Person: Macdonald, Francis
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Macdonald
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Macdonald, Francis
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Publication Persistence of a Freshwater Surface Ocean After a Snowball Earth(Geological Society of America, 2017-05-01) Yang, Jun; Jansen, Malte; Macdonald, Francis; Abbot, DorianGeochemical data from cap carbonates deposited above Cryogenian glacial deposits have been widely used to infer the conditions that prevailed in the aftermath of snowball Earth. However, the time scale over which these carbonates were deposited and the degree to which they record the chemistry of a globally well-mixed ocean have remained poorly constrained. During deglaciation, a large volume of meltwater entered the ocean, creating two distinct layers: the fresh, hot, and light upper layer, and the salty, cold, and dense lower layer. Here we estimate the ocean mixing time scale based on energetic constraints. We find that the mixing time scale is 104–105 yr, with a best estimate of ∼5 × 104 yr, or up to 100 times longer than that of the modern ocean. Mixing of the surface temperature anomaly implies a delayed sea-level rise of 40–50 m associated with pure thermal expansion. This result reconciles geological, geochemical, and paleomagnetic data from basal Ediacaran cap carbonates with physical oceanographic theory. In particular, our model suggests that (1) the cap dolostones formed predominantly in a freshwater environment; (2) the waters in which the dolostones formed were not well mixed with saline deep water, allowing for large geochemical differences between the cap dolostones and the deep ocean; and (3) the cap carbonate sequences formed in a two-phase transgression that lasted >104 yr, which is consistent with both local sea-level records and the preservation of magnetic excursions and reversals.Publication Doushantuo-Type Microfossils From Latest Ediacaran Phosphorites of Northern Mongolia(Geological Society of America, 2017-10-19) Anderson, Ross P.; Macdonald, Francis; Jones, David; McMahon, Sean; Briggs, DerekPhosphorites of the latest Ediacaran upper Khesen Formation in the Khuvsgul Group of northern Mongolia preserve a newly discovered, three-dimensionally phosphatized Doushantuo-type microfossil assemblage. Eight genera include the second occurrence of the putative multicellular fossil animal embryo Megasphaera outside South China, the Doushantuo-Pertatataka–type acanthomorphic acritarchs h Cavaspina, and Variomargosphaeridium, and the possible alga Archaeophycus yunnanensis. The assemblage occurs in the lowermost phosphorite horizon in foreland basin deposits on the Khuvsgul terrane; lithostratigraphic and δ13C correlation with the Zavkhan terrane of southwestern Mongolia establishes a latest Ediacaran age for the fossiliferous phosphorites. Thus, this is the youngest Doushantuo-type assemblage yet reported. It extends the range of Megasphaera, and fills a gap in the record of phosphatized embryo-like forms between the ca. 600 Ma Doushantuo Weng’an biota and Cambrian examples. The Khesen fossil assemblage emphasizes the potential of Mongolian phosphorites to provide new paleontological data on the Ediacaran-Cambrian transition, and to resolve the phylogenetic debate surrounding Megasphaera embryo-like taxa.Publication Tropical Weathering of the Taconic Orogeny as a Driver for Ordovician Cooling(Geological Society of America, 2017-06-09) Swanson-Hysell, Nicholas; Macdonald, FrancisThe Earth's climate cooled through the Ordovician Period leading up to the Hirnantian glaciation. Increased weatherability of silicate rocks associated with topography generated on the Appalachian margin during the Taconic orogeny has been proposed as a mechanism for Ordovician cooling. However, paleogeographic reconstructions typically place the Appalachian margin within the arid subtropics, outside of the warm and wet tropics where chemical weathering rates are highest. In this study, we reanalyze the paleomagnetic database and conclude that Ordovician constraints from cratonic Laurentia are not robust. Instead, we use paleomagnetic data from well-dated volcanic rocks in the accreting terranes to constrain Laurentia's position given that the Appalachian margin was at, or equatorward of, the paleolatitude of these terranes. To satisfy these