Abstract

The formation of sedimentary strata was examined within a temperate fjord, Icy Bay, Alaska. Radioisotopes ( 234 Th, 210 Pb) were used to develop a chronology of sediment deposition and accumulation from cores collected in August 1994 and July 1995. Sedimentary structures were described and attributed to specific sedimentologic, oceanographic, and glacial processes. Annual sedimentologic processes were summarized by examination of sedimentary structures seen in x-radiographs and by developing a monthly chronology using 234 Th. During the months of May-July 1995, deposition of sediment varied from highly non-steady-state at ice-proximal stations to steady-state at a mid-fjord station. Deposition rates systematically decreased down fjord from >0.3 cm d (super -1) to <0.02 cm d (super -1) . Laminated muds were forming daily in Icy Bay at upper-fjord stations, and bioturbated sediments dominated in the lower fjord. Diamicton beds, containing biogenic structures, were formed in Icy Bay during the winter, when sediment deposition rates were lower. Sediment profiles of 210 Pb activity indicate a non-steady-state flux of particulate 210 Pb to the seabed, precluding measurement of decadal-scale sediment accumulation rates using this isotope. The dynamic relationship between deposition rates, distance from the glacier terminus, and the production of sedimentary structures were explored by examining longer cores (kasten, piston) that contain the history of ice recession in Icy Bay over the past few decades. These cores display changes in strata formation resulting from progressive glacial retreat, as indicated by down-core increases in the percentage of core length represented by mud layers, laminated sediments, and the relative concentration of sand to silt.

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