Diversity and origin of carbonates in a volcanic-CO2 boosted continental setting:
the modern Laguna Pastos Grandes (Bolivian Altiplano)

Defended on the 4th December 2020

Funding: Total

Supervisor: Emmanuelle Vennin ; cosupervisor: Christophe Durlet

Started in February 2017

Abstract

The production of sedimentary carbonates in continental settings frequently results from the recycling (e.g. dissolution) of ancient carbonates, available at the surface or much deeper, releasing chemical elements necessary for carbonate precipitation (e.g. Ca⁺⁺, Mg⁺⁺, HCO₃^–), with the incorporation of atmospheric carbon. Even in the absence of ancient carbonates, continental carbonates may be produced through the superficial to deep alteration of magmatic or mantellic rocks. This type of carbonate production predominated during the Precambrian, when sedimentary carbonate rocks were less available, or during later periods with abundant production and alteration of oceanic crust. Today, sedimentary carbonates are rarely produced in large quantities in purely volcanic environments. The volcanic caldera of Pastos Grandes (Bolivia; 4450 m above sea level) is a notable exception. Here, although no ancient carbonates are present in the watershed or, presumably, in the substratum, diversified carbonate deposits are produced in an extensive palustrine-to-lacustrine system (120 km²) within the caldera, from calcium and carbon supplied by the hydrothermal springs. However, the diversity of these carbonate deposits has not been exhaustively documented in the few existing studies, and the sources of the chemical elements have not yet been characterized. To achieve these goals, a pluridisciplinary approach was developed, with: (i) the cartography and description of Holocene to present-day deposits; (ii) the hydrochemical analysis of parent fluids; (iii) mineralogical, geochemical, and elemental analyses of carbonates (and clays). Results show that Pastos Grandes carbonates contain calcium released by the alteration of underlying volcanic rocks, and carbon from the rise of magmatic and mantellic CO₂, which stimulate carbonate production. The diversified calcitic macrofacies, with microbialites, concretions, and muds, result from different palustrine-to-lacustrine sedimentary environments, influenced by three hydrochemical poles: (i) hydrothermal springs; (ii) ephemeral and perennial pools fed by waters of hydrothermal and meteoric origin; (iii) the ephemeral central lake fed by meteoric waters, where evaporites dominate. The δ¹³C and δ¹⁸O values from these carbonates show great variation, distributed over a broad range, not documented in other modern continental systems. This morphological and isotopic diversity is explained by several factors: (1) the initial chemical and isotopic composition of the three hydrochemical environments; (2) a high-altitude, arid climate, subject to wind, evaporation, and temperature variation; (3) diversified microbial mats, which are more or less mineralized and affected by their environment. The temporal evolution of these processes is also broached with the study of Holocene carbonates. During wetter climatic episodes, also highlighted elsewhere in the Altiplano, carbonate sedimentation occurs in a lacustrine setting, under a shallow water depth, punctuated by hydrothermal vents. The lower δ¹⁸O values in Holocene and present-day carbonates from Pastos Grandes are interpreted as the result of a low water residence time, induced by underground leakage.

Keywords

continental carbonates, CO2, hydrology, geochemistry, Bolivian Altiplano

jury

Anneleen Foubert (université de Fribourg, Suisse), reviewer
Daniel Aritzegui (université de Genève, Suisse), reviewer
Eric Verrechia (université de Lausanne, Suisse), examiner
Emmanuelle Vennin (université Bourgogne Franche-Comté),
supervisor
Christophe Durlet (université Bourgogne Franche-Comté),
cosupervisor
Eric Gaucher (Total SA, CSTJF, Pau), invited
Aurélien Virgone (Total SA, CSTJF, Pau), invited

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kc_raw_content:

Diversity and origin of carbonates in a volcanic-CO2 boosted continental setting:
the modern Laguna Pastos Grandes (Bolivian Altiplano)

 

Defended on the 4th December 2020

Funding: Total

Supervisor: Emmanuelle Vennin ; cosupervisor: Christophe Durlet

Started in February 2017

 