allochthonous data, Laurentia must have moved toward the equator during the Ordovician such that the Appalachian margin was within 10° of the equator by 465 Ma. This movement into the tropics coincided with the collision and exhumation of the Taconic arc system, recorded by a shift in neodymium isotope data from shale on the Appalachian margin to more juvenile values. This inflection in detrital neodymium isotope values precedes a major downturn in global seawater strontium isotopic values by more than one million years, as would be predicted from a change in weathering input and the relatively long residence time of strontium in the ocean. These data are consistent with an increase in global weatherability associated with the tropical weathering of mafic and ultramafic lithologies exhumed during the Taconic arc-continent collision. A Taconic related increase in weatherability is a viable mechanism for lowering atmospheric CO2levels through silicate weathering contributing to long-term Ordovician cooling.Publication Phosphatized Early Cambrian Archaeocyaths and Small Shelly Fossils (SSFs) of Southwestern Mongolia(Elsevier BV, 2019-01) Pruss, Sara; Dwyer, Camille; Smith, Emily; Macdonald, Francis; Tosca, NicholasArchaeocyaths are an enigmatic group of calcifying sponges prevalent in early Cambrian (Terreneuvian to Series 2) successions around the world and preserved predominantly in reefal buildups, but also in adjacent reworked deposits. Here we report exceptionally preserved phosphatized archaeocyaths and small shelly fossils from phosphatized reef flank deposits at the top of the Salaagol Formation of southwestern Mongolia. Recent chemostratigraphic age models suggest that these archaeocyaths are among the earliest reported in the Terreneuvian Stage 2 (Tommotian). These fossils provide a window into the mechanisms of archaeocyath phosphatization, a generally rare mode of archaeocyath preservation. To assess the composition and nature of phosphatization, fossil assemblages were examined in insoluble residue and thin section. These archaeocyaths are preserved as phosphatic internal molds in residue, and both phosphatized and unphosphatized archaeocyaths are present in thin section. The occurrence of internal molds and complementary mineralogical data suggest that the decay of organic material within the archaeocyaths created the necessary redox conditions for apatite nucleation. We propose that, shortly after death, this assemblage was transported to a deeper water environment, and that the presence of organic matter in a low oxygen setting led to abundant phosphatization of archaeocyaths.Publication Triple oxygen and multiple sulfur isotope constraints on the evolution of the post-Marinoan sulfur cycle(Elsevier BV, 2016) Crockford, Peter W.; Cowie, Benjamin; Johnston, David; Hoffman, Paul; Sugiyama, Ichiko; Pellerin, Andre; Bui, Thi Hao; Hayles, Justin; Halverson, Galen P.; Macdonald, Francis; Wing, Boswell A.Triple oxygen isotopes within post-Marinoan barites have played an integral role in our understanding of Cryogenian glaciations. Reports of anomalous View the MathML source values within cap carbonate hosted barites however have remained restricted to South China and Mauritania. Here we extend the View the MathML source anomaly to northwest Canada with our new measurements of barites from the Ravensthroat cap dolostone with a minimum View the MathML source value of −0.75‰. For the first time we pair triple oxygen with multiple sulfur isotopic data as a tool to identify the key processes that controlled the post-Marinoan sulfur cycle. We argue using a dynamic 1-box model that the observed isotopic trends both in northwest Canada and South China can be explained through the interplay between sulfide weathering, microbial sulfur cycling and pyrite burial. An important outcome of this study is a new constraint placed on the size of the post-Marinoan sulfate reservoir (≈0.1% modern), with a maximum concentration of less than 10% modern. Through conservative estimates of sulfate fluxes from sulfide weathering and under a small initial sulfate reservoir, we suggest that observed isotopic trends are the product of a dynamic sulfur cycle that saw both the addition and removal of the View the MathML source anomaly over four to five turnovers of the post-Marinoan marine sulfate reservoir.Publication Initiation of Snowball