AUTHOR=Hindshaw Ruth S. , Rickli Jörg , Leuthold Julien 

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

JOURNAL=Frontiers in Earth Science

VOLUME=Volume 7 - 2019

YEAR=2019

URL=https://www.frontiersin.org/journals/earth-science/articles/10.3389/feart.2019.00316

DOI=10.3389/feart.2019.00316

ISSN=2296-6463

ABSTRACT=Magnesium and lithium stable isotope ratios (dMg and dLi) 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 glaciated 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 dLi, the average dissolved river water value (+19.2±2.5‰, 2SD) was higher than bedrock, river sediment and mineral dLi values, implying a fractionation process. 
For dMg, 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 dMg values (-1.63 to +0.06‰). 
The river dMg 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 dMg values approximately 0.80‰ lower than those measured in the Leverett River and could potentially 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 dMg values exist, preferential weathering of individual silicate minerals in addition to carbonate should be considered when interpreting dissolved dMg values.