Chemical weathering under the Greenland Ice Sheet

Abstract

To contrast continental and alpine subglacial weathering regimes and thereby assess the role of large ice masses in chemical weathering, borehole and outlet water samples were collected from multiple locations on a major, land-terminating outlet of the Greenland Ice Sheet. Boreholes, reaching ice depths to 824 m, were drilled to the bed with hot-water methods in four areas of the ice sheet ablation zone along a 45 km transect extending inland from the outlet terminus. The bulk chemical composition of these samples shows substantially less infl uence of sulfi des and carbonates than found in alpine glaciers, suggesting that the sediment under this region of the ice sheet has become depleted of accessory minerals. The waters show wide variability in chemical composition over both large and small temporalspatial scales, suggesting large ranges in length of subglacial water storage and in rates of abrasion and comminution of subglacial earth materials. The dissolved solids concentrations found in the Greenland Ice Sheet are comparable to and in some cases exceed those of alpine glaciers, suggesting that large ice masses are capable of generating substantial dissolved loads through silicate weathering mechanisms. INTRODUCTION AND FIELD CONTEXTThe effects of glaciation on chemical weathering are critical to explaining the role of ice ages in Earth's geochemical cycles. Studies of alpine glaciers suggest that dissolved loads in glacial outlet streams are comparable to nonglacial rivers (Anderson et al., 1997). However, the weathering mechanisms differ, with subaerial weathering dominated by the alteration of silicate minerals to clays, and subglacial weathering controlled chiefl y by the high abundance of reactive surfaces in freshly comminuted material (Anderson, 2005). In many glacial systems, carbonate dissolution and sulfi de oxidation produce most of the dissolved products in outlet waters. This can be true even in glaciers on silicate bedrock, as the abrasion and comminution of subglacial sediments allows fast-reacting accessory materials to be exposed (Hosein et al., 2004;Raiswell, 1984;Tranter et al., 2002).Most measurements of glacial water chemistry have been performed in alpine environments, so it remains to be seen whether the weathering of fast-reacting accessory minerals from freshly comminuted surfaces plays as important a role in continental ice sheets. The size of ice sheets allows for larger and more complex subglacial hydrologic pathways and for longer transport distances for subglacial sediment. Subglacial water and sediment may therefore spend more time in contact with each other underneath larger bodies of ice. Existing data show a positive correlation between ice body size and the proportion of dissolved load derived from silicate weathering (Wadham et al., 2010). Limited data suggest that saturation limits on carbonate dissolution allow sulfi de-mediated silicate dissolution to dominate the chemistry of Antarctic ice stream till pore waters . Here we present major ion and dissolved silica chemist...

Combined diurnal variations of discharge and hydrochemistry of the Isunnguata Sermia outlet, Greenland Ice Sheet

“…The relative abundances of cation species are comparable to measurements taken at the Isunnguata Sermia terminus in the summer of 2011 (Graly et al, 2014). The SO 2− 4 /alkalinity ratios are diminished compared to those measured in 2011, but are comparable to those found in samples collected in 2010 and 2012.…”

“…Hot water boreholes along a transect from the outlet to 40 km upstream have provided data regarding water pressure (Meierbachtol et al, 2013), ice temperature (Harrington et al, 2015), subglacial water chemistry (Graly et al, 2014), and mass balance between subglacial sediment and rock (Graly et al, 2016). More limited data sets from Isunnguata Sermia's terminal and lateral outlets were reported for the 2010, 2011, and 2012 seasons (Graly et al, 2014). The work reported here is based on samples and data collected over a 6-day period from 16 to 21 July 2013.…”

“…Na-and K-dominated waters likely form in settings where water-rock interactions occur only over a limited time, such that cation exchange occurs on fresh feldspar and mica surfaces, but complete silicate dissolution and clay precipitation do not occur (Blum and Stillings, 1995;Graly et al, 2014). The lack of constituents derived from reactive accessory minerals such as pyrite (i.e., SO 2− 4 ) implies that the waters were reacting with sediment that has been depleted of accessory minerals.…”

“…The lack of constituents derived from reactive accessory minerals such as pyrite (i.e., SO 2− 4 ) implies that the waters were reacting with sediment that has been depleted of accessory minerals. Such accessory mineral depletion can occur if sediment residence time in the subglacial system is sufficiently long (Graly et al, 2014). Sampling of sediment beneath ice boreholes has shown the greatest chemical depletion in portions of the ice sheet most influenced by distributed flow (Graly et al, 2016).…”

“…Seven measurements of dissolved solute chemistry taken from samples collected over the course of 2 days in 2011 at the terminus of Insunnguata Sermia, a major landterminating West Greenland outlet glacier, potentially show a direct relationship between solute concentration and discharge (Graly et al, 2014;Landowski, 2012). In this limited time series, solute concentration appears to peak at midafternoon, while discharge is high, and minimal in the early morning hours, with total variation of < 20 %.…”