Processes controlling the Si-isotopic composition in the Southern Ocean and application for paleoceanography

[摘要] Southern Ocean biogeochemical processes have an impact on global marineprimary production and global elemental cycling, e.g. by likely controllingglacial-interglacial pCO₂ variation. In this context, the naturalsilicon isotopic composition (δ³⁰Si) of sedimentary biogenicsilica has been used to reconstruct past Si-consumption:supply ratios in thesurface waters. We present a new dataset in the Southern Ocean from a IPY-GEOTRACES transect (Bonus-GoodHope) whichincludes for the first time summer δ³⁰Si signatures ofsuspended biogenic silica (i) for the whole water column at three stationsand (ii) in the mixed layer at seven stations from the subtropical zone upto the Weddell Gyre. In general, the isotopic composition of biogenic opalexported to depth was comparable to the opal leaving the mixed layer and didnot seem to be affected by any diagenetic processes during settling, even ifan effect of biogenic silicadissolution cannot be ruled out in the northern part of the Weddell Gyre. We develop a mechanistic understanding ofthe processes involved in the modern Si-isotopic balance, by implementing amixed layer model. We observe that the accumulated biogenic silica (sensu Rayleighdistillation) should satisfactorily describe the δ³⁰Sicomposition of biogenic silica exported out of the mixed layer, within thelimit of the current analytical precision on the δ³⁰Si. Thefailures of previous models (Rayleigh and steady state) become apparentespecially at the end of the productive period in the mixed layer, whenbiogenic silica production and export are low. This results from (1) ahigher biogenic silica dissolution:production ratio imposing a lower netfractionation factor and (2) a higher Si-supply:Si-uptake ratio supplyinglight Si-isotopes into the mixed layer. The latter effect is especiallyexpressed when the summer mixed layer becomes strongly Si-depleted, togetherwith a large vertical silicic acid gradient, e.g. in the Polar Front Zoneand at the Polar Front.

[学科分类] 地球化学与岩石