CO 



u 



-z. 



LU 

 O 

 O 



CD 



CP 

 O 



O 



CM 



I 

 -O 



a. 



100 ^150 200 250 



\ 



\ 



YEARS BEFORE PRESENT 



0.0 0.5 1.0 1.5 2.0 2.5 3.0 



MASS DEPTH 



Figure 3. Top: During this time the biogenic silica content 

 (diatom frustules) has varied nearly 30 percent. Such 

 fluctuations may result from changes in productivity 

 through time, perhaps induced by changes in 

 hydrothermal inputs of nutrients as tectonic shifts 

 alter the sublacustrine "plumbing" of the Yellowstone 

 systems. Other hypothesized causes include climatic 

 changes, forest fire history and changing food web 

 dynamics; bottom: A Pb-210 profile from a core collected in 

 the deep basin in West Thumb. Sedimentation rates have been 

 relatively constant here at approximately 22 mg dry sediment 

 cm~2-Y~l over the last 100 to 120 years. 



85 



