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AMolecular and Carbon Isotopic Analysis of Specific Biological Markers: Evidence for Previous HitDistinguishingTop between Marine and Lacustrine Depositional Environments in Sedimentary Basins of Brazil

J. M. Guthrie, L. A. F. Trindade, C. B. Eckardt, and T. Takaki

A suite of organic-rich, immature shales (TOCs ranging between 0.7 to 28.4 %) from various basins throughout Brazil were selected for carbon isotopic (^dgr13C) analysis of individual biological compounds in order to characterize marine and lacustrine depositional environments. Carbon isotopic compositions of pristane, phytane, n-heptadecane; 28, 30-bisnorhopane; 17^agr(H), 21ß(H)-hopane, and gammacerane, together with other commonly used biomarker parameters show variations that are related to sources of organic matter and to depositional processes in the paleoenvironment.

Plots relating hopane/sterane ratios to the ^dgr13C compositions of pristane, phytane, n-heptadecane; 28, 30-bisnorhopane, and 17^agr(H), 21ß(H)-hopane show that these compounds in lacustrine shales are consistently more depleted in 13C than in marine shales. For example, the ^dgr13C values of pristane and phytane in lacustrine shales vary from -28 to -32 ^pmil and in marine shales from -22 to -24 ^pmil. The correlation between high abundance of hopanes relative to steranes with depleted 13C compositions in these compounds is supporting evidence for a more abundant input of methanogenic bacteria in these lacustrine shales.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California