The Silicon Isotopic Composition of Surface Waters in the Atlantic and Indian Sectors of the Southern Ocean

We report here the silicon isotopic composition (δSi) of dissolved silicon (DSi) from 42 surface water samples from the Drake Passage, the Weddell Gyre, other areas south of the Southern Boundary of the Antarctic Circumpolar Current (ACC), and the ACC near the Kerguelen Plateau, taken between the beginning of February and the end of March 2007. From the beginning to end of the cruise (ANTXXIII/9), DSi diminished in the Antarctic by 50 μmol L while concentrations of nitrate + nitrite and phosphate showed no net decline, indicating that the high seasonal Si/N removal ratios well known for the Southern Ocean may be more related to the strength of the silicate pump in the Southern Ocean than to the instantaneous Si/N uptake ratio of diatoms. The δSi of DSi in samples containing more than 20 μM DSi were strongly negatively correlated to DSi concentrations, supporting the use of δSi as a proxy for DSi removal. The “open system” fractionation observed, ε = −1.2 ± 0.11‰, agrees well with results from previous work in other areas, and the estimate of the initial δSi of DSi of +1.4‰ is not far off observations of the δSi of DSi in Winter Water (WW) in this area. Results were used to model DSi draw down in the past from the δSi of sediment cores, although isotopic fractionation during silica dissolution appeared to influence the δSi of some surface water samples, inviting further study of this phenomenon.