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Reconstruction of paleoenvironments using carbon stable isotopes of specific compounds

RECONSTRUCTION OF PALEOENVIRONMENTS USING CARBON STABLE ISOTOPES OF SPECIFIC COMPOUNDS

EUGENIO V. SANTOS NETO

PETROBRAS Research and Development Center Cidade Universitária, Ilha do Fundão, 21949-900 Rio de Janeiro (RJ), Brazil.

Presented by ALCIDES N. SIAL

Representative samples from two contrasting paleoenvironments, e.g., lacustrine and marine-evaporitic, were analyzed using the Isotope Ratio Monitoring-Gas Chromatography-Mass Spectrometry (IRM-GC-MS) technique to assess the carbon isotopic compositions of saturated hydrocarbons. Those compounds were previously separated from the bulk organic extracts using molecular sieving.

Systematic variations in the abundance of 13C in saturated hydrocarbons were used to assess the conditions of the lacustrine (Neocomian) and lagoonal to restricted marine (Aptian) paleoenvironments of the Pendência and Alagamar Formations, respectively, from the Potiguar Basin, Northeast Brazil.

In the lacustrine section, the isotopic compositions of compounds formed in the photic zone, e.g., pristane, -carotane, and some n-alkanes vary only slightly with depth, suggesting consistent conditions at the water surface. However, there are dramatic changes in the isotopic compositions of hopanoids produced by bacteria that inhabited the lower part of the Pendência paleolake. The C values of those hopanoids indicate an effective stratification of the water column and an active recycling of methane.

In the marine-restricted section, C values of 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. Those variations can be correlated with global changes in the isotopic compositions of marine dissolved inorganic carbon associated with the oceanic anoxic event that occurred in the Upper Aptian (OAE-1). The hopanoids are the most depleted in 13C among saturated hydrocarbons and reflect contributions from chemoautotrophs, and in extreme cases from methanotrophs. The covariations between de C of pristane and phytane, and these acyclic isoprenoids and -carotane suggest a common origin from phytoplankton. Gammacerane has C values compatible with a derivation from bacterivorous ciliates feeding on algae and cyanobacteria growing under variable paleoenvironmental conditions.

The objectives of the present study are to identify biogeochemical processes involv ed in the production, reworking, and preservation of the organic matter, and consequently to improve the understanding of the lacustrine and marine-evaporitic sections of the Potiguar Basin, Northeast Brazil. (May 19, 2000)

*E-mail: eugenio@cenpes.petrobras.com.br

Publication Dates

  • Publication in this collection
    05 Jan 2001
  • Date of issue
    Dec 2000
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