Pham, and 7 co-authors report that trace elements and isotopes (TEIs) were measured as part of the GEOTRACES PANDORA cruise (July–August 2012, R/V L'Atalante), among them Rare Earth Elements (REEs) as pertinent tracers of land-ocean inputs and water mass transformations. This work discusses results of 19 dissolved REE (dREE) profiles measured using a tri-spike method in the Coral Sea and inside and at the exits of the Solomon Sea, a semi-enclosed sea with complex topography and straits. Overall, dREEs –except the insoluble Ce– show nutrient like profiles, i.e. depleted at the surface and enriched at depth.
Illustrative Nd concentrations range from ~5 pmol/kg at the surface to>25 pmol/kg at 5000 meter depth. However, local dREE enrichments are observed, mostly in the Straits (Indispensable, Solomon and Vitiaz Straits) and along the island coasts. A box model allows calculating and discussing the fate of the dREEs in the different water layers flowing through the Solomon Sea. Finally, subtle variations revealed by La, Ce, Eu anomalies and the normalized light versus heavy REE ratio (expressed as Ndn/ Ybn) allows the identification of specific mechanisms affecting the distribution of the different dREEs.
The positive Eu anomaly observed in the surface layers reflects the basaltic origin of external inputs, consistent with the intensive weathering and/or volcanic activity affecting the surrounding islands. These data also confirm that the distributions of heavy dREEs (like Yb) are better correlated to the dSi concentrations than that of the other REEs.
This study presented vertical profiles of dREE concentrations for 143 samples and 21 stations collected in the Coral and Solomon Seas as part of the PANDORA cruise (summer 2012). Although the profile shapes display common dREE features -i.e. surface depleted and enriched at depth- the dREE patterns and anomalies could be discussed and lead to interesting results. A pronounced dissolved positive dLa anomaly is estimated (dLa*/ dLa varying between 3 and 6). Despite the interesting correlation between dBa and dLa in our data, we could not demonstrate that this anomaly was linked to the Ba-Barite cycle. This specific lanthanum behavior likely relates more to the “tetrad” effect. The future analysis of particulate La and biogenic particulate Ba may bring further information. A ubiquitous europium positive anomaly is also found in the dREE patterns, with higher dEu/dEu* values in the upper layers. This striking Fig. 8. Relationship between a) dNd and b) dYb (in pmol/kg) versus dSi (in μmol/kg) in Surface-Thermocline and Intermediate-Bottom layers in the Solomon Sea. V.Q. Pham, et al. Chemical Geology 524 (2019) 11–36 feature agrees with previous observations (Grenier et al., 2013, 2018; Zhang et al., 2008) and suggests that the enrichment calculated with the box model is of basaltic origin, consistent with the geology of the surrounding islands. The lithogenic character of this source is later supported by dCe anomaly. dHREE behavior seem to be mostly influenced by the Si cycle as suggested by Akagi (2013) and also observed by Grenier et al. (2018). However, HREE are also scavenged by adsorption on the settling particles, which explains differential fates of both tracers in the upper layers (dSi < 20 μM).
The box model allowed us to establish the net dNd (dREE) fluxes of the waters flowing equatorward through the Solomon Sea. Significant enrichment is observed for the Lower Thermocline Layer (between 200 and 500 m; Ftotal PANDORA=145 ± 46 tons of Nd per year). This enrichment is of main interest, as these waters will eventually be advected till the equatorial eastern Pacific through the lower part of the EUC (Slemons et al., 2010; Grenier et al., 2011). By contrast, the lack of clear net enrichment in the upper thermocline is surprising in this area bounded by continental margins and with many potential sources. However, the large error bars prevent us from drawing conclusions about what is happening in other layers. This points to the complexity of boundary exchange processes and to the need of better understanding the removal processes of the oceanic dissolved material. The ongoing analysis of the associated particles and Nd isotopic composition will also help to refine our understanding of these net exchange fluxes.
This article is part of a special issue entitled: “Cycles of trace elements and isotopes in the ocean – GEOTRACES and beyond” - edited by Tim M. Conway, Tristan Horner, Yves Plancherel, and Aridane G. González.
Managed & Built by I2M Consulting, LLC | © 2010-
