Geochemical Evidence of Biotic Influence in Brazilian Pre-Salt Carbonates
The Cretaceous pre-salt lacustrine carbonates in the Santos Basin, offshore Brazil, are as geologically enigmatic as they are economically valuable. Much debate has centered lately on the extent to which microbes may play a role in their genesis. In marine carbonates, biotic and abiotic calcites can be distinguished based on their isotopic and elemental compositions. In this study, we applied these geochemical tests to thin sections and bulk core samples from well BM-S-22 Guarani-1ST (3-ESSO-004-SPS). Modern calcite-producing taxa often show positive linear covariance in carbon and oxygen isotope compositions. These trends are anchored on the most positive end at abiotic carbonate compositions and extend to more negative values in carbonates precipitated faster. The Guarani samples analyzed in this study show less variation in carbon and oxygen isotope values than many modern calcifiers (like echinoderms, corals, algae), though the range is somewhat consistent with others (like foraminifera). Modern calcite-producing taxa show enrichment in Sr over modern calcite cements as shown on a plot of Mg/Ca vs Sr/Ca. The Guarani samples show this same linear covariance between Mg/Ca and Sr/Ca as well as Sr enrichment, suggesting a biotic influence on their precipitation. These isotopic and elemental data along with scanning electron microscope images showing fossilized microbes seem to be undeniable evidence that biological processes played at least some role in the origin of Brazilian pre-salt lacustrine carbonates.
Hasiuk, F. & Kaczmarek, S.E. (2015, June). Geochemical Evidence of Biotic Influence in Brazilian Pre-Salt Carbonates. Paper presented at the American Association of Petroleum Geologists Annual Convention and Exhibition (May 31 – June 3, 2015), Denver, CO.
Virtual Commons Citation
Hasiuk, Franciszek and Kaczmarek, Stephen E. (2015). Geochemical Evidence of Biotic Influence in Brazilian Pre-Salt Carbonates. In Geological Sciences Faculty Publications. Paper 14.
Available at: http://vc.bridgew.edu/geology_fac/14