Weddell sea contributes little to silicate enrichment in Antarctic Bottom Water

Abstract

The enrichment of silicate in Antarctic Bottom Water (AABW) is associated in the Weddell Sea with a small but significant depletion in oxygen. Silicate fluxes from the sediment (33–210 and 210–480 mmol m −2 y −1 in deep-sea and shelf sediments, respectively) are far too low to account for the Si enrichment in AABW. Moreover, the ratio of silicate to nitrate and consequently to oxygen fluxes across the sediment-water interface (0.42 molar ratio Si/O 2 flux) contrasts with the observed ratio of silicate enrichment to oxygen depletion in AABW (2.9 molar ratio). Silicate data in the Weddell Sea can be explained largely by conservative mixing: Si concentration and potential temperature of the cold bottom water formed by the overflow of Ice Shelf Water (ISW) from the Filchner Depression follow the conservative mixing line of ISW (θ–2.0°C) with the lower end of Warm Deep Water (θ 0.4°C) until this cold water reaches a depth of over 2000 m and flows under the AABW. The composition of the so-formed Weddell Sea Bottom Water is determined mainly by vertical mixing with AABW flowing into the Weddell Sea north of Kapp Norvegia (θ down to –0.7/–0.8°C). However, in situ dissolution of Si causes additional enrichments in the bottom water up to 20 μM towards the center of the Weddell Gyre, implying longer residence times. Siliceous sediments in the Enderby Basin produce far higher Si fluxes than the sediments of the Weddell Sea. It is likely that the Si enrichment in the AABW flowing into the Weddell Sea is mainly produced in the Enderby Basin by dissolution in the water column and in the sediments. Thus, although the Weddell Sea produces 70% ( Carmack, 1977, A voyage of discovery, pp. 15–41) of the cold endmember of the AABW, it contributes little to its silicate enrichment.