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Department of Geological Sciences, University of Florida, PO Box 112120, Gainesville, FL 32611-2120, USA jaeger{at}geology.ufl.edu
Glacial systems release sediment to the marine environment over a range of time periods, including short-term seasonal, tidal, and diurnal scales. Often, the sedimentary record providing the highest temporal resolutions of short-term processes is found in relatively inaccessible areas such as proximal to glacier termini or below floating ice sheets. To assess the importance of short-term glacial and oceanographic processes in creating glacimarine strata, it is necessary to evaluate strata production over similar time scales. Time-series coring, sediment traps, or bathymetric profiles may be difficult to perform in these harsh settings. Coupling the observations of sedimentary structures seen in core X-ray radiographs with sound chronologies allows for the evaluation of short-term glacimarine sedimentation. For seasonal time-scales, the highly particle-reactive radioisotope 234Th (24 day half-life) can be used to measure the rates of strata production under both steady and non-steady sediment deposition. To create appropriate age-depth relationships, two approaches are used: a rigorous and exacting mathematical model developed from a steady-state transport-reaction equation; and the less exacting CIC (constant initial concentration) and CRS (constant rate of supply or constant flux) point transformations. These methods are used for developing chronologies of cores collected in 1995 in Icy Bay, Alaska at 4, 12, and 32 km from the tidewater terminus of the Guyot Glacier. Examples are given of the boundary conditions that must be satisfied to use each approach in developing age-depth relationships. For the two cores collected closest to the terminus, age-depth relationships can be generated using all three approaches, whereas the presence of bioturbation in the most ice-distal sample severely complicates matters. The high (
0.5 cm d–1) but non-steady sedimentation rates at the ice-proximal station create a high-resolution sedimentary record. The controls on sedimentation at this location are evaluated by examining sedimentary structures (alternating light and dark laminae and beds) seen in digitized X-ray radiographs. The CRS method is used to convert depth to time in X-ray radiographs. The down-core pixel intensities of the grey-scale X-ray radiograph positives are used to create a time series of sedimentation reflected in proxy data (the bulk density of the sediments). By using Blackman-Tukey spectral analyses and relatively new wavelet techniques to evaluate periodic processes, sedimentation at this location can be related to seasonal variability in meltwater production, fortnightly tidal influences on particle settling rates, and episodic precipitation-induced sediment deposition.
