Geotimes
Highlights
Carbonates
Evan K. Franseen and Katherine A. Giles

At an ever-increasing rate, new technologies and techniques are being utilized and integrated with more traditional methods to gain a better understanding of carbonate systems and the factors that control their development. Although not comprehensive, the following categories exemplify major focus areas of research in 2002.

Stratigraphy-Sedimentology. Examples under this broad category included a tectonic mechanism for tepee formation in peritidal carbonates (Pratt, Sedimentary Geology, v. 153, p. 57-64), factors associated with the demise of reefs at the Frasnian/Famennian boundary (special issue of Palaeogeography, Palaeoclimatology, Palaeoecology, v.181, p. 1-374), and a comprehensive study of Cenozoic carbonates in Southeast Asia and their implications for equatorial carbonate development (Wilson, Sedimentary Geology, v. 147, p. 295-428). Carbonate slope settings continued to receive attention, especially toward understanding triggering mechanisms and sediment composition of gravity flow deposits (Drzewiecki and Simo, Sedimentary Geology, v. 146, p. 155-189) and quantifying the geometry and sediment fabric of linear slopes (Adams and others, Sedimentary Geology, v. 154, p. 11-30). One study demonstrated the utility of 3-D seismic data in showing that Paleozoic carbonate buildups, once thought to be chaotically arranged, were oriented into a mosaic of ridges (Elvebakk and others, Sedimentary Geology, v. 152, p. 7-17). A study of the Phanerozoic suggested that poleward movement of the major continents over the past 540 million years decreased the low-latitude shelf area available for accumulation of shallow water carbonates, shifted carbonate sedimentation to deep-ocean settings and may have facilitated development of open-ocean planktonic calcifiers (Walker and colleagues, Journal of Geology, v. 110, p. 75-87).

Research on carbonate production included the use of biostratigraphy and carbon isotopes to document environmental changes responsible for variations in carbonate production rates (Morettini and colleagues, Palaeogeography, Palaeoclimatology, Palaeoecology, v.184, p. p. 251-273). A plate tectonic control was invoked for altering magnesium-calcium ratios of Cretaceous seawater, which in turn altered the abundance of major carbonate producers and chemical composition of biological calcite (Steuber, Geology, v. 30, p. 286-288). Modeling of carbonate production on the Great Bahama Bank suggests that for large platforms with long residence times, the shallowest areas may become sinks, not sources of sediment (Demicco and Hardie, Journal of Sedimentary Research, v. 72, p. 849-857). Another investigation hypothesized that the close spatial and temporal coincidence of eolian siltstone and algal dominance in the late Paleozoic could be due to nutrient seeding of ecosystems by high atmospheric dust loads (Soreghan and Soreghan, Journal of Sedimentary Research, v. 72, p. 457-461).

Progress continued for improving correlation and resolution of correlations of Neoproterozoic through Cenozoic carbonate systems. Methods included carbon isotope anomalies, carbon and oxygen isotopes, biostratigraphy, cyclostratigraphy and astronomic calibration, neodymium/neodymium and samarium/neodymium isotopes, uranium series dating of pedogenic carbonates, and sequence stratigraphy. The improved dating and correlations provided insight into controls on carbonate systems, including palaeoceanographic conditions (for example, Buonocunot and colleagues, Sedimentology, v. 49, p. 1321-1337; Ludwig and Paces, Geochimica et Cosmochimica Acta, v. 66, p. 487-506; Booler and Tucker, Sedimentary Geology, v. 146, p. 225-247).

New tools and approaches to quantitative data collection, from micro to macro scale, were also utilized. One study demonstrated a new application of a fluid-inclusion technique for quantifying water depth of ancient carbonate platforms (Mallarino and others, Geology, v. 30, p. 783-786). Total stations, GPS systems, photogrammetry, ground-based laser ranging, and satellite or airborne remote sensing platforms are being used to collect accurate 3-D data at various scales to provide quantitative data on rates and patterns of facies changes of modern and ancient carbonate systems (for example, Rankey and Morgan, Geology, v. 30, p. 583-586; Bellian and colleagues, AAPG Annual Meeting, Official Program, v. 11, p. A16; Acker and others, International Journal of Remote Sensing, v. 23, p. 2853-2868).

Microbial Carbonate. Microbial systems are receiving increasingly greater attention with the newly recognized links of these biota to Earth's earliest life forms and potential extraterrestrial forms of life. Current focus is on morphologic and geochemical recognition of microbialites and calibration of sediment production rates of microbial-related carbonate sediments and boundstones (for example, Pasteris and others, Nature, v. 420, p. 476-477; Arp and colleagues, Geology, v. 30, p. 579-580).

Diagenesis. Last year showed a resurge of interest in the processes of dolomitization, in response to many newly discovered dolomite hydrocarbon reservoirs and interest in maintaining production from old fields. Hydrothermal dolomite has received renewed attention based on new geochemical datasets. Questions concerning other current diagenetic paradigms were raised from studies of cores from the Neogene of the Bahamas (for example, Machel and Lonnee, Sedimentary Geology, v. 152, p. 163-171; Aranburu and others, Sedimentology, v. 49, p. 875-890; Melim and Scholle, Sedimentology, v. 49, p. 1207-1228; Melim and others, Marine Geology, v. 185, p. 27-53).

Paleoclimate. Paleoclimate studies remain a top priority, with most research focused on constraining the temporal and spatial variation in fundamental climatic parameters such as mean temperature and rainfall. Studies utilizing carbonate rocks primarily use oxygen and carbon isotopes within paleosol carbonate nodules, lacustrine carbonates and speleothems to constrain these parameters. Primary emphasis has been to calibrate these systems to Quaternary ice-core-derived datasets in order to assess their applicability to more ancient systems (for example, Budd and colleagues, Palaeogeography, Palaeoclimatology, Palaeoecology, v. 188, p. 249-273; Tabor and Montañez, Geology, v. 30, p. 1127-1130; Tabor and others, Geochimica et Cosmochimica Acta, v. 66, p. 3093-3107; Hoffman and colleagues, Geology, v. 30, p. 286-288).

Ocean Water Chemistry. Research into changes in ocean water chemistry through geologic time has focused on discerning the nature of ocean water during major biotic crisis intervals in order to determine if there is a link. Studies primarily utilize trends in oxygen, carbon and strontium isotopes and phosphate levels in carbonate rocks and carbonate skeletal material (for example, Longinelli and others, Earth and Planetary Science Letters, v. 203, p. 445-459; Sephton and others, Geological Society of America Special Paper, 356, p. 455-462; Wilson and others, Geology, v. 30, p. 607-610; Montañez, Proceedings of the National Academy of Sciences, v. 99, p. 15852-15854).

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Franseen is a senior scientist at the Kansas Geological Survey and Giles is a professor at New Mexico State University. E-mail: evanf@kgs.ukans.edu and kgiles@nmsu.edu.

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