Alan D. Rooney

A Cryogenian chronology: Two long-lasting synchronous Neoproterozoic glaciations

The snowball Earth hypothesis predicts globally synchronous glaciations that persisted on a multimillion year time scale. Geochronological tests of this hypothesis have been limited by a dearth of reliable age constraints bracketing these events on multiple cratons. Here we present four new Re-Os geochronology age constraints on Sturtian (717–660 Ma) and Marinoan (635 Ma termination) glacial deposits from three different paleocontinents. A 752.7 ± 5.5 Ma age from the base of the Callison Lake Formation in Yukon, Canada, confirms nonglacial sedimentation on the western margin of Laurentia between ca. 753 and 717 Ma. Coupled with a new 727.3 ± 4.9 Ma age directly below the glacigenic deposits of the Grand Conglomerate on the Congo craton (Africa), these data refute the notion of a global ca. 740 Ma Kaigas glaciation. A 659.0 ± 4.5 Ma age directly above the Maikhan-Uul diamictite in Mongolia confirms previous constraints on a long duration for the 717–660 Ma Sturtian glacial epoch and a relatively short nonglacial interlude. In addition, we provide the first direct radiometric age constraint for the termination of the Marinoan glaciation in Laurentia with an age of 632.3 ± 5.9 Ma from the basal Sheepbed Formation of northwest Canada, which is identical, within uncertainty, to U-Pb zircon ages from China, Australia, and Namibia. Together, these data unite Re-Os and U-Pb geochronological constraints and provide a refined temporal framework for Cryogenian Earth history.

Re-Os geochronology and coupled Os-Sr isotope constraints on the Sturtian snowball Earth

Significance The causal mechanisms of global glaciations are poorly understood. The transition to a Neoproterozoic Snowball Earth after more than 1 Gy without glaciation represents the most dramatic episode of climate change in the geological record. Here we present new Re-Os geochronology, which, together with existing U-Pb ages, reveal that the glacial period in northwest Canada lasted ∼55 My. Additionally, we present an original method to track tectonic influences on these climatic perturbations with a high-resolution coupled Os-Sr isotope curve across the transition from an ice-free world to a Neoproterozoic Snowball Earth. The data indicate that increases in mantle-derived, juvenile material emplaced onto continents and subsequently weathered into the oceans led to enhanced consumption and sequestration of CO2 into sediments. After nearly a billion years with no evidence for glaciation, ice advanced to equatorial latitudes at least twice between 717 and 635 Mya. Although the initiation mechanism of these Neoproterozoic Snowball Earth events has remained a mystery, the broad synchronicity of rifting of the supercontinent Rodinia, the emplacement of large igneous provinces at low latitude, and the onset of the Sturtian glaciation has suggested a tectonic forcing. We present unique Re-Os geochronology and high-resolution Os and Sr isotope profiles bracketing Sturtian-age glacial deposits of the Rapitan Group in northwest Canada. Coupled with existing U-Pb dates, the postglacial Re-Os date of 662.4 ± 3.9 Mya represents direct geochronological constraints for both the onset and demise of a Cryogenian glaciation from the same continental margin and suggests a 55-My duration of the Sturtian glacial epoch. The Os and Sr isotope data allow us to assess the relative weathering input of old radiogenic crust and more juvenile, mantle-derived substrate. The preglacial isotopic signals are consistent with an enhanced contribution of juvenile material to the oceans and glacial initiation through enhanced global weatherability. In contrast, postglacial strata feature radiogenic Os and Sr isotope compositions indicative of extensive glacial scouring of the continents and intense silicate weathering in a post–Snowball Earth hothouse.

