--> Mechanisms and Products of Mineralization in Modern and Recent Microbialites From Great Salt Lake – Implications for the Fossil Record

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Mechanisms and Products of Mineralization in Modern and Recent Microbialites From Great Salt Lake – Implications for the Fossil Record

R. Bourillot, A. Pace, A. Bouton, E. Vennin, C. Dupraz, S. Galaup, I. Bundeleva, P. Patrier, C. Thomazo, P. Sansjofre, Y. Yokoyama, M. Franceschi, Y. Anguy, L. Pigot, A. Virgone and P. Visscher

Bordeaux-INP, Ensegid, Georessources & Environnement, Pessac, France
Department of Geological Sciences, University of Stockholm, Stockholm, Sweden
Université de Bourgogne, Biogéosciences, Dijon, France
Université de Poitiers, Hydrasa, Poitiers, France
Université de Bretagne Occidentale, Laboratoire Domaines Océaniques, Brest, France
Department of Earth and Planetary Sciences, Atmosphere and Ocean Research Institute, Tokyo, Japan
ENSAM, I2M, Talence, France
Total, Pau, France
University of Connecticut, Center for Integrative Geosciences, Groton, Connecticut, United States

September 7, 2016 Wednesday 2:00 P.M. AAPG/SEG International Conference & Exhibition, Cancun, Mexico

Abstract

Abstract

Microbialites are organosedimentary deposits resulting from the mineralization of benthic microbial mats. Since ca. 3.4 Ga, microbialites are a major component of the sedimentary record. The cycling of elements (C, N, O, P, etc.) operated by microbial mats is supposed to have a played a key role on the evolution of the hydrosphere and atmosphere. Modern and past hypersaline environments (e.g. lakes, salina, lagoons) are generally rich in microbialites. The authigenic minerals produced in microbial mats provide a unique sedimentary archive of the chemical and biological changes in these environments. Several works have shown that the lithification of modern hypersaline mats depends on three factors: (1) the physicochemistry of the environment (e.g. pH; light; temperature); (2) microbial metabolisms (e.g. photosynthesis, sulfate reduction) and (3) the properties of extracellular organic matrices (EOM). However, the relative influence of those three factors varies greatly between sites of study.

Great Salt Lake (GSL) is a vast (ca. 4500 km2), shallow (max depth <10 m) and hypersaline lake (50-285 g.l-1) which develops in a N-S graben. Its margins are covered by mobile ooid and pellet sands surrounding microbialites, themselves covered by soft microbial mats. This work investigates the transition from microbial mats to microbialites, in order to identify the processes and products of mineralization in these structures. Microbial metabolisms were characterized by measuring dissolved oxygen and sulfate reduction, while the nature and composition of minerals was determined by XRD and micro-FTIR. Then, the spatial relationships between organic matrices, bacteria and minerals were observed with high-resolution microscopic techniques (CLSM; cryo-SEM; environmental SEM).

Our results show that GSL microbialites mineralize in three steps. The mineralization develops along the sharp vertical chemical gradient produced by microbial activity. An initial poorly crystallized Mg-Si phase nucleates on the EOM. Then, aragonite precipitates probably in association with EOM alteration. Dolomite finally precipitates, possibly on highly degraded organic matrices. The interpretation of this paragenesis will help in the interpretation of microbial processes in ancient, hypersaline lakes.

AAPG Datapages/Search and Discovery Article #90260 © 2016 AAPG/SEG International Conference & Exhibition, Cancun, Mexico, September 6-9, 2016