A Multi-Proxy Sedimentary Record of Hubbard Glacier Meltwater Production, Disenchantment Bay, Alaska

Alaskan glaciers have experienced significant volume loss during a period of pronounced changes in regional oceanography and climatology. To test how meltwater production varies in response to climate forcing, a multi-decadal proxy record of meltwater discharge was established for the Hubbard Glacier. 33 piston cores and 8 multicores were collected from 24 stations to measure annual sediment deposition patterns over the last several decades. Sedimentary facies, physical properties, and short-lived radioisotopes were used to delineate summertime sediment deposition rates spanning fortnightly to annual time scales, as tidal and seasonal forcing create distinct laminated (cyclopels) and annually varved deposits. Magnetic susceptibility (MS), sortable silt, and suspended sediment floc characteristics were used as proxies of meltwater plume competence. We hypothesized that inter-annual variations in MS (related to an increased ability to transport magnetite in a more competent plume), coarsening in grain size and the floc limit (a measure of the amount and coarseness of silt carried in suspension by the meltwater plume) would show increasing plume competence and capacity in response to more melting as regional temperatures increase. Complete varves were identified from cores for up to 22 years (1981-2003) and the period of highest and most variable summer layer thickness coincides with El Niño-dominated conditions in the North Pacific from 1990-1995 and a warm phase of the Pacific Decadal Oscillation (PDO). The velocity and competence of the meltwater plume increased in the period 1987-1992, likely associated with the numerous ENSO events that resulted in higher winter precipitation. Since then, there appears to have been no secular coarsening in any of the sediment proxies investigated in this study, implying that the amount of suspended sediment and volume of meltwater produced from the Hubbard have not increased in the past 20 years but rather fluctuated associated with short-term changes in regional climate.