygen maxima > 260 μM. In the surface layer, H 4SiO 4 ranged from 1.5 to μM and increased continuously with depth to 158.7 μM in the southernmost bottom waterand to 141 μM in the bottom water of the Crozet Basin. There is a linear relation between H 4SiO 4 and salinity, except inthe eastern Enderby Basin, where the bottom water is enriched in H 4SiO 4. At the eastern flank of the Crozet Plateau, a western boundary current was observed carrying deep water withabout 2.3 nM Al and 155–156 μM H 4SiO 4. This indicates a net input of H 4SiO 4 and no net input of Al during water transport from the western Weddell Basin to the Crozet-Kerguelen Passage. At the southernmost 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 reported for the western Weddell Sea, where the water originates. Also the Al values in the salinity maximum of the Circumpolar Deep Water are lower than expected from the intrusion of saline, Al-rich North Atlantic Deep Water. Both observations suggest that scavenging of A1, possibly by diatoms, occurs in parts of the Southern Ocean. In the Crozet Basin deep waters, H 4SiO 4 behaves conservatively, while increased concentrations of dissolved A1 in bottom water are due to input from the sediments. In the eastern Endergy Basin, excess H 4SiO 4 in bottom waters indicates input from the sediment, without such an input of A1. This is explained by the higher concentrations of H 4SiO 4 and lower dissolved A1 in pore water of the Enderby Basin, compared to the Crozet Basin. The A1/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 biogenic silica-rich surface sediment in the Enderby Basin acts as a filter for A1, diffusiong upward from pore water, whereas the low A1/Si ratioss in biogenic silica support a marked H 4SiO 4 flux into the bottom waters. In the Crozet Basin sediments, with less biogenic silica and more A1-releasing terrigenous material, some dissolved A1 can escape into bottom water, but the higher Al Si ratios in biogenic silica suppress release of H 4SiO 4.