Benthic anoxia, intermittent photic zone euxinia and elevated productivity during deposition of the Lower Permian, post-glacial fossiliferous black shales of the Paraná Basin, Brazil

Abstract

Here, the Lower Permian, post-glacial fossiliferous Lontras black shales from the Paraná Basin (southern Brazil) are studied using integrated palynological, geochemical and petrographic methods for the first time in order to decipher the prevalent palaeoenvironmental conditions during their sedimentation. These black shales were deposited in a restricted marine environment. Inorganic geochemical data (U/Th ratios, authigenic uranium, molybdenum), organic geochemical data (total organic carbon, biomarkers) and framboid pyrite size distributions point to predominantly anoxic/euxinic bottom-water conditions. Moreover, the presence of aryl isoprenoids and maleimide biomarkers indicates that euxinia in the water column was intermittently present in the photic zone. The onset of anoxic conditions was caused by elevated productivity in the basin, which was related to deglaciation, marine transgression and the increased delivery of terrestrial nutrients. The presence of a positive organic carbon isotope excursion indicates that the black shale deposition resulted from increased productivity and the expansion of anoxic and nitrogen- and phosphate-enriched waters into the shallow photic zone. The high values of δ15N (exceeding 9‰) may be related to the deglaciation-driven sea-level rise and advection of denitrified water mass from the Panthalassic Ocean to the intracratonic Paraná Basin. Prolonged periods of sea-floor anoxia/euxinia excluded potential scavengers and bioturbators, thus enhancing the preservation of numerous fossil taxa, including fish, sponges, insects and their larval cases, and conodont apparatuses. The intermittent photic zone euxinia may also have contributed to the mass mortality of fish populations, the fossils of which are very well-preserved in these black shales.