AGE DATING SEDIMENT TO ESTABLISH GEOCHRONOLOGY OF MID-ATLANTIC FLOODPLAINS AND SEDIMENT WAITING TIME DISTRIBUTIONS (Invited Presentation)

Floodplains are critical environments for the retention of sediment, nutrients, and carbon and their associated biogeochemical cycling. The ontogeny of floodplains in the eastern US has been the focus of significant research, but many questions remain. The volume of “legacy” sediment (post-settlement alluvium) that is currently being eroded and transported by the channel remains a key uncertainty. Although it is evident that the source of the alluvium is at least partially anthropogenic, the volumes of post-settlement alluvium and variations in its spatial distribution, sedimentation rate and its overall contribution to sediment supply have been less well-studied.

We dated floodplain deposits exposed in eroding streambanks of the South River, VA, and Difficult Run, VA using fallout radionuclides (Pb-210, Cs-137), optically stimulated luminescence (OSL), and radiocarbon dating to determine sediment ages and construct sediment waiting time distributions. Ages from fallout radionuclides document sedimentation from the early 1900s to the present. OSL dates span approximately 200 to 17,000 years old. We performed a Weibull analysis of non-exceedance to construct a waiting time distribution of floodplain sediment for the South River. The mean waiting time for floodplain sediment is 4641 years, while the median is approximately 1130 years. When the floodplain waiting time distribution is combined with the waiting time distribution for in-channel sediment storage (available from previous studies), the median waiting time shifts to approximately 56 years, suggesting that quantifying sediment waiting times for both channel and floodplain storage is critical in advancing knowledge of particle trajectories through watersheds. The geochronology suggests that only approximately 30% of the sediment entering the channel through bank erosion is “legacy” alluvium of post-settlement age (and approximately 70% is older and thus not anthropogenic).