ALBERT

Description: The composition of seawater changed dramatically during the initial rise of atmospheric oxygen in the earliest Paleoproterozoic, but the emerging view is that atmosphere-ocean system did not experience an irreversible transition to a well-oxygenated state. Instead, it has been suggested that the oxygen content of the atmosphere-ocean system decreased considerably after ca. 2.06 billion years ago (Ga), which resulted in a crash in marine sulfate concentrations. The end of the deposition of major granular iron formations at ca. 1.85 Ga has been linked either to the development of extensive euxinic conditions along continental shelves or a decrease in hydrothermal flux. The record of oceanic redox state is not well constrained for the period between ca. 2.06 Ga, the end of the Lomagundi positive carbon isotope excursion, and ca. 1.88 Ga when major granular iron formations appeared. We address this gap by presenting new iron-speciation, major and trace element data, as well as sulfur, organic carbon, and molybdenum isotopic data for greenschist facies organic matter-rich mudrocks (ORMs) of the ca. 1.93 Ga Bravo Lake Formation, Piling Group, Baffin Island. The iron speciation data suggest deposition of the Bravo Lake Formation under a euxinic (anoxic and sulfidic) water column. Trace metal enrichments and Mo isotope data suggest extensive marine euxinia ca. 90 million years before the disappearance of large-scale, economic granular iron formations. The addition of new Mo data in this time interval is important, as it contributes to filling in the sparse Proterozoic record. Lastly, this work provides further support for the idea that there was widespread anoxia shortly after the end of the Lomagundi Event.

Description: The partial pressure of oxygen in Earth’s atmosphere has increased dramatically through time, and this increase is thought to have occurred in two rapid steps at both ends of the Proterozoic Eon (∼2.5–0.543 Ga). However, the trajectory and mechanisms of Earth’s oxygenation are still poorly constrained, and little is known...

Description: Shortly after the initial rise of atmospheric oxygen in the Paleoproterozoic Era, a major perturbation occurred in the global carbon cycle, which is manifested as a long-lived positive carbon isotope excursion recorded in ~2.22–2.06 Ga carbonate rocks, known as the Lomagundi Event. Beyond its significance for evolving seawater composition, this geochemical event can be used as an indirect age marker in Paleoproterozoic sedimentary successions. Documenting further occurrences of this event in other Paleoproterozoic carbonate rocks confirms that the event was global and reflects ambient seawater composition. This event, however, has only been documented in two successions in Canada, despite the ubiquity of Paleoproterozoic-aged rocks on the Canadian Shield. Our study focuses on metacarbonate rocks from the Paleoproterozoic-aged Murmac Bay Group on the southwestern margin of the Rae craton in northern Saskatchewan. Measured 13 C values (up to 7.8) fall within the range of the Lomagundi Event, but most values are relatively low, suggesting CO 2 loss (decarbonation) altered the 13 C values. Stable carbon isotope data coupled with major element geochemical data allowed us to account for the degree to which the values have changed due to metamorphic CO 2 loss. We also compare 13 C values from micro-drilled (dolomite and calcite phases) with whole-rock 13 C values from the same samples to characterize the 13 C composition of pre-metamorphic carbonate minerals. Both measured and corrected 13 C values place the Murmac Bay Group metacarbonates within the range of values that characterize the Lomagundi Event and indicate, for the first time, the presence of the Lomagundi Event on the Rae craton.

Description: The proximity and positions of cratons constituting the western Canadian Shield prior to and during the Rhyacian Period (2.30–2.05 Ga) are poorly known. In the absence of paleomagnetic data, stratigraphic correlation and detrital zircon isotopic data from sedimentary successions can be used to constrain relative craton positions during their time of deposition. The Murmac Bay Group, a multiply deformed metasedimentary succession located on the Rae craton margin in Canada, provides an opportunity to test hypotheses regarding its nearest cratonic neighbors during deposition. The polydeformed nature of the Murmac Bay Group, however, presents challenges in determining detailed stratigraphic relationships in the upper succession, which lacks distinct marker beds. Provenance analysis from detrital zircon geochronology provides one strategy for overcoming these challenges. Previous U-Pb geochronology indicates the lower succession was deposited 〈2.32 Ga, and the upper succession was deposited between 〈2.17 Ga and 〉1.94 Ga. We provide new U-Pb detrital zircon ages for the upper succession, including a new maximum depositional age at 〈2.00 Ga. We integrate our new data with published detrital zircon ages and compare them with published and public-domain igneous crystallization ages stored within a large geochronological database to identify potential provenance locations. While there is no known local source for the ca. 2.17 Ga age population, potential 2.17 Ga sources are found on the neighboring Slave craton and Buffalo Head–Chinchaga domain. Geographically, sources for the remaining dominant age populations (e.g., 2.33 Ga) exist either locally, or between the potential sources for ca. 2.17 Ga zircon grains and our study area. Our results support the interpretation that the Rae and Slave cratons were already amalgamated during upper Murmac Bay Group deposition (〈2.00 to 〉1.94 Ga). This provides an important constraint on the timing of Rae-Slave amalgamation.

