Guy M. Narbonne

Ferruginous Conditions Dominated Later Neoproterozoic Deep-Water Chemistry

Earths surface chemical environment has evolved from an early anoxic condition to the oxic state we have today. Transitional between an earlier Proterozoic world with widespread deep-water anoxia and a Phanerozoic world with large oxygen-utilizing animals, the Neoproterozoic Era [1000 to 542 million years ago (Ma)] plays a key role in this history. The details of Neoproterozoic Earth surface oxygenation, however, remain unclear. We report that through much of the later Neoproterozoic (<742 ± 6 Ma), anoxia remained widespread beneath the mixed layer of the oceans; deeper water masses were sometimes sulfidic but were mainly Fe2+-enriched. These ferruginous conditions marked a return to ocean chemistry not seen for more than one billion years of Earth history.

Life after snowball: The oldest complex Ediacaran fossils

Newly discovered fronds of the Ediacaran index fossil Charnia from the Drook Formation of southeastern Newfoundland are the oldest large, architecturally complex fossils known anywhere. Two species are present: Charnia masoni , originally described from Charnwood Forest in central England and now known worldwide, may have ranged through as much as 30 m.y. of Ediacaran time, and C. wardi sp. nov., a new species of Charnia that consists of slender fronds to nearly 2 m in length, is the longest Ediacaran fossil yet described anywhere. These fossils, which are present midway between the glacial diamictites of the Gaskiers Formation (ca. 595 Ma) and the classic Ediacaran assemblage of the Mistaken Point Formation (565 ± 3 Ma) 1500 m higher in the same section, provide our first glimpse of complex megascopic life after the meltdown of the “snowball Earth” glaciers.

The Ediacaran Period: A New Addition to the Geologic Time Scale

The International Union of Geological Sciences has approved a new addition to the geologic time scale: the Ediacaran Period. The Ediacaran is the first Proterozoic period to be recognized on the basis of chronostratigraphic criteria and the first internationally ratified, chronostratigraphically defined period of any age to be introduced in more than a century. In accordance with procedures established by the International Commission on Stratigraphy, the base of the Ediacaran Period is defined by a Global Stratotype Section and Point (GSSP) placed at the base of the Nuccaleena Formation cap carbonate directly above glacial diamictites and associated facies at Enorama Creek in the Flinders Ranges of South Australia. Its top is defined by the initial GSSP of the Cambrian Period. The new Ediacaran Period encompasses a distinctive interval of Earth history that is bounded both above and below by equally distinctive intervals. Both chemostratigraphic and biostratigraphic data indicate that the subdivision of the period into two or more series is feasible, and this should be a primary objective of continuing work by the Ediacaran Subcommission of the ICS.

Ediacaran remains from intertillite beds in northwestern Canada

A turbidite sequence within the Twitya Formation (Windermere Supergroup) in the ackenzie Mountains, northwestern Canada, has yielded an assemblage composed of simple centimetric annuli and discs, which are interpreted as biogenic and referred to Nimbia occlusa Fedonkin, Vendetta ? sp., and Irridinitus ? sp. They occur below a glaciomarine diamictite about 2.5 km below the base of the Cambrian, and they may represent the oldest Ediacaran fossils now known. The find has implications for models that relate the evolution of the Ediacara fauna to the waning of the Varanger glaciation.

Neoproterozoic of the Mackenzie Mountains, northwestern Canada

Neoproterozoic strata of the Mackenzie Mountains are more than 10 km thick, and comprise two supergroups of differing style. The Mackenzie Mountains Supergroup (4–6 km thick), an epicratonic succession of mainly shallow-water siliciclastic and carbonate strata, contains detrital zircons dated at 1080 Ma and is cut by dykes and plugs dated at 780-778 Ma. The overlying Windermere Supergroup (5–7 km thick) comprises basal rift deposits and glacial diamictites that are overlain by three, kilometre-scale, siliciclastic to carbonate “Grand Cycles” deposited on a passive margin. “Grand Cycles” in the eastern Mackenzie Mountains were deposited in a shallow-water setting, whereas contemporaneous cycles in the western Mackenzies were deposited on a continental slope; mapping of successive shelf-margin positions reveals southwestward progradation. The Windermere Supergroup is unconformably overlain by the Ingta Formation, which contains the Proterozoic-Cambrian boundary. Subtrilobite Cambrian strata are approximately 1.5 km thick, and are overlain by trilobite-bearing Lower Cambrian carbonates of the Sekwi Formation. Fossils are abundant and biostratigraphically useful in these Neoproterozoic strata. Acritarchs, carbonaceous megafossils, and microbial structures occur throughout the entire succession, but are especially well preserved in the Little Dal Group of the Mackenzie Mountains Supergroup. Edicara-type fossils first appear below the second Windermere glacial unit (Ice Brook diamictite), and range upwards through 2.5 km of strata to near the top of the Windermere; three distinctive assemblages corresponding to the Twitya, Sheepbed, and Blueflower formations can be recognized. The Neoproterozoic-Cambrian boundary occurs within the Ingta Formation, and is marked by an abrupt change from simple, subhorizontal burrows (Planolites-Torrowangea Assemblage) to complex feeding burrows of the Phycodes Assemblage. Small shelly fossils of Nemakit-Daldyn aspect occur in the upper part of the Ingta Formation, slightly above the basal Cambrian boundary. The presence of two glacial diamictites in the Windermere and numerous sequence boundaries and flooding surfaces throughout the succession provides opportunities for regional and global event correlation. The palaeomagnetic record shows three hair-pin curves that should be recognizable in continents attached to Laurentia. Carbonate interbeds throughout the succession typically yield relatively unaltered carbon isotope ratios; C and Sr chemostratigraphy shows marked excursions that appear to be globally correlatable.

