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1. New multicellular marine macroalgae from the early Tonian of northwestern Canada

   https://doi.org/10.1130/G48508.1  

   Maloney, Katie M.; Halverson, Galen P.; Schiffbauer, James D.; Xiao, Shuhai; Gibson, Timothy M.; Lechte, Maxwell A.; Cumming, Vivien M.; Millikin, Alexie E.G.; Murphy, Jack G.; Wallace, Malcolm W.; et al (March 2021, Geology)

   Abstract Molecular phylogenetic data suggest that photosynthetic eukaryotes first evolved in freshwater environments in the early Proterozoic and diversified into marine environments by the Tonian Period, but early algal evolution is poorly reflected in the fossil record. Here, we report newly discovered, millimeter- to centimeter-scale macrofossils from outer-shelf marine facies of the ca. 950–900 Ma (Re-Os minimum age constraint = 898 ± 68 Ma) Dolores Creek Formation in the Wernecke Mountains, northwestern Canada. These fossils, variably preserved by iron oxides and clay minerals, represent two size classes. The larger forms feature unbranching thalli with uniform cells, differentiated cell walls, longitudinal striations, and probable holdfasts, whereas the smaller specimens display branching but no other diagnostic features. While the smaller population remains unresolved phylogenetically and may represent cyanobacteria, we interpret the larger fossils as multicellular eukaryotic macroalgae with a plausible green algal affinity based on their large size and presence of rib-like wall ornamentation. Considered as such, the latter are among the few green algae and some of the largest macroscopic eukaryotes yet recognized in the early Neoproterozoic. Together with other Tonian fossils, the Dolores Creek fossils indicate that eukaryotic algae, including green algae, colonized marine environments by the early Neoproterozoic Era. 

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2. Preservation of early Tonian macroalgal fossils from the Dolores Creek Formation, Yukon

   https://doi.org/10.1038/s41598-022-10223-x  

   Maloney, Katie M.; Schiffbauer, James D.; Halverson, Galen P.; Xiao, Shuhai; Laflamme, Marc (December 2022, Scientific Reports)

   Abstract The rise of eukaryotic macroalgae in the late Mesoproterozoic to early Neoproterozoic was a critical development in Earth’s history that triggered dramatic changes in biogeochemical cycles and benthic habitats, ultimately resulting in ecosystems habitable to animals. However, evidence of the diversification and expansion of macroalgae is limited by a biased fossil record. Non-mineralizing organisms are rarely preserved, occurring only in exceptional environments that favor fossilization. Investigating the taphonomy of well-preserved macroalgae will aid in identifying these target environments, allowing ecological trends to be disentangled from taphonomic overprints. Here we describe the taphonomy of macroalgal fossils from the Tonian Dolores Creek Formation (ca. 950 Ma) of northwestern Canada (Yukon Territory) that preserves cm-scale macroalgae. Analytical microscopy, including scanning electron microscopy and tomographic x-ray microscopy, was used to investigate fossil preservation, which was the result of a combination of pyritization and aluminosilicification, similar to accessory mineralization observed in Paleozoic Burgess Shale-type fossils. These new Neoproterozoic fossils help to bridge a gap in the fossil record of early algae, offer a link between the fossil and molecular record, and provide new insights into evolution during the Tonian Period, when many eukaryotic lineages are predicted to have diversified. 

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3. Thallium isotopic evidence for the Tonian rise and Cryogenian fall of Neoproterozoic oxygen levels

   https://doi.org/10.1130/G53625.1  

   Webb, Lucy C; Macdonald, Francis A; Halverson, Galen P; Ostrander, Chadlin M; Nielsen, Sune G; Sperling, Erik A (January 2026, Geology)

