The silicon isotopic composition of surface waters in the Atlantic and Indian sectors of the Southern Ocean

We report here the silicon isotopic composition (delta Si-30) 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 mu mol L-1 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 delta Si-30 of DSi in samples containing more than 20 mu M DSi were strongly negatively correlated to DSi concentrations, supporting the use of delta Si-30 as a proxy for DSi removal. The "open system" fractionation observed, epsilon = -1.2 +/- 0.11 parts per thousand, agrees well with results from previous work in other areas, and the estimate of the initial delta Si-30 of DSi of + 1.4 parts per thousand is not far off observations of the delta Si-30 of DSi in Winter Water (WW) in this area. Results were used to model DSi draw down in the past from the delta Si-30 of sediment cores, although isotopic fractionation during silica dissolution appeared to influence the d 30 Si of some surface water samples, inviting further study of this phenomenon. (C) 2011 Elsevier Ltd. All rights reserved.