Abstract:
Hydrothermal systems played a pivotal role in life’s emergence and early evolution. To better understand this role, examining fundamental interactions between hydrothermal processes, minerals, and microbial life in deep time is paramount. Microbial biosignatures are the only available tool to investigate these interactions directly on the rock record. Chapter 1 of this thesis reviews the Precambrian record of microbial biosignatures in deep-sea hydrothermal sulfides. The literature survey reveals that this record is scarce, and an insufficient understanding of taphonomic processes under sulfidic hydrothermal conditions limits its exploration. Aiming to tackle this problem, we integrate experimental and analytical techniques from mineralogy, geochemistry, and microbiology (e.g., reflected light microscopy, Raman spectroscopy, secondary ion mass spectrometry, and microbial culturing). This geobiological approach, detailed in Chapter 2, contributes to a more robust reconstruction of the deep-time evolution of life in hydrothermal systems.
Chapter 3 reports experiments simulating the sulfidic diagenesis of magnetite nanoparticles under hydrothermal conditions. Magnetite rapidly dissolved across all experimental setups and transformed to pyrite at pH ~7 and 80°C. The results highlight that magnetite sulfidation to pyrite is a critical taphonomic process in these environments. Chapter 4 experimentally demonstrates that the hydrothermal sulfidation of abiogenic and biogenic magnetite yields pyrite with various distinct morphologies, including framboid-like spheroids. Notably, framboid-like pyrite, commonly considered a potential fingerprint of microbial sulfur cycling, was exclusively produced from the hydrothermal sulfidation of biogenic (i.e., organic matter-associated) magnetite. Thus, framboid-like pyrite is also a taphonomic fingerprint of biogenic iron minerals, providing a target for reconstructing the evolution of microbial iron cycling in deep time. Chapter 5 presents textural and in-situ sulfur and iron isotope analyses of euhedral and framboidal pyrite from the ~390 Ma metamorphic Rammelsberg massive sulfide deposit. Combined δ34S, Δ33S, and δ56Fe analyses record evidence of microbial sulfur and iron cycling despite extensive XIalteration
of the precursor sediment by sulfidic and iron-rich fluids and later greenschist metamorphism. Thus, coupled textural and in-situ stable isotope analysis could trace microbial sulfur and iron cycling in some of Earth’s oldest rocks, including greenschist facies hydrothermal sulfides from the Pilbara Craton and Barberton Greenstone Belt.
Chapter 6 synthesizes how the results of this thesis substantially improve our understanding of the taphonomy of microbial biosignatures in hydrothermal sulfides. Combined textural and in-situ sulfur and iron isotope analyses of pyrite emerge as a promising approach for detecting microbial biosignatures throughout the geological record. This, in turn, is fundamental for understanding life’s emergence and evolution in hydrothermal systems and other environments on early Earth.
Early Earth