Earth with volcanic sulfur aerosol emissions(Wiley-Blackwell, 2017) Macdonald, Francis; Wordsworth, RobinWe propose that the first Neoproterozoic Snowball Earth event, the Sturtian glaciation, was initiated by the injection of sulfate aerosols into the stratosphere. Geochronological data indicate that the Natkusiak magmatic assemblage of the Franklin large igneous province coincided with onset of the Sturtian glaciation. The Natkusiak was emplaced into an evaporite basin and entrained significant quantities of sulfur, which would have led to extensive SO2 and H2S outgassing in hot convective plumes. The largest of these plumes could have penetrated the tropopause, leading to stratospheric sulfate aerosol formation and an albedo increase sufficient to force a Snowball. Radiative forcing was maximized by the equatorial location of the Franklin and the cool Neoproterozoic background climate, which would have lowered the tropopause height, increasing the rate of stratospheric aerosol injection. Our results have implications for understanding Phanerozoic mass extinction events, exoplanet habitability, and aerosol perturbations to the present-day climate.Publication Oxygen, facies, and secular controls on the appearance of Cryogenian and Ediacaran body and trace fossils in the Mackenzie Mountains of northwestern Canada(Geological Society of America, 2015) Sperling, Erik A.; Carbone, Calla; Strauss, Justin Vincent; Johnston, David; Narbonne, Guy M.; Macdonald, FrancisThe causes behind the appearance of abundant macroscopic body and trace fossils at the end of the Neoproterozoic Era remain debated. Iron geochemical data from fossiliferous Ediacaran successions in Newfoundland suggested that the first appearances correlated with an oxygenation event. A similar relationship was claimed to exist in the Mackenzie Mountains, Canada, although later stratigraphic studies indicated that the sections analyzed for geochemistry were incorrectly correlated with those hosting the fossils. To directly connect fossil occurrences with geochemistry in the Mackenzie Mountains, we conducted a multiproxy iron, carbon, sulfur, and trace-element geochemical analysis of stratigraphic sections hosting both the Cryogenian “Twitya discs” at Bluefish Creek as well as Ediacaran fossils and simple bilaterian traces at Sekwi Brook. There is no clear oxygenation event correlated with the appearance of macroscopic body fossils or simple bilaterian burrows; however, some change in environment—a potential partial oxygenation—is correlated with increasing burrow width higher in the Blueflower Formation. Data from Sekwi Brook suggest that these organisms were periodically colonizing a predominantly anoxic and ferruginous basin. This seemingly incongruent observation is accommodated through accounting for differing time scales between the characteristic response time of sedimentary redox proxies versus that for ecological change. Thus, hypotheses directly connecting ocean oxygenation with the appearance of macrofossils need not apply to all areas of a heterogeneous Ediacaran ocean, and stably oxygenated conditions on geological time scales were not required for the appearance of these Avalon-assemblage Ediacaran organisms. At least in the Mackenzie Mountains, the appropriate facies for fossil preservation appears to be the strongest control on the stratigraphic distribution of macrofossils.Publication Updated Definition and Correlation of the Lower Fifteenmile Group in the Central and Eastern Ogilvie Mountains(Yukon Geological Survey, 2012) Halverson, Galen P.; Macdonald, Francis; Strauss, Justin Vincent; Smith, Emily; Cox, Grant M.; Hubert-Théou, LucieOngoing mapping, chemostratigraphy, geochronology, and stratigraphic analysis of Neoproterozoic successions in the Ogilvie Mountains requires redefinition and correlation of the Fifteenmile Group across the Proterozoic inliers in Yukon. Here we present new stratigraphic logs through the lower Fifteenmile Group in the Coal Creek and Hart River inliers. Based on these data and new observations, we propose redefinition of the lower Fifteenmile Group. A succession dominated by sandstone, mapped as unit PPD1 in the Hart River inlier, is now recognized at the base of the Fifteenmile Group in the Coal Creek inlier. These strata unconformably overlie the Pinguicula Group and transition upward into a distinctive carbonate