OCEANIC FLUXES FROM PROGLACIAL AND DEGLACIAL WATERSHEDS IN WESTERN GREENLAND

Weathering in western Greenland occurs in two distinct environments: proglacial watersheds that extend from the margin of the Greenland Ice Sheet (GIS) and derive water from ice melt, and deglacial watersheds that develop on terrains unconnected to the GIS and derive water from annual precipitation. Proglacial and deglacial watersheds currently provide equal amounts of runoff in western Greenland. These watersheds may contribute different solute fluxes to the oceans depending on exposure age, climate, and weathering environment. We test this hypothesis by comparing chemical compositions of streams in four deglacial watersheds (Sisimiut, Nerumaq, Qorlortoq, Kangerlussuaq) and one proglacial watershed (Watson River Akuliarusiarsuup Kuua River; AKR) along a ~160 km transect from the coast to the GIS. Recent work found that weathering reactions in the deglacial watersheds shift from being dominated by carbonate dissolution inland to sulfide oxidation near the coast. Silicate weathering, based on increased Si, Na and K concentrations, is a minor source of solutes to deglacial streams and is less extensive near the GIS than the coast, where older moraines experience greater precipitation. In general, specific conductivity (SpC: 48-301 μS/cm) and pH (7.0-8.2) increase inland as precipitation decreases and fresh mineral surfaces become more common. The AKR, in contrast, has lower average SpC (11.9 uS/cm) and pH (6.86) than the deglacial streams. Low SpC reflects dilution by ice melt and short residence time of water in the subglacial system. Proglacial flow is enriched in Si compared to deglacial flow particularly near headwaters, indicating higher silicate weathering rates in the pro- and sub-glacial systems. Low pH values indicate: 1) equilibration with atmospheric CO2 in the supraglacial system near headwaters, and 2) acid production generated by sulfide oxidation in the hyporheic zone identified by elevated SO4 concentrations. However, Ca, Mg and HCO3 are the dominant ions over the length of the AKR indicating that dissolution of carbonate is the predominant form of weathering. Our results indicate the two types of watersheds provide distinct fluxes of solutes to the oceans that are likely to change as ice sheets retreat and advance with changing climate

Hydrologic exchange and chemical weathering in a proglacial watershed near Kangerlussuaq, west Greenland

The exchange of proglacial river water with active layer pore water could alter water chemical compositions in glacial outwash plains and oceanic solute fluxes. To evaluate effects of this exchange, we sampled Watson River and adjacent pore water during the 2013 melt season at two sandurs in western Greenland; one in Sandflugtdalen and the other near the confluence with Søndre Strømfjord. We measured temperature, specific conductivity, and head gradients between the river and bank over a week-long period at Sandflugtdalen, as well as sediment hydraulic conductivity and chemical compositions of waters from both sites. Specific conductivity of pore water is four to ten times greater than river water as solutes are concentrated from weathering reactions, cryoconcentration, and evaporation. Pore water compositions are predominantly altered by carbonate dissolution and sulfide mineral oxidation. High concentrations of HCO3 and SO4 result from solute recycling and dissolution of secondary Ca-Mg carbonate/sulfate salts initially formed by near-surface evaporation in the summer and at depth by freeze-in of the active layer and cryoconcentration in the winter. High hydraulic conductivity (10−5 to 10−4 m/s) and diurnal fluctuations of river stage during our study caused exchange of river and pore water immediately adjacent to the river channel, with a net loss of river water to the bank. Pore water \u3e6 m from the river continuously flowed away from the river. Approximately 1–8% of the river discharge through the Sandflugtdalen was lost to the river bank during our 6.75 day study based on calculations using Darcy’s Law. Although not sampled, some of this water should discharge to the river during low river stage early and late in the melt season. Elevated pore water solute concentrations in sandurs and water exchange at diurnal and seasonal frequency should impact fluxes of solutes to the ocean, although understanding the magnitude of this effect will require long-term evaluation throughout the melt season

