New paleoenvironmental proxies for the Irati black shales (Paraná Basin, Brazil) based on acidic NSO compounds revealed by ultra-high resolution mass spectrometry

Abstract

Variations of the acidic NSO (Nitrogen, Sulphur, and Oxygen) compound composition of the Lower Permian Irati black shales and Serra Alta shales were assessed by Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) using electrospray ionization (ESI) in negative ion mode to test their significance for the regional paleoenvironmental reconstruction by comparison with known features in the northeastern and central-eastern Paraná Basin, Brazil. The high abundance of the S1Ox class for the basal Irati black shales in the northeastern basin reflects a sulfide-rich environment, whereas high O>2 classes in the Serra Alta shales indicate a high input of terrestrial organic matter deposited in oxic waters. Here, eight parameters based on O1 and O2 compounds are suggested as new paleoenvironmental proxies: phenol index (%DBE 4; O1 class); C27/C28 DBE 4 (O1 class); C27/C28 DBE 5 (O1 class); Even/OddFA; TARFA Odd (terrigenous/aquatic ratio); TARFA Even; C36 hopanoic acid index; and hopanoic/steranoic acids ratio. Higher values of the phenol index, the TARFA and Even/OddFA indicate higher land plant input during the final black shale deposition. Variations of C27/C28 DBE 4 and C27/C28 DBE 5, the first being based on the distribution of methylated isoprenoidyl phenols, can be used to reconstruct paleosalinity; here higher values indicate higher salinity. The C36 hopanoic acid index is higher for the marine hypersaline samples from the northeastern basin, while a significant bacterial biomass signal is stored as a higher hopanoic/steranoic acids ratio for samples from the central-eastern basin.