Jan-Peter Duda

Testing the preservation of biomarkers during experimental maturation of an immature kerogen

Lipid biomarkers have been extensively applied for tracing organisms and evolutionary processes through Earths history. They have become especially important for the reconstruction of early life on Earth and, potentially, for the detection of life in the extraterrestrial realm. However, it is not always clear how exactly biomarkers reflect a paleoecosystem as their preservation may be influenced by increasing temperatures (T) and pressures (P) during burial. While a number of biomarker indices reflecting thermal maturity have been established, it is often less well constrained to which extent biomarker ratios used for paleoreconstruction are compromised by T and P processes. In this study we conducted hydrous pyrolysis of Green River Shale (GRS) kerogen in gold capsules for 2–2400 h at 300°C to assess the maturation behaviour of several compounds used as life tracers and for the reconstruction of paleoenvironments ( n -alkanes, pristane, phytane, gammacerane, steranes, hopanes and cheilanthanes). Lignite samples were maturated in parallel with the GRS kerogen to obtain exact vitrinite reflectance data at every sampling point. Our experiment confirms the applicability of biomarker-based indices and ratios as maturity indicators (e.g. total cheilanthanes/hopanes ratio; sterane and hopane isomerization indices). However, several biomarker ratios that are commonly used for paleoreconstructions (e.g. pristane/phytane, pristane/ n -C 17 , phytane/ n -C 18 and total steranes/hopanes) were considerably affected by differences in the thermal degradation behaviour of the respective compounds. Short-term experiments (48 h) performed at 400°C also revealed that biomarkers >C 15 (especially steranes and hopanes) and ‘biological’ chain length preferences for n -alkanes are vanished at a vitrinite reflectance between 1.38 and 1.83% R O . Our data highlight that ‘thermal taphonomy’ effects have to be carefully considered in the interpretation of biomarkers in ancient rocks and, potentially, extraterrestrial materials.

A Rare Glimpse of Paleoarchean Life: Geobiology of an Exceptionally Preserved Microbial Mat Facies from the 3.4 Ga Strelley Pool Formation, Western Australia.

Paleoarchean rocks from the Pilbara Craton of Western Australia provide a variety of clues to the existence of early life on Earth, such as stromatolites, putative microfossils and geochemical signatures of microbial activity. However, some of these features have also been explained by non-biological processes. Further lines of evidence are therefore required to convincingly argue for the presence of microbial life. Here we describe a new type of microbial mat facies from the 3.4 Ga Strelley Pool Formation, which directly overlies well known stromatolitic carbonates from the same formation. This microbial mat facies consists of laminated, very fine-grained black cherts with discontinuous white quartz layers and lenses, and contains small domical stromatolites and wind-blown crescentic ripples. Light- and cathodoluminescence microscopy, Raman spectroscopy, and time of flight—secondary ion mass spectrometry (ToF-SIMS) reveal a spatial association of carbonates, organic material, and highly abundant framboidal pyrite within the black cherts. Nano secondary ion mass spectrometry (NanoSIMS) confirmed the presence of distinct spheroidal carbonate bodies up to several tens of μm that are surrounded by organic material and pyrite. These aggregates are interpreted as biogenic. Comparison with Phanerozoic analogues indicates that the facies represents microbial mats formed in a shallow marine environment. Carbonate precipitation and silicification by hydrothermal fluids occurred during sedimentation and earliest diagenesis. The deciphered environment, as well as the δ13C signature of bulk organic matter (-35.3‰), are in accord with the presence of photoautotrophs. At the same time, highly abundant framboidal pyrite exhibits a sulfur isotopic signature (δ34S = +3.05‰; Δ33S = 0.268‰; and Δ36S = -0.282‰) that is consistent with microbial sulfate reduction. Taken together, our results strongly support a microbial mat origin of the black chert facies, thus providing another line of evidence for life in the 3.4 Ga Strelley Pool Formation.

Depositional dynamics of a bituminous carbonate facies in a tectonically induced intra-platform basin: the Shibantan Member (Dengying Formation, Ediacaran Period)

Abstract The Ediacaran Shibantan Member (Dengying Formation, South China) is characterised by a black, laminated bituminous limestone facies with diverse Ediacara-type organisms and trace fossils. However, still little is known about the evolution of the Shibantan basin and its sedimentary dynamics. A detailed sedimentological characterisation revealed that the Shibantan Member was deposited on a carbonate ramp system linked to an intra-platform basin. Evaporitic dolomites of the Hamajing Member (representing sub- to supratidal inner ramp environments above fair weather wave base) are sharply overlain by black, laminated limestones of the lower Shibantan Member, representing a subtidal lower- to middle ramp environment close to the storm wave base. This facies-shift implies a sudden local deepening event, probably due to a tectonically induced increase in subsidence. The Shibantan basin was subsequently filled as evidenced by the gradual transition into dark wavy dolomites deposited in a subtidal middle-ramp environment (i.e. between storm- and fair weather wave bases) and, eventually, the upper Dengying Formation (i.e. the Baimatuo Member), which represents the same depositional environments as the Hamajing Member. Sedimentation in the Shibantan basin was highly dynamic as evidenced by a distinct slumping horizon and mass-flow deposits, partly possibly due to synsedimentary tectonic processes. A microbial mat associated biota which includes Ediacara-type fossils is restricted to the lower Shibantan Member. The close spatial relationship between allochthonous event deposits and autochthonous fossil associations in this setting implies that the event deposits were not only important for the preservation of fossils, but probably also for the supply of nutrients.