Description: The proximity and positions of cratons constituting the western Canadian Shield prior to and during the Rhyacian Period (2.30–2.05 Ga) are poorly known. In the absence of paleomagnetic data, stratigraphic correlation and detrital zircon isotopic data from sedimentary successions can be used to constrain relative craton positions during their time of deposition. The Murmac Bay Group, a multiply deformed metasedimentary succession located on the Rae craton margin in Canada, provides an opportunity to test hypotheses regarding its nearest cratonic neighbors during deposition. The polydeformed nature of the Murmac Bay Group, however, presents challenges in determining detailed stratigraphic relationships in the upper succession, which lacks distinct marker beds. Provenance analysis from detrital zircon geochronology provides one strategy for overcoming these challenges. Previous U-Pb geochronology indicates the lower succession was deposited 〈2.32 Ga, and the upper succession was deposited between 〈2.17 Ga and 〉1.94 Ga. We provide new U-Pb detrital zircon ages for the upper succession, including a new maximum depositional age at 〈2.00 Ga. We integrate our new data with published detrital zircon ages and compare them with published and public-domain igneous crystallization ages stored within a large geochronological database to identify potential provenance locations. While there is no known local source for the ca. 2.17 Ga age population, potential 2.17 Ga sources are found on the neighboring Slave craton and Buffalo Head–Chinchaga domain. Geographically, sources for the remaining dominant age populations (e.g., 2.33 Ga) exist either locally, or between the potential sources for ca. 2.17 Ga zircon grains and our study area. Our results support the interpretation that the Rae and Slave cratons were already amalgamated during upper Murmac Bay Group deposition (〈2.00 to 〉1.94 Ga). This provides an important constraint on the timing of Rae-Slave amalgamation.

Description: During the Sturtian and Marinoan "snowball Earth" episodes, ice cover is thought to have extended from polar to tropical latitudes. We test the supposition that such an extreme glacial climate, not repeated in the subsequent ~635 m.y. of Earth history, would have reduced the vigor of the hydrologic cycle and thus diminished sediment flux to the oceans. With 〉500 sediment accumulation rates to characterize Sturtian and Marinoan deposits, we find median accumulation rates at least four to 15 times slower than expected for Phanerozoic glaciomarine deposits as characterized by 〉10,000 rates. Our comparison is conservative with respect to time span, latitude, and distance from the ice margin. Phanerozoic accumulation rates decrease systematically when averaged over longer time spans. Comparisons were drawn, therefore, at 5 and 57 m.y. time spans to match minimum Marinoan and Sturtian durations, respectively. Cenozoic glaciomarine accumulation also slows with increasing latitude from temperate to polar climates and with increasing distance from the ice margin. After accounting for time span, snowball Earth deposits at low latitude are found to be thinner than would be expected either for high-latitude Cenozoic glacial deposits or for very distal glaciomarine abyssal muds with ice-rafted debris. The rate discrepancy is not readily attributed to overestimates of the total Marinoan or Sturtian durations. If sediment fluxes during warm melt intervals did approach Phanerozoic rates, these intervals must have occupied a much smaller proportion of snowball Earth episodes than in younger glacial climates.

Description: Geological evidence indicates that grounded ice sheets reached sea level at all latitudes during two long-lived Cryogenian (58 and ≥5 My) glaciations. Combined uranium-lead and rhenium-osmium dating suggests that the older (Sturtian) glacial onset and both terminations were globally synchronous. Geochemical data imply that CO 2 was 10 2 PAL (present atmospheric level) at the younger termination, consistent with a global ice cover. Sturtian glaciation followed breakup of a tropical supercontinent, and its onset coincided with the equatorial emplacement of a large igneous province. Modeling shows that the small thermal inertia of a globally frozen surface reverses the annual mean tropical atmospheric circulation, producing an equatorial desert and net snow and frost accumulation elsewhere. Oceanic ice thickens, forming a sea glacier that flows gravitationally toward the equator, sustained by the hydrologic cycle and by basal freezing and melting. Tropical ice sheets flow faster as CO 2 rises but lose mass and become sensitive to orbital changes. Equatorial dust accumulation engenders supraglacial oligotrophic meltwater ecosystems, favorable for cyanobacteria and certain eukaryotes. Meltwater flushing through cracks enables organic burial and submarine deposition of airborne volcanic ash. The subglacial ocean is turbulent and well mixed, in response to geothermal heating and heat loss through the ice cover, increasing with latitude. Terminal carbonate deposits, unique to Cryogenian glaciations, are products of intense weathering and ocean stratification. Whole-ocean warming and collapsing peripheral bulges allow marine coastal flooding to continue long after ice-sheet disappearance. The evolutionary legacy of Snowball Earth is perceptible in fossils and living organisms.

