Carbon export in the naturally iron-fertilized Kerguelen area of the Southern Ocean based on the 234 Th approach

• Published in: Biogeosciences
• Main Authors: Planchon, F, Ballas, D, Cavagna, A-J, Bowie, AR, Davies, D, Trull, T, Laurenceau-Cornec, EC, Van Der Merwe, P, Dehairs, F
• Format: Article in Journal/Newspaper
• Language: English
• Published: Copernicus GmbH 2015
• Subjects: Earth Sciences; Oceanography; Chemical Oceanography; Austral; Kerguelen; Southern Ocean
• Online Access: https://doi.org/10.5194/bg-12-3831-2015; http://ecite.utas.edu.au/103857

This study examined upper-ocean particulate organic carbon (POC) exportusing the 234 Th approach as part of the second KErguelen Ocean andPlateau compared Study expedition (KEOPS2). Our aim was to characterize thespatial and the temporal variability of POC export during austral spring(OctoberNovember 2011) in the Fe-fertilized area of the Kerguelen Plateau region.POC export fluxes were estimated at high productivity sites over anddownstream of the plateau and compared to a high-nutrient low-chlorophyll (HNLC) area upstream of the plateau in order to assess the impact ofiron-induced productivity on the vertical export of carbon. Deficits in 234 Th activities were observed at all stations in surfacewaters, indicating early scavenging by particles in austral spring. 234 Th export was lowest at the reference station R-2 and highest in therecirculation region (E stations) where a pseudo-Lagrangian survey wasconducted. In comparison 234 Th export over the central plateau and northof the polar front (PF) was relatively limited throughout the survey.However, the 234 Th results support that Fe fertilization increasedparticle export in all iron-fertilized waters. The impact was greatest in therecirculation feature (34 fold at 200 m depth, relative to the referencestation), but more moderate over the central Kerguelen Plateau and in thenorthern plume of the Kerguelen bloom (~2-fold at 200 m depth). The C : Th ratio of large (>53 μm) potentially sinkingparticles collected via sequential filtration using in situ pumping (ISP)systems was used to convert the 234 Th flux into a POC export flux. TheC : Th ratios of sinking particles were highly variable (3.1 0.1 to10.5 0.2 μmol dpm −1 ) with no clear site-relatedtrend, despite the variety of ecosystem responses in the fertilized regions.C : Th ratios showed a decreasing trend between 100 and 200 m depthsuggesting preferential carbon loss relative to 234 Th possibly due toheterotrophic degradation and/or grazing activity. C : Th ratios of sinkingparticles sampled with drifting sediment traps in most cases showed verygood agreement with ratios for particles collected via ISP deployments(>53 μm particles). Carbon export production varied between 3.5 0.9 and11.8 1.3 mmol m −2 d −1 from the upper 100 m and between1.8 0.9 and 8.2 0.9 mmol m −2 d −1 from the upper200 m. The highest export production was found inside the PF meander with arange of 5.3 1.0 to 11.8 1.1 mmol m −2 d −1 overthe 19-day survey period. The impact of Fe fertilization is highest insidethe PF meander with 2.94.5-fold higher carbon flux at 200 m depth incomparison to the HNLC control station. The impact of Fe fertilization wassignificantly less over the central plateau (stations A3 and E-4W) and in thenorthern branch of the bloom (station F-L) with 1.62.0-fold highercarbon flux compared to the reference station R. Export efficiencies (ratioof export to primary production and ratio of export to new production) wereparticularly variable with relatively high values in the recirculationfeature (6 to 27 %, respectively) and low values (1 to 5 %,respectively) over the central plateau (station A3) and north of the PF(station F-L), indicating spring biomass accumulation. Comparison with KEOPS1results indicated that carbon export production is much lower during theonset of the bloom in austral spring than during the peak and decliningphases in late summer.