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Abstract. The first KErguelen Ocean and Plateau compared Study (KEOPS1), conducted in the naturally iron-fertilised Kerguelen bloom, demonstrated that fecal material was the main pathway for exporting carbon to the deep ocean during summer (January-February 2005), suggesting a limited role of direct export via phytodetrital aggregates. The KEOPS2 project reinvestigated this issue during the spring bloom initiation (October-November 2011), when zooplankton communities may exert limited grazing pressure, and further explored the link between carbon flux, export efficiency and dominant sinking particles depending upon surface plankton community structure. Sinking particles were collected in polyacrylamide gel-filled and standard free-drifting sediment traps (PPS3/3), deployed at six stations between 100 and 400 m, to examine flux composition, particle origin and their size distributions. Results revealed an important contribution of phytodetrital aggregates (49 ± 10 and 45 ± 22 % of the total number and volume of particles respectively, all stations and depths averaged). This high contribution dropped when converted to carbon content (30±16 % of total carbon, all stations and depths averaged), with cylindrical fecal pellets then representing the dominant fraction (56 ± 19 %).At 100 and 200 m depth, iron-and biomass-enriched sites exhibited the highest carbon fluxes (maxima of 180 and 84 ± 27 mg C m −2 d −1 , based on gel and PPS3/3 trap collection respectively), especially where large fecal pellets dominated Published by Copernicus Publications on behalf of the European Geosciences Union. 1008 E. C. Laurenceau-Cornec et al.: The importance of sinking particle types to carbon export over phytodetrital aggregates. Below these depths, carbon fluxes decreased (48±21 % decrease on average between 200 and 400 m), and mixed aggregates composed of phytodetritus and fecal matter dominated, suggesting an important role played by physical aggregation in deep carbon export.Export efficiencies determined from gels, PPS3/3 traps and 234 Th disequilibria (200 m carbon flux/net primary productivity) were negatively correlated to net primary productivity with observed decreases from ∼ 0.2 at low-iron sites to ∼ 0.02 at high-iron sites. Varying phytoplankton communities and grazing pressure appear to explain this negative relationship. Our work emphasises the need to consider detailed plankton communities to accurately identify the controls on carbon export efficiency, which appear to include small spatio-temporal variations in ecosystem structure.

As part of the GEOTRACES Bonus-GoodHope (BGH) expedition (January–March 2008) in the Atlantic sector of the Southern Ocean, particulate organic carbon (POC) export was examined from the surface to the mesopelagic twilight zone using water column distributions of total ²³⁴Th and biogenic particulate Ba (Ba_xs). Surface POC export production was estimated from steady state and non steady state modelling of ²³⁴Th fluxes, which were converted into POC fluxes, using the POC/²³⁴Th ratio of large, potentially sinking particles (> 53 μm) collected via in situ pumps. Deficits in ²³⁴Th activities were observed at all stations from the surface to the bottom of the mixed layer, yielding ²³⁴Th export fluxes from the upper 100 m of 496 ± 214 dpm m⁻² d⁻¹ to 1195 ± 158 dpm m⁻² d⁻¹ for the steady state model and of 149 ±517 dpm m⁻² d⁻¹ to 1217 ± 231 dpm m⁻² d⁻¹ for the non steady state model. Using the POC/²³⁴Th_p ratio of sinking particles (ratios varied from 1.7 ± 0.2 μmol dpm⁻¹ to 4.8 ± 1.9 μmol dpm^−1) POC export production at 100 m was calculated to range between 0.9 ± 0.4 and 5.1 ± 2.1 mmol C m⁻² d⁻¹,assuming steady state and between 0.3 ± 0.9 m⁻² d⁻¹ and 4.9 ± 3.3 mmol C m⁻² d⁻¹, assuming non steady state. From the comparison of both approaches, it appears that during late summer export decreased by 56 to 16% for the area between the sub-Antarctic zone and the southern Antarctic Circumpolar Current Front (SACCF), whereas it remained rather constant over time in the HNLC area south of the SACCF. POC export represented only 6 to 54% of new production, indicating that export efficiency was, in general, low, except in the vicinity of the SACCF, where export represented 56% of new production. Attenuation of the POC sinking flux in the upper mesopelagic waters (100–600 m depth interval) was evidenced both, from excess ²³⁴Th activities and from particulate biogenic Ba (Ba_xs) accumulation. Excess ²³⁴Th activities, reflected by ²³⁴Th/²³⁸U ratios as large as 1.21 ± 0.05, are attributed to remineralisation/disaggregation of ²³⁴Th-bearing particles. The accumulation of excess ²³⁴Th in the 100–600 m depth interval ranged from 458 ± 633 dpm m⁻² d⁻¹ to 3068 ± 897 dpm m⁻² ...

International audienceWe examined phytoplankton community responses to natural iron fertilisation at 32 sites over and downstream from the Kerguelen Plateau in the Southern Ocean during the austral spring bloom in October–November 2011. The community structure was estimated from chemical and isotopic measurements (particulate organic carbon – POC; ¹³C-POC; particulate nitrogen – PN; ¹⁵N-PN; and biogenic silica – BSi) on size-fractionated samples from surface waters (300, 210, 50, 20, 5, and 1 μm fractions). Higher values of ¹³C-POC (vs. co-located ¹³C values for dissolved inorganic carbon – DIC) were taken as indicative of faster growth rates and higher values of ¹⁵N-PN (vs. co-located ¹⁵N-NO₃ source values) as indicative of greater nitrate use (rather than ammonium use, i.e. higher f ratios).Community responses varied in relation to both regional circulation and the advance of the bloom. Iron-fertilised waters over the plateau developed dominance by very large diatoms (50–210 μm) with high BSi / POC ratios, high growth rates, and significant ammonium recycling (lower f ratios) as biomass built up. In contrast, downstream polar frontal waters with a similar or higher iron supply were dominated by smaller diatoms (20–50 μm) and exhibited greater ammonium recycling. Stations in a deep-water bathymetrically trapped recirculation south of the polar front with lower iron levels showed the large-cell dominance observed on the plateau but much less biomass. Comparison of these communities to surface water nitrate (and silicate) depletions as a proxy for export shows that the low-biomass recirculation feature had exported similar amounts of nitrogen to the high-biomass blooms over the plateau and north of the polar front. This suggests that early spring trophodynamic and export responses differed between regions with persistent low levels vs. intermittent high levels of iron fertilisation

We report silicon isotopic determinations for USGS rock reference materials BHVO‐1 and BHVO‐2 using a Nu Plasma multi‐collector (MC)‐ICP‐MS, upgraded with a new adjustable entrance slit, to obtain medium resolution, as well as a stronger primary pump and newly designed sampler and skimmer cones (“B” cones). These settings, combined with the use of collector slits, allowed a resolution to be reached that was sufficient to overcome the 14N16O and 14N2 interferences overlying the 30Si and the 28Si peaks, respectively, in an earlier set‐up. This enabled accurate measurement of both δ30Si and δ29Si. The δ value is expressed in per mil variation relative to the NBS 28 quartz reference material. Based on data acquired from numerous sessions spread over a period of six months, we propose a recommended average δ30Si of −0.33 ± 0.05‰ and −0.29 ± 0.11‰ (2se) for BHVO‐1 and BHVO‐2, respectively. Our BHVO grand mean silicon isotope composition (δ30Si =−0.31 ± 0.06‰) is significantly more negative than the only published value for BHVO‐2, but is in very good agreement with the recently established average value of ocean island basalts (OIB), confirming the conclusion that the OIB reservoir has a distinct isotopic composition from the solar reservoir as sampled by chondrites.