Reading the Siberian record: Unravelling biological and environmental dynamics in the Ediacaran Khatyspyt Lagerstätte
Mineralogy, Petrology and Geochemistry
Final Report Abstract
The Ediacaran (635–541 Ma) records the global occurrence of macroscopic complex organisms (Ediacara-type fossils, carbonaceous compression fossils) prior to the celebrated "Cambrian Explosion" of metazoan life. However, still little is known about the ecosystems in which these organisms thrived. The Khatyspyt Formation (c. 544 Ma, Arctic Siberia, Russia) is one of only two carbonate settings with Ediacara-type fossils and abundant carbonaceous compression fossils. Rocks of the formation furthermore contain well preserved molecular fossils (i.e., “organic biomarkers”), allowing for integrative studies on biological communities and environmental conditions. This project focused on exploring the fundamental geobiology of the Khatyspyt Formation through the combined analysis of sedimentary facies, paleontological evidence and (kerogen-bound) molecular fossils. In the Khatyspyt Formation, fossils of macroscopic complex organisms occur in black and commonly laminated limestones. The limestones are bituminous, with total organic carbon (TOC) contents of 0.15–2.22 wt.%. Laminated architectures of the sedimentary facies are due to the presence of alternating event beds and microbial mat layers plus diagenetic pressure solution rims. Detailed analysis of sedimentary features revealed that the sediments were deposited in subtidal environments, close to storm wave base. This is similar to the Ediacaran Shibantan Member (<551 Ma, South China) – the second carbonate setting with Ediacara-type fossils. The organic matter is thermally immature (oil window maturity: Rock-Eval Tmax = 428–445°C, free bitumen C31 hopane αβ 22S/(S+R) = 0.55–0.62, free bitumen C29 sterane ααα 20S/(S+R) = 0.47–0.52) and shows little effect of biodegradation. Petrographic observations, exterior/interior experiments, and the internal consistency of independent maturity proxies point to the syngeneity of contained lipid biomarkers. This is further confirmed by the similarity between rock extracts (free bitumen, mineral-occluded bitumen) and kerogen pyrolysates (via catalytic hydropyrolysis, HyPy). The organic inventory of the Khatyspyt Formation thus provides a robust archive that complements the sedimentary and paleontological records. All analyzed samples display certain biomarker features that are characteristic of Ediacaran rocks. Abundant hopanes, cyclohexylalkanes, and methyl-branched alkanes indicate bacterial contributions, likely including cyanobacteria and anaerobic bacteria. Steranes support the presence of eukaryotes and show typical distributions for the Ediacaran (i.e., dominance of stigmastane), probably reflecting high inputs of chlorophyte algae. Potential traces of 24- isopropylcholestane (24-ipc) and 26-methylstigmastane (26-mes) indicate that demosponges were possibly present but not widespread in the Khatyspyt environments. Some samples additionally contain a putative 2-methylgammacerane. Hopane/sterane ratios are generally ≥2.73, supporting previous findings that Ediacara-type organisms flourished in bacterially dominated environments. There are also profound differences in the biomarker assemblages related to different environmental facies. For instance, varying abundances of gammacerane indicate changing degrees of stratification. Relative abundances of C35 homohopanes and regular sterane isomers (αββ R+S vs. ααα S+R) point to fluctuations in salinity and oxygenation. It therefore appears that organisms in the Khatyspyt Formation were confronted with dynamically changing environmental conditions – very similar to environments of the Shibantan Member in South China. In summary, the Khatyspyt ecosystem was bacterially dominated. Detailed examination of the facies revealed an intricate relationship between microbial mat growth and event deposition. Organisms in Khatyspyt environments apparently faced dynamically changing environmental conditions (stratification, salinity, oxygenation). These varying and unstable conditions may have provoked evolutionary responses that were critical for the further diversification of complex macroscopic life on Earth.
Publications
- (2019): On mysterious creatures and their enigmatic habitats – exploring carbonate ecosystems with Ediacara-type organisms. Oral presentation, 90th Annual Meeting of the Paläontologische Gesellschaft (Paleontological Society), 15 –18 September 2019, Munich, Germany. ISBN 978-3-946705-07-9
Duda, J.-P., Love, G.D., Reitner, J., Zhu, M., Grazhdankin, D.V.
- (2019): Reading the Siberian record: Unravelling the geobiology of the Ediacaran Khatyspyt Lagerstätte (Arctic Siberia, Russia). 29th International Meeting on Organic Geochemistry (IMOG), 1–6 September 2019, Gothenburg, Sweden
Duda, J.-P., Rogov, V.I., Melnik, D.S., Love, G.D., Blumenberg, M., Grazhdankin, D.V.
(See online at https://doi.org/10.3997/2214-4609.201903056)
