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A geologically rapid Neoproterozoic oxygenation event is commonly linked to the appearance of marine animal groups in the fossil record. However, there is still debate about what evidence from the sedimentary geochemical record—if any—provides strong support for a persistent shift in surface oxygen immediately preceding the rise of animals. We present statistical learning analyses of a large dataset of geochemical data and associated geological context from the Neoproterozoic and Palaeozoic sedimentary record and then use Earth system modelling to link trends in redox-sensitive trace metal and organic carbon concentrations to the oxygenation of Earth’s oceans and atmosphere. We do not find evidence for the wholesale oxygenation of Earth’s oceans in the late Neoproterozoic era. We do, however, reconstruct a moderate long-term increase in atmospheric oxygen and marine productivity. These changes to the Earth system would have increased dissolved oxygen and food supply in shallow-water habitats during the broad interval of geologic time in which the major animal groups first radiated. This approach provides some of the most direct evidence for potential physiological drivers of the Cambrian radiation, while highlighting the importance of later Palaeozoic oxygenation in the evolution of the modern Earth system.

The Bylot basins of northeastern Canada and northwestern Greenland comprise the Borden, Aston-Hunting, Fury and Hecla, and Thule basins. This system of late Mesoproterozoic (c. 1.27–1.0 Ga) sedimentary basins preserves an important record of present day northeastern Laurentia coincident with the emplacement of the Mackenzie large igneous province, the Shawinigan and Ottawan phases of the Grenville Orogeny, and the development of the Midcontinent Rift. However, establishing correlations between the sedimentary successions of the Bylot basins has been hindered by the absence of robust chronostratigraphic constraints. As a result, the degree to which these basins were interconnected, whether they share a common tectonostratigraphic history, and how their sedimentary patterns relate to regional tectonic events remain open questions. Recent Re–Os geochronology from organic-rich strata has yielded depositional ages from the Borden (1048 and 1046 Ma) and Fury and Hecla (1087 Ma) basins, which we integrate with existing models for the depositional history of these basins to derive three tectonostratigraphic assemblages from the Bylot basins. We project our refined tectonostratigraphic framework for the Borden and Fury and Hecla successions to Greenland to establish a testable hypothesis for how the Thule Supergroup fits into this tectonostratigraphic picture. [ABSTRACT FROM AUTHOR]

Microorganisms that transform and oxidize organic material (that is, heterotrophs) play a fundamental role in the geochemical cycling of key elements in the ocean. Through their growth and activity, heterotrophic microorganisms degrade much of the organic matter produced by phytoplankton in the surface ocean, leading to the regeneration and redistribution of nutrients and carbon back into the water column. However, most organic matter is physically too large to be taken up directly by heterotrophic microorganisms. Consequently, many heterotrophs secrete exoenzymes that break down large molecules outside the cell into smaller substrates that can then be directly taken up by the cell. The complex nature of the biochemical systems that microorganisms use to secrete these enzymes suggests that they were unlikely to have been present in the earliest heterotrophs. In a pre-exoenzyme ocean, heterotrophic microorganisms would only be able to access a small fraction of organic matter such that most dead phytoplankton biomass would have passed directly through the water column and settled onto the seafloor. Here we synthesize existing geobiological evidence to examine the fate of organic matter in the absence of exoenzymes in early oceans. We propose that on an Earth before exoenzymes, organic matter preservation, metal availability and phosphorus recycling would have operated differently than they do on the contemporary Earth.

Correlations within and between Precambrian basins are heavily reliant on precise dating of volcanic units (i.e., tuff beds and lava flows) in the absence of biostratigraphy. However, felsic tuffs and lavas are rare or absent in many basins, and direct age determinations of Precambrian basaltic lavas have proven to be challenging. In this paper, we report the first successful application of 40Ar/39Ar dating to pyroxene from a Neoproterozoic basalt unit, the Keene Basalt in the Officer Basin of central Australia. 40Ar/39Ar analyses of igneous pyroxene crystals yielded an age of 752 ± 4 Ma (mean squared weighted deviation = 0.69, p= 72%), which is underpinned by 40Ar/39Ar plagioclase age (753.04 ± 0.84 Ma) from the basalt. This age is significant because the Keene Basalt is one of the very few extrusive igneous rocks identified within the Neoproterozoic successions of central Australia and is potentially an important time marker for correlating the Neoproterozoic stratigraphy within, and beyond, the central Australian basins. Our geochronological and geochemical data show that the Keene Basalt, which is characterized by enriched elemental and Nd‐Pb isotopic signatures, is strikingly similar to, and coeval with, the 755 ± 3 Ma Mundine Well Dolerite in northwestern Australia. Here we suggest that both are part of the same large igneous province (~6.5 × 105km2) related to breakup of the supercontinent Rodinia. This study demonstrates the potential of pyroxene 40Ar/39Ar geochronology to date ancient flood basalts and to provide pivotal time constraints for stratigraphic correlations of Precambrian basins. Ancient basaltic lavas erupted on Earth's crust hold critical clues about the tectonic and igneous history of geologic past and are important for correlating Precambrian stratigraphy and reconstructing supercontinents. However, constraining the crystallization age of basaltic lavas proves challenging due to the dearth of commonly used uranium‐lead geochronometers such as zircon in such rocks. In this research, we applied 40Ar/39Ar dating of pyroxene and plagioclase to establish the eruption age of a critical basalt lava suite, the Keene Basalt, in the Neoproterozoic Officer Basin in central Australia. The new age, combined with geochemical constraints, contributes to conclusively defining the genetic affinity and geodynamic setting of the basalt unit. Application of pyroxene 40Ar/39Ar dating to terrestrial basalts represents an important technical breakthrough, and we report the first successful result for a Precambrian basalt. The robustly dated Keene Basalt is an important time marker for correlating the Neoproterozoic successions and associated glacial deposits in Australia and worldwide. The new age and geochemistry of the basalt are used to test its relationship to known Neoproterozoic igneous events. Our data, integrated with existing evidence, reveals a 755‐ to 750‐Ma large igneous province related to Rodinia breakup. We report the first application of pyroxene 40Ar/39Ar dating to a Precambrian basaltic lava, the Keene Basalt in central AustraliaThe Keene Basalt is correlated with the Mundine Well Dolerite, both being part of a 755‐ to 750‐Ma LIPThe robustly dated Keene Basalt is an important time marker for correlating Neoproterozoic successions in Australia and worldwide

