Hot Topic Article:
Fresh Insight into the Dolomite Problem
Jay Chapman
In 1785, James Hutton presented the theory of uniformitarianism, which states that Earth has a long geologic history and that one can gain insight into this history by examining modern processes. His assertion that geologic processes have remained uniform throughout time is one of the cornerstones of modern geology. It is precisely in this light that the "Dolomite Problem" has plagued and inspired scientists for the last half-century.
Dolomite, a calcium-magnesium carbonate, is found abundantly in Paleozoic and older strata, but is conspicuously rare in younger rocks. There are only a handful of locations where dolomite is forming today. In order to explain the rapid disappearance of dolomite in the rock record, scientists need to understand how dolomite forms, but researchers have struggled to produce dolomite in the laboratory at temperatures and pressures realistic to its natural formation. The crux of the "Dolomite Problem" is to determine how dolomite forms and with that information, understand why it was once so prevalent, but exceptionally uncommon today.
A false color SEM photomicrograph of two different types of microorganisms (green and yellow) attached to a basalt substrate. Each cell is 1 micron long. The yellow microbe is interpreted as a methanogen with nucleated dolomite shown in orange. Attachment filaments are shown in pink. Image courtesy of Dr. Jennifer Roberts, with color enhancement by Jeffrey Horowitz. Click on image for larger photo. |
Historically, scientists have considered the "Dolomite Problem" a geochemical concern and some have spent lifetimes pursuing geochemical solutions. The latest research though suggests that microorganisms are responsible for mediating dolomite precipitation. Jennifer Roberts, of the University of Kansas, says that the increasing overlap between the work of microbiologists and geochemists is understandable because, "microorganisms are little geochemists" themselves and "it's often easy to distill their activities down to a single chemical reaction."
In the April 2004 issue of Geology, Dr. Roberts, Philip Bennett, at the University of Texas, and others reported that methanogens (methane producing microbes) in a freshwater aquifer, and in similar laboratory conditions, nucleated dolomite on their cell walls. Previous studies of microbial dolomite precipitation have focused on environments where dolomite has formed in the past, namely saline, magnesium-rich waters. The discovery of biologically mediated dolomite in a freshwater, low-magnesium environment came as a pleasant surprise and significant discovery. "The recognition of freshwater microbial dolomite now expands immensely the range of environmental conditions conducive to dolomite precipitation," says Judith McKenzie of the Swiss Federal Institute of Technology in Zurich, another leading researcher of the "Dolomite Problem."
The new research has also "expanded the range of microorganisms that will participate in the reaction, adding methanogens to the list," says Dr. Bennett. Sulfate-reducing bacteria are the only other group of microbes proven to precipitate dolomite. "Constraining the geochemical and microbiological parameters necessary for dolomite formation is an important stepping stone in understanding the thermodynamics of dolomite solubility," says Dr. Roberts. Understanding dolomite solubility is a stepping stone to understanding the origin of massive dolomite deposits in the rock record and the lack thereof today.
It is highly unlikely that methanogens, or any other microorganism, directly deposited enough dolomite to account for the ancient deposits, but they may have still been essential to the formation of this enigmatic mineral. Dr. McKenzie reports that, "although the nucleation of dolomite crystals is mediated by microbes, once nucleated they can continue to grow and mature inorganically." Another possibility, the replacement hypothesis, suggests the massive deposits originated as calcium carbonate and were biologically altered by microbes into dolomite, a process proposed for the formation of the Dolomite Mountains in Italy.
Whatever the possibilities are of finally cracking the "Dolomite Problem, the continued "study of these microbial processes in natural environments, combined with simulation experiments under controlled conditions, provides us with a powerful tool to calibrate processes related to microbial activity in ancient systems linking the geosphere with the biosphere," says Dr. McKenzie. Linking the geosphere and biosphere is what biogeoscience is all about.
References
1. Roberts, J.A., Bennett, P.C., González, L.A., Macpherson, G.L., and Milliken, K.L., 2004, Microbial precipitation of dolomite in methanogenic groundwater: Geology, v. 32, n. 4, p. 227-280.
Contacts
Jay Chapman jchapman@geosociety.org
Jennifer Roberts
Philip Bennett
|