Paleoecology of the oldest known animal communities: Ediacaran assemblages at Mistaken Point, Newfoundland

Abstract Ediacaran fossils at Mistaken Point, southeastern Newfoundland (terminal Neoproterozoic; 565–575 Ma) represent the oldest known animal communities. In contrast to most Phanerozoic fossil assemblages, in which postmortem transportation, bioturbation, and the accumulation of hardparts obscure community relationships, all fossils in the Mistaken Point assemblages were sessile, soft-bodied organisms that show no evidence of mobility in life or transportation after death. Mistaken Point assemblages are spectacularly preserved on large bedding planes as in situ census populations of hundreds to thousands of fossils, recording the living soft-bodied benthic community at the moment it was smothered by volcanic ash. This unique preservation style allows ecological tests routinely conducted in modern communities (e.g., species richness, abundance, “biomass,” diversity, and evenness, as well as statistical tests of nearest-neighbor interactions) to be applied to the fossil communities. Observed patterns of community variability are consistent with the theory that Mistaken Point fossil surfaces are “snapshots” recording different stages of ecological succession, progressing from communities of low-level feeders (e.g., pectinates and spindles) to frond-dominated communities with complex tiering and spatial structure. The presence of diverse slope communities at Mistaken Point suggests that the deep sea was colonized rapidly during the evolution of complex organisms. Species richness, abundance, and diversity values, as well as levels of intraspecific interaction, all fall within the typical range observed in modern slope communities. These structural similarities imply that ecological processes present in Ediacaran communities at Mistaken Point were strikingly similar to the processes that operate in modern deep-sea animal communities.

Taphonomic Control on Microstructure in Early Neoproterozoic Reefal Stromatolites and Thrombolites

Abstract Early Neoproterozoic reefs of the Little Dal Group, Northwest Territories, are built by stromatolites and thrombolites containing calcified filamentous cyanobacteria and interstitial cement. Micritic and microcrystalline carbonate grew in or on extracellular cyanobacterial sheaths, preserving filaments when mineralization was early relative to sheath degradation, or grumeaux when mineralization was later. Filamentous microstructure is volumetrically predominant in the reefs; less common are micritic and grumelous microstructures already known from late Proterozoic stromatolites and Phanerozoic thrombolites. Textural intergradation of filamentous-calcimicrobial microstructure with these non-filamentous microstructures reflects microstructural variation developed through differential preservation at the scale of individual filaments and laminae. Textural gradients from filaments to grumeaux, and from calcimicrobial to stromatolitic and thrombolitic microstructure types, imply that a wide variety of microbialite microstructure types can be derived from a single progenitor community. This suggests that taphonomic variables may be as important in the development of microbialite microstructure as the biology of the microbial mat community. It also challenges recent suggestions that the Neoproterozoic increase in thromboids was related to the rise of multicellular organisms. These conclusions have broad implications for the interpretation of fossil microbialites, many of which might have been more closely related in origin than hitherto suspected.

Ediacaran epifaunal tiering

Epifaunal tiering, the subdivision of vertical space within a community, is a fundamental attribute of Phanerozoic suspension-feeding communities. This paper documents tiering, including the presence of meter-tall organisms, in Neoproterozoic Ediacaran communities. Ediacaran tiering was studied from three exceptionally preserved deep-water communities at Mistaken Point, Newfoundland, which contain in situ census populations of hundreds to thousands of organisms. Tiering consists of overlapping populations of dominant organisms and is characterized by gradational, rather than abrupt, tier boundaries. At least three tiers are apparent: a lower level 0–8 cm above the seafloor, an intermediate level between 8–22 cm above the seafloor, and an upper level that extends as high as 120 cm. Tier boundaries are relatively consistent between communities, but the constituent organisms in each level are variable, suggesting that some Ediacaran taxa could fill different tiers interchangeably. Development of a tiered epifaunal structure is consistent with suspension feeding or absorbing dissolved nutrients directly from seawater. Despite the common occurrence of tall organisms, all communities share a similar population structure in which biomass is concentrated in the basal 10 cm above the seafloor. Comparison with shallow-water Ediacaran localities suggests that the observed tiering structure is typical of Ediacaran communities. Ediacaran tierers also show the fundamental subdivision between organisms and/or colonies that fed along their entire length and those that developed a specialized feeding apparatus, implying that the features of Phanerozoic tiered skeletal ecosystems were first initiated in soft-bodied communities in the late Neoproterozoic.

Burrowing below the basal Cambrian GSSP, Fortune Head, Newfoundland

The range of Treptichnus pedum , the index trace fossil for the Treptichnus pedum Zone, extends some 4 m below the Global Standard Stratotype-section and Point for the base of the Cambrian Period at Fortune Head on the Burin Peninsula in southeastern Newfoundland. The identification of zigzag traces of Treptichnus isp., even further below the GSSP than T. pedum in the Fortune Head section, and in other terminal Proterozoic successions around the globe, supports the concept of a gradational onset of three-dimensional burrowing across the Proterozoic–Cambrian boundary. Although T. pedum remains a reasonable indicator for the base of the Cambrian Period, greater precision in the stratotype section can be achieved by a detailed re-evaluation of the stratigraphic ranges and the morphological variation of ichnotaxa included in the T. pedum Zone.

The Ediacaran Period

This book is an essential reference for all geoscientists, including researchers, students, and petroleum and mining professionals. The presentation is non-technical and illustrated with numerous colour charts, maps and photographs.