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| Titel |
Late summer particulate organic carbon export and twilight zone remineralisation in the Atlantic sector of the Southern Ocean |
| VerfasserIn |
F. Planchon, A.-J. Cavagna, D. Cardinal, L. André, 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 ; 10, no. 2 ; Nr. 10, no. 2 (2013-02-06), S.803-820 |
| Datensatznummer |
250017502
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| Publikation (Nr.) |
 copernicus.org/bg-10-803-2013.pdf |
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| Zusammenfassung |
| 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 (Baxs). 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/234Thp 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 (Baxs) 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 d−1, assuming steady state. Using the
POC/234Thp ratio of sinking particles (> 53 μm),
this 234Th accumulation flux was converted into a POC remineralisation
flux which ranged between 0.9 ± 1.2 mmol C m−2 d−1 and
9.2 ± 2.9 mmol C m−2 d−1. Mesopelagic particulate
biogenic Ba has been reported to reflect bacterial degradation of organic
matter and to be related to oxygen consumption and bacterial carbon
respiration. We observed that the highest Baxs contents (reaching
up to > 1000 pM), in general, occurred between 200 and 400 m.
Depth-weighted average mesopelagic Baxs (meso-Baxs)
values were converted into respired C fluxes, which ranged between 0.23 and
6.4 mmol C m−2 d−1, in good agreement with 234Th-based
remineralisation fluxes. A major outcome from this study is the observed
significant positive correlation between POC remineralisation as estimated
from meso-Baxs contents and from 234Th excess
(R2 = 0.73; excluding 2 outliers). Remineralisation of POC in the
twilight zone was particularly efficient relative to POC export resulting in
negligible bathypelagic (> 600 m) POC export fluxes in the sub-Antarctic
zone, the Polar Front zone and the northern Weddell Gyre, while the
subtropical zone as well as the vicinity of the SACCF had significant deep
POC fluxes. |
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