Organic matter preservation in the presence of variable mineralogical matrices – clues to the interpretation of Precambrian biosignatures
by Lena Weimann
Date of Examination:2025-04-22
Date of issue:2025-05-09
Advisor:Prof. Dr. Volker Thiel
Referee:Prof. Dr. Volker Thiel
Referee:Prof. Dr. Jan-Peter Duda
Persistent Address:
http://dx.doi.org/10.53846/goediss-11257
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Abstract
English
Hydrothermal environments, such as terrestrial hot spring settings, harbored life on the early Earth. However, the scarcity of Precambrian rocks, particularly hydrothermal deposits, hinders the reconstruction of the biological history of these early Earth systems. To overcome this challenge, it is essential to investigate known ancient hydrothermal deposits and combine these investigations with laboratory experiments simulating conditions to which biosignatures are exposed. Therefore, this thesis focuses on (i) the investigation of carbonaceous matter (CM) within hydrothermally deposited black bedded barite from the approximately 3.5 billion-year-old Dresser Formation in Western Australia, and (ii) the incorporation of molecular organic matter within siliceous hot spring deposits through laboratory experiments. To answer whether CM in the hydrothermally deposited black bedded barite from the Dresser Formation is syngenetic to the host rock and of biogenic origin. Furthermore, to investigate if the presence of silica in a hydrothermal environment impacts the preservation potential of organic molecules. Overall, this thesis seeks to provide insights into the preservation potential of biosignatures in hydrothermal environments and discuss the implications for biosignature preservation in Precambrian rocks. The first study (chapter 2) investigates the presence of CM in ~ 3.5 Ga hydrothermal black bedded barites from the Dresser Formation (Pilbara Craton, Western Australia). Raman spectroscopy mapping revealed three distinct populations of CM: (i) CM at single growth bands of barite crystals (most frequent), (ii) CM within the barite matrix, and (iii) CM in 50–300 µm wide secondary quartz veins that cross-cut the black bedded barite. Calculating maximum metamorphic temperatures based on the Raman spectra showed that CM in secondary quartz veins was introduced after the main metamorphic event, which occurred ~ 3.3 Ga ago. Further analysis using near edge X-ray absorption fine structure (NEXAFS) and solid-state nuclear magnetic resonance (NMR) analysis revealed a high aromatic nature of CM (~ 65 %), together with an aliphatic component (~ 35 %). Stable carbon isotope (δ13C) measurements using secondary ion mass spectrometry (SIMS) showed a wide range of depleted values (− 33.4 ‰ to − 16.5 ‰), consistent with a biogenic origin of CM. In conclusion the CM within the barite is syngenetic to the host rock and formed at least partly by microorganisms using multiple metabolic pathways. Different pathways for the input of CM into the barite are discussed, including the cycling of biological organic material within the hydrothermal system. The second study (chapter 3) is a re-evaluation of the Raman spectroscopy data from the first study using an alternative fitting method. This showed that the CM in the secondary quartz veins had been exposed to peak metamorphic temperatures of approximately 260 °C. This confirms that the quartz vein-associated CM is a secondary addition that entered the system after the peak metamorphic event, making it at least 200 million years younger than the barite-hosted CM. The third study (chapter 4) examines the effect of laboratory heating on archaeal lipids (archaeol and isoprenoid glycerol dialkyl glycerol tetraethers (isoGDGTs)) in the presence and absence of silica. The experiments involved heating single compounds as well as a total lipid extract (TLE) from a microbial mat at 250 °C and 300 bar for 14 days in closed gold capsules. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) were used to examine the mineral phase before and after heating. This showed that heating with single organic compounds led to the formation of small silica microspheres (~ 10 µm), whereas heating with the TLE resulted in irregularly shaped silica grains. Gas chromatography-mass spectrometry (GC–MS) analysis revealed an enhanced decomposition of archaeol in the presence of silica, while isoGDGTs were unaffected by it. However, other organic compounds within the microbial mat TLE seemed to exert a buffering or shielding mechanism on isoGDGTs, increasing their preservation potential. In contrast, archaeol was not affected in the same way. These heating experiments showed that the preservation of archaeal lipids depends on (i) the silica matrix, (ii) their molecular structure, and (iii) the presence of other organic compounds. The fourth study (chapter 5) examines the molecular-level changes and transformations for selected 13C-depleted isoprenoid hydrocarbons and n-tricosenes within the microbial mat TLE during heating in the presence and absence of silica. GC–MS analysis revealed that heating caused n-tricos-10(Z)-ene to partially transform into n-tricos-10(E)-ene and n-tricosane, while unsaturated pentamethylicosane (PMI∆) was partially hydrogenated to pentamethylicosane (PMI). Additionally, a small portion of acyclic isoprenoids containing functional groups or double bonds (such as phytanol, phytene, PMI∆) were transformed into isoprenoid thiophenes. Furthermore, a tetracyclic isoprenoid hydrocarbon with a highly negative δ13C value was identified in the original hydrocarbon fraction of the TLE. The results show that silica enhanced the degradation of the described compounds with isoprenoid parts, likely due to hydroxyl radicals. In summary (chapter 6), the individual chapter findings provide insight into the preservation potential of biosignature in hydrothermal environments. Hydrothermal deposits from the Archean can preserve primary biosignatures, but it is essential to consider the possibility of secondary introduction of organic material. However, the presence of a siliceous matrix can enhance the decomposition of certain organic molecular compounds, such as archaeol, PMI, and isoprenoid thiophenes. To gain a comprehensive understanding of biosignature preservation, combining results from ancient and modern samples with laboratory simulations is essential. Such a multidisciplinary approach provides valuable insights into the overall significance and interpretation of biosignatures in Precambrian rocks
Keywords: Precambrian; Organic matter; Early life; Biomarkers; Silica; Barite
