From: PO3::"gordonr@CC.UMANITOBA.CA" "Richard Gordon" 9-MAY-1996 12:17:01.67 To: Multiple recipients of list DIATOM-L CC: Subj: travertine and stromatolites I received the following "tutorial" from Barbara Winsborough, in response to a question. She's agreed to share it with everyone. -Dick Gordon At 11:30 AM 5/7/96 -0500, bwinsbor@tpoint.net wrote: HHello and thanks for the message. To answer your question about the similarity between travertine and stromatolites...both are biogenic to some degree or other. Stromatolites form in hypersaline, normal marine, brackish and fresh, carbonate-rich water. What they all have in common is that there are certain organisms associated with the microbial mat (surface of said stromatolite) that are responsible for generating the stromatolite shape, and they do so either by trapping and binding grains, from 1 micron up to sand size in the case of marine forms (diatoms are significant trappers in the Shark Bay stromatolites for example); or by precipitating carbonate, as in the case of freshwater forms. This precipitation can be active as in the case of some algae and cyanobacteria, or more passive as in the case of the stalks of diatoms which provide a substrate for calcite nucleation. There is a crate of literature on this subject. There are lots of diatoms on every stromatolite I have ever examined, which makes sense- they are epiphytic on the bigger filaments and epipelic and epilithic on the carbonate grains- it is hard to separate the habitats since some stroms accumulate carbonate layers daily and the diatoms no doubt do a lot of travelling. To get a stromatolite there has to be a very delicate balance that keeps the weed flora and fauna away and allows for a very stable although by some definitions a rather "stressed" environment, like hypersaline water in Shark Bay or the tremendous currents and meter tall sand waves that regularly bury the giant Bahaman stromatolites, or the salinity of the Great Salt Lake stromatolites. In Mexico the stromatolites at Cuatro Cienegas are in low nutrient, high sulfate water. That excludes all but the hearty. >At any rate, travertines are much more restricted in their occurrence, >groundwater charged with CO2 and carbonate being necessary for travertine >formation. There is hot and cold travertine but when you look at them both the >diatoms are pretty much the same species. They just dont grow at all until the >water cools off. Travertines have many facies. Most abundant are facies >associated with spring discharge, where topography determines the overall >shape and degassing, current, humidity, illumination, etc determine >small-scale facies, such as calcified moss, laminated cascade and waterfall >deposits, pisoids, dams, ooids, calcite rafts, bubbles, encrusted stems, >calcified microbial mats etc. There are different organisms associated with >each facies, and some are much more biogenic than others, but the facies >traditionally considered the most abiogenic, like pisoids and bubbles often >have a coating of diatoms growing on their surface within minutes of >formation! There are also travertine stromatolites, that grow in specialized >habitats where the light, degassing, ppt & ? decrease (often in shelter caves) >to the extent that precipitation that would otherwise go on in a rampant way, >is restricted to encrustation around organisms living on the surface mat. This >makes it a stromatolite, because the organisms are responsible for the shape >of the deposit. At the moment I am writing a paper about diatom preservaton in >ancient travertines (which is quite good). I hope this answered your question, >sorry to go on and on, it is a pet topic of mine. -- >Barbara Winsborough bwinsbor@tpoint.net Richard Gordon, Department of Radiology, University of Manitoba ON104, HSC, 820 Sherbrook Street, Winnipeg, MB R3A 1R9 Canada Phone: (204) 789-3828, Fax: (204) 787-2080, E-mail: GordonR@cc.UManitoba.ca