Abstract
Abstract. Southern Ocean biogeochemical processes have an impact on global marine primary production and global elemental cycling, e.g. by likely controlling glacial-interglacial pCO2 variation. In this context, the natural silicon isotopic composition (δ30Si) of sedimentary biogenic silica has been used to reconstruct past Si-consumption:supply ratios in the surface waters. We present a new dataset in the Southern Ocean from a IPY-GEOTRACES transect (Bonus-GoodHope) which includes for the first time summer δ30Si signatures of suspended biogenic silica (i) for the whole water column at three stations and (ii) in the mixed layer at seven stations from the subtropical zone up to the Weddell Gyre. In general, the isotopic composition of biogenic opal exported to depth was comparable to the opal leaving the mixed layer and did not seem to be affected by any diagenetic processes during settling, even if an effect of biogenic silica dissolution cannot be ruled out in the northern part of the Weddell Gyre. We develop a mechanistic understanding of the processes involved in the modern Si-isotopic balance, by implementing a mixed layer model. We observe that the accumulated biogenic silica (sensu Rayleigh distillation) should satisfactorily describe the δ30Si composition of biogenic silica exported out of the mixed layer, within the limit of the current analytical precision on the δ30Si. The failures of previous models (Rayleigh and steady state) become apparent especially at the end of the productive period in the mixed layer, when biogenic silica production and export are low. This results from (1) a higher biogenic silica dissolution:production ratio imposing a lower net fractionation factor and (2) a higher Si-supply:Si-uptake ratio supplying light Si-isotopes into the mixed layer. The latter effect is especially expressed when the summer mixed layer becomes strongly Si-depleted, together with a large vertical silicic acid gradient, e.g. in the Polar Front Zone and at the Polar Front.
Highlights
In the Southern Ocean, the deep nutrient-rich waters ascend into the surface layer south of the Polar Front (PF), and are returned to the subsurface northward before the available pools of nitrogen and phosphorus are fully used by phytoplankton
There is a 30Si enrichment associated with the northward decreasing Si(OH)4 gradient across the Polar Front (PF), as observed earlier for δ30SiSi(OH)4 (Varela et al, 2004; Cardinal et al, 2005) and Si(OH)4 concentrations (Brzezinski et al, 2001; Queguiner and Brzezinski, 2002). δ30SibSiO2 values are systematically lighter than δ30SiSi(OH)4 values in agreement with the preferential uptake of 28Si by diatoms
With a palaeoceanography perspective this study has attempted to assess the controls upon, and the seasonal evolution of, biogenic silica isotopic compositions in the mixed layer and its transfer across the water column. This data set acquired in late austral summer highlights two main points: 1. A large latitudinal variation in δ30SibSiO2 is observed in the mixed layer across the meridional BGH transect, contrasting with a narrower variation for δ30SiSi(OH)4
Summary
In the Southern Ocean, the deep nutrient-rich waters ascend into the surface layer south of the Polar Front (PF), and are returned to the subsurface northward before the available pools of nitrogen and phosphorus are fully used by phytoplankton. During silicic acid consumption by diatoms, the lighter Si isotope (28Si) is preferentially consumed, leaving the silicic acid pool enriched in heavy Si-isotope (30Si) (De La Rocha et al, 1997) Such preferential incorporation of 28Si into biogenic silica (bSiO2) is defined by a fractionation factor (30ε), which is equivalent to the ratio of the reaction rates of the heavy (30k) and light (28k) Si-isotopes (=30k:28k −1, reported in permil units, ‰). The calibration of this proxy in the modern ocean has still not been fully achieved and processes such as Si(OH) supply and bSiO2 dissolution can bias the expected relationship between Si(OH) concentration and δ30Si composition (Demarest et al, 2009). None of the existing isotopic fractionation models are capable of reproducing these differences This highlights the significant importance of fully understanding the different processes involved in contemporary Si-isotopic balances before applying this proxy to reconstruct past ocean environments
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