Abstract

The production of sedimentary carbonates in continental settings frequently results from the recycling (e.g. dissolution) of ancient carbonates, available at the surface or much deeper, releasing chemical elements necessary for carbonate precipitation (e.g. Ca⁺⁺, Mg⁺⁺, HCO₃^-), with the incorporation of atmospheric carbon. Even in the absence of ancient carbonates, continental carbonates may be produced through the superficial to deep alteration of magmatic or mantellic rocks. This type of carbonate production predominated during the Precambrian, when sedimentary carbonate rocks were less available, or during later periods with abundant production and alteration of oceanic crust. Today, sedimentary carbonates are rarely produced in large quantities in purely volcanic environments. The volcanic caldera of Pastos Grandes (Bolivia; 4450 m above sea level) is a notable exception. Here, although no ancient carbonates are present in the watershed or, presumably, in the substratum, diversified carbonate deposits are produced in an extensive palustrine-to-lacustrine system (120 km²) within the caldera, from calcium and carbon supplied by the hydrothermal springs. However, the diversity of these carbonate deposits has not been exhaustively documented in the few existing studies, and the sources of the chemical elements have not yet been characterized. To achieve these goals, a pluridisciplinary approach was developed, with: (i) the cartography and description of Holocene to present-day deposits; (ii) the hydrochemical analysis of parent fluids; (iii) mineralogical, geochemical, and elemental analyses of carbonates (and clays). Results show that Pastos Grandes carbonates contain calcium released by the alteration of underlying volcanic rocks, and carbon from the rise of magmatic and mantellic CO₂, which stimulate carbonate production. The diversified calcitic macrofacies, with microbialites, concretions, and muds, result from different palustrine-to-lacustrine sedimentary environments, influenced by three hydrochemical poles: (i) hydrothermal springs; (ii) ephemeral and perennial pools fed by waters of hydrothermal and meteoric origin; (iii) the ephemeral central lake fed by meteoric waters, where evaporites dominate. The δ¹³C and δ¹⁸O values from these carbonates show great variation, distributed over a broad range, not documented in other modern continental systems. This morphological and isotopic diversity is explained by several factors: (1) the initial chemical and isotopic composition of the three hydrochemical environments; (2) a high-altitude, arid climate, subject to wind, evaporation, and temperature variation; (3) diversified microbial mats, which are more or less mineralized and affected by their environment. The temporal evolution of these processes is also broached with the study of Holocene carbonates. During wetter climatic episodes, also highlighted elsewhere in the Altiplano, carbonate sedimentation occurs in a lacustrine setting, under a shallow water depth, punctuated by hydrothermal vents. The lower δ¹⁸O values in Holocene and present-day carbonates from Pastos Grandes are interpreted as the result of a low water residence time, induced by underground leakage.

 

Keywords

continental carbonates, CO2, hydrology, geochemistry, Bolivian Altiplano

 

jury

Anneleen Foubert (université de Fribourg, Suisse), reviewer
Daniel Aritzegui (université de Genève, Suisse), reviewer
Eric Verrechia (université de Lausanne, Suisse), examiner
Emmanuelle Vennin (université Bourgogne Franche-Comté),
supervisor
Christophe Durlet (université Bourgogne Franche-Comté),
cosupervisor
Eric Gaucher (Total SA, CSTJF, Pau), invited
Aurélien Virgone (Total SA, CSTJF, Pau), invited

extrait:

lien_externe:

titre:

Diversité et origine des carbonates dans un système continental boosté par du CO2 volcanique : l’exemple moderne de la lagune de Pastos Grandes (Altiplano bolivien)

date_de_debut:

février 2017

titre_these:

Fonctionnement sédimentologique et diagénétique d'un couplage carbonates-silicates-matière organique sous influence volcanique et mantellique en contexte continental. Le cas de la lagune de Pastos Grandes (Bolivie)

date_de_debut_these:

février 2017

nom:

Bougeault

date_de_debut_these_numerique:

201702