UC Irvine

A geochronology time series provides a powerful tool for elucidating sedimentary processes such as episodic deposition and diffusive mobility of particle-reactive constituents. Depth distributions of 210Pb and 137Cs from Skan Bay, Alaska were determined for sediment cores collected in 1980, 1984, 1987, and 1990. Sediment X-radiographs reveal distinct layers indicating that sediments were not continuously mixed by bioturbation. However, the geochronology time series is inconsistent with an undisturbed, steady-state sediment column. Profiles from 1980, 1984, and 1990 reveal subsurface regions in which 210Pb activity is relatively constant. In addition, the depth of the primary 137Cs maximum (reflecting the 1963 peak in atmospheric bomb testing) does not increase in a regular fashion between 1980 and 1990. The 210Pb and 137Cs geochronologies can be reconciled by removing the effects of an instantaneous depositional event. The average 210Pb sedimentation rate (corrected for episodic deposition) in cores that were collected over a ten year period (0.241 ± 0.006 g cm-2 yr-1) is in excellent agreement with the average 137Cs sedimentation rate (0.258 ± 0.008 g cm-2 yr-1) calculated from three stratigraphic markers [peak fallout (1963), first appearance in the sediment record (1952), and the Chernobyl accident (1986)]. The mobility of bomb-derived 137Cs under in situ conditions was evaluated by a time-dependent numerical model applied to the 137Cs time series. The model indicates that bomb-derived cesium is immobile in Skan Bay sediments with a solid-liquid distribution coefficient (Kd of ≥ 105 (ml g-1). © 1994.