The Late Devonian was a significant period in Phanerozoic reef evolution. Reef complexes reached their acme in the Middle Devonian and then declined in numbers and complexity thereafter. This change was accompanied by a shift in reef composition characterized by an increase in calcified microbes in the reef milieu. Late Devonian Nisku Formation reefs in the Cynthia Basin at Meekwap, Alberta are composed of calcimicrobes (Renalcis, Epiphyton, Girvanella, Rothpletzella, and Wetheredella), stromatoporoids, and corals. Accessory organisms include brachiopods, benthic foraminifera, molluscs, sponges, bryozoans, and crinoids. Calcimicrobes generate dorsal or ventral crusts on stromatoporoids, and form oncolites in open marine facies. Dorsal crusts are either Girvanella and Rothpletzella, or non-calcimicrobial, spongiostromate or cemented calcisilt encrustations. Ventral crusts are cryptic and composed of Renalcis and Epiphyton. Oncolites comprise layers of Girvanella and Rothpletzella and occur in lagoon and upper-foreslope facies. Girvanella and Rothpletzella are thought to have been photosynthetic because of their preference for interpreted well-lit settings (oncolites and dorsal surfaces) and possible competition for light as expressed by alternating layers of Girvanella and Rothpletzella as well as domal, accretionary growths of either Girvanella or Rothpletzella in oncolites. Renalcis and Epiphyton are viewed as non-photosynthetic or light sensitive because they are rare on upper surfaces and are instead found in cryptic environments. The abundance of calcimicrobes in Meekwap limestones is interpreted to have reflected elevated nutrient levels. Nutrients were likely terrestrially sourced and brought onto the shelf via fluvial runoff and submarine groundwater discharge. Geometry of the Cynthia Basin, as well as the presence of local nutrient sources at Meekwap is thought to have enhanced more regional nutrification via ocean upwelling. The change in the composition of reefs throughout the Late Devonian is attributed to paleoenvironmental changes, such as a colder climate and falling ocean temperatures, as well as increasing nutrient levels, prior to the Late Devonian mass extinction. / Thesis (Master, Geological Sciences & Geological Engineering) -- Queen's University, 2010-12-21 12:24:17.569

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This is an investigation of new vertebrate fossil material from the Late Devonian locality of Ivanovka, Uryup River, Siberia. This bone bed material, circa 375 million years in age, represents a unique opportunity to fill a gap in our understanding of Late Devonian diversity, biogeography, and vertebrate evolution: Siberia, at the time, was an independent continent, and yet its fauna remains virtually unknown in comparison with the other paleocontinents, Euramerica and Gondwana. Using synchrotron microtomographic scanning, a non-destructive technique that has never, to our knowledge, been applied to bone bed material, we obtained 3D image stacks that were then modelled to yield triangle meshes representing the bones in three dimensions. These meshes could then be identified, described, and interpreted. Many of the discovered bones belong to the poorly known genus Megistolepis Obruchev 1955, potentially allowing a radical increase in knowledge regarding this taxon. Other material includes lungfish and possible fragments of limbed tetrapods, though the evidence of the latter is scarce. A discussion of the advantages and disadvantages of synchrotron microtomography for the study of bone bed material concludes the paper.

It has been hypothesized that authigenic carbonate minerals, formed within the pore spaces of marine siliciclastic formations during early diagenesis, may have had a substantial influence on the global carbon cycle, particularly in times of low oxygen in Earth history. According to this idea, alkalinity is generated via anaerobic organic matter degradation, resulting in carbonate oversaturation and the precipitation of low δ13C carbonate cements. If a substantial amount of 13C-depleted carbonate was sequestered in this authigenic sink, the δ13C of dissolved inorganic carbon (DIC) in the global ocean would be driven to more positive values without significant organic carbon burial - a signal which would be recorded in marine carbonates. Research presented herein tests this hypothesis from newly acquired lithostratigraphic and coupled stable carbon and oxygen isotope data of Upper Devonian limestone and black shale formations preserved within the Western Canada Sedimentary Basin. The Late Devonian includes a mass-extinction event, and is characterized by pervasive ocean anoxia and a dramatic reduction in platformal carbonate sediment deposition. As such, it has been hypothesized to represent an ideal time for the emergence of an active authigenic carbonate sink. Results show that both basinal shale (Besa River and Exshaw formations) and platform carbonates (Wabamun Group and its equivalents), record a δ13C signal that is within the expected range of Devonian seawater (3‰ to -2‰), signifying that precipitated authigenic carbonate had no influence on the isotopic composition of DIC. It was observed, however, that evaporitic depositional settings can accumulate carbonate sediment with low δ13C values (down to -8.4‰), potentially caused by local water column organic matter respiration during prolonged water-mass residence in a restricted marginal marine setting. If such depositional environments were globally pervasive, such as during during global sea-level lows, it is plausible that the carbon isotope mass balance would be affected. / Graduate / 2021-06-18

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