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Microstructure and mineral composition of Holocene stromatolites from Lagoa Vermelha, a hypersaline lagoon in Brazil; insights into laminae genesis

Carolina N. Keim, Helisson Nascimento dos Santos, Carolina Souza Santiago, Simone Pennafirme, Reiner Neumann, Juergen Schnellrath, Inaya Lima, Mirian A. C. Crapez and Marcos Farina
Microstructure and mineral composition of Holocene stromatolites from Lagoa Vermelha, a hypersaline lagoon in Brazil; insights into laminae genesis
Journal of Sedimentary Research (August 2020) 90 (8): 887-905

Abstract

Stromatolites are domes, columns, or nearly flat crusts of laminated sedimentary rocks, usually consisting of Ca-Mg carbonates. Stromatolites result from lithification of microbial mats, which are benthic microbial ecosystems where microorganisms arrange themselves in layers according to their physiology. Despite a century of research, the hypothesis of stromatolite genesis by lithification of microbial mats remains controversial, and a convincing explanation for how stromatolites arise from microbial mats is still lacking. In this work, we analyze in detail a stromatolite from Lagoa Vermelha, a coastal hypersaline lagoon in Rio de Janeiro State, Brazil. The stromatolite presents a laminated core and thrombolitic regions at the periphery. Both thrombolitic and laminated facies consist of fine-grained authigenic minerals with minor contributions of bioclasts and quartz grains. X-ray diffraction shows aragonite, high-magnesium calcite (HMC) containing about 17% MgCO (sub 3) , a very-high-Mg calcite (VHMC) containing 29-46% MgCO (sub 3) , and small amounts of quartz and pyrite. Scanning electron microscopy of polished samples coupled to energy-dispersive X-ray analysis (EDS) showed that each lamina was composed of 1-4 distinct mineral phases embedded within each other, indicating sequential steps of precipitation of Ca-Mg carbonates under distinct biogeochemical conditions. The coexistence of different phases in a single lamina suggests that several processes contribute to mineral deposition as the incipient stromatolite laminae are left behind by microorganisms from the lower layers of the microbial mat when they grow and/or move upwards.


ISSN: 1527-1404
EISSN: 1938-3681
Serial Title: Journal of Sedimentary Research
Serial Volume: 90
Serial Issue: 8
Title: Microstructure and mineral composition of Holocene stromatolites from Lagoa Vermelha, a hypersaline lagoon in Brazil; insights into laminae genesis
Affiliation: Universidade Federal do Rio de Janeiro, Instituto de Microbiologia Paulo de Goes, Rio de Janeiro, Brazil
Pages: 887-905
Published: 20200819
Text Language: English
Publisher: Society for Sedimentary Geology, Tulsa, OK, United States
References: 70
Accession Number: 2020-067494
Categories: Sedimentary petrology
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 2 tables, sketch map
S22°55'58" - S22°55'58", W42°23'35" - W42°23'35"
Secondary Affiliation: Centro de Tecnologia Mineral-CETEM, BRA, BrazilUniversidade Federal Fluminense, BRA, Brazil
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2020, American Geosciences Institute. Reference includes data from GeoScienceWorld, Alexandria, VA, United States. Reference includes data supplied by SEPM (Society for Sedimentary Geology), Tulsa, OK, United States
Update Code: 202019
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