Controlling factors and impacts of river-borne neodymium isotope signatures and rare earth element concentrations supplied to the Canadian Arctic Archipelago

Mélanie Grenier (1) , K.A. Brown (2) , M. Colombo (3) , M. Belhadj (1) , I. Baconnais (4) , V. Pham (1) , M. Soon (3) , P.G. Myers (5) , C. Jeandel (1) , R. François (3)

1 LEGOS - Laboratoire d'études en Géophysique et océanographie spatiales
2 IOS - Institute of Ocean Sciences [Sidney]
3 UBC - University of British Columbia
4 U of S - University of Saskatchewan [Saskatoon, Canada]
5 University of Alberta

Determining the factors controlling the neodymium (Nd) isotopic compositions (expressed as ε Nd) and rare earth element (REE) concentrations of rivers is imperative to improve our understanding of the distribution of these water mass tracers in the ocean. Here we present the first measurements of <0.45 μm-filtrate REE concentrations (i.e., the concentration of truly dissolved, nano-particulate and colloidal REEs passing through a 0.45 μm filter, hereafter referred to as dissolved) and ε Nd in rivers draining into the Canadian Arctic Archipelago (CAA). Results show a large variation in both REE concentrations (Nd = [8; 37, 260] pmol/kg) and ε Nd ([−32.1; −15.1]) in the thirteen rivers sampled. Dissolved REE concentrations increase with increasing fractions of metamorphic rocks and ice cover in the rivers' watersheds, while ε Nd and PAAS-normalized REE patterns are inherited from the watershed's bedrock lithology. The four rivers draining watersheds composed of Precambrian metamorphic rocks exhibit the highest REE concentrations, the least radiogenic ε Nd , and shale-normalized patterns enriched in light REEs. The remaining nine rivers drain mainly or exclusively sedimentary bedrock and have lower REE concentrations, more radiogenic ε Nd and generally show heavy REE enrichment, with variable negative cerium (Ce) anomalies. The presence of ice sheets in the drainage area, and the ionic strength, pH, and dissolved organic carbon concentration of river water are the main factors determining riverine dissolved REE concentrations by controlling the formation and stabilization of colloids. We estimate a flow of dissolved Nd transported by Arctic rivers discharging into the CAA equivalent to the Nd flux of seawater entering the CAA. Depending on the percentage of Nd removal during mixing with seawater, the impact of river water on the ε Nd of seawater exiting the CAA could be modest (if 90% removal) or more significant (if 90% removal). This riverine contribution could potentially impart a climatesensitive ε Nd signature to the seawater reaching Baffin Bay, with possible implications for the use of ε Nd in paleoceanographic reconstructions of water mass distribution.

Mots clés: Rare Earth Elements; neodymium isotopes; bedrock geology; dissolved riverine inputs; Canadian Arctic Archipelago; Rare Earth Elements; river geochemistry

Domaines: Géochimie; Océanographie

Mélanie Grenier, K.A. Brown, M. Colombo, M. Belhadj, I. Baconnais, et al.. Controlling factors and impacts of river-borne neodymium isotope signatures and rare earth element concentrations supplied to the Canadian Arctic Archipelago. Earth and Planetary Science Letters, 2022, 578, pp.117341. ⟨10.1016/j.epsl.2021.117341⟩. ⟨hal-03805808⟩