Mg and Li Stable Isotope Ratios of Rocks, Minerals, and Water in an Outlet Glacier of the Greenland Ice Sheet

Abstract

Magnesium and lithium stable isotope ratios (δ 26 Mg and δ 7 Li) have shown promise as tools to elucidate biogeochemical processes both at catchment scales and in deciphering global climate processes. Nevertheless, the controls on riverine Mg and Li isotope ratios are often difficult to determine as a myriad of factors can cause fractionation from bulk rock values such as secondary mineral formation and preferential weathering of isotopically distinct mineral phases. Quantifying the relative contribution from carbonate and silicate minerals to the dissolved load of glacierized catchments is particularly crucial for determining the role of chemical weathering in modulating the carbon cycle over glacial-interglacial periods. In this study we report Mg and Li isotope data for water, river sediment, rock, and mineral separates from the Leverett Glacier catchment, West Greenland. We assess whether the silicate mineral contributions to the dissolved load, previously determined using radiogenic Sr, Ca, Nd, and Hf isotopes, are consistent with dissolved Mg and Li isotope data, or whether a carbonate contribution is required as inferred previously for this region. For δ 7 Li, the average dissolved river water value (+19.2 ± 2.5 , 2SD) was higher than bedrock, river sediment, and mineral δ 7 Li values, implying a fractionation process. For δ 26 Mg, the average dissolved river water value (−0.30 ± 0.14 , 2SD) was within error of bedrock and river sediment and within the range of mineral δ 26 Mg values (−1.63 to +0.06). The river δ 26 Mg values are consistent with the mixing of Mg derived from the same mineral phases previously identified from radiogenic isotope measurements as controlling the dissolved load chemistry. Glacier fed rivers previously measured in this region had δ 26 Mg values ∼0.80 lower than those measured in the Leverett River which could be caused by a larger contribution from garnet (−1.63) dissolution compared to Leverett. This study highlights that dissolved Mg and Li isotope ratios in the Leverett River are affected by different processes (mixing and fractionation), and that since variations in silicate mineral δ 26 Mg values exist, preferential weathering of individual silicate minerals should be considered in addition to carbonate when interpreting dissolved δ 26 Mg values.

Ge/Si and Ge Isotope Fractionation During Glacial and Non-glacial Weathering: Field and Experimental Data From West Greenland

“…Then, element-to-Na ratios can be used to assess the dissolution of different silicate and trace minerals and the precipitation of secondary phases (Figure 4). As noted in previous studies, the dissolved composition of the streams draining the Leverett and Russell glaciers is for the most part consistent with the dissolution of dominant gneissic bedrock minerals, such as plagioclase, K-feldspar, and hornblende, with some additional contribution of Ca from either trace carbonates or Ca-rich silicates [ Figure 4B proposed preferential leaching of glacially ground biotite in the Leverett catchment Auqué et al, 2019;Hindshaw et al, 2019). Since biotite and other micas typically exhibit higher Ge/Si ratios than other silicates (Mortlock and Froelich, 1987;Filippelli et al, 2000;Kurtz et al, 2002), their preferential weathering could result in elevated Ge/Si of the dissolved phase, as proposed previously for glacial streams in Alaska (Anders et al, 2003).…”

“…The chemical composition of the AK, QK, and Watson rivers, as well as of the Isunngua streams at similar locations to those sampled here, has been previously studied in detail by a number of researchers (e.g., Wimpenny et al, 2010, Wimpenny et al, 2011Graly et al, 2014;Hindshaw et al, 2014, Hindshaw et al, 2019Yde et al, 2014;Hatton et al, 2019a). In general, the majority of weathering reactions are thought to occur in the subglacial environment, characterized by a large reactive mineral surface area due to glacial comminution and relatively short (but seasonally variable) water transit times, similar to many other glacial systems (Tranter and Wadham, 2014).…”

“…The study site is located in southwest Greenland, in the proglacial area between the Isunnguata Sermia, Russell, and Leverett glaciers to the east and the settlement of Kangerlussuaq in the west (Figure 1), a region which has been the focus of many previous geochemical studies on chemical weathering and non-traditional stable isotope dynamics (e.g., Wimpenny et al, 2010, Wimpenny et al, 2011Graly et al, 2014;Hindshaw et al, 2014;Yde et al, 2014;Graly et al, 2016;Rickli et al, 2017;Stevenson et al, 2017;Hatton et al, 2019a;Auqué et al, 2019;Hindshaw et al, 2019). The local geology has been described in detail previously (e.g., Van Gool et al, 2002;Engström and Klint, 2014) and a good summary was recently provided by Hindshaw et al (2019), and references therein. Briefly, the bedrock underneath the Leverett and Russel glaciers is composed of two distinct Archean orthogneiss units with different degrees of metamorphic alteration, intruded by a mafic dyke swarm.…”

“…In the main river samples, an increase in Na* and K indicated the dissolution of a small amount of silicates, such as plagioclase (saturation index (SI) of −3.8 to −5.7) and K-feldspar (SI of −1.5 to −2.7), both dominant in the local bedrock (Hindshaw et al, 2014). The dissolved Ca/Na* increased slightly from around 1.1-1.6 mol/mol ( Figure 4B), perhaps reflecting the dissolution of a small amount of trace calcite or garnet (Hindshaw et al, 2019). In contrast, K/Na* remained constant within uncertainty around 0.7 mol/mol ( Figure 4C), which suggests negligible dissolution of biotite, consistent with a K-mica SI of 2.2-6.7.…”

Controls on δ26Mg variability in three Central European headwater catchments characterized by contrasting bedrock chemistry and contrasting inputs of atmospheric pollutants

“…Silicate weathering typically results in low δ 26 Mg values of runoff, and high δ 26 Mg values in clay minerals [21,22]. Recent studies, however, have shown that the direction of Mg fractionation during weathering can be mineral-specific [23][24][25][26][27][28]. Also soil exchange processes can be isotopeselective [29].…”