The tracer signature of Antarctic Bottom Water and its spread in the Southwest Indian Ocean: Part II—Dissolution fluxes of dissolved silicate and their impact on its use as a chemical tracer

Abstract

Using a combination of dissolved silicate, oxygen and a simple ternary mixing model we have been able to make comparisons of dissolved silicate and oxygen anomalies in the Southwest Indian Ocean and distinguish between remineralisation and diagenetic release as sources of Antarctic Bottom Water's (AABW) enhanced dissolved silicate concentrations. Combining theses data with transit times deduced from CFC measurements (see Part 1) we have gone on to make estimates of the diagenetic and remineralisation fluxes of silicate to the deep waters of the region. This provides a novel approach to the determination of such fluxes and as such our results should help to provide tighter constraints on future estimates of the global silicate balance. Results show that diagenetic release is generally between 75 and 780 mmol m −2 yr −1, in agreement with results from sediment core studies. Such a flux equates to a maximum silicate excess, over and above that expected from simple mixing of the deep water masses, of 22 μmol kg −1, some 15% of the measured concentration. Silicate release from remineralisation is lower, with values <316 mmol m −2 yr −1, generally equating to <10% of the dissolved silicate concentration. The dissolved silicate anomalies broadly reflect the flow patterns of AABW as it moves away from the Enderby Basin, as discussed in Part 1, but the effect is masked by large localised variations. High total anomalies are observed in the region adjacent to the Conrad Rise (∼30 μmol kg −1) and these are possibly attributable to high productivity associated with the Polar Front. Other high spots include the deep flows north of the Conrad Rise, to the west of Crozet Island and in the Crozet-Kerguelen Gap (up to 31 μmol kg −1). This variability indicates caution is needed when using silicate as a quantitative tool in areas of high siliceous productivity and/or sedimentation.