Cable, J. E.. Louisiana State University, jcable@unix1.sncc.lsu.edu
Smoak, J. E.. University of Florida, smoak@lsu.edu
Brenner, M. University of Florida, brenner@nervm. nerdc.ufl.edu
Schelske, C. L.. University of Florida, schelsk@nersp.nerdc.ufl.edu

 
BERYLLIUM-7 AS AN INDICATOR OF LAKE SEDIMENT MIXING
 
Paleolimnological studies in shallow lakes provide insights to historical changes in lake trophic state and sediment and nutrient accumulation. These investigations depend on reliable techniques for generating sediment age/depth relationships for timing of recent environmental changes. Pb-210 (half-life=22.3 yr) dating is used to establish sediment chronologies over the last 100-150 years. Despite the widespread use of this dating technique, many studies provide no independent check on modeled ages. In some cases, where uppermost deposits are mixed, Pb-210 models may yield inaccurate sediment accumulation rates. Be-7 (half-life=53 d) activity in uppermost deposits may be used to assess sediment mixing and thereby avoid the inappropriate application of Pb-210 dating models. Short-lived Be-7 should be undetectable in sediments >~1 year old, and its presence in older, i.e. deeper deposits, is an indicator of mixing. Sediment cores from some shallow Florida lakes display Be-7 activity at depths considerably deeper, i.e. older than those predicted by the crs Pb-210 dating model. For instance, in Lake Apopka, Be-7 activity was detected at 27 cm below the sediment-water interface. This depth corresponds to a Pb-210 age of ~19 years, in which Be-7 activity should be undetectable. We demonstrate the utility of Be-7 for evaluating recent sediment mixing in several Florida lakes.
 
Day: Friday, Feb. 5
Time: 11:30 - 11:45am
Location: Sweeney Center
 
Code: CS63FR1130S