Sunday, July 6, 2008

Science week starts, microbialites get probed...

Greg Drushcel and Ben Cowie lower the microelectrode to the microbialite field

by Greg Druschel
Department of Geology
University of Vermont

Sediment porewater redox chemistry in lakes reflects the activity of microorganisms that utilize oxygen, nitrate, manganese, iron, and sulfate, giving deeper sediment porewater a different chemical signature compared to the water column. Of particular note for our research, sediment porewater organisms include sulfate reducers that generate hydrogen sulfide as a product of their metabolism. At Pavilion Lake, the location of microbialites is highly heterogeneous and they have an interesting morphology, including vent-like structures that are often associated with flow in other systems. The ‘vent’ structures observed with the microbialites have not thus far been noted to be associated with any detectable flow, but it has been hypothesized that diffuse flow may be an important part of these structures. Allyson Brady and Greg Slater have found evidence in the microbialites themselves for sulfate reducing bacteria, further suggesting that these organisms may be somehow linked to microbialite formation and morphology. We are investigating if this diffuse flow may cause upwelling of porewaters, or mixing of groundwaters with deeper porewaters, that may contain low concentrations of hydrogen sulfide or other reduced forms of manganese or iron associated with sediment porewater microbes. Hydrogen sulfide reactions may additionally change the pH of the water and potentially cause changes in carbonate precipitation, and some microbialites have been observed to include bands of different colors often associated with iron and manganese precipitates which can also affect water chemistry and associated carbonate precipitation or dissolution. We are using voltammetric microelectrodes that can measure oxygen and hydrogen sulfide (in addition to iron, manganese, and many other sulfur forms) to investigate if any of these reduced chemical forms may be present in and around the vent structures.

Yesterday, Greg Slater and Dale Andersen carried a sampling pole with a voltammetric microelectrode down to a field of microbialites at 70 feet deep in the 3 Poles region of Pavilion Lake. The electrode was attached to a potentiostat and computer on a pontoon boat where I was constantly monitoring the computer screen for changes in redox chemistry at the tip of the microelectrode. Dale and Greg placed the sampling pole in the sediment and placed the voltammetric microelectrode (a little smaller than the size of a small pencil) inside 5 different microbialite vent structures at different levels and outside the microbialites to see if there were any changes in redox chemistry associated with diffuse flow through the microbialite structures. The redox chemistry within these 5 vents was no different than the redox chemistry outside the vents, which was fully oxygenated. Our hypothesis that the occurrence and morphology of the microbialites is affected by a link with sediment porewater chemistry was not supported by measurements at these sites, but we shall keep thinking, working, and looking to unlock the secrets of these fascinating formations!

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