Abstract
Abstract. Silicon isotopic signatures (δ30Si) 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). δ30Si 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 δ30Si 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 δ30Si 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 δ30Si 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 δ30Si constraints, net biogenic silica production (representative of annual export), at the Greenwich Meridian is estimated to be 5.2 ± 1.3 and 1.1 ± 0.3 mol Si m−2 for the Antarctic Zone and Polar Front Zone, respectively. This is in good agreement with previous estimations. Furthermore, summertime Si-supply into the mixed layer of both zones, via vertical mixing, is estimated to be 1.6 ± 0.4 and 0.1 ± 0.5 mol Si m−2, respectively.
Highlights
In the Southern Ocean, deep nutrient-rich waters ascend into the surface layer and are returned equatorward as subsurface waters, before the available nitrogen pool is fully used by phytoplankton
Between 7 February and 24 March 2008, the International Polar Year (IPY) BONUS-GoodHope (BGH) cruise aboard the R/V Marion Dufresne covered a transect from Cape Town (South Africa) up to 58◦ S in the Southern Ocean roughly centred on the 0◦ meridian (Fig. 1a)
The different water masses are defined by their potential temperature, salinity, and O2 properties (Fig. 2): Upper Circumpolar Deep Water (UCDW; oxygen minimum), Lower Circumpolar Deep Water (LCDW; deep salinity maximum), Antarctic Intermediate
Summary
In the Southern Ocean, deep nutrient-rich waters ascend into the surface layer and are returned equatorward as subsurface waters, before the available nitrogen pool is fully used by phytoplankton. This contrasts with silicon (in the form of silicic acid, Si(OH)4), which is much more depleted by diatom growth and exported along the same pathway (Sarmiento et al, 2004). Deciphering the relative importance of these different processes, which control the nutrient distribution in the Southern Ocean, and understanding how they affect nutrient export to the lower latitudes is necessary to better constrain the role of the Southern Ocean in global biogeochemical cycles of the past, present, and future (Sarmiento et al, 2004; Sigman et al, 2010)
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