740 Ma vase-shaped microfossils from Yukon, Canada: Implications for Neoproterozoic chronology and biostratigraphy

Biostratigraphy underpins the Phanerozoic time scale, but its application to pre-Ediacaran strata has remained limited because Proterozoic taxa commonly have long or unknown stratigraphic ranges, poorly understood taphonomic constraints, and/or inadequate geochronological context. Here we report the discovery of abundant vase-shaped microfossils from the Callison Lake dolostone of the Coal Creek inlier (Yukon, Canada) that highlight the potential for biostratigraphic correlation of Neoproterozoic successions using species-level assemblage zones of limited duration. The fossiliferous horizon, dated here by Re-Os geochronology at 739.9 ± 6.1 Ma, shares multiple species-level taxa with a well-characterized assemblage from the Chuar Group of the Grand Canyon (Arizona, USA), dated by U-Pb on zircon from an interbedded tuff at 742 ± 6 Ma. The overlapping age and species assemblages from these two deposits suggest biostratigraphic utility, at least within Neoproterozoic basins of Laurentia, and perhaps globally. The new Re-Os age also confirms the timing of the Islay δ13Ccarbonate anomaly in northwestern Canada, which predates the onset of the Sturtian glaciation by >15 m.y. Together these data provide global calibration of sedimentary, paleontological, and geochemical records on the eve of profound environmental and evolutionary change.

Redox heterogeneity of subsurface waters in the Mesoproterozoic ocean

A substantial body of evidence suggests that subsurface water masses in mid-Proterozoic marine basins were commonly anoxic, either euxinic (sulfidic) or ferruginous (free ferrous iron). To further document redox variations during this interval, a multiproxy geochemical and paleobiological investigation was conducted on the approximately 1000-m-thick Mesoproterozoic (Lower Riphean) Arlan Member of the Kaltasy Formation, central Russia. Iron speciation geochemistry, supported by organic geochemistry, redox-sensitive trace element abundances, and pyrite sulfur isotope values, indicates that basinal calcareous shales of the Arlan Member were deposited beneath an oxygenated water column, and consistent with this interpretation, eukaryotic microfossils are abundant in basinal facies. The Rhenium-Osmium (Re-Os) systematics of the Arlan shales yield depositional ages of 1414±40 and 1427±43 Ma for two horizons near the base of the succession, consistent with previously proposed correlations. The presence of free oxygen in a basinal environment adds an important end member to Proterozoic redox heterogeneity, requiring an explanation in light of previous data from time-equivalent basins. Very low total organic carbon contents in the Arlan Member are perhaps the key--oxic deep waters are more likely (under any level of atmospheric O2) in oligotrophic systems with low export production. Documentation of a full range of redox heterogeneity in subsurface waters and the existence of local redox controls indicate that no single stratigraphic section or basin can adequately capture both the mean redox profile of Proterozoic oceans and its variance at any given point in time.

Anoxia in the terrestrial environment during the late Mesoproterozoic

A significant body of evidence suggests that the marine environment remained largely anoxic throughout most of the Precambrian. In contrast, the oxygenation history of terrestrial aquatic environments has received little attention, despite the significance of such settings for early eukaryote evolution. To address this, we provide here a geochemical and isotopic assessment of sediments from the late Mesoproterozoic Nonesuch Formation of central North America. We utilize rhenium-osmium (Re-Os) geochronology to yield a depositional age of 1078 ± 24 Ma, while Os isotope compositions support existing evidence for a lacustrine setting. Fe-S-C systematics suggest that the Nonesuch Formation was deposited from an anoxic Fe-rich (ferruginous) water column. Thus, similar to the marine realm, anoxia persisted in terrestrial aquatic environments in the Middle to Late Proterozoic, but sulfidic water column conditions were not ubiquitous. Our data suggest that oxygenation of the terrestrial realm was not pervasive at this time and may not have preceded oxygenation of the marine environment, signifying a major requirement for further investigation of links between the oxygenation state of terrestrial aquatic environments and eukaryote evolution.

