to 3 meters in what would appear to be actively accumulating 

 sediments (Blackwell, pers. comm. 1987, see also Otis et al . 

 1977 ) . Present sediment deposition rates determined by Pb-210 

 dating in Yellowstone Lake are on the order of 10 to 20 mg 

 cm~2'Y~l, or roughly a mm per year. These offsets frequently 

 extended to the sediment surface, the implication being that 

 these "faults" represent contemporary events and that the bottom 

 experiences major shifts in response to continuing tectonic 

 activity. 



ROV observations of the bottom revealed steep topography, 

 sediment slumping and "outcrops" of exposed sediment strata. If 

 our estimates of deposition rates apply, these sediments are 

 young, no more than a few hundred to a couple of thousand years 

 old at most. They appeared to be very well lithified, however, 

 in contrast to sediments collected in other deep areas of the 

 lake in cores nearly a meter in length. The sediments in 

 Yellowstone are a diatomaceous ooze consisting of up to 50% to 

 60% biogenic silica and having an organic carbon content of 3%. 



It is possible that these exposed "outcrops" represent older 

 sediments or that they have undergone accelerated diagenesis and 

 lithif ication due to heating from below. 



Geothermal activity in nearshore areas was much more readily 

 observable and more dramatic. In Mary Bay, for example, gas 

 ebullition from submerged fumaroles was widespread and extremely 

 active. Curtains of gas bubbles, consisting largely of carbon 

 dioxide, were observed emanating from barren sandy sediments 

 which reached temperatures of nearly 100° C at 4 or 5 cm below 

 the sediment surface. Submerged fumaroles were also found in 

 Sedge Bay. Sediment temperatures here were cooler, however, and 

 macrophyte (primarily aquatic mosses) and attached algal growth 

 was well established and appeared, in some instances, to be 

 enhanced by the high CO2 content of the fumarole gas. 



Growth of a Thiothrix-type organism was commonly observed 

 as white filamentous material covering rocks and plants growing 

 in fumarole effluents containing hydrogen sulfide. The absence 

 of these sulfide oxidizers around some gas vents is assumed to 

 indicate the absence of hydrogen sulfide as well. In some 

 shallow areas, dense and colorful microbial mats grew in bands 

 approximately 1 meter in width and extended for 10' s of meters 

 across silty-sand bottoms. These mats apparently followed 

 subsurface thermal features generating steep thermal and chemical 

 gradients. 



Preliminary Observations on the Chemistry of Submerged 

 Hydrothermal Springs in a Nearshore Region: Sedge Bay 



While submerged fumaroles were the most prevalent feature 

 observed in these shallow near shore areas, occasional small 

 hydrothermal springs and seeps occurred in association with gas 



88 



