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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