Neoproterozoic stratigraphy of the Zavkhan terrane of Mongolia: The backbone for Cryogenian and early Ediacaran chemostratigraphic records

The Neoproterozoic Tsagaan-Olom Group is exposed in the Zavkhan Terrane of southwestern Mongolia and hosts unique geochemical, paleoclimate, and paleontological records that have become central to our understanding of this pivotal interval of Earth history. New sedimentological, stratigraphic, geochronological, and geochemical data provide context for and further develop these records. Detrital zircon provenance indicates that Neoproterozoic strata of the Zavkhan Terrane were derived from basement with age peaks between 1950 to 2100 and 2400 to 2600 Ma. At ∼800 Ma, the Zavkhan Terrane transformed from an active arc and back-arc complex to a rifted ribbon continent with passive margins on both sides. Deposition was accommodated by extension, which is recorded with syn-sedimentary normal faulting and alluvial fan deposition in the Zavkhan and Khasagt formations. Passive margin sedimentation in the overlying Tsagaan-Olom Group begins with the glacigenic Maikhan-Uul Formation, which consists of two massive diamictite units separated by clast-poor graded beds of the middle member. Detrital zircon at the base of the middle member of the Maikhan-Uul Formation were dated with U-Pb chemical abrasion isotope-dilution thermal ionization mass spectrometry and constrained its age to <729.8 ± 1.4 Ma. This, along with chemostratigraphy and Re/Os geochronological constraints from the overlying Taishir Formation, supports our correlation of the Maikhan-Uul Formation with the ∼717 to 660 Ma Sturtian glaciation. The Taishir Formation was deposited on a carbonate ramp in four large-scale sequence tracts that thin to the southwest. The Taishir Formation preserves a large negative δ13C excursion referred to as the Taishir excursion that covaries in carbonate and organic carbon isotopes in limestone sections. A dolomitization front at the top of the Taishir Formation also results in depleted δ13C values, however, these are related to local processes and do not represent a global Trezona excursion. Although δ13C values in the Ol Formation are highly variable along strike, 0.70756 initial strontium isotope values in limestone of the upper Ol Formation are consistent with earliest Ediacaran values. A sandstone-filled karst surface at the top of the Shuurgat Formation that overlies the Ol Formation defines the top of the Tsagaan-Olom Group and is interpreted to mark a major unconformity. Carbon and strontium isotope values in the uppermost Shuurgat Formation are also consistent with early Ediacaran values and suggest that most of the late Ediacaran Period is missing in the Zavkhan Terrane of Mongolia. Carbon isotope profiles from sections preserved as limestone and dolostone display large differences and indicate that isotopic data from dolomites should be used with caution. With our new data and correlations, we construct composite Cryogenian and Ediacaran carbon and strontium isotope curves from limestone-dominated successions in Mongolia, and then integrate additional geochronological and geochemical data sets from around the globe.

Stratigraphy and geochronology of the Tambien Group, Ethiopia: Evidence for globally synchronous carbon isotope change in the Neoproterozoic

The Neoproterozoic Era was an interval characterized by profound environmental and biological transitions. Existing age models for Neoproterozoic nonglacial intervals largely have been based on correlation of carbonate carbon isotope values, but there are few tests of the assumed synchroneity of these records between basins. In contrast to the ash-poor successions typically targeted for Neoproterozoic chemostratigraphy, the Tonian to Cryogenian Tambien Group (Tigray region, Ethiopia) was deposited in an arc-proximal basin where volcanic tuffs suitable for U-Pb geochronology are preserved within the mixed carbonate-siliciclastic sedimentary succession. The Tambien Group culminates in a diamictite interpreted to correlate to the ca. 717–662 Ma Sturtian snowball Earth glaciation. New physical stratigraphic data and high-precision U-Pb dates from intercalated tuffs lead to a new stratigraphic framework for the Tambien Group that confirms identification of negative δ13C values from Assem Formation limestones with the ca. 800 Ma Bitter Springs carbon isotope stage. Integration with data from the Fifteenmile Group of northwestern Canada constitutes a positive test for the global synchroneity of the Bitter Spring Stage and constrains the stage to have started after 811.51 ± 0.25 Ma and to have ended before 788.72 ± 0.24 Ma. These new temporal constraints strengthen the case for interpreting Neoproterozoic carbon isotope variation as a record of large-scale changes to the carbon cycle and provide a framework for age models of paleogeographic change, geochemical cycling, and environmental evolution during the radiation of early eukaryotes.

