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Carbonates
The University of Kansas Carbonate Research Group

Carbonates are impressionable. Their character is greatly affected by biological, chemical and physical inputs, and thus they provide a powerful record of subsurface history, ocean and atmospheric chemistry, paleoenvironment, ecosphere and paleoclimate. We have made recent advances in understanding their diagenesis, elucidating the roles that microbes play, predicting stratigraphic response to environmental variables and using geochemistry to help reconstruct characteristics of Earth's past environment.

Depositional environments

A renaissance continued in Belize, a current focus of modern carbonate sedimentation research, with core-based studies of lagoonal mud banks by Mazzullo and collaborators (Sedimentology, v. 73, p. 743-770) and of isolated platforms by Gischler (Sedimentary Geology, v. 159, p. 113-132; Gischler and colleagues, Palaios, v. 18, p. 236-255) and other researchers. Reef studies also centered more on Quaternary history than the present surface with an issue of Sedimentary Geology, v. 159, p. 1-132) devoted to Late Quaternary Reef Development.

Process and stratigraphic response

Quantification of sea-level changes was a common theme at the Global Sedimentary Geology Project symposium, "Cretaceous carbonate platforms: modeling and quantification" (Palaeogeography, Palaeoclimatology, Palaeoecology, v. 200, p. 1-265). A highlight in this issue was a report on Strasser's effort to quantify all the parameters in high-frequency sequence stratigraphy (Hillg{{umlaut a}}rtner and Strasser, p. 43-63. The role of nutrients and climate was another focus. Schlager (International Journal of Earth Sciences, v.92, p. 445-464) introduced a classification for Phanerozoic benthic carbonate production systems. Mutti and Hallock (International Journal of Earth Sciences, v.92, p.465-475) summarized proxies that can be used to determine nutrient fluxes to constrain paleooceanic controls.

Research continued on heterozoan-dominated systems. One study demonstrated high rates of production in the aphotic zone (Corda and Brandano, Sedimentary Geology, v. 161, p. 55-70). Studies of carbonate sequences in ramp settings demonstrated the interaction of glacioeustacy and tectonism (for example Al-Tawil and colleagues, SEPM Special Publication 78, p. 219-237), the building and filling internal architecture of high-frequency sequences (McKirahan and colleagues, SEPM Special Publication 78, p. 95-114) and mound evolution (Murillo-Muñetón and Dorobek, Journal of Sedimentary Research, v. 73, p. 869-886). Weber and colleagues (SEPM Special Publication 78, p. 351-394) developed a supersequence-scale stratigraphic framework for the Tengiz oil field.

Geochemistry

Morse and colleagues (Geochimica et Cosmochimica Acta, v. 67, p. 2819-2826) revisited the controversy of calcium-carbonate precipitation in whitings and used the results from experimental studies on the kinetics of calcium carbonate to argue against their homogeneous nucleation in these fish. Various studies provided new data on secular changes in seawater magnesium-calcium (Hardie, Geology, v. 31, p. 785-788; Dickson, 12th Bathurst Meeting Abstracts, p. 29). Frank and Fielding (Geology, v. 31, p. 1101-1104) presented evidence of a marine origin for Precambrian carbonate-hosted magnesite deposits. Jian and colleagues Nature, v. 426, p. 822-826) used carbon-isotope data to argue for the involvement of methane hydrate degradation in the formation of Precambrian cap carbonates, while Ridgwell and colleagues (Science, v. 302, p. 859-862) used a carbonate-precipitation model to explain glaciations and cap carbonates requiring little input from methane hydrate. Saltzman (Geology, v. 31, p.151-154) used carbon isotopes to propose that Pennsylvanian glaciations were triggered by changes in ocean circulation.

Paleoenvironmental diagenesis and diagenetic processes

The diagenetic record can now be used to reconstruct surface paleoenvironments not otherwise recorded by sediment deposition. This evolving field has produced records of climate, sea level, tectonic rates and, last year, evidence for changes in ocean circulation and nutrient supply along hardgrounds (Mutti and Bernoulli, Journal of Sedimentary Research, v.73, p. 296-308).

Recent research has led to major revisions of commonly accepted diagenetic models. Csoma and Goldstein (Abstracts, 22nd IAS Meeting, p. 35) studied several examples of mixing zones with calcite and aragonite precipitation, rather than dolomitization or dissolution. Surprisingly, mixing ratio was unimportant. New evidence has been amassed on microbial influences on diagenetic reactions, and Sanders (Journal of African Earth Sciences, v. 36, p. 99-134) showed that such site-specific processes may control dissolution and precipitation in the marine realm. Trenton-Black River hydrocarbon discoveries are leading to a resurgent focus on linking hydrothermal porosity development to tectonic setting (Newell and colleagues, SEPM Special Publication 78, p. 333-350).

Impact of microbes

Microbial mineralization was reported in many marine and other systems last year. Microbes drive precipitation by changing bulk-water chemistry through metabolic activity or by concentrating metals and nucleating crystals on cell walls and associated exopolysaccharides (EPS). Arp and colleagues (Journal of Sedimentary Research, v. 73, p. 105-127) described microbial calcium-carbonate precipitation from an alkaline system in Indonesia that is EPS-mediated rather than photosynthesis-driven. In low-temperature dolomite precipitation, both metabolic activity and microbial surface controls are important. Van Lith and colleagues (Sedimentology, v. 50, p. 237-245; Geobiology v. 1, p.71-79) evaluated the importance of cell-wall nucleation by sulfate-reducing bacteria. Roberts Rogers and colleagues (Geochimica et Cosmochimica Acta Supplement, 13th V.M. Goldschmidt Conference, p. 400) showed experimentally that methanogens nucleate and precipitate ordered, stoichiometric dolomite in dilute groundwater with a magnesium to calcium ratio of less than 1.

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The University of Kansas Carbonate Research Group: Paul Enos, Evan K. Franseen, Robert H. Goldstein, Luis A. González and Jennifer A. Roberts.

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