Person:

Strauss, Justin Vincent

The Neoproterozoic was an era of great environmental and biological change, but a paucity of direct and precise age constraints on strata from this time has prevented the complete integration of these records. We present four high-precision U-Pb ages for Neoproterozoic rocks in northwestern Canada that constrain large perturbations in the carbon cycle, a major diversification and depletion in the microfossil record, and the onset of the Sturtian glaciation. A volcanic tuff interbedded with Sturtian glacial deposits, dated at 716.5 million years ago, is synchronous with the age of the Franklin large igneous province and paleomagnetic poles that pin Laurentia to an equatorial position. Ice was therefore grounded below sea level at very low paleolatitudes, which implies that the Sturtian glaciation was global in extent.

Mineralogical, petrographic and sedimentological observations document early diagenetic talc in carbonate-dominated successions deposited on two early Neoproterozoic (~ 800–700 million years old) platform margins. In the Akademikerbreen Group, Svalbard, talc occurs as nodules that pre-date microspar cements that fill molar tooth structures and primary porosity in stromatolitic carbonates. In the upper Fifteenmile Group of the Ogilvie Mountains, NW Canada, the talc is present as nodules, coated grains, rip-up clasts and massive beds that are several meters thick. To gain insight into the chemistry required to form early diagenetic talc, we conducted precipitation experiments at 25 °C with low-SO4 synthetic seawater solutions at varying pH, Mg2+ and SiO2(aq). Our experiments reveal a sharp and reproducible pH boundary (at ~ 8.7) only above which does poorly crystalline Mg-silicate precipitate; increasing Mg2+ and/or SiO2(aq) alone is insufficient to produce the material. The strong pH control can be explained by Mg-silica complexing activated by the deprotonation of silicic acid above ~ 8.6–8.7. FT-IR, TEM and XRD of the synthetic precipitates reveal a talc-like 2:1 trioctahedral structure with short-range stacking order. Hydrothermal experiments simulating burial diagenesis show that dehydration of the precipitate drives a transition to kerolite (hydrated talc) and eventually to talc. This formation pathway imparts extensive layer stacking disorder to the synthetic talc end-product that is identical to Neoproterozoic occurrences. Early diagenetic talc in Neoproterozoic carbonate platform successions appears to reflect a unique combination of low Al concentrations (and, by inference, low siliciclastic input), near modern marine salinity and Mg2+, elevated SiO2(aq), and pH > ~ 8.7. Because the talc occurs in close association with microbially influenced sediments, we suggest that soluble species requirements were most easily met through microbial influences on pore water chemistry, specifically pH and alkalinity increases driven by anaerobic Fe respiration.

A mechanistic understanding of relationships between global glaciation, a putative second rise in atmospheric oxygen, the Shuram carbon isotope excursion, and the appearance of Ediacaran-type fossil impressions and bioturbation is dependent on the construction of accurate geological records through regional stratigraphic correlations. Here we integrate chemo-, litho-, and sequence-stratigraphy of fossiliferous Ediacaran strata in northwestern Canada. These data demonstrate that the FAD of Ediacara-type fossil impressions in northwestern Canada occur within a lowstand systems tract and above a major sequence boundary in the informally named June beds, not in the early Ediacaran Sheepbed Formation from which they were previously reported. This distinction is substantiated by δ13Ccarb chemostratigraphy of the Sheepbed carbonate, which overlies the Sheepbed Formation, and the Gametrail Formation, which overlies the June beds. The Sheepbed carbonate hosts heavy δ13Ccarb values whereas the Gametrail Formation contains a large δ13Ccarb excursion, which we correlate with the globally recognized Shuram excursion. Stratigraphically above the Gametrail excursion, the first bilaterian burrows are present in the basal Blueflower Formation. Together, these data allow us to construct an age model for Ediacaran strata in northwestern Canada and conclude that a purported shift in Fe speciation in the Sheepbed Formation significantly predates the shift recorded above the ca. 582 Ma Gaskiers glaciation in Newfoundland and the first appearance of Ediacaran biota. The Gametrail excursion shares many characteristics with Shuram negative δ13Ccarb excursion: 1) δ13Ccarb and δ18Ocarb covary; 2) δ13Ccarb and δ13Corg do not covary; 3) the excursion is developed during a transgressive systems tract and recovers in an highstand systems tract; and 4) values in some sections are well below mantle δ13C input values but are variable between sections. We relate regional lateral variability in the magnitude and character of this excursion to condensation and diachronous deposition during the transgression and local authigenic carbonate production. In light of these observations, we explore a variety of models for the genesis of the Shuram excursion and suggest that the location and amount of authigenic carbonate production played a role in the excursion.

The 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.