Silicon pool dynamics and biogenic silica export in the Southern Ocean inferred from Si-isotopes

• Main Authors: Fripiat, François, André, Luc, Cardinal, Damien, Cavagna, Anne-Julie, Dehairs, Frank, Speich, Sabrina
• Format: Article in Journal/Newspaper
• Language: English
• Published: 2011
• Subjects: Océanographie physique et chimique; Paléontologie et paléoécologie; Antarctic; Greenwich; Indian; Southern Ocean; The Antarctic; Weddell; Antarc*; North Atlantic Deep Water; North Atlantic
• Online Access: http://hdl.handle.net/2013/ULB-DIPOT:oai:dipot.ulb.ac.be:2013/184302; https://dipot.ulb.ac.be/dspace/bitstream/2013/184302/3/doi_167929.pdf

Silicon isotopic signatures (δ 30Si) 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). δ 30Si 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 δ 30Si 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 δ 30Si 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 δ 30Si 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 δ 30Si constraints, net biogenic silica production (representative of annual export), at the Greenwich Meridian is ...