Year: 1997
Type: article
Abstract: Over the last decade, the use of geochemistry and especially stable isotopes in stratigraphy hasbecome commonplace. Recent reviews of the stratigraphic applications of the carbon, oxygen,sulfur and strontium isotopic systems can be found in Holser (1996), Popp et al. (1997), McArthur(1994), Veizer et al. (1997a,b) and Strauss (1997).Many stable isotope studies have focused on the Permian/Triassic boundary and the lower part ofthe Lower Triassic. However, much less attention has been paid to the remainder of the Triassic.The Triassic segment of the widely cited Phanerozoic curves (Holser, 1988; Burke et al., 1982;Veizer, 1997b) is poorly constrained for both carbon and strontium isotopes, although a number ofpapers treated various aspects of origin, paleoclimate or diagenesis using carbon and oxygenisotopes from Triassic carbonates (Scherer, 1977; Henrich and Zankl, 1986; Frisia-Bruni andWeissert, 1989; Spotl and Burns, 1991; Litnerova, 1992; Bernasconi, 1993; Bellanca, 1995; Al-Aasm, 1995; Mutti and Weissert, 1995; Loreau, 1995).There are basically two approaches employed for reconstructing pre-Jurassic secular variations ofcarbon isotope ratios, discussed in Grossman (1994) and Scholle (1995). The first considers onlythe data recorded from carefully selected carbonate components, like marine cements and mainlynonluminescent brachiopod shells, which are more likely to preserve primary signatures. Thesecond one relies on the use of whole-rock micritic limestones or dolomites assuming, throughmass-balance criteria, that inorganic carbon isotopes are affected to only a small degree bydiagenetic alteration in carbonate rich and organic carbon poor sediments; oxygen isotopes havebeen, in most cases, reset during diagenesis.The scarce occurence of thick shelled brachiopods in Triassic deposits seriously limits theacquisition of high-confidence carbon and oxygen isotope data. In addition, due to the currentcorrelation problems that are posed (mainly to the Lower and Middle Triassic), an isotopic curvewith a good chronostratigraphic coverage must be obtained from sections that are well calibrated bybiostratigraphical means.One of the few studies that present in detail carbon isotope variations along measured Triassicprofiles are given by Steuber (1991) and Simon and Steuber (1993). The data were obtained on aMiddle Triassic to Lower Jurassic section from the Helicon Mountains, Greece. No well definedfluctuations were observed in d13C values of the carbonates, instead, a gradual increase in d13Cvalues of organic matter from Ladinian to Norian was found. More recently, Bohm and Gawlick(1997) reported a positive carbon isotope excursion in the Upper Triassic based on data obtainedon a section from the Northern Calcareous Alps.During the last three years, we have studied the carbonate carbon and oxygen isotope variations inseveral marine Triassic sections, under close biostratigraphic control, in order to detect anysystematic fluctuations that would potentially serve for chemostratigraphical correlations. Herebelow we briefly overview some Triassic carbon isotope events, some of them already describedelsewhere, others are newly described.
Source: IRIS
Authors: Viorel Atudorei, Aymon Baud
Language: English
Subfield: Paleontology
Field: Earth and Planetary Sciences
Domain: Physical Sciences
Open Access status: green
OpenAlex ID: W2519372247