A HIGH RESOLUTION 241AM AND 210PB CHRONOLOGY OF THE EUTROPHIC LAKE BONNY IN LAKELAND, FL

²⁴¹Am is a decay product of ²⁴¹Pu (t_1/2 = 14.4 yr) – a direct fission product from the Cold War Era nuclear testing of 1953 to 1963 – and is thus a valuable geochronometer. While ¹³⁷Cs and ²¹⁰Pb are more commonly used as sediment dating tools, ²⁴¹Am has the capability to validate ²¹⁰Pb and ¹³⁷Cs dating models in environments where ¹³⁷Cs is mobile. We apply radionuclide chronometers to study sedimentation in Lake Bonny (Lakeland, FL), a eutrophic lake with sediments that are depleted in swelling clays and highly enriched in organic matter (mean LOI = 66%). ¹³⁷Cs can be geochemically mobile in soils that are deficient in both potassium and swelling clays and where uptake by vegetation is prevalent (Drexler et al., 2016).

Our high resolution chronology of Lake Bonny using ²⁴¹Am and ²¹⁰Pb CRS dating models shows significant migration of ¹³⁷Cs. The concentration maxima of ¹³⁷Cs (18.2 Bq/kg at 50cm depth) and ²⁴¹Am (3.4 Bq/kg at 54 cm depth) possess similar peak shapes, but ²⁴¹Am displays a tighter full width at half maximum. The total mass accumulation rate is 0.012 g/(cm²*yr) from 1903 to 1971, collapses to 0.0076 g/(cm²*yr) from 1971 to 1989 and makes a partial recovery to 0.0092 g/(cm²*yr) from 1989 to 2020. Interestingly, ²²⁶Ra activities peak to a maximum of 122.1 Bq/kg in 1958 but are approximately half that in both the decades before and after the peak. We hypothesize that ²²⁶Ra levels were elevated due to groundwater amendments to the lake, which have been documented in nearby watersheds. Our study shows how ²¹⁰Pb and ²⁴¹Am together can be used to understand changes in sedimentation and groundwater inputs in this eutrophic system.