interval; together, these comprise the informally defined Gibben formation. The shallowing-upward carbonate sequence contains abundant oolitic grainstone and packstone and microbial laminated dolostone. It is capped by a distinct interval of mud-cracked maroon mudstone, siltstone, and fine-grained sandstone that forms the base of what we informally define as the Chandindu formation. The mud-cracked shale transitions upwards into interbedded shale, coarse-grained sandstone, and minor carbonate. The overlying informally defined Reefal assemblage consists of up to 1 km of complexly interbedded carbonate and shale, with variable truncation beneath the major angular unconformity at the base of the Callison Lake Dolostone. The lower Fifteenmile Group (now informally PPD1 through the Chandindu formation) likely correlates with the Hematite Creek Group in the Wernecke Mountains.Publication Early Neoproterozoic Basin Formation in Yukon, Canada: Implications for the Make-Up and Break-Up of Rodinia(Geological Association of Canada, 2012) Macdonald, Francis; Halverson, Galen P.; Strauss, Justin Vincent; Smith, Emily; Cox, Grant; Sperling, Erik A.; Roots, Charles F.Geological mapping and stratigraphic anaylsis of the early Neoproterozoic Fifteenmile Group in the western Ogilvie Mountains of Yukon, Canada, has revealed large lateral facies changes in both carbonate and siliciclastic strata. Syn-sedimentary NNW-side-down normal faulting during deposition of the lower Fifteenmile Group generated local topographic relief and wedge-shaped stratal geometries. These strata were eventually capped by platformal carbonate after the establishment of a NNW-facing stromatolitic reef complex that formed adjacent to the coeval Little Dal Group of the Mackenzie Mountains, Northwest Territories. Correlations between specific formations within these groups are tested with carbon isotope chemostratigraphy. As there are no known 830-780 Ma stratigraphic successions south of 62°N, the basin-forming event that accommodated the Fifteenmile and Little Dal Groups of the Ogilvie and Mackenzie Mountains and equivalent strata in the Shaler Supergroup of Victoria Island was restricted to the northwest margin of Laurentia. Therefore, this event does not represent widespread rifting of the entire western margin of Laurentia and instead we propose that these strata were accommodated in a failed rift generated by localized subsidence associated with the emplacement of the coeval Guibei (China) and Gairdner (Australia) large igneous provinces. The northern margin of Laurentia was reactivated by renewed extension at ca. 720 Ma associated with the emplacement of the Franklin large igneous province. Significant crustal thinning and generation of a thermally subsiding passive margin on the western margin of Laurentia may not have occurred until the late Ediacaran.Publication Possible early foraminiferans in post-Sturtian (716-635 Ma) cap carbonates(Geological Society of America, 2011) Bosak, Tanja; Lahr, Daniel J. G.; Pruss, Sara B.; Macdonald, Francis; Gooday, Andrew J.; Dalton, Lilly; Matys, Emily D.Foraminifera are an ecologically important group of modern heterotrophic amoeboid eukaryotes whose naked and testate ancestors are thought to have evolved ∼1 Ga ago. However, the single-chambered agglutinated tests of these protists appear in the fossil record only after ca. 580 Ma, coinciding with the appearance of macroscopic and mineralized animals. Here we report the discovery of small, slender tubular microfossils in the Sturtian (ca. 716–635 Ma) cap carbonate of the Rasthof Formation in Namibia. The tubes are 200–1300 μm long and 20–70 μm wide, and preserve apertures and variably wide lumens, folds, constrictions, and ridges. Their sometimes flexible walls are composed of carbonaceous material and detrital minerals. This combination of morphologic and compositional characters is also present in some species of modern single-chambered agglutinated tubular foraminiferans, and is not found in other agglutinated eukaryotes. The preservation of possible early Foraminifera in the carbonate rocks deposited in the immediate aftermath of Sturtian low-latitude glaciation indicates that various morphologically modern protists thrived in microbially dominated ecosystems, and contributed to the cycling of carbon in Neoproterozoic oceans much before the rise of complex animals.