Application of Fallout Pb-210 Geochronology to River-Floodplain Systems

Lowland floodplains store suspended sediment carried over bank during floods. Because floodplain sedimentation may account for the majority of the net sediment accumulation within sand-bedded river-floodplain systems, it is important to develop a means to quantify this process. Recent studies have demonstrated that the natural radionuclide Pb-210 can be used to determine floodplain accumulation rates over decadal-to-century timescales with a Constant Input Concentration and Constant Sedimentation rate model (CICCS). To investigate the application of the CICCS method to a large, dynamic, pristine river system with extensive forested floodplains, 265 vertical cores were collected throughout the floodplains along ~2,000 km of the Beni and Mamore Rivers in northern Bolivia. Rather than measuring bulk-averaged down-core Pb-210 activity to estimate average accumulation rate according to the standard CICCS procedure, the cores were instead sectioned vertically to obtain a high-resolution record of Pb-210 activity and sediment accumulation. Over 95% of these profiles portray a history of episodic deposition with decadal recurrence intervals, and few profiles exhibit the steady-state accumulation assumed by the CICCS model. Because these discrete deposition events are large, few of the cores ever reach supported background Pb-210 activity at depth, another requirement of CICCS. Furthermore, the input activity of Pb-210 is not constant, both at a site and down-channel. The Pb-210 content of sediment varies directly with fine sediment content (typically clay), which is highly variable with depth for many of the cores. Without a discretely sampled, clay-normalized Pb-210 activity profile, it is difficult to evaluate the suitability of CICCS for each core. To accommodate these complexities, a more labor intensive approach is proposed for dating sedimentation using discrete down-core, clay-normalized measurements of Pb-210 activity, coupled with a geomorphic model of Pb-210 input during large floods. Combined with a field determination of supported Pb-210 activity, collection of multiple terrace cores to determine the meteoric input of Pb-210, and a technique for independently dating the "meteoric cap" above discrete depositional events, this approach to Pb-210 geochronology provides a convenient and robust approach to dating floodplain sedimentation to a near-annual precision. In addition to the Bolivian pilot study, additional examples will be presented from recent floodplain sampling campaigns on the Sacramento River, California (80 cores), and the Strickland River, Papua New Guinea (190 cores).