Identifying weathering sources and processes in an outlet glacier of the Greenland Ice Sheet using Ca and Sr isotope ratios

Chemical and isotope data (εCa, δCa, Sr/Sr, δO) of river water samples were collected twice daily for 28 days in 2009 from the outlet river of Leverett Glacier, West Greenland. The water chemistry data was combined with detailed geochemical analysis and petrography of bulk rock, mineral separates and sediment samples in order to constrain the mineral weathering sources to the river. The average isotopic compositions measured in the river, with 2SD of all the values measured, were εCa=+4.0±1.4, δCa=+0.60±0.10‰ and Sr/Sr=0.74243±0.00327. Based on changes in bulk meltwater discharge, the hydrochemical data was divided into three hydrological periods. The first period was marked by the tail-end of an outburst event and was characterised by water with decreasing suspended sediment concentrations (SSC), ion concentrations and pH. During the second hydrological period, discharge increased whilst Sr/Sr decreased from 0.74550 to 0.74164. Based on binary mixing diagrams using Sr/Sr with Na/Sr, Ca/Sr and εCa, this is interpreted to reflect an increase in reactive mineral weathering, in particular epidote, as the water residence time decreases. The decrease in water residence time is a result of the evolution from a distributed (long water residence time) to a channelised (short water residence time) subglacial drainage network. The third hydrological period was defined as the period when overall discharge was decreasing. This hydrological period was marked by prominent diurnal cycles in discharge. During this period, significant correlations between δCa and SSC and δO were observed which are suggestive of fractionation during adsorption. This study demonstrates the potential of radiogenic Ca to both identify temporally changing mineral sources in conjunction with Sr/Sr values and to separate source and fractionation effects in δCa values.