EPISODIC DEPOSITION AND CS-137 IMMOBILITY IN SKAN BAY SEDIMENTS - A 10-YEAR PB-210 AND CS-137 TIME-SERIES

A geochronology time series provides a powerful tool for elucidating s edimentary processes such as episodic deposition and diffusive mobilit y of particle-reactive constituents. Depth distributions of Pb-210 and Cs-137 from Skan Bay, Alaska were determined for sediment cores colle cted in 1980, 1984, 1987, and 1990. Sediment X-radiographs reveal dist inct layers indicating that sediments were not continuously mixed by b ioturbation. However, the geochronology time series is inconsistent wi th an undisturbed, steady-state sediment column. Profiles from 1980, 1 984, and 1990 reveal subsurface regions in which Pb-210 activity is re latively constant. In addition, the depth of the primary Cs-137 maximu m (reflecting the 1963 peak in atmospheric bomb testing) does not incr ease in a regular fashion between 1980 and 1990. The Pb-210 and Cs-137 geochronologies can be reconciled by removing the effects of an insta ntaneous depositional event. The average Pb-210 sedimentation rate (co rrected for episodic deposition) in cores that were collected over a t en year period (0.241 +/- 0.006 g cm-2 yr-1) is in excellent agreement with the average Cs-137 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 acci dent (1986)]. The mobility of bomb-derived Cs-137 under in situ condit ions was evaluated by a time-dependent numerical model applied to the Cs-137 time series. The model indicates that bomb-derived cesium is im mobile in Skan Bay sediments with a solid-liquid distribution coeffici ent (K(d)) of greater-than-or-equal-to 10(5) (ml g-1).