Subglacial hydrological controls on meltwater 87Sr/86Sr and the strontium flux from the Greenland Ice Sheet

Tuesday, 16 December 2014
Benjamin Linhoff, Massachusetts Institute of Technology, Cambridge, MA, United States, Matthew A Charette, WHOI, Woods Hole, MA, United States, Jemma L Wadham, School of Geographical Sciences, University of Bristol, Bristol, United Kingdom and Bernhard Peucker-Ehrenbrink, Woods Hole Oceanographic Institution, Woods Hole, MA, United States
As an end member of physical weathering, glaciers produce highly abraded, fine-grained mineral surfaces, which can readily undergo chemical weathering. Consequently, extensive chemical weathering can occur subglacially in water-filled cavities and till. Glacial weathering products may therefore alter the trace element and isotope composition of the ocean on glacial-interglacial time scales. For example, major changes in Earth’s glacial history may influence changes in marine 87Sr/86Sr, potentially as the result of radiogenic strontium being released from glacial weathering products. We investigate the 87Sr/86Sr of glacial meltwater discharging a large outlet glacier of the Greenland Ice Sheet during a 76-day time series sampling campaign in 2012. In order to assess potential chemical changes that might occur when glacial meltwater enters the ocean, samples were also collected across the freshwater-saltwater mixing zone in the downstream fjord. In glacial meltwater, substantial variations in 87Sr/86Sr were found in samples collected 12 hours apart (0.755 – 0.742) as well as between mid May and late July (0.755 – 0.733). Strontium concentrations ranged from 250 nM during low flows in May to 51 nM in late July during high flow. Fjord samples showed conservative mixing between glacial meltwater and seawater. These temporal variations are likely the result of changing subglacial water transit times and incongruent weathering of minerals at the ice bed. The large seasonal change likely reflects the evolution of subglacial meltwater flow paths from a groundwater- and linked cavity-dominated system in the spring to an efficient channelized system in mid summer. Daily changes likely reflect varying contributions from channelized and linked cavity groundwater systems controlled by surface meltwater inputs. Our findings suggest that 87Sr/86Sr in glacial meltwater is sensitive to the seasonal evolution of subglacial flowpaths and daily changes in meltwater sources.