Seasonal variations, origin, and fate of settling diatoms in the Southern Ocean tracked by silicon isotope records in deep sediment traps

Abstract

The Southern Ocean plays a pivotal role in the control of atmospheric CO 2 levels, via both physical and biological sequestration processes. The biological carbon transfer to the ocean interior is tightly coupled to the availability of other elements, especially iron as a trace-limiting nutrient and dissolved silicon as the mineral substrate that allows diatoms to dominate primary production. Importantly, variations in the silicon cycling are large but not well understood. Here we use δ 30 Si measurements to track seasonal flows of silica to the deep sea, as captured by sediment trap time series, for the three major zones (Antarctic, AZ; Polar Frontal, PFZ; and Sub-Antarctic, SAZ) of the open Southern Ocean. Variations in the exported flux of biogenic silica (BSi) and its δ 30 Si composition reveal a range of insights, including that (i) the sinking rate of BSi exceeds 200 m d À1 in summer in the AZ yet decreases to very low values in winter that allow particles to remain in the water column through to the following spring, (ii) occasional vertical mixing events affect the δ 30 Si composition of exported BSi in both the SAZ and AZ, and (iii) the δ 30 Si signature of diatoms is well conserved through the water column despite strong BSi and particulate organic carbon (POC) attenuation at depth and is closely linked to the Si consumption in surface waters. With the strong coupling observed between BSi and POC fluxes in PFZ and AZ, these data provide new constraints for application to biogeochemical models of seasonal controls on production and export.

The fate of diatom valves in the Subantarctic and Polar Frontal Zones of the Southern Ocean: Sediment trap versus surface sediment assemblages

“…2). These sinking speeds agree well with those previously estimated by Closset et al (2015) for site 54°S and fall within the range of settling speeds of marine snow and faecal aggregates in the literature (Turner, 2002). consistent with the composition of the sinking diatom assemblages that were overwhelmingly dominated by the heavily silicified Fragilariopsis kerguelensis (up to 96% of the diatom assemblage; Fig.…”

“…This latter concept would also help to explain the high POC registered during summer 1999-2000, since export food webs tend to lose more organic matter via downward flux (high export efficiency) than retention food webs (low export efficiency) (Dugdale and Goering, 1967;Eppley and Peterson, 1979). In considering these features of our results we hypothesize that the higher valve fragmentation observed in the samples during summer 1999-2000 could be due to other factors different from an intense "grazing event" as suggested previously (Closset et al, 2015). Alternatively, we venture that our sample processing technique may have selectively fragmented the valves of the weakly silicified species of the "high export…”

“…This is despite the fact that the two A C C E P T E D M A N U S C R I P T ACCEPTED MANUSCRIPT 37 zonal systems host dramatically different phytoplankton communities. The poor silica particle content registered by the sediment traps at the 47°S site (Table 4) suggest a secondary role for diatoms in the control of carbon export in this region (Closset et al 2015;Rigual-Hernández et al 2015b). Thus, our data indicate that a system dominated by non-siliceous phytoplankton can export the same amount of POC as a diatomdominated ecosystem such as the PFZ.…”

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

“…Similar to marine work (Varela et al, 2004;Closset et al, 2015), lake sediment traps have shown the δ 30 Si signature of diatoms (δ 30 Si diatom ) to be resilient to dissolution and preserved through the water column into the sediment record (Panizzo et al, 2016). Accordingly, δ 30 Si diatom from lacustrine sediment cores can be used to reconstruct past changes in biogeochemical cycling at the catchment/drainage basin scale.…”

“…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.…”