33 



Laboratory Analyses 



Sediment samples were subdivided in the Paleoecology 

 Laboratory of the Florida Museum of Natural History for diatom 

 analyses, estimation of bulk sediment accumulation rates by 21 Op 5 

 assay, and determination of percent organic matter. 



Sediment samples for diatom analyses were cleaned using 

 hydrogen peroxide and potassium dichromate (Van der Werff 1955). 

 Digested samples were diluted with deionized water in 400-ml 

 beakers and settled overnight. The supernatant solutions were 

 removed by vacuum aspiration and the process of dilution, settling 

 and aspiration was repeated until the dichromate color was no longer 

 visible. Cleaned samples were suspended in 60 ml of water and 

 settled onto coverslips in evaporation trays (Battarbee 1973). Dried 

 coverslips were mounted on glass slides using Hyrax mounting 

 medium. 



A minimum of 500 diatom valves was counted and identified on 

 a single slide of each sample using an American Optical Microstar 

 microscope at 1500X with a dark-phase condenser. Each diatom 

 valve was identified to the lowest taxonomic level possible using 

 standard diatom floras including those of Patrick and Reimer (1966- 

 1975) and Hustedt (1930,1930-1966). 



The concentration of diatoms (D CONC) in the sediment samples 

 was calculated with the following formula: 



D-CONC = C(PVD)-1 



where D-CONC = number of frustules g-^ dry weight of sediment 

 C = number of valves counted 

 P = proportion of settling tray area counted 



