Axis 1 Si and C cycles in the Southern Ocean
The main research on this axis has been developed through two international projects: KEOPS-2 and SAZ.
1.1 KEOPS-2 Project
KEOPS-2 project studies a naturally Fe fertilized phytoplankton bloom in the Southern Ocean (Kerguelen Plateau,). This seasonal bloom is dominated by diatoms, a group of algae that requires silicon to build their glass frustule. Our isotopic approaches of the silicon cycle highlight the importance of summer silicon supply to the mixed layer from below with a silica production regime that can be compared to upwelling ecosystems (Closset et al., 2014). New data from early summer together with previous works on the same region in late summer has allowed us to propose a seasonal budget of the silicon cycle above the Kerguelen Plateau, one of the specific objectives of MuSiCC. This budget includes winter and summer Si nutrient supply from below, bulk and net silica production as well silica export (cf. figure).Other main outcomes of this study are:
- The Kerguelen Plateau sustain biogenic silica production regimes similar to those observed in upwelling systems;
- There is no significant silicification by diatoms below the euphotic zone and the dissolution are relatively constant;
- We observe that the Fe-rich region is characterised by higher Si:N uptake ratios compared to HNLC area. This suggests that natural iron fertilization over long time scales does not necessarily decrease Si:N diatoms uptake ratios.
- Summer supply represents 38% of new nutrient Si supply and should not be neglected.
- In contrast to initial assumption, the silicic acid leakage hypothesis as one of the drivers of glacial – interglacial atmospheric CO2 changes cannot be justified by a change in Si:N uptake ratio due to increase Fe supply in glacials.
1.2 SAZ sediment trap Project
SAZ Project is led by colleagues from Antarctic & Climate Common Research Centre in Hobart (Tasmania, Australia). We use silicon isotopes (δ30Si) 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 d30Si composition reveal a range of insights (Closset et al., 2015), including that:
- The sinking rate of diatoms exceeds 200 m/d 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,
- Occasional vertical mixing events in the surface can be tracked by deep δ30Si composition of exported diatoms in both the SAZ and AZ,
- The δ30Si signature of diatoms is well conserved through the water column despite strong silica 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 diatoms and POC fluxes in PFZ and AZ, these data provide new constraints for application to biogeochemical models of seasonal controls on carbon production and export.
- This data confirms the applicability of this isotopic proxy for paleo-oceanographic studies.
- It can also be used to estimate diatom’s settling rate.
- Such impacts are directly relevant to the efficiency of the carbon biological pump, the biological-control process transferring carbon from the surface to the deep ocean where carbon will be stored for centuries to millennia.
Axis 2 Si cycle at continent – ocean interfaces
In Delvaux et al. (2013), we analysed seasonal variations of Si isotopic signatures (δ30Si signature) of dissolved silicon (DSi) in the Scheldt (Belgium) freshwater estuary and in its main tributaries. We show that in this highly anthropogenised watershed, except during diatom bloom in spring/summer, the δ30Si is likely to controlled by land-use at the sub-basin scale. In Hughes et al. (2012), we show that the silicon fluxes and isotopic composition are modified by damming.
Scientific impact: δ30Si can be used as a proxy sensitive to anthropogenic pressures on Si cycle.
In collaboration with Indian colleagues, a set of 18 estuaries have been sampled at both dry and monsoon season to study silicon cycle. The main results, that still need to be completed and interpreted with other parameters and statistical methods show that:
(i) During dry period, average δ30Si is 1.9 ± 0.4 ‰ (n= 58) which is almost 1 ‰ heavier than the world river supply to the ocean estimated so far. Indeed, contrary to a typical weathering control on riverine δ30Si as generally observed, there is no positive correlation between Dissolved Silicon (DSi) content and δ30Si.
(ii) The δ30Si of monsoonal headwater samples are not significantly different from those of the dry period (1.64±0.55 ‰ and 2.01±0.61, respectively) despite there is almost no discharge during dry period. This indicates that the Si source to estuary is insensitive to the season even though it appears to be controlled by weathering as seen on the inverse relationship δ30Si vs. 1/DSi. Then, within estuary during monsoon, δ30Si is little changed and seems to be mainly controlled by mixing.
- Silicon does not appear to be a limiting nutrient in Indian estuaries, which may favour the lack of eutrophication along the Indian coast,
- In dry period, there is a non-conservative behaviour of silicon, likely taken up by diatoms in the mid/upper estuaries, while in wet period there is generally a conservative behaviour,
- Silicon isotopes of rivers (i.e. upstream estuaries) are more related to weathering and/or watershed characteristics (e.g. maybe land-use) than seasonal discharge.