Summary
Highlights
Maurice Black's expedition to the Bahamas in 1930 led to the discovery of modern stromatolites composed of carbonate sediments and organic mats. The mats, now known to be formed by cyanobacteria (formerly blue-green algae), facilitate sediment accumulation and mineral precipitation. Cyanobacteria perform aerobic photosynthesis, consuming carbon dioxide and producing oxygen, which played a crucial role in the Great Oxidation Event, making Earth's atmosphere oxidizing.
The speaker introduces stromatolites as layered rocks created by microbes interacting with their environment, highlighting their abundance in the Precambrian era. He shares a personal anecdote about discovering stromatolites on the Isle of Islay and discusses a stromatolite boulder he found, to be showcased later. The presentation then moves to examples of extensive stromatolite horizons in Precambrian carbonate successions in East Greenland and Svalbard.
The discussion shifts to the meaning of stromatolite shapes and the history of their classification. The term 'stromatolite' was first used in 1908, meaning 'layered rock.' Early Russian scientists, followed by others in various countries, developed a binomial nomenclature for stromatolites, leading to an intricate taxonomy. However, this classification was not widely accepted in the West due to reproducibility issues and its failure to reflect evolutionary trends or environmental influences.
Despite a 'golden period' of research in the mid-70s, stromatolite formation and shape control remain largely unexplained. The speaker recounts a workshop in 1986 that aimed to address this mystery, leading to an international geological correlation program. He then delves into the early history of stromatolite study, starting with 19th-century observations by Steel and Hall, and Charles Walcott's comparison of ancient stromatolites to modern lake structures in Mexico and Montana.
A significant breakthrough occurred in 1965 with the identification of fossil microbial cells in the Gunflint Chert, confirming the microbial origin of stromatolites. The speaker explains the complex interaction between microbes, sediment trapping, and mineral precipitation in stromatolite formation. He introduces thrombolites, a type of microbial rock emerging late in the Precambrian, characterized by clotted fabric rather than lamination, and the broader concept of 'microbialites' encompassing both stromatolites and thrombolites.
The talk discusses the dramatic reduction in stromatolite abundance after the Cambrian period, attributed to the evolution of grazing animals. This led to the misconception, or 'meme,' that stromatolites were exclusively intertidal. The speaker challenges this view, citing Phil Playford's 1967 paper showing stromatolites forming deep-water reefs in the Devonian of Australia. Further discoveries by Paul Hoffman and others in subtidal environments further disproved the intertidal meme, revealing that stromatolites can thrive in diverse settings, including deep-water and freshwater, especially where grazers are absent.
The speaker revisits his Scottish case study on Islay, showing how environmental changes, such as tidal currents, influenced stromatolite morphology. He contrasts his findings with prevailing models of sedimentation, introducing the 'tidal flat and island model' as a better fit for complex coastlines. He also corrects a past misinterpretation of 'anhydrite nodules' in Scottish rocks, acknowledging the influence of Roy McGregor and the Greenlands formations. He then delves into the geochemistry of Precambrian stromatolites, focusing on carbon isotope composition and using techniques like cathode luminescence to distinguish between detrital grains and precipitated layers.
The speaker shares an anecdote about his collaboration with Andy Knoll, a renowned paleobiologist. He highlights Knoll's work examining black chert samples from Svalbard, which revealed exquisitely preserved filamentous and coccoid microfossils. The Dracone Formation in Svalbard, with its diverse sub-environments, offers stunning examples of preserved microbial life, including columnar stromatolites, in the intertidal and subtidal zones. He also discusses other microbial fossils found in Svalbard, such as polybusterus bipartitum, a tidal flat organism, and diverse plankton in subtidal mudstones, emphasizing the rarity of such preservation in Precambrian rocks.
The speaker discusses conical stromatolites, or 'konophytons,' comparing them to structures formed by gas-lifted mats in modern Antarctic lakes, although noting key differences. He describes his visit to Mauritania, where he observed large domal and conical stromatolites. He highlights a particularly bizarre conical stromatolite called 'jacobphyton,' characterized by petal-like concentric branches, whose formation is linked to changing sea levels and turbulent conditions.
The speaker addresses the difficulty in explaining stromatolite shapes, mentioning mathematical growth models and the challenge of discerning biogenic from abiotic structures in very ancient rocks, such as the 3.4-billion-year-old stromatolites from Western Australia. He then discusses 'manganese nodules' as a contemporary example of inorganic structures that some scientists consider 'bacterial stromatolites.' He concludes by advocating for laboratory studies of cultured microbial mats as a promising avenue for future research. Finally, he presents the 'sawn rock' from earlier in the talk, emphasizing the intricate and enigmatic nature of stromatolites, and suggests two books related to stromatolites.