(Table 1) Water column silicic acid concentrations and Si isotopic composition during Marion Dufresne cruise MD166 off South Africa

• Main Authors: Fripiat, François, Cavagna, Anne-Julie, Dehairs, Frank, Speich, Sabrina, André, Luc, Cardinal, Damien
• Format: Dataset
• Language: English
• Published: PANGAEA 2011
• Subjects: BONUS-GOODHOPE; GIPY6; Colorimetry; CTD/Rosette; CTD-RO; Date/Time of event; DEPTH; water; Event label; GEOTRACES; Global marine biogeochemical cycles of trace elements and their isotopes; International Polar Year (2007-2008); IPY; Latitude of event; Longitude of event; Marion Dufresne (1995); MD_Large_2; MD_Large_3; MD_Large_4; MD_Large_5; MD_Large_6; MD_Large_7; MD_Super_1; MD_Super_2; MD_Super_3; MD_Super_4; MD_Super_5; MD166; Multi-collector inductively coupled plasma mass spectrometer (MC-ICP-MS); off South Africa; Sample comment; Silicic acid; δ30Si; error; Antarc*; Antarctic; International Polar Year; North Atlantic Deep Water; North Atlantic; Southern Ocean
• Online Access: https://doi.pangaea.de/10.1594/PANGAEA.809699; https://doi.org/10.1594/PANGAEA.809699

Silicon isotopic signatures (d30Si) of water column silicic acid (Si(OH)4) were measured in the Southern Ocean, along a meridional transect from South Africa (Subtropical Zone) down to 57° S (northern Weddell Gyre). This provides the first reported data of a summer transect across the whole Antarctic Circumpolar Current (ACC). d30Si variations are large in the upper 1000 m, reflecting the effect of the silica pump superimposed upon meridional water transfer across the ACC: the transport of Antarctic surface waters northward by a net Ekman drift and their convergence and mixing with warmer upper-ocean Si-depleted waters to the north. Using Si isotopic signatures, we determine different mixing interfaces: the Antarctic Surface Water (AASW), the Antarctic Intermediate Water (AAIW), and thermoclines in the low latitude areas. The residual silicic acid concentrations of end-members control the d30Si alteration of the mixing products and with the exception of AASW, all mixing interfaces have a highly Si-depleted mixed layer end-member. These processes deplete the silicic acid AASW concentration northward, across the different interfaces, without significantly changing the AASW d30Si composition. By comparing our new results with a previous study in the Australian sector we show that during the circumpolar transport of the ACC eastward, the d30Si composition of the silicic acid pools is getting slightly, but significantly lighter from the Atlantic to the Australian sectors. This results either from the dissolution of biogenic silica in the deeper layers and/or from an isopycnal mixing with the deep water masses in the different oceanic basins: North Atlantic Deep Water in the Atlantic, and Indian Ocean deep water in the Indo-Australian sector. This isotopic trend is further transmitted to the subsurface waters, representing mixing interfaces between the surface and deeper layers. Through the use of d30Si constraints, net biogenic silica production (representative of annual export), at the Greenwich Meridian is ...