The Tonian and Cryogenian periods (ca. 1000–635.5 Ma) witnessed important biological and climatic events, including diversification of eukaryotes, the rise of algae as primary producers, the origin of Metazoa, and a pair of Snowball Earth glaciations. The Tonian and Cryogenian will also be the next periods in the geological time scale to be formally defined. Time-calibrating this interval is essential for properly ordering and interpreting these events and establishing and testing hypotheses for paleoenvironmental change. Here, we briefly review the methods by which the Proterozoic time scale is dated and provide an up-to-date compilation of age constraints on key fossil first and last appearances, geological events, and horizons during the Tonian and Cryogenian periods. We also develop a new age model for a ca. 819–740 Ma composite section in Svalbard, which is unusually complete and contains a rich Tonian fossil archive. This model provides useful preliminary age estimates for the Tonian succession in Svalbard and distinct carbon isotope anomalies that can be globally correlated and used as an indirect dating tool.

The global biosphere is commonly assumed to have been less productive before the rise of complex eukaryotic ecosystems than it is today1. However, direct evidence for this assertion is lacking. Here we present triple oxygen isotope measurements (∆17O) from sedimentary sulfates from the Sibley basin (Ontario, Canada) dated to about 1.4 billion years ago, which provide evidence for a less productive biosphere in the middle of the Proterozoic eon. We report what are, to our knowledge, the most-negative ∆17O values (down to −0.88‰) observed in sulfates, except for those from the terminal Cryogenian period2. This observation demonstrates that the mid-Proterozoic atmosphere was distinct from what persisted over approximately the past 0.5 billion years, directly reflecting a unique interplay among the atmospheric partial pressures of CO2and O2and the photosynthetic O2flux at this time3. Oxygenic gross primary productivity is stoichiometrically related to the photosynthetic O2flux to the atmosphere. Under current estimates of mid-Proterozoic atmospheric partial pressure of CO2(2–30 times that of pre-anthropogenic levels), our modelling indicates that gross primary productivity was between about 6% and 41% of pre-anthropogenic levels if atmospheric O2was between 0.1–1% or 1–10% of pre-anthropogenic levels, respectively. When compared to estimates of Archaean4–6and Phanerozoic primary production7, these model solutions show that an increasingly more productive biosphere accompanied the broad secular pattern of increasing atmospheric O2over geologic time8. Triple oxygen isotope measurements of 1.4-billion-year-old sedimentary sulfates reveal a unique mid-Proterozoic atmosphere and demonstrate that gross primary productivity in the mid-Proterozoic was between 6% and 41% of pre-anthropogenic levels.

On timescales significantly greater than 105years, atmospheric pCO2is controlled by the rate of mantle outgassing relative to the set-point of the silicate weathering feedback. The weathering set-point has been shown to depend on the distribution and characteristics of rocks exposed at the Earth’s surface, vegetation types and topography. Here we argue that large-scale climate impacts caused by changes in ocean circulation can also modify the weathering set-point and show evidence suggesting that this played a role in the establishment of the Antarctic ice sheet at the Eocene–Oligocene boundary. In our simulations, tectonic deepening of the Drake Passage causes freshening and stratification of the Southern Ocean, strengthening the Atlantic meridional overturning circulation and consequently raising temperatures and intensifying rainfall over land. These simulated changes are consistent with late Eocene tectonic reconstructions that show Drake Passage deepening, and with sediment records that reveal Southern Ocean stratification, the emergence of North Atlantic Deep Water, and a hemispherically asymmetric temperature change. These factors would have driven intensified silicate weathering and can thereby explain the drawdown of carbon dioxide that has been linked with Antarctic ice sheet growth. We suggest that this mechanism illustrates another way in which ocean–atmosphere climate dynamics can introduce nonlinear threshold behaviour through interaction with the geologic carbon cycle.