SEASONAL EVOLUTION AND SPATIAL DISTRIBUTION OF WEATHERING IN WESTERN GREENLAND

Through physical weathering, the Greenland Ice Sheet (GIS) produces sediments which are subsequently chemically weathered in three types of watersheds: 1) deglacial watersheds that are physically disconnected from the GIS and drain local precipitation, 2) proglacial watersheds that are hydrologically connected to the GIS, and 3) subglacial watersheds that form beneath the GIS. Chemical weathering in the glacial foreland may be important to atmospheric CO2 drawdown and oceanic fluxes of solutes, yet no holistic study exists that compares solute sources across all types of watersheds and through the melt season. Consequently, we investigated spatiotemporal changes in weathering through the 2013 ablation season from a transect of watersheds spanning the coast to the GIS in western Greenland. We sampled one proglacial (PG) watershed, from which we also assess subglacial (SG) weathering, one inland deglacial (IDG) and one coastal deglacial (CDG) watershed. A simple stoichiometric mass balance quantifies solute sources in each watershed. The principal solute source is trace carbonates in all watersheds; however, IDG has more carbonate (61 vs 36 mol%) and less silicate (3 vs 14 mol%) weathering than CDG. PG has similar carbonate (41 mol%) and silicate weathering (16 mol%) proportions to CDG, despite proximity to IDG. Weathering of biotite decreases from 12 mol% at PG to 3 mol% at CDG along an exposure age gradient, consistent with more radiogenic 87Sr/86Sr in waters at PG (0.73556) than DGC (0.71114). Carbonate weathering decreases and biotite + silicate weathering increases downstream through PG, reflecting increased weathering. Solute sources change little through time or space at IDG, but at PG, silicate weathering increases and carbonate weathering decreases as flow increases through the melt season, consistent with increased contributions of SG waters with long residence times in distributed channels. Thus, the evolution of SG through time and connections between subglacial reservoirs and main flow paths plays an important role in weathering at PG. As the GIS retreats, deglacial watersheds will constitute a greater fraction of the weathering flux and thus increased silicate weathering should alter solute fluxes to the oceans and increase atmospheric CO2 drawdown

Exposure age and climate controls on weathering in deglaciated watersheds of western Greenland

Fine-grained sediments deposited by retreating glaciers weather faster than the global average and this weathering can impact the global carbon cycle and oceanic fluxes of nutrients and radiogenic isotopes. Much work has focused on subglacial and proglacial weathering of continental ice sheets, but little is known about weathering and resulting fluxes from deglacial watersheds, which are disconnected from the ice sheets and discharge only annual precipitation and permafrost melt. We investigate the effects of exposure age and precipitation on weathering intensity in four deglacial watersheds on Greenland that form a transect from the coast near Sisimiut toward the Greenland Ice Sheet (GrIS) near Kangerlussuaq based on evaluations of major ion compositions, Sr isotope ratios, and mineral saturation states of waters and sediments. The transect is underlain by Archean orthogneiss and is characterized by gradients in moraine ages (∼7.5–8.0 ky inland to ∼10 ky at the coast) and water balance (−150 mm/yr inland to +150 mm/yr at the coast). Anion compositions are generally dominated by HCO3, but SO4 becomes increasingly important toward the coast, reflecting a switch from trace carbonate dissolution to sulfide mineral oxidation. Coastal watersheds have a higher proportion of dissolved silica, higher Na/Cl, Si/Ca, and lower Ca/Sr ratios than inland watersheds, indicating an increase in the relative proportion of silicate weathering and an increase in the extent of weathering toward the coast. More extensive weathering near the coast is also apparent in differences in the 87Sr/86Sr ratios of stream water and bedload (Δ87Sr/86Sr), which decreases from 0.017 inland to 0.005 at the coast, and in increased saturation states relative to amorphous SiO2 and quartz. The steep weathering gradient from inland to coastal watersheds reflects enhanced weathering compared to that expected from the 2 to 3 ky difference in exposure age caused by elevated coastal precipitation. The gradient of weathering with exposure age, water budget and distance from the ice sheet indicates that oceanic and atmospheric fluxes will change as continental glaciers retreat, precipitation patterns across the deglacial region readjust, and the relative proportion of deglacial to proglacial runoff increases