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| Titel |
Processes controlling the Si-isotopic composition in the Southern Ocean and application for paleoceanography |
| VerfasserIn |
F. Fripiat, A.-J. Cavagna, F. Dehairs, A. Brauwere, L. André, D. Cardinal |
| 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 ; 9, no. 7 ; Nr. 9, no. 7 (2012-07-06), S.2443-2457 |
| Datensatznummer |
250007179
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| Publikation (Nr.) |
 copernicus.org/bg-9-2443-2012.pdf |
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| Zusammenfassung |
| Southern Ocean biogeochemical processes have an impact on global marine
primary production and global elemental cycling, e.g. by likely controlling
glacial-interglacial pCO2 variation. In this context, the natural
silicon isotopic composition (δ30Si) of sedimentary biogenic
silica has been used to reconstruct past Si-consumption:supply ratios in the
surface waters. We present a new dataset in the Southern Ocean from a IPY-GEOTRACES transect (Bonus-GoodHope) which
includes for the first time summer δ30Si signatures of
suspended biogenic silica (i) for the whole water column at three stations
and (ii) in the mixed layer at seven stations from the subtropical zone up
to the Weddell Gyre. In general, the isotopic composition of biogenic opal
exported to depth was comparable to the opal leaving the mixed layer and did
not seem to be affected by any diagenetic processes during settling, even if
an effect of biogenic silica
dissolution cannot be ruled out in the northern part of the Weddell Gyre. We develop a mechanistic understanding of
the processes involved in the modern Si-isotopic balance, by implementing a
mixed layer model. We observe that the accumulated biogenic silica (sensu Rayleigh
distillation) should satisfactorily describe the δ30Si
composition of biogenic silica exported out of the mixed layer, within the
limit of the current analytical precision on the δ30Si. The
failures of previous models (Rayleigh and steady state) become apparent
especially at the end of the productive period in the mixed layer, when
biogenic silica production and export are low. This results from (1) a
higher biogenic silica dissolution:production ratio imposing a lower net
fractionation factor and (2) a higher Si-supply:Si-uptake ratio supplying
light Si-isotopes into the mixed layer. The latter effect is especially
expressed when the summer mixed layer becomes strongly Si-depleted, together
with a large vertical silicic acid gradient, e.g. in the Polar Front Zone
and at the Polar Front. |
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