Using Organic and Inorganic Studies to Understand Microbial Carbonate Laminae Formation in the Modern Environments of Lagoa Salgada, Rio de Janeiro, SE Brazil
Bahniuk, Anelize1; Matsuda, Nilo; Anjos, Sylvia C.; Vasconcelos, Crisogono; and McKenzie, Judith
Sedimentological and biogeochemical studies of modern stromatolites are of key importance for interpreting past metabolisms and paleoenvironment conditions of ancient counterparts. Here modern stromatolites from Lagoa Salgada (Brazil) have been investigated using "clumped" isotope methodology and associated biomarkers to delineate paleo-environmental conditions. In addition, radiocarbon (14C) dating was performed in specific stromatolite to determinate the growth ration. By combing these data with macroscopic and microscopic sedimentological analyses, it was possible to define distinctive units of stromatolitic laminae formation. The lower unit began about 2300 years ago in a water body open to the ocean with a strong terrestrial influence, as indicated by distinctive biomarkers trapped as intracrystalline organic matter. Cementation under marine conditions was the dominant process stabilizing dolomite and quartz grains. Between 1980 and 1130 yr. ago, the stromatolites recorded a transitional unit wherein the water had an increased meteoric signal but δ13C values became strongly enriched (+11 to +17 ‰ PDB). The upper unit, dated between 592 and 200 yr. ago, represents a fully microbial-influenced system. The mineralogy of this system is characterized by an association of Mg-calcite, Ca-dolomite and authigenic clays forming microbialite laminae with a positive δ13C value indicating meteoric influence. The carbonate precipitation temperatures based in clumped isotopes measurements range between 23 and 37o C, with values that systematically increase towards the top of each unit, suggesting the lagoon restriction. The average growth rate obtained of the Lagoa Salgada stromatolite is 1 cm/100 yr. This rate is lower than some microbialites described in the literature and it may be related to the precipitation mechanisms, amalgamation versus trapping and binding.
AAPG Search and Discovery Article #90166©2013 AAPG International Conference & Exhibition, Cartagena, Colombia, 8-11 September 2013