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| Titel |
Reconstructing the Nd oceanic cycle using a coupled dynamical – biogeochemical model |
| VerfasserIn |
T. Arsouze, J.-C. Dutay, F. Lacan, C. Jeandel |
| 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 ; 6, no. 12 ; Nr. 6, no. 12 (2009-12-04), S.2829-2846 |
| Datensatznummer |
250004216
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| Publikation (Nr.) |
 copernicus.org/bg-6-2829-2009.pdf |
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| Zusammenfassung |
| The decoupled behaviour observed between Nd isotopic composition (Nd IC,
also referred as εNd) and Nd concentration cycles has led
to the notion of a "Nd paradox". While εNd behaves in
a quasi-conservative way in the open ocean, leading to its broad use as a
water-mass tracer, Nd concentration displays vertical profiles that increase
with depth, together with a deep-water enrichment along the global
thermohaline circulation. This non-conservative behaviour is typical of
nutrients affected by scavenging in surface waters and remineralisation at
depth. In addition, recent studies suggest the only way to reconcile both
concentration and Nd IC oceanic budgets, is to invoke a "Boundary
Exchange" process (BE, defined as the co-occurrence of transfer of
elements from the margin to the sea with removal of elements from the sea by
Boundary Scavenging) as a source-sink term. However, these studies do not
simulate the input/output fluxes of Nd to the ocean, and therefore prevents
from crucial information that limits our understanding of Nd decoupling. To
investigate this paradox on a global scale, this study uses for the first
time a fully prognostic coupled dynamical/biogeochemical model with an
explicit representation of Nd sources and sinks to simulate the Nd oceanic
cycle. Sources considered include dissolved river fluxes, atmospheric dusts
and margin sediment re-dissolution. Sinks are scavenging by settling
particles. This model simulates the global features of the Nd oceanic cycle
well, and produces a realistic distribution of Nd concentration (correct
order of magnitude, increase with depth and along the conveyor belt, 65%
of the simulated values fit in the ±10 pmol/kg envelop when compared
to the data) and isotopic composition (inter-basin gradient,
characterization of the main water-masses, more than 70% of the simulated
values fit in the ±3 εNd envelop when compared to
the data), though a slight overestimation of Nd concentrations in the deep
Pacific Ocean may reveal an underestimation of the particle fields by the
biogeochemical model. Our results indicate 1) vertical cycling
(scavenging/remineralisation) is absolutely necessary to simulate both
concentration and εNd, and 2) BE is the dominant Nd source
to the ocean. The estimated BE flux (1.1×1010 g(Nd)/yr) is much higher
than both dissolved river discharge (2.6×108 g(Nd)/yr) and atmospheric
inputs (1.0×108 g(Nd)/yr) that both play negligible role in the water
column but are necessary to reconcile Nd IC in surface and subsurface
waters. This leads to a new calculated residence time of 360 yrs for Nd in
the ocean. The BE flux requires the dissolution of 3 to 5% of the annual
flux of continental weathering deposited via the solid river discharge to
the continental margin. |
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