and Mg have also been analyzed on a few samples via AAS with 

 agreement to within 10%. Dissolved silica (APHA 1985) and 

 ammonium (Koroleff 1969) were determined colorimetrically. Total 

 dissolved inorganic carbon was analyzed via the syringe stripping 

 method of Stainton (1973) on a Gow Mac 150 gas chromatograph. 



Water for radon-222 analysis was collected with a 30 liter 

 niskin sampler into 20 liter evacuated sample bottles. Radon was 

 extracted and counted following Broecker (1965). Supported 

 radon-222 concentrations have been determined on re-extracted 

 samples following ingrowth and are less than 0.06 dpm/1. Water 

 column temperature profiles were measured with a YSI in situ 

 probe. 



RESULTS AND DISCUSSION 



ROV Observations 



In 1987 direct observations of the bottom of Yellowstone 

 Lake were made for the first time using a ROV. Over a period of 

 six days, 22 dives were made in Mary Bay, Sedge Bay, West Thumb, 

 and in the deepest portion of the lake near Stevenson Island. 

 All of these locations represented areas suspected of having a 

 high probability of hydrothermal activity based upon geothermal 

 gradient data, surface observations of gas ebullition, or 

 proximity to onshore thermal areas. With the exception of one 

 dive, evidence of geothermal activity was observed at all sites. 



In the deeper depositional areas of the lake, the most 

 frequently observed features were small (approximately a few cm 

 in diameter) depressions or openings in bottom sediments from 

 which an occasional gas bubble was emitted. These small holes 

 were also frequently surrounded by a white mat or precipitate 

 assumed to result from the growth of a Thiothrix -like organism 

 commonly observed in shallow waters in association with sulfide 

 emanations. Gas bubbles were not always seen. It was assumed 

 that, based upon the loose, flocculent nature of these 

 sediments, that some relatively recent and persistent physical 

 disturbance would be required to prevent covering over and 

 filling in with sediment. Occasionally, warm water was 

 observed flowing from a fissure or hole in the bottom, creating a 

 shimmering effect against the backdrop of cooler waters. The 

 most dramatic example of this was observed at over 98 meters in a 

 narrow >112 meter deep depression in the main basin of the lake 

 near Stevenson Island. This small deep defile represented a 

 sounding more than 15 meters deeper than any before recorded. 

 Sediments throughout these regions were warmer than bottom waters 

 by approximately 1° C, and warmer still near presumed thermal 

 features. 



Seismic profiles of the bottom in this region indicated 

 apparent downfaulting and offsets in sediment stratigraphy of up 



