Isotopic biogeochemistry of the Neocomian lacustrine and Upper Aptian marine-evaporitic sediments of the Potiguar Basin, Northeastern Brazil

Systematic variations in the abundances of in kerogens and in saturated hydrocarbons were used to assess the paleoenvironments of the Neocomian lacustrine and Aptian lagoonal to restricted marine-evaporitic sediments of the Pendência and Alagamar Formations, respectively, from the Potiguar Basin, Brazil. In the Pendência Formation, the isotopic compositions of compounds formed in the photic zone, i.e., pristane, β-carotane, and some of the n-alkanes, vary only slightly with depth, suggesting invariable conditions in the surface environment. However, dramatic changes occur in the carbon-isotopic compositions of hopanoid biomarkers produced by organisms inhabiting the lower part of the water column of the Pendência paleolake. Parts of the section having finely laminated sediments with no burrowing coincide with the most significant depletion of the hopanoids relative to algal products, indicating effective stratification of the water column and active recycling of methane. In the Alagamar Formation, isotopic compositions of the n-alkanes vary systematically with depth but not with carbon number, indicating dominance of a single source for all n-alkanes, independent of carbon number, and secular variations in the isotopic composition of that source. The latter variations can be correlated with global changes in the isotopic composition of marine dissolved inorganic carbon associated with the oceanic anoxic event that occurred in the Upper Aptian (OAE-1, [Arthur, M. A. and Schlanger, S. O. (1979) AAPG Bull. 63, 870–885]). The 17α(H),21β(H)-C30 hopane is the most depleted in among the compounds and reflects contributions from chemoautotrophs, and in extreme cases from methanotrophs. The covariations between the δ values of pristane and phytane, and these acyclic isoprenoids and β-carotane suggest a common origin from phytoplankton. Gammacerane has δ values compatible with a derivation from bacterivorous ciliates feeding on algae and cyanobacteria growing under variable paleoenvironmental conditions.