Reconstructing the Nd oceanic cycle using a coupled dynamical – biogeochemical model

Abstract

Abstract. The decoupled behaviour observed between Nd isotopic composition (Nd IC, also referred as ε Nd ) and Nd concentration cycles has led to the notion of a "Nd paradox". While ε Nd 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)Correspondence to: T. Arsouze (arsouze@ldeo.columbia.edu) and isotopic composition (inter-basin gradient, characterization of the main water-masses, more than 70% of the simulated values fit in the ±3 ε Nd 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 ε Nd , and 2) BE is the dominant Nd source to the ocean. The estimated BE flux (1.1×10 10 g(Nd)/yr) is much higher than both dissolved river discharge (2.6×10 8 g(Nd)/yr) and atmospheric inputs (1.0×10 8 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.

Neodymium isotopic composition of the oceans: A compilation of seawater data

“…The Nd concentration vertical gradient is maximum in the Pacific, less pronounced in the Austral and Indian, much more moderated in the Atlantic, and not the dominant feature in the Mediterranean and the Arctic. The Nd concentration vertical gradient increase along the global thermohaline circulation has been attributed to the cumulative effect of particle remineralization as deep water age (as for nutrients) [Siddall et al, 2008;Arsouze et al, 2009]. The present compilation also suggests a possible impact of the size of each basin (i.e.…”

“…The major process involved in Nd removal is considered to be adsorption onto organic and Fe-Mn oxide coatings of sedimenting particles [Sholkovitz et al, 1994;Haley et al, 2004]. The oceanic residence time of Nd is of the order of 300 to 1000 years [Tachikawa et al, 2003;Arsouze et al, 2009], which is shorter than the mean oceanic mixing time. These features have led to use the Nd IC as a tracer of oceanic circulation for the modern and past ocean [e.g.…”

“…The oceanic Nd IC distribution is heterogeneous, controlled both by Nd sources and sinks, and by the oceanic circulation. Besides river and aeolian inputs, Boundary Exchange -dissolution of Nd from margin sediments associated to removal of dissolved Nd from seawater -seems to dominate Nd sources and sinks [Tachikawa et al, 2003;Lacan and Jeandel, 2005b;Arsouze et al, 2009]. The major process involved in Nd removal is considered to be adsorption onto organic and Fe-Mn oxide coatings of sedimenting particles [Sholkovitz et al, 1994;Haley et al, 2004].…”

A boundary exchange influence on deglacial neodymium isotope records from the deep western Indian Ocean

“…Arsouze et al, 2007;Jeandel et al, 2007;Arsouze et al, 2009). This approximation is likely to be true along most of the Madagascan margin, whereas northwards of WIND 24B volcanic sediment appears to form a greater part of the detrital mixture ( Figure 2a) and may contribute preferentially to boundary exchange.…”

“…Lacan and Jeandel, 2005 and references therein). However, modelling studies at a global scale have reached different conclusions on the relative importance of water mass advection and boundary exchange as controls on the Nd isotopic distribution of seawater Jones et al, 2008;Siddall et al, 2008;Arsouze et al, 2009;Rempfer et al, 2011). Uncertainty in the models is due to (i) the strong sensitivity of models to poorly constrained boundary conditions (surface inputs, scavenging, boundary exchange); (ii) a lack of understanding of the mechanism for boundary exchange; and (iii) an inherent difficulty in distinguishing between the controls on a global scale because the detrital Nd isotopic composition of the boundaries generally changes along the deep water flow path in a similar manner to that expected for Atlantic-Pacific water mass mixing.…”