Evidence of Hydrothermal Activity Within the Lake 



In 1974, Morgan et al . (1977) undertook the first studies of 

 thermal gradients within the lake floor in order to study the 

 existence of the mantle derived intrusives in the upper crust 

 beneath Yellowstone. More recently, Morgan and Blackwell have 

 significantly increased the data base on thermal gradients within 

 the lake sediments and have calculated for the major portion of 

 the lake, the flow of heat resulting from the gradients observed. 

 For the Rockies in general, the average heat flow is 60 MW 

 per m . in Yellowstone Lake the range is from 100 to over 40,000 

 MW m~2 (personal communication, Morgan, Northern Arizona State 

 University, 1987). The regions of highest heat flux are in Mary 

 Bay, the site of a relatively recent explosion crater (Wold et 

 al. 1977), southwest West Thumb and in deep water east of 

 Stevenson Island. Temperature gradients in Mary Bay, for 

 example, reach as high as a 50° C increase in temperature per 

 meter depth into the sediment. The magnitude of these gradients 

 and heat fluxes are indicative of hydrothermal activity 

 throughout much of the lake. The topography of the lake bottom 

 is complex and poorly known. At least some of the high tempera- 

 ture anomalies appear to be associated with downfaulted sediment 

 structure and small fissures or troughs on the lake floor 

 (personal communication, Morgan, Northern Arizona State Univer- 

 sity, 1987). Continuous bottom water temperature recorders 

 placed at several locations in the lake by the Morgan and 

 Blackwell group in conjunction with the thermal gradient studies 

 showed a gradual and steady increase of ~ 1° C in water tempera- 

 ture during the winter ice cover period from December to May. 



METHODS 



Direct observations were made in both shallow and deep water 

 via a MiniRover MKI . To facilitate the measurement of thermal 

 anomalies, the movable claw on the ROV was replaced with a 

 rotating drum to which was mounted an " 40 cm aluminum rod 

 holding a high precision well logging thermister loaned to us by 

 Dr. Paul Morgan of N. Arizona State University. The drum and rod 

 were configured so that the thermister could be held vertically 

 and then rotated through 90 degrees to the horizontal, allowing 

 the probe to be inserted up to 5 to 10 cm in the sediment. 



Water samples of hydrothermal solutions were collected by 

 SCUBA divers using a tube (with attached temperature probe) 

 connected to a peristaltic pump at the surface or by using hand 

 held 50 ml syringes. Samples of microbial mats were similarly 

 collected using sterile 10 ml disposable plastic syringes. 



Samples for chemical analysis were held on ice until brought 

 back to the laboratory. The cations: Na, K, Mg, Ca and the 

 anions: CI and SO4 were separated via single column ion 

 chromatography on a dual channel Wescan Mdl 261 IC. To date, Ca 



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