Exploring interacting influences on the silicon isotopic composition of the surface ocean: a case study from the Kerguelen Plateau

Abstract

Abstract. This study presents six new water column profiles of the silicon isotopic composition (δ 30 Si) of dissolved silicon (DSi) from the Atlantic and Indian sectors of the Southern Ocean and a variable depth box model of silica cycling in the mixed layer that was constructed to illuminate the evolution of surface ocean δ 30 Si over the full course of a year. In keeping with previous observations, δ 30 Si values ranged from +1.9 to +2.4 ‰ in the mixed layer (ML), +1.2 to +1.7 ‰ in Winter Water (WW), and +0.9 to +1.4 ‰ in Circumpolar Deep Water (CDW). These data also confirmed the occurrence of diminished values for ML δ 30 Si at low DSi concentrations in early austral autumn on the Kerguelen Plateau. The box model was used to investigate whether these low, post-growing season values of δ 30 Si were related to input of DSi to the ML from basalt weathering, biogenic silica dissolution (with or without isotopic fractionation), the onset of winter mixing, or some combination of the three. Basalt weathering and fractionation during biogenic silica dissolution could both lower ML δ 30 Si below what would be expected from the extent of biological uptake of DSi. However, the key driver of the early autumn decrease in δ 30 Si appears to be the switch from bloom growth (with net removal of DSi and net accumulation of biogenic silica (BSi) biomass) to steady state growth (when slow but continuing production of BSi prevented significant net increase in DSi concentrations with diffusive input of DSi from WW but not decrease in ML δ 30 Si towards WW values). Model results also indicated that fractionation during dissolution has only a negligible effect on the δ 30 Si of BSi exported throughout the course of the year. However, seasonal changes in export efficiency (e.g. favouring the export of bloom BSi versus the export of BSi produced during other times of the year) should strongly influence the δ 30 Si of BSi accumulating in marine sediments.Finally, the choice for the parameterisation of the mixing between the ML and the WW in terms of δ 30 Si (i.e. constant or allowed to vary with the seasonal migration of the thermocline) is critical to take into account in box model simulations of the silica biogeochemical cycle. Altogether, these results suggest that as a paleoceanographic proxy, δ 30 Si may more reflect the dominant mode of production of the BSi that is exported (i.e. bloom versus steady state growth) rather than strictly the extent of DSi utilisation by diatoms.

New Insights Into Processes Controlling the δ30Si of Sinking Diatoms: A Seasonally Resolved Box Model Approach

“…Previous studies have suggested that the effects of fractionation during dissolution should be considered when interpreting paleoceanographic reconstructions based on δ 30 Si (e.g., Coffineau et al, 2014). This simple model, combined with the sediment trap time series, gives the opportunity to test the effect of silicon isotope fractionation during dissolution of BSi in the water column.…”

“…To date, only three papers have attempted to describe the seasonal Si biogeochemical and isotopic cycle in the Southern Ocean using modelling approaches (Coffineau et al, 2014;De Brauwere et al, 2012;Fripiat et al, 2012). None of them consider the history of particles during their descent through the water column nor do they integrate the seasonality of parameters that control the recycling of BSi at depth.…”

The silicon stable isotope distribution along the GEOVIDE section (GEOTRACES GA-01) of the North Atlantic Ocean

“…The intermediate layers of the section were represented by LSW, Mediterranean Water (MW), Subarctic Intermediate Water (SAIW), and Polar Intermediate Water (PIW). LSW is the last stage of the transformation of SPMWs and forms in the Labrador and Irminger seas (e.g., Pickart et al, 2003;de Jong and de Steur, 2016;Fröb et al, 2016). MW enters the North Atlantic from the Mediterranean Sea through the Strait of Gibraltar (Ambar and Howe, 1979;Baringer and Price, 1997).…”

“…4c) extending to depths below 2000 m, reflecting the presence of LSW. This water mass, which contributes to NADW, is formed by deep convection in the Labrador and Irminger seas (e.g., de Jong and de Steur, 2016) and spreads across the North Atlantic at intermediate to middle depths (see Fig. 1).…”

“…Significant progress has been made in this regard by fifteen years' worth of work in the Southern Ocean (Varela et al, 2004;Cardinal et al, 2005;De La Rocha et al, 2011;Fripiat et al, 2011), in the North, Equatorial and South Pacific (De La Rocha et al, 2000;Reynolds et al, 2006;Beucher et al, 2008Beucher et al, , 2011de Souza et al, 2012a), and recently in the Arctic Ocean (Varela et al, 2016), in conjunction with various models (De La Rocha and Bickle, 2005;Reynolds, 2009;Coffineau et al, 2014), not the least of which are global circulation models (Wischmeyer et al, 2003;de Souza et al, 2014de Souza et al, , 2015Holzer and Brzezinski, 2015). It is now widely understood that fractionation of silicon isotopes during uptake and biomineralization of silica in surface waters increasingly elevates the δ 30 Si DSi in surface waters (De La Rocha et al, 1997;Sutton et al, 2013).…”