TIDAL RHYTHMITES IN ALASKAN FJORD SEDIMENT: ESTABLISHING A PALEOMELTWATER RECORD

Climate change has accelerated glacier melting in Alaska over the past two decades. Direct measurement of meltwater discharge from tidewater glaciers is logistically difficult because these glaciers terminate in the sea as vertical cliffs. Deep water tidal rhythmites preserved in fjord basins adjacent to glaciers may provide a high-resolution discharge proxy. To assess this potential, two jumbo piston cores were collected from Muir Inlet, Glacier Bay National Park, Southeast Alaska aboard the R/V Maurice Ewing in 2004 (EW0408). Rhythmite depositional processes have been previously studied in these highly turbid, macrotidal fjords and the sediment sources, Muir and McBride Glaciers, have well documented retreat histories.

Each core is over 17 m long and dominated by rhythmically laminated mud with silt. Cores were subsampled with OPD-style u-channels and scanned for magnetic susceptibility and bulk density. To measure small-scale density differences between silt and mud laminae the u-channels were x-rayed, and the radiographs were digitized at 600 dpi. An Interactive Data Language (IDL) program was developed to collect and organize downcore pixel intensity profiles from x-ray negatives. In a profile of laminae thicknesses, breaks in the core and dropstones can easily be excluded. Downcore profiles are an average of multiple cross-core pixel intensities. Tidal cyclicity is evident in both x-radiographs and pixel profiles. Spring-neap packages, representing 2-week periods during summer are clearly visible because of prominent bounding silt laminae that produce peak pixel intensities. Time-series analysis can be performed once the profiles are converted from constant-depth to constant-time. Strong seasonality is marked by winter gravelly mud beds and spring plankton blooms that are used to determine annual sedimentation rates.