Behavior of rare earth elements and yttrium during simulation of arctic estuarine mixing between glacial-fed river waters and seawater and the impact of inorganic (nano-)particles

Abstract

Rivers are the major source of many trace elements to the oceans. As a consequence of climate change, increasing volumes of glacial meltwater enter the oceans after being affected by estuarine processes. Although the behavior of high field strength elements such as the rare earth elements and yttrium (REY) has been intensively studied in tropical, temperate and boreal estuaries, little is known about arctic estuarine mixing of glacial-fed river waters that are poor in organic nanoparticles and colloids (NPCs), but rich in inorganic NPCs. Here we provide the first data set from estuarine mixing experiments with glacial-fed river waters and seawater. The glacial-fed river water endmembers originate from southern Iceland (sampled in 2010 and 2013) and from West Greenland (sampled in 2013); these endmembers are rich in NPCs and larger particles of (glassy) volcanic ash and of ultra-fine rock flour, respectively.For the mixing experiments, 0.2 μm-filtered glacial-fed river water was mixed with seawater in different ratios to cover the full range of estuarine low- to high-salinity conditions. All 2010 freshwater endmembers show higher concentrations of individual REY (due to their high NPC load) than the respective seawater endmember, whereas the 2013 freshwater endmembers have higher light REY, but lower heavy REY concentrations (due to significantly less NPCs). Admixture of minute amounts of seawater (5% and 10% seawater (SW) admixture, i.e. salinities of ~1.7 psu and of ~3.4 psu, respectively) already has a strong impact on REY concentrations and REY signatures. In all experiments, a large amount of REY (e.g., up to 98.2% of Nd and 98.2% of Yb) is removed at very low salinities. At intermediate to high salinities, however, remobilization of REY from aggregated NPCs occurs if NPC-rich river waters are used in the experiments, whereas if the endmember is poor in NPCs, the REY mix almost conservatively. The REY removal is dependent on the amount of NPCs present in the glacial-fed river waters; NPC-rich river water shows larger REY removal than NPC-poorer river water. After the initial drop in REY concentrations under low-salinity conditions, the experiment with glacial-fed river water from Greenland reveals that heavy REY mix almost conservatively, but light REY are again remobilized from aggregated NPCs. Our results suggest that aggregated NPCs composed of rock flour (Greenland) and volcanic ash (Iceland) may cause different trace element behavior during estuarine processes. However, all experiments show similar Y-Ho fractionation during estuarine mixing, indicating that the actual type of particle is not a major constraint on Y-Ho behavior, but rather the availability of particle surfaces. Estuarine processes may cause the development of negative Ce anomalies during remobilization of REY at higher salinity if Ce(IV) compounds of low solubility are present in the (nano-)particle load. However, if only Ce(III) is present, such as in fresh mafic volcanic ash, no fractionation of Ce from its trivalent REY neighbors occurs.