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| Titel |
Carbon export in the naturally iron-fertilized Kerguelen area of the Southern Ocean based on the 234Th approach |
| VerfasserIn |
F. Planchon, D. Ballas, A.-J. Cavagna, A. R. Bowie, D. Davies, T. Trull, E. C. Laurenceau-Cornec, P. van der Merwe, F. Dehairs |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1726-4170
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| Digitales Dokument |
URL |
| Erschienen |
In: Biogeosciences ; 12, no. 12 ; Nr. 12, no. 12 (2015-06-23), S.3831-3848 |
| Datensatznummer |
250117997
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| Publikation (Nr.) |
 copernicus.org/bg-12-3831-2015.pdf |
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| Zusammenfassung |
This study examined upper-ocean particulate organic carbon (POC) export
using the 234Th approach as part of the second KErguelen Ocean and
Plateau compared Study expedition (KEOPS2). Our aim was to characterize the
spatial and the temporal variability of POC export during austral spring
(October–November 2011) in the Fe-fertilized area of the Kerguelen Plateau region.
POC export fluxes were estimated at high productivity sites over and
downstream of the plateau and compared to a high-nutrient low-chlorophyll (HNLC) area upstream of the plateau in order to assess the impact of
iron-induced productivity on the vertical export of carbon.
Deficits in 234Th activities were observed at all stations in surface
waters, indicating early scavenging by particles in austral spring.
234Th export was lowest at the reference station R-2 and highest in the
recirculation region (E stations) where a pseudo-Lagrangian survey was
conducted. In comparison 234Th export over the central plateau and north
of the polar front (PF) was relatively limited throughout the survey.
However, the 234Th results support that Fe fertilization increased
particle export in all iron-fertilized waters. The impact was greatest in the
recirculation feature (3–4 fold at 200 m depth, relative to the reference
station), but more moderate over the central Kerguelen Plateau and in the
northern plume of the Kerguelen bloom (~2-fold at 200 m depth).
The C : Th ratio of large (>53 μm) potentially sinking
particles collected via sequential filtration using in situ pumping (ISP)
systems was used to convert the 234Th flux into a POC export flux. The
C : Th ratios of sinking particles were highly variable (3.1 ± 0.1 to
10.5 ± 0.2 μmol dpm−1) with no clear site-related
trend, despite the variety of ecosystem responses in the fertilized regions.
C : Th ratios showed a decreasing trend between 100 and 200 m depth
suggesting preferential carbon loss relative to 234Th possibly due to
heterotrophic degradation and/or grazing activity. C : Th ratios of sinking
particles sampled with drifting sediment traps in most cases showed very
good agreement with ratios for particles collected via ISP deployments
(>53 μm particles).
Carbon export production varied between 3.5 ± 0.9 and
11.8 ± 1.3 mmol m−2 d−1 from the upper 100 m and between
1.8 ± 0.9 and 8.2 ± 0.9 mmol m−2 d−1 from the upper
200 m. The highest export production was found inside the PF meander with a
range of 5.3 ± 1.0 to 11.8 ± 1.1 mmol m−2 d−1 over
the 19-day survey period. The impact of Fe fertilization is highest inside
the PF meander with 2.9–4.5-fold higher carbon flux at 200 m depth in
comparison to the HNLC control station. The impact of Fe fertilization was
significantly less over the central plateau (stations A3 and E-4W) and in the
northern branch of the bloom (station F-L) with 1.6–2.0-fold higher
carbon flux compared to the reference station R. Export efficiencies (ratio
of export to primary production and ratio of export to new production) were
particularly variable with relatively high values in the recirculation
feature (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 KEOPS1
results indicated that carbon export production is much lower during the
onset of the bloom in austral spring than during the peak and declining
phases in late summer. |
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