QUANTIFICATION OF SOLUTE EVOLUTION IN GLACIAL MELTWATERS

The Herbert, Eagle, and Mendenhall Glaciers are outlet glaciers of the Juneau Icefield. They are located approximately 25 km north of Juneau, Alaska, in the Coast Mountains Complex—a series of Mesozoic granodioritic and tonalitic sills, bounded on the west by late Paleozoic/early Mesozoic metasedimentary units: slates, quartzites, schists, and phyllites, with interlayered beds of marble, layered gneiss, and amphibolite. These temperate wet-based glaciers cause substantial physical weathering producing large volumes of rock flour and glacial till. Rock flour has an extremely high amount of reactive surface area that permits increased rates of chemical weathering. This study examines the nature of chemical weathering in these three glacial environments. Water samples were taken at several points along the meltwater channels, as well as at nearby meteoric streams. Ice, rainwater, and rock samples were also collected. Initial analysis of major-ion chemistry of these waters reveals a sharp divide between meltwaters and meteoric waters. Meltwaters tend to be very dilute (less than 25 µS specific conductance), whereas meteoric waters show a higher concentration of solutes (40-120 µS). The chemistry of both meltwater and meteoric water in these systems is dominated by calcium and bicarbonate, and a molar comparison of Acid Neutralizing Capacity vs. (Ca2+ + Mg2+) shows a close 2:1 ratio. This suggests that the strongest influence on these waters is the weathering of carbonates—even though carbonates only represent a trace amount of the overall bedrock. Suspended sediments were collected for X-ray and SEM analysis. Their mineralogy will constrain the weathering reactions occurring in this system. Combined water chemistry and rock type analysis will permit a mass-balance approach to solute evolution in these waters. Oxygen and deuterium isotope analysis will be used as an indicator of water source, and strontium isotope analysis should suggest a source of Sr+ (carbonate dissolution or siliciclastic weathering). Laboratory dissolution experiments on freshly-powdered rock and suspended sediments will provide a quantification of weathering rate.