Controlled hydroxyapatite biomineralization in an ~810 million-year-old unicellular eukaryote

New analyses show that 811 million-year-old microfossils from the Yukon are the oldest evidence of eukaryotic biomineralization. Biomineralization marks one of the most significant evolutionary milestones among the Eukarya, but its roots in the fossil record remain obscure. We report crystallographic and geochemical evidence for controlled eukaryotic biomineralization in Neoproterozoic scale microfossils from the Fifteenmile Group of Yukon, Canada. High-resolution transmission electron microscopy reveals that the microfossils are constructed of a hierarchically organized interwoven network of fibrous hydroxyapatite crystals each elongated along the [001] direction, indicating biological control over microstructural crystallization. New Re-Os geochronological data from organic-rich shale directly below the fossil-bearing limestone constrain their age to <810.7 ± 6.3 million years ago. Mineralogical and geochemical variations from these sedimentary rocks indicate that dynamic global marine redox conditions, enhanced by local restriction, may have led to an increase in dissolved phosphate in pore and bottom waters of the Fifteenmile basin and facilitated the necessary geochemical conditions for the advent of calcium phosphate biomineralization.

Coupled Re-Os and U-Pb geochronology of the Tonian Chuar Group, Grand Canyon

Well-preserved strata of the late Tonian Chuar Group exposed in the Grand Canyon host fossil evidence for the development of eukaryotic predation, the presence of unique biomarkers, and large changes in C, S, and Mo isotope chemostratigraphy. Despite the importance of this critical succession, few radioisotopic age constraints are available to place these records into a global context. Here, we couple high-precision U-Pb chemical abrasion−isotope dilution−thermal ionization mass spectrometry (CA-ID-TIMS) on zircon crystals with the rhenium-osmium (Re-Os) sedimentary and sulfide geochronometer to refine the temporal framework of this pivotal interval of Earth history. Zircons recovered from a tuff within the uppermost Walcott Member of the Kwagunt Formation yield a weighted mean 206Pb-238U age of 729.0 ± 0.9 Ma (mean square of weighted deviates [MSWD] = 0.86), differing significantly from the previous air-abrasion upper-intercept age of 742 ± 6 Ma on zircons from this same horizon. Organic-rich carbonates from the Carbon Canyon Member of the Galeros Formation yield a model 1 Re-Os age of 757.0 ± 6.8 Ma (MSWD = 0.47, n = 8), and an initial Os isotope (187Os/188Os [Osi]) composition of 1.13 ± 0.02. The radiogenic Osi value from this horizon suggests that the basin was restricted from the open ocean during deposition of the Carbon Canyon Member, in agreement with sedimentological and stratigraphic data. The Re-Os geochronology of marcasite (FeS2) nodules from the Awatubi Member of the Kwagunt Formation yield a model 1 age of 751.0 ± 7.6 Ma (MSWD = 0.37, n = 5), with an Osi of 0.44 ± 0.01. This Re-Os date is interpreted to constrain the growth of the marcasite nodules in the Awatubi Member during deposition. The formation of sulfides and the less radiogenic Osi value are consistent with an influx of sulfate-laden seawater to the basin during deposition of the Kwagunt Formation. Attempts to apply the Re-Os geochronometer to the Walcott and Tanner Members of the Chuar Group failed to yield meaningful ages, despite elevated Re enrichments (>20 ng/g). The Re-Os data from these units yielded negative Osi values, which suggest disturbance to the Re-Os system. The low Os abundances (typically <100 pg/g) relative to the amount expected based on the elevated Re abundances suggest leaching of Os due to oxidative weathering on geologically recent time scales. Finally, the Carbon Canyon Member provides a useful case study for quantifying how input uncertainties in the Re-Os geochronometer propagate into the resulting age uncertainty, and we discuss the protocols that will yield the best improvement in age precision for future studies. The U-Pb and Re-Os geochronology presented here illustrates the power of coupling these systems and the importance of recent improvements in both methods. Our analysis suggests that for our data, the most efficient way of reducing uncertainties in the presented Re-Os dates is through improved precision of measured Os values.