Samples of dolomite and limestone from the Permian Irati Formation collected in the Paraná Basin, southern Brazil, have been analyzed for δC13 and δO18. The δC13 ranges from +18.3‰ to −17.4‰ PDB, while the δO18 ranges from −2.6‰ to −12.5‰ PDB. In some quarries where the exposures are especially good, a large variation in δC13 can be found. The lower, dense gray dolomite has light carbon (−17 to +2.6‰), whereas the overlying intermediate zone of interbedded organic-rich shale and black dolomite has dolomite containing heavy carbon (+4.8 to +14.9‰PDB).
We believe that the dolomite represents diagenetically altered limestone, and that light CO2 produced by decomposing organic matter was involved in the diagenesis of the lower units, while heavy CO2 produced by the isotopic exchange between CO2 and methane was involved in the alteration of the upper, organic-rich layers.
The original environment of deposition may be represented by the few limestone outcrops from the southern part of the basin, which have somewhat light (−2.6 to −7.6‰) carbon and light (−6.7 to −11.1‰) oxygen that may be the result of precipitation in a lake or inland sea.