Controls on the solute geochemistry of subglacial discharge from the Russell Glacier, Greenland Ice Sheet determined by radiogenic and stable Sr isotope ratios

Andrews, M. Grace and Jacobson, Andrew D. (2018) Controls on the solute geochemistry of subglacial discharge from the Russell Glacier, Greenland Ice Sheet determined by radiogenic and stable Sr isotope ratios. Geochimica et Cosmochimica Acta, 239, 312-329. (doi:10.1016/j.gca.2018.08.004).

Abstract

We used radiogenic and stable Sr isotope ratios (87Sr/86Sr and δ88/86Sr) to examine controls on solute acquisition in subglacial discharge from the Russell Glacier, a land-terminating lobe of the Greenland Ice Sheet (GrIS) located in western Greenland. The study focused on two melt seasons in 2014 and 2015. Subglacial discharge 87Sr/86Sr ratios are 13,000 ppm higher than those measured for bedload and suspended sediment digests, and are more similar to those of bedload sediment leachates. These results point to the preferential dissolution of minerals with high 87Sr/86Sr ratios. Analyses of mineral separates from bulk rocks demonstrate that biotite, chlorite, hornblende, and K-feldspar have relatively high 87Sr/86Sr ratios. Subglacial discharge δ88/86Sr values are ∼0.10‰ higher than those for bedload and suspended sediment digests. Isotope fractionation during secondary mineral formation and/or adsorption cannot account for the difference between subglacial discharge and bedrock δ88/86Sr values, as suspended and bedload sediment leachates and digests produced similar δ88/86Sr values and are within the range for bulk silicate Earth. Consistent with the interpretation of 87Sr/86Sr ratios, we attribute the difference to the preferential dissolution of minerals with high δ88/86Sr values. Mineral separates display a wide range of δ88/86Sr values (∼0.40‰). Those having high δ88/86Sr values include hornblende and K-feldspar, as well as apatite and titanite. Taken together, the preferential weathering of predominately silicate minerals explains the Sr isotope geochemistry of subglacial discharge.

Subglacial discharge from the Russell Glacier feeds the proglacial Akuliarusiarsuup Kuua River (AKR). Along a 32 km transect of the AKR from the GrIS margin toward the coast, riverine 87Sr/86Sr ratios increase from ∼0.722 to ∼0.747 in an approximately step-wise pattern that corresponds to point-source inputs of additional subglacial discharge. Major cation concentrations and 87Sr/86Sr ratios minimally vary along lengths of the transect with no hydrological inputs. This suggests that proglacial chemical weathering is negligible and likely does not contribute significantly to GrIS solute fluxes.

In general, this study supports the recent contention that silicate mineral weathering dominates the solute geochemistry of GrIS subglacial discharge in contrast to valley glaciers, which typically show substantial contributions from carbonate and sulfide weathering regardless of primary bedrock composition. Ice sheet subglacial chemical weathering may therefore have a greater impact on long-term atmospheric CO2 drawdown than previously realized.

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