The information encoded in Neoproterozoic sedimentary sulfide
Dr David Johnston, Harvard University, USA
The sulfur isotopic composition of sedimentary sulfides, especially when paired with similar metrics in the carbon cycle, anchor interpretations of Earth history. These reconstructions are predicated on having a quantitative understanding of how microbial / geochemical / environmental information is transferred to mineral phases like pyrite. Much of this focus has in the past fallen on the microbial metabolisms that both drive redox transformations (largely in marine sediments) and consequently impart much of the observed isotopic effect. Therefore, variance in geological isotope records – for instance through the Ediacaran and across the Shuram excursion - could be and are inverted to tell stories of microbial evolution / innovation and/or Earth surface change. In parallel was an attempt to understand how boundary conditions (like sulfate concentrations) might come to influence the isotope record preserved in marine sediments. The obvious goal is to construct and apply an interpretative framework that accounts for both approaches, while also including the physics of sediment diagenesis. In working toward this goal, we first developed a quantitative context for understanding early diagenetic sulfur isotope signals through reaction-transport modelling. As a case study, we provide results from a geochemically well-characterized system (Aarhus Bay, Denmark). Importantly, a major result of that work was the requirement for large and invariant intrinsic fractionations associated with sulfate reduction (approximating an equilibrium effect of 70‰). These predictions for near equilibrium behavior apply over a range of different depositional environments (e.g., the California borderland basins). Together, these conclusions now stand in opposition to geological storylines calling upon evolving biogeochemistry as the root of changing historical isotope records. This new interpretive context allows for alternative explanations for Neoproterozoic records.
D.T. Johnston1, J. Hemingway1, B.C. Gill2, I. Halevy3
1Harvard University 2Virginia Tech, 3Weitzmann Institute)
The Tonian carbonate factory and the long-term evolution of the Precambrian CaCO3 cycle
Dr Nick Tosca, University of Oxford, UK
Numerous hypotheses have sought to identify what perturbations to the global carbon cycle fueled Earth system change during the Neoproterozoic Era (1000–541 million years ago, Ma). Nevertheless, a lack of constraints on ocean-atmosphere carbon chemistry has precluded mechanistic links between biology, climate, and the lithosphere. Field observations, combined with microanalytical data and new experimental constraints, show that early Neoproterozoic seawater featured elevated alkalinity in the presence of high atmospheric pCO2, which sustained excessive marine CaCO3 supersaturation. These data also show that between ~800–750 Ma, this inorganic carbon reservoir was halved. Without pelagic calcification, marine CaCO3 supersaturation and pCO2 would have been modulated principally by CaCO3 precipitation kinetics; thus, secular changes in kinetic inhibitors to CaCO3 deposition may have destabilized the global carbon cycle, facilitating extreme Neoproterozoic climatic instability.
Effects of sulphur cycle imbalance on the emergence and radiation of complex life
Professor Graham Shields, University College London, UK
The Neoproterozoic Era witnessed a succession of biological innovations that culminated in diverse animal body plans and behaviours during the Ediacaran-Cambrian radiations. Intriguingly, this interval is also marked by perturbations to the global carbon cycle, as evidenced by anomalous fluctuations in climate and carbon isotopes. The Neoproterozoic isotope record has defied parsimonious explanation because sustained 12C-enrichment (low δ13C) in seawater seems to imply that substantially more oxygen was consumed by organic carbon oxidation than could possibly have been available. We propose a solution to this problem, in which carbon and oxygen cycles maintained dynamic equilibrium during negative excursions because surplus oxidant could be generated through bacterial reduction of sulfate that originated from evaporite weathering. Coupling of evaporite dissolution with pyrite burial drove a positive feedback loop whereby net oxidation of marine organic carbon could sustain greenhouse forcing of chemical weathering, nutrient input and ocean margin euxinia. Our proposed framework is particularly applicable to the late Ediacaran ‘Shuram’ isotope excursion that directly preceded the emergence of energetic metazoan metabolisms during the Ediacaran-Cambrian transition. However, in this discussion, we outline how sulfur cycle imbalance might potentially have affected the global carbon cycle at other times, too, leading to climatic, ocean redox and biotic changes on a global scale.
Late Ediacaran evolution of complex life and environment
Dr Shuhai Xiao, Virginia Tech, USA
The development of controlled motility among bilaterian animals is a manifestation of morphological, neural, and behavioral complexity. It is also a transformative innovation that redefined animals in ecological engineering and geobiological interactions (e.g., substrate and agronomic revolutions). It has been hypothesized that heterogenous distribution of food may have been an evolutionary stimulus for the evolution of bilaterian motility. Here I would like to explore the possibility that dynamic and heterogeneous redox conditions in late Ediacaran oceans may have been an alternative or additional driving force leading to the evolution of bilaterian motility.
Trace fossils indicate that the earliest motile bilaterians appeared in late Ediacaran (ca. 560-540 Ma), and ichnofossil diversity increased terminal Ediacaran (ca. 550-540 Ma) when many Ediacara-type organisms went extinct. Geochemical data indicate that the late Ediacaran is characterized by a major expansion of oceanic anoxia, resulting in temporally dynamic and spatially heterogeneous redox conditions in shallow shelves where animals lived. While such an expansion of anoxia may have driven some organisms to extinction, it may have also stimulated others to develop evolutionary innovations.
The observation that Ediacaran ichnofossils are closely associated with cyanobacterial mats offers insights into the possible relationship between animal motility and redox conditions. Because of the dynamic redox conditions in the water column and in the cyanobacterial mats, animals exploring localized oxygen oases must also be able to maneuver in and out these oases. This may have been an environmental stimulus that drove the evolution of motility among early benthic bilaterian animals.