Biotic vs. Abiotic Origin of Brazilian Presalt “Microbialites”: A Test for Geochemistry?

Abstract

Microbialites are often difficult to identify in the fossil record due to pervasive early diagenesis that can erase distinct diagnostic criteria (e.g. bacterial filaments and microtubules). While these visual cues may be destroyed, geochemical signature of biological processes may remain. Two geochemical proxies are widely regarded as suggestive of a biotic origin. Using isotopic geochemistry, when d18C and d18O covary along a linear trend away from “primary” values, but with a steeper slope than found during burial diagenesis, it suggests increasing kinetic fractionation by biological processes. Bulk stable isotope data show no clear co-varying trend suggesting either that there is none or that bulk sampling mixed end-member components. Elemental ratios from biotic calcite (high and low magnesium) show a similar kinetically induced enrichment in Sr/Ca relative to abiotic calcite cements (high and low magnesium). Retention of “primary” d18O in bulk stable isotope data suggest that early diagenesis occurred? in a closed system and that “primary” elemental ratios (Sr/Ca and Mg/Ca) could also be preserved. Elemental data are being collected on micro-drilled samples from conventional core of Presalt lacustrine microbialites in the BM-S-22 Guarani-1ST (3-ESSO-004-SPS) well of the Santos Basin, Brazil. These analyses will supplement previous stable isotope data and ascertain to what extent elemental signatures of biotic carbonate exist. The results of this work will be combined with other petrographic studies to provide additional tools for establishing role of microbes, if any, in the development of the Presalt lacustrine reservoirs than by visual inspection alone.

AAPG Datapages/Search and Discovery Article #90189 © 2014 AAPG Annual Convention and Exhibition, Houston, Texas, USA, April 6–9, 2014