Seasonal evolution of water source contributions to the subglacial outflow from a land-terminating Greenland ice sheet outlet glacier: Insight from a new isotope mixing model (Invited)
Abstract
Little is currently known about the subglacial hydrological system beneath the Greenland ice sheet (GrIS). Evidence that a large fraction of annual surface meltwater may drain to the bed of the GrIS suggests that portions of the GrIS subglacial hydrological system may undergo a seasonal evolution, akin to those present beneath alpine glaciers, with significant geophysical and biogeochemical implications. The interaction of surface meltwater with the glacier bed alters its chemical composition from dilute snow- and ice-melt to chemically-enriched subglacial discharge waters. In theory, variations in solute concentrations could be used to infer the evolution of the subglacial drainage network by differentiating water source contributions. There is ample evidence, however, that chemical mixing models which rely on bulk, non-conservative properties such as electrical conductivity, are unreliable. Here we present results from a novel multi-component isotope mixing model to quantify the relative contributions of surface snow, glacial ice melt, and basal melt to the bulk meltwater discharge at a small (~ 5 km2) land-terminating outlet glacier along the western margin of the GrIS. This model utilizes radioactive and stable isotopes (7Be, 222Rn, 18O, D) as passive flow tracers coupled with stream discharge and meteorological measurements. Each of these tracers has a unique and predictable signal for the different source waters that ultimately contribute to the subglacial discharge. We also use the radioactive nature of 7Be (half-life 53.3 d) to constrain the meltwater transit time from the glacier surface to the ice margin at the onset of the summer melt season. Finally, we compare our model results to those obtained from using conservative ionic species as tracers in order to assess the applicability of this approach for hydrograph separation. Results illustrate (1) the potential of using radon and stable water isotopes to provide quantitative estimates of the snow, ice, and basal flow components, and (2) the presence of a constant basal flow component which is diluted first by snow-melt and then by increasing amounts of glacial ice melt as the season progresses. These results are consistent with the hypothesis that the Greenland ice sheet subglacial drainage system undergoes a seasonal evolution from a distributed drainage system to a more efficient channelized drainage system. The development of an isotope mixing model that can successfully differentiate water source contributions would complement existing geophysical methods used to study seasonally-evolving subglacial hydrological systems.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2010
- Bibcode:
- 2010AGUFM.C11B..04B
- Keywords:
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- 1827 HYDROLOGY / Glaciology