6 research outputs found
Seasonal and interannual variability in the hydrology and geochemistry of an outlet glacier of the Greenland Ice Sheet
Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution February 2016In the spring and summer within the ablation zone of the Greenland Ice Sheet (GrIS), meltwater
drains to the ice sheet bed through an evolving network of efficient channelized and inefficient
distributed drainage systems. Distributed system drainage is a key component in stabilizing GrIS
velocity on interannual time scales and controlling geochemical fluxes. During the spring and
summer of 2011 and 2012, I conducted fieldwork at a large outlet glacier in southwest Greenland
underlain by metamorphic silicate rocks. Data collected from a continuous 222Rn monitor in the
proglacial river were used as a component of a mass balance model. I demonstrated that Jdis, the
222Rn fraction derived from the distributed system, was >90% of the 222Rn flux on average, and
therefore, 222Rn can be used as a passive flow tracer of distributed system drainage. Supraglacial
meltwater runoff estimated using two independent models was compared with ice velocity
measurements across the glacier’s catchment. Major spikes of Jdis occurred after rapid
supraglacial meltwater runoff inputs and during the expansion of the subglacial channelized
system. While increases in meltwater runoff induced ice acceleration, they also resulted in the
formation of efficient subglacial channels and increased drainage from the distributed system,
mechanisms known to cause slower late summer to winter velocities. Sr, U, and Ra isotopes and
major and trace element chemistry were used to investigate the impact of glacial hydrology on
subglacial weathering. Analysis of partial and total digestions of the riverine suspended load
(SSL) found that trace carbonates within the silicate watershed largely controlled the 87Sr/86Sr
ratio in the dissolved load. Experiments and sampling transects downstream from the GrIS
demonstrated that δ234U in the dissolved phase decreased with increasing interaction with the
SSL. The (228Ra/226Ra) value of the dissolved load was significantly higher than that of the SSL
and therefore, was not the result of the source rock material but of extensive mineral surface
weathering and the faster ingrowth rate of 228Ra (t1/2=5.75 y) relative to 226Ra (t1/2=1600 y). In
summary, extensive, repeated cycles of rapid supraglacial meltwater runoff to subglacial
drainage networks leads to increased distributed system drainage and mineral weathering.Funding for this work was provided by the U.S. National Science Foundation Arctic Natural
Sciences Program (ANS-1256669); Woods Hole Oceanographic Institution Arctic Research
Initiative, Ocean Ventures Fund, and Ocean Climate Change Institute; United Kingdom Natural
Environment Research Council studentship (NE/152830X/1); the Carnegie Trust, Edinburgh
University Development Trust
Large subglacial source of mercury from the southwestern margin of the Greenland Ice Sheet
The Greenland Ice Sheet is currently not accounted for in Arctic mercury budgets, despite large and increasing annual runoff to the ocean and the socio-economic concerns of high mercury levels in Arctic organisms. Here we present concentrations of mercury in meltwaters from three glacial catchments on the southwestern margin of the Greenland Ice Sheet and evaluate the export of mercury to downstream fjords based on samples collected during summer ablation seasons. We show that concentrations of dissolved mercury are among the highest recorded in natural waters and mercury yields from these glacial catchments (521–3,300 mmol km−2 year−1) are two orders of magnitude higher than from Arctic rivers (4–20 mmol km−2 year−1). Fluxes of dissolved mercury from the southwestern region of Greenland are estimated to be globally significant (15.4–212 kmol year−1), accounting for about 10% of the estimated global riverine flux, and include export of bioaccumulating methylmercury (0.31–1.97 kmol year−1). High dissolved mercury concentrations (~20 pM inorganic mercury and ~2 pM methylmercury) were found to persist across salinity gradients of fjords. Mean particulate mercury concentrations were among the highest recorded in the literature (~51,000 pM), and dissolved mercury concentrations in runoff exceed reported surface snow and ice values. These results suggest a geological source of mercury at the ice sheet bed. The high concentrations of mercury and its large export to the downstream fjords have important implications for Arctic ecosystems, highlighting an urgent need to better understand mercury dynamics in ice sheet runoff under global warming
Utility of 222Rn as a passive tracer of subglacial distributed system drainage
This paper is not subject to U.S. copyright. The definitive version was published in Earth and Planetary Science Letters 462 (2017): 180-188, doi:10.1016/j.epsl.2016.12.039.Water flow beneath the Greenland Ice Sheet (GrIS) has been shown to include slow-inefficient (distributed) and fast-efficient (channelized) drainage systems, in response to meltwater delivery to the bed via both moulins and surface lake drainage. This partitioning between channelized and distributed drainage systems is difficult to quantify yet it plays an important role in bulk meltwater chemistry and glacial velocity, and thus subglacial erosion. Radon-222, which is continuously produced via the decay of 226Ra, accumulates in meltwater that has interacted with rock and sediment. Hence, elevated concentrations of 222Rn should be indicative of meltwater that has flowed through a distributed drainage system network. In the spring and summer of 2011 and 2012, we made hourly 222Rn measurements in the proglacial river of a large outlet glacier of the GrIS (Leverett Glacier, SW Greenland). Radon-222 activities were highest in the early melt season (10–15 dpm L−1), decreasing by a factor of 2–5 (3–5 dpm L−1) following the onset of widespread surface melt. Using a 222Rn mass balance model, we estimate that, on average, greater than 90% of the river 222Rn was sourced from distributed system meltwater. The distributed system 222Rn flux varied on diurnal, weekly, and seasonal time scales with highest fluxes generally occurring on the falling limb of the hydrograph and during expansion of the channelized drainage system. Using laboratory based estimates of distributed system 222Rn, the distributed system water flux generally ranged between 1–5% of the total proglacial river discharge for both seasons. This study provides a promising new method for hydrograph separation in glacial watersheds and for estimating the timing and magnitude of distributed system fluxes expelled at ice sheet margins.U.S. National Science Foundation Arctic Natural Sciences Program (ANS-1256669); Woods Hole Oceanographic Institution Arctic Research Initiative, Ocean Ventures Fund, and Ocean Climate Change Institute; United Kingdom Natural Environment Research Council studentship (NE/152830X/1); the Carnegie Trust, Edinburgh University Development Trust