Opening up a window into ecosystems with Ediacara-type organisms: preservation of molecular fossils in the Khatyspyt Lagerstätte (Arctic Siberia)

The Khatyspyt Formation in Arctic Siberia is one of only two carbonate settings with Ediacara-type fossils. As a potential hydrocarbon source rock, it contains abundant molecular fossils that may help to expand our understanding of these ecosystems. Unfortunately, however, the molecular fossil record in geological materials is commonly biased by secondary processes such as thermal maturation, migration of bitumen compounds or surface contamination. In this study, we evaluate the preservation of molecular fossils in a sample from the Khatyspyt Formation and elucidate their paleobiological meaning. Our results reveal that the organic matter is remarkably immature (oil window maturity) and shows little effect of biodegradation. Petrographic observations, exterior/interior experiments, and the similarity between free bitumen, mineral-occluded bitumen, and kerogen pyrolysate point to the syngeneity of the molecular fossils. Abundant hopanes, cyclohexylalkanes, and methyl-branched alkanes indicate a bacterial source of the organic matter, likely including cyanobacteria and anaerobic bacteria. At the same time, a carbonaceous compression fossil on top of the sample and abundant steranes indicate the presence of eukaryotes. The steranes show typical distributions for the Ediacaran (i.e., dominance of stigmastane). Given the exceptional preservation of the body fossils, trace fossils, and molecular fossils, the Khatyspyt Formation can be considered a fossil lagerstätte sensu Seilacher (1970: Begriff und Bedeutung der Fossil-Lagerstätten. Neues Jahrbuch für Geologie und Paläontologie, Monatshefte: 34–39). The combined analysis of sedimentary facies, paleontology (body, trace, and molecular fossils), and biogeochemistry will provide a more complete understanding of ecosystems with Ediacara-type fossils.KurzfassungDie Khatyspyt-Formation im arktischen Sibirien ist eines von lediglich zwei karbonatischen Ablagerungsräumen mit Ediacara-Fossilien. Gleichzeitig ist sie auch ein potentielles Kohlenwasserstoff-Muttergestein. Nicht zuletzt aufgrund der daher reichlich enthaltenen molekularen Fossilien birgt sie das Potential, unser Verständnis dieser Ökosysteme zu verbessern. Das Inventar an in geologischen Materialien erhaltenen molekularen Fossilien ist jedoch häufig durch diverse sekundäre Prozesse, wie z. B. thermische Maturierung, die sekundäre Migration von Kohlenwasserstoffen oder Oberflächen-Kontamination, verfälscht. In dieser Studie evaluieren wir die Erhaltung molekularer Fossilien in einer Probe der Khatyspyt-Formation und diskutieren ihre paläobiologische Bedeutung. Unsere Ergebnisse zeigen, dass das organische Material eine bemerkenswert niedrige Reife aufweist (Ölfenster) und kaum durch Biodegradation beeinflusst wurde. Petrographische Beobachtungen, Exterieur/Interieur-Experimente, und die Gleichheit zwischen freiem Bitumen, mineral-gebundenem Bitumen und Kerogen-Pyrolysat unterstreichen die Syngenität der enthaltenen molekularen Fossilien. Hopane, Cyclohexane, und and methylverzweigte Alkane deuten auf die Anwesenheit von Bakterien, wahrscheinlich inklusive Cyanobakterien und anaeroben Bakterien. Gleichzeitig belegen ein organisch erhaltenes Makrofossil auf der Probenoberseite und häufige Sterane, welche ein für das Ediacarium typisches Verteilungsmuster aufweisen (Dominanz von Stigmastan), die Existenz von Eukaryoten. Aufgrund der außergewöhnlichen Erhaltung von Körper-, Spuren- und molekularen Fossilien kann die Khatyspyt-Formation als Fossillagerstätte sensu Seilacher (1970: Begriff und Bedeutung der Fossil-Lagerstätten. Neues Jahrbuch für Geologie und Paläontologie, Monatshefte: 34–39) interpretiert werden. Die zukünftig gemeinschaftliche Analyse von sedimentärer Fazies, Paläontologie (Körper-, Spuren- und molekulare Fossilien) und Biogeochemie wird ein kompletteres Bild von Ökosystemen mit Ediacara-Fossilien ermöglichen.