Description: Iron formations (IF) represent an iron-rich rock type that typifies many Archaean and Proterozoic supracrustal successions and are chemical archives of Precambrian seawater chemistry and post-depositional iron cycling. Given that IF accumulated on the seafloor for over two billion years of Earth's early history, changes in their chemical, mineralogical, and isotopic compositions offer a unique glimpse into environmental changes that took place on the evolving Earth. Perhaps one of the most significant events was the transition from an anoxic planet to one where oxygen was persistently present within the marine water column and atmosphere. Linked to this progressive global oxygenation was the evolution of aerobic microbial metabolisms that fundamentally influenced continental weathering processes, the supply of nutrients to the oceans, and, ultimately, diversification of the biosphere and complex life forms. Many of the key recent innovations in understanding IF genesis are linked to geobiology, since biologically assisted Fe(II) oxidation, either directly through photoferrotrophy, or indirectly through oxygenic photosynthesis, provides a process for IF deposition from mineral precursors. The abundance and isotope composition of Fe(II)-bearing minerals in IF additionally suggests microbial Fe(III) reduction, a metabolism that is deeply rooted in the Archaea and Bacteria. Linkages among geobiology, hydrothermal systems, and deposition of IF have been traditionally overlooked, but now form a coherent model for this unique rock type. This paper reviews the defining features of IF and their distribution through the Neoarchaean and Palaeoproterozoic. This paper is an update of previous reviews by Bekker et al. (2010, 2014) that will improve the quantitative framework we use to interpret IF deposition. In this work, we also discuss how recent discoveries have provided new insights into the processes underpinning the global rise in atmospheric oxygen and the geochemical evolution of the oceans.

Description: Geobiology explores how Earth's system has changed over the course of geologic history and how living organisms on this planet are impacted by or are indeed causing these changes. For decades, geologists, paleontologists, and geochemists have generated data to investigate these topics. Foundational efforts in sedimentary geochemistry utilized spreadsheets for data storage and analysis, suitable for several thousand samples, but not practical or scalable for larger, more complex datasets. As results have accumulated, researchers have increasingly gravitated toward larger compilations and statistical tools. New data frameworks have become necessary to handle larger sample sets and encourage more sophisticated or even standardized statistical analyses. In this paper, we describe the Sedimentary Geochemistry and Paleoenvironments Project (SGP; Figure 1), which is an open, community-oriented, database-driven research consortium. The goals of SGP are to (1) create a relational database tailored to the needs of the deep-time (millions to billions of years) sedimentary geochemical research community, including assembling and curating published and associated unpublished data; (2) create a website where data can be retrieved in a flexible way; and (3) build a collaborative consortium where researchers are incentivized to contribute data by giving them priority access and the opportunity to work on exciting questions in group papers. Finally, and more idealistically, the goal was to establish a culture of modern data management and data analysis in sedimentary geochemistry. Relative to many other fields, the main emphasis in our field has been on instrument measurement of sedimentary geochemical data rather than data analysis (compared with fields like ecology, for instance, where the post-experiment ANOVA (analysis of variance) is customary). Thus, the longer-term goal was to build a collaborative environment where geobiologists and geologists can work and learn together to assess changes in geochemical signatures through Earth history. With respect to the data product, SGP is focused on assembling a well-vetted and comprehensive dataset that is tractable to multivariate statistical analyses accounting for multiple geological and methodological biases. Phase 1 of the project, which focused on the Neoproterozoic and Paleozoic, has been completed. Future phases will capture a broader range of geologic time, data types, and geography. The database contains tens of thousands of unpublished data points provided by consortium members, as well as detailed metadata that go beyond what is contained in papers. In many cases, these represent measurements that are tangential to a given published study but still of high utility to database studies; these allow the community to address questions that would be impossible to answer solely with the published data. For instance, in order to use a proxy such as Mo/TOC (total organic carbon) ratios in mudrocks deposited under a euxinic water column, the full suite of trace metal, iron speciation, and total organic carbon data is needed. Likewise, geospatial information is required to account for sampling biases, and many statistical learning approaches cannot accept, or have difficulty with, incomplete geological predictor variables. Ultimately, it is this complete data matrix that will allow for SGP’s most insightful analyses.