Glacial effects on weathering processes: New insights from the elemental and lithium isotopic composition of West Greenland rivers

Abstract

Greenland is by far the dominant source of glacial runoff to the oceans but the controls on the chemical and isotopic composition of this runoff are poorly known. To better constrain glacial effects on weathering processes, we have conducted elemental and lithium isotope analyses of glacial and non-glacial rivers in gneiss catchments in West Greenland. The glacial rivers have high total suspended solids (0.5 g l− 1) and low total dissolved solids (12 μScm− 1) relative to the non-glacial rivers, and they contain a higher proportion of dissolved Ca2+ and K+ because of subglacial, preferential, weathering of trace carbonates and biotite. The glacial rivers also have high SO42− because of the oxidation of trace sulphides under the ice. Both glacial and non-glacial rivers have high δ7Li (respectively, ∼ 26‰ and ∼ 30‰) relative to the rocks from which the Li is derived (∼ 8‰). Saturation state modelling suggests that this is due to the formation of Fe-oxyhydroxides in the non-glacial rivers, with preferential uptake of 6Li during inner sphere sorption of Li+ on the Fe-oxyhydroxide surface. Glacial rivers, however, are undersaturated with respect to clay minerals and Fe-oxyhydroxides. Nevertheless, leaching of suspended sediments indicates that ∼ 65% of the Li in these sediments is associated with Fe-oxyhydroxide phases, and the δ7Li value of this Li is low, ∼ 5‰. These results suggest that these Fe-oxyhydroxides formed under the ice, as a product of sulphide oxidation, with preferential uptake of 6Li onto the mineral surface. Solubilisation of Li from these Fe-oxyhydroxide phases is unlikely to represent a significant flux of Li to the oceans. Moreover, because the difference between the δ7Li values of glacial vs non-glacial rivers is small, glaciation has not had a significant impact on the Li isotopic composition of the riverine flux delivered to the oceans in the past, even at the height of the last deglaciation.