Assessing Possibilities and Limitations for Biomarker Analyses on Outcrop Samples: A Case Study on Carbonates of the Shibantan Member (Ediacaran Period, Dengying Formation, South China)

The Shibantan Member (Dengying Formation, South China) represents one of only two carbonate settings with Ediacara-type organisms and offers a rare opportunity to study the biogeochemistry of these ecosystems. To evaluate possibilities and limitations for future biomarker studies on fossil-bearing outcrop samples of the Shibantan Member, we analysed the spatial distribution of hydrocarbons in extractable organic matter (i.e. bitumen) on a millimetre scale. Our study demonstrates that the sample and most likely also other rocks from the same setting are contaminated with petroleum-derived compounds that bear the potential for erroneous interpretations in palaeo-reconstructions. The contamination was revealed by distribution patterns and amounts of extractable n-alkanes and acyclic isoprenoids. The contamination is linked to the external weathering surfaces but also to cracks within the rock, and the extent most likely depends on concentration gradients between these contamination sources. Here we show that contamination can successfully be distinguished from syngenetic signals obtained from non-extractable organic matter (i.e. kerogen) using catalytic hydropyrolysis (HyPy). However, we observed that decalcification is necessary to achieve sufficient yields of kerogen-bound hydrocarbons and to avoid artificial alteration of the biomarker signals due to matrix effects.

The taphonomic fate of isorenieratene in Lower Jurassic shales-controlled by iron?

Fossil derivatives of isorenieratene, an accessory pigment in brown-colored green sulfur bacteria, are often used as tracers for photic zone anoxia through Earths history, but their diagenetic behavior is still incompletely understood. Here, we assess the preservation of isorenieratene derivatives in organic-rich shales (1.5-8.4 wt.% TOC) from two Lower Jurassic anoxic systems (Bächental oil shale, Tyrol, Austria; Posidonia Shale, Baden-Württemberg, Germany). Bitumens and kerogens were investigated using catalytic hydropyrolysis (HyPy), closed-system hydrous pyrolysis (in gold capsules), gas chromatography-mass spectrometry (GC-MS) and gas chromatography combustion isotope ratio-mass spectrometry (GC-C-IRMS). Petrography and biomarkers indicate a syngenetic relationship between bitumens and kerogens. All bitumens contain abundant isorenieratane, diverse complex aromatized isorenieratene derivatives, and a pseudohomologous series of 2,3,6-trimethyl aryl isoprenoids. In contrast, HyPy and mild closed-system hydrous pyrolysis of the kerogens yielded only minor amounts of these compounds. Given the overall low maturity of the organic matter (below oil window), it appears that isorenieratene and its abundant derivatives from the bitumen had not been incorporated into the kerogens. Accordingly, sulfur cross-linking, the key mechanism for sequestration of functionalized lipids into kerogens in anoxic systems, was not effective in the Jurassic environments studied. We explain this by (i) early cyclization/aromatization and (ii) hydrogenation reactions that have prevented effective sulfurization. In addition, (iii) sulfide was locally removed via anoxygenic photosynthesis and efficiently trapped by the reaction with sedimentary iron, as further indicated by elevated iron contents (4.0-8.7 wt.%) and the presence of abundant pyrite aggregates in the rock matrix. Although the combined processes have hampered the kerogen incorporation of isorenieratene and its derivatives, they may have promoted the long-term preservation of these biomarkers in the bitumen fraction via early defunctionalization. This particular taphonomy of aromatic carotenoids has to be considered in studies of anoxic iron-rich environments (e.g., the Proterozoic ocean).

Early life processes: A geo- and astrobiological approach

The search for potential extraterrestrial life has been a hot topic for a long time. In almost the same manner, the origin and early diversification of life on Earth has been of great interest for many people in and outside academia for decades. Fundamental problems related to both issues, as for instance the identification of potential biosignatures and habitable environments, are strikingly similar. This scientific and technological overlap gave rise to the field of astrobiology (e.g. Dick & Strick 2004; Farmer 2011). Most aspects of astrobiology are inherently linked to the field of geobiology (e.g. Reitner & Thiel 2011; Knoll et al. 2012). Examples include the search for biosignatures, the reconstruction of early life processes from the rock record and the investigation of recent analogues for past environments. One of the few differences is that astrobiology also includes the investigation of extraterrestrial materials. In either case, however, research projects are commonly rather disciplinethan problem-specific, and all scientific approaches have their technical and interpretative limitations. Aiming at stimulating discussion across different scientific disciplines, the international symposium ‘Dating the origin of Life: Present-Day Molecules and First Fossil Record’ was held at the Göttingen Academy of Sciences & Humanities (GASH) in October 2014. This symposium was financed by the Deutsche Forschungsgemeinschaft (RE 665/39-1), the GASH, and the Courant Research Centre of Geobiology (Reitner et al. 2014; Fig. 1). This special issue of the International Journal of Astrobiology attempts to reflect the scientific spirit of this meeting.