Si/C decoupling in the world ocean: is the Southern Ocean different?

This paper deals with the decoupling between the biogeochemical cycles of biogenic silica (BSiO2) and of particulate organic carbon (POC) in the Southern Ocean (five study sites situated north, within and south of the Polar Front) compared to the rest of the world ocean (four study sites situated in the Atlantic and in the Pacific oceans). In the surface layer the highest annual flux of BSiO2 is recorded in the Southern Ocean (about 3 mol m−2 yr−1) and the highest flux of POC in the Equatorial Pacific (about 25 mol C m−2 yr−1). Si:C molar ratios in the surface layers range from 0.04 for the rest of the world ocean to 0.25 for the Southern Ocean, reflecting the variable contribution of diatoms to phytoplankton. The Southern Ocean is typified with high Si:C ratios both in the water column (ratios between 2.6 and 10.4 for the rain rate) and in sediments (ratios between 13 and 39 for the long-term burial). The lowest Si:C ratios are reported for the Bermuda Atlantic Time Series Station (0.55 and 0.57 for the rain rate and the sediment, respectively). Whatever may be the study site, the Si:C ratio increases with depth. Relative to the surface production ratio, Si:C increases by a factor of 24–28 for the rain rate, and by a factor of 106–111 for the accumulation rate. In the water column the Si:C increase vs. depth is according to the equation: (Si:C)z=a(Si:C)0b/zc, where (Si:C)0 is the ratio value for surface waters, a, b, and c being fitting parameters for sites situated outside the Southern Ocean. Taking into account the uncertainties in BSiO2 and POC flux determinations for the different study sites, our equation reasonably predicts the vertical variations of the Si:C ratio for the Southern Ocean. As far as the decoupling between the Si and C cycle is concerned, the Southern Ocean behavior is not different from the rest of the world ocean.