DOI: 10.18739/a2vd6p488
title: Major ion and isotope data for meltwaters, rocks, and sediments, Russell Glacier, West Greenland, 2014-2015

description: The overall goal of the project was to test the hypothesis that rivers draining the Greenland Ice Sheet (GrIS) evade carbon dioxide (CO2) to the atmosphere. Previous studies first conducted by our group and later by others show that water emerging from beneath the Russell Glacier in West Greenland has a higher partial pressure of CO2(pCO2) compared to atmospheric equilibrium. This CO2 may originate from subglacial microbial metabolism, as well as carbonate weathering under closed system conditions. During downstream transport, the chemical weathering of glacial till may sequester excess CO2 as HCO3, allowing the remainder to evade to the atmosphere. With this initial insight, key questions concern if, how, and to what extent CO2 evasion will either increase or decrease as the ice sheet decays in a warmer world. Research activities tested five hypotheses designed to elucidate how microbial activity, hydrologic processes, and chemical weathering regulate the rate and extent of CO2 evasion in the modern-day. The project culminated in two peer-reviewed publications noted below. In one study, we investigated seasonal and interannual dissolved C cycling at the margin of the Russell Glacier. By synthesizing isotopic analyses of water (δ18O) and C (δ13C and Δ14C) with geomicrobiological observations, we found evidence for previously unknown connections between the GrIS’s supraglacial and subglacial dissolved C cycles. Supraglacial streams have variable concentrations of dissolved organic carbon (DOC) and are the dominant source of DOC in subglacial discharge. Supraglacial stream dissolved inorganic carbon (DIC) concentrations are uniform and sourced from a spatially and temporally constant mixture of organic C (~25%) respired by aerobic heterotrophs inhabiting the GrIS surface and dissolved atmospheric C (~75%). Supraglacial inputs account for ~50% of subglacial discharge DIC. The remaining subglacial DIC derives from carbonate weathering and microbial CO2 production, with the latter attributable to abundant anaerobic heterotrophic communities observed in subglacial discharge. Furthermore, we found that supraglacial streams deliver young DOC to the subglacial environment during snowmelt and rain events. These pulses of organic C may drive heterotrophic microbial respiration, with the cumulative effect being a seasonal shift in the source of basal DIC, from microbial- to carbonate-dominated. In a second study, we used major ion measurements, as well as radiogenic and stable Strontium (Sr) isotope ratios (87Sr/86Sr and δ88/86Sr), to examine controls on solute acquisition in subglacial discharge from the Russell Glacier. One goal was to understand the fate of CO2 produced by subglacial carbonate weathering and microbial metabolism. 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 about 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 δ 88/86Sr 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 δ 88/86Sr 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 controls the transformation of CO2 to HCO3 in this setting. This study went on to examine the compositional evolution of water during subsequent downstream transport. 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 stepwise 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 later study supports the 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. Files include data published in Andrews et al. (2018) and Andrews and Jacobson (2018): Andrews M. G., Jacobson A. D., Osburn M. R., and Flynn T. M. (2018) Dissolved carbon dynamics in meltwaters from the Russell Glacier, Greenland Ice Sheet. Journal of Geophysical Research-Biogeosciences 123, 2922-2940. Andrews M. G. and Jacobson A. 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.