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

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 pCO 2 variation. In this context, the natural silicon isotopic composition (δ 30 Si) 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 (BonusGoodHope) which includes for the first time summer δ 30 Si 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 δ 30 Si composition of biogenic silica exported out of the mixed layer, within the limit of the current analytical precision on the δ 30 Si. 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.

Coupling of the distribution of silicon isotopes to the meridional overturning circulation of the North Atlantic Ocean

“…2), that is intermediate between that of AABW, þ1. 2‰ (Cardinal et al, 2005;de Souza et al, 2012b;Fripiat et al, 2012), and the northern component waters, 4 þ1.7‰. The higher fraction of Si(OH) 4 from the north in the deep eastern basin below 4000 m is consistent with the heavier δ 30 Si (OH) 4 values in that basin compared to the values in the deep western basin (Fig.…”

“…These waters experience little biological consumption before subducting and turning northward at abyssal depths resulting in the AABW having a relatively light δ 30 Si(OH) 4 signature of þ1. 2‰ (Cardinal et al, 2005;de Souza et al, 2012b;Fripiat et al, 2012).…”

“…), and the lightest δ 30 Si(OH) 4 in the Southern Ocean, þ1. 2‰ (Cardinal et al, 2005;de Souza et al, 2012b;Fripiat et al, 2012), compared to an average of þ1.3 to þ1.4‰ for most of the deep ocean (Beucher et al, 2008;Reynolds, 2009). The contrasting isotopic signatures between the two polar oceans arise through dynamics that are unique to each region (Fig.…”

A Review of the Stable Isotope Bio-geochemistry of the Global Silicon Cycle and Its Associated Trace Elements

“…The influence of environment on δ 30 Si composition in diatoms has yet to be shown under laboratory controlled conditions. Data from in-situ/natural settings, have not documented a species effect (e.g., Cardinal et al, 2007;Fripiat et al, 2012;Closset et al, 2015) although if anything this clearly demonstrates the need to better understand the variability of δ 30 Si in diatoms.…”

“…However, more recent studies show that this fractionation factor may be more variable, and potentially species-specific, based on both field studies (e.g., Varela et al, 2004;Beucher et al, 2007;Ehlert et al, 2012;Fripiat et al, 2012) and culture experiments (Sutton et al, 2013).…”