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
