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.

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Keywords: radon Greenland glacier proglacial river meltwater 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 226 Ra, accumulates in meltwater that has interacted with rock and sediment. Hence, elevated concentrations of 222 Rn 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 222 Rn 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 222 Rn mass balance model, we estimate that, on average, greater than 90% of the river 222 Rn was sourced from distributed system meltwater. The distributed system 222 Rn 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 222 Rn, 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. Published by Elsevier B.V.

Thesis AbstractIn 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 222 Rn monitor in the proglacial river were used as a component of a mass balance model. I demonstrated that J dis , the 222 Rn fraction derived from the distributed system, was >90% of the 222 Rn flux on average, and therefore, 222 Rn 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 J dis 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 87 Sr/ 86 Sr ratio in the dissolved load. Experiments and sampling transects downstream from the GrIS demonstrated that δ 234 U in the dissolved phase decreased with increasing interaction with the SSL. The ( 228 Ra/ 226 Ra) 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 228 Ra (t 1/2 =5.75 y) relative to 226

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In an area of elevated nitrate (NO3) groundwater concentrations in the northern Chihuahuan Desert in central New Mexico (United States), a large reservoir of nitrate was found in the subsoil of an arroyo floodplain. Nitrate inventories in the floodplain subsoils ranged from 10,000 to 38,000 kg NO3-N/ha—over twice as high as any previously measured arid region. The floodplain subsoil NO3 reservoir was over 100 times higher than the adjacent desert (59–95 kg NO3-N/ha). Chloride mass balance calculations of subsoils indicate arroyo floodplain subsoils have undergone negative recharge since 2600–8600 yr ago, while the surrounding desert has had negative recharge since 13,000–17,000 yr ago. Compared to the adjacent desert, plant communities are larger and more abundant in the floodplain, though subsoil NO3 is apparently not utilized. We demonstrate that NO3 accumulates in the subsoil of the floodplain through evaporation of monsoon season precipitation funneled into the arroyo. Through a one-dimensional vadose zone model, we show that the NO3 inventories in the arroyo floodplain could be acquired 8 to 75 times faster than through atmospheric deposition through the lateral movement