   Abstract Estimating dissolved oxygen (O2) concentrations in seawater during the Neoproterozoic is central to testing hypotheses about the role of O2 in animal evolution. Here we apply the thallium (Tl) isotope redox proxy to samples stratigraphically below the ca. 810-million-year-old (Ma) Bitter Springs Carbon Isotope Excursion and spanning the interval between the two Snowball Earth glaciations (ca. 662–650 Ma) to constrain the evolution of Neoproterozoic bottom water redox conditions. Thallium isotopes can be used to reconstruct the global extent of oxygenated oceanic bottom waters because the primary control on seawater Tl isotope compositions (ε205Tl) over million-year time scales is changes in the amount of 205Tl removal by Mn oxides on the seafloor. Samples spanning an ~20-m.y. period preceding the Bitter Springs excursion from the Tonian Reefal Assemblage (n = 18/30) yield ε205Tlauth values lower than global oceanic inputs (ε205Tl ~–2±), with some samples approaching the modern seawater ε205Tl value of –6±. These sustained low ε205Tlauth values require enhanced burial of Mn oxides elsewhere on the seafloor, which we interpret as evidence for the oxygenation of the deep ocean in the Tonian. In contrast, the majority of samples from the Cryogenian Hay Creek Group (n = 13/16) yield ε205Tlauth values similar to global oceanic inputs, suggesting that the deep ocean was not ventilated at this time. This indicates that Earth’s deep ocean was not gradually oxygenated throughout the Neoproterozoic, but rather experienced intervals of increased and decreased O2 concentrations. 

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4. Localized and Transient Oxygenation of Shallow Oceans of Southwestern Laurentia at the Ediacaran–Cambrian Boundary

   https://doi.org/10.1111/gbi.70039  

   Chanchai, Watsawan; Nelson, Lyle L; Smith, Emily F; Hardisty, Dalton S; Lonsdale, Mary C; Burke, Janet E; Lau, Kimberly V (January 2026, Geobiology)

   ABSTRACT The Ediacaran–Cambrian boundary, which precedes one of the most significant biotic diversification events in Earth's history, is associated with a global negative carbon isotope excursion termed the BAsal Cambrian carbon isotope Excursion (BACE). Late Ediacaran and early Cambrian changes in shallow marine oxygenation have been proposed to relate to the BACE as well as metazoan extinction and radiation. However, reconstructing paleoredox conditions at the Ediacaran–Cambrian boundary is limited by challenges in correlating carbonate strata due to sparse stratigraphic markers and non‐unique chemostratigraphic correlations. These imprecise correlations have led to uncertainty in how redox changes across the BACE should be interpreted in relation to broader regional and global environmental patterns. Here, we present redox reconstructions from southwestern Laurentian carbonate successions that record the BACE, including the limestone‐dominated Deep Spring Formation, southwestern USA, and the dolostone‐dominated La Ciénega Formation, northern Mexico. We combine local (carbonate‐bound iodine, I/(Ca + Mg) and cerium anomaly, Ce/Ce*) and global (carbonate‐associated uranium isotopes, δ²³⁸U_carb) redox proxies to investigate marine oxygenation in relation to the BACE. Contrary to previous suggestions that a global ocean oxygenation event coincided with the BACE, we do not observe a shift in δ²³⁸U_carbconcurrent with the carbon isotope excursion in either section. The δ²³⁸U_carbvalues differ between two sections, likely reflecting distinct diagenetic offsets attributed to different diagenetic U reduction, but together provide a minimal constraint on the carbonate δ²³⁸U value and suggest a more anoxic ocean compared to today. The local proxy results at both sites suggest widespread low‐oxygen surface waters with a transient and localized interval of shallow marine oxygenation at one site that coincides with the nadir of the BACE. Persistently low I/(Ca + Mg) ratios, below values observed in today's oxygenated oceans, suggest a broadly redox‐stratified surface ocean. Negative Ce anomalies in the La Ciénega Formation were recorded during the BACE nadir, suggesting a short‐lived interval of local oxygenation within otherwise low‐oxygen conditions. In sum, we do not find evidence for major, widespread oxygenation coincident with the BACE, but a continuation of low‐oxygen conditions punctuated by a short‐lived oxygenation event in the shallow oceans. These brief fluctuations in oxygen levels, in turn, may have played a role in the onset of behavioral complexity among bilaterian invertebrates during this critical transition. 

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5. Late inception of a resiliently oxygenated upper ocean

   https://doi.org/10.1126/science.aar5372  

   Lu, Wanyi; Ridgwell, Andy; Thomas, Ellen; Hardisty, Dalton S.; Luo, Genming; Algeo, Thomas J.; Saltzman, Matthew R.; Gill, Benjamin C.; Shen, Yanan; Ling, Hong-Fei; et al (May 2018, Science)

   Rising oceanic and atmospheric oxygen levels through time have been crucial to enhanced habitability of surface Earth environments. Few redox proxies can track secular variations in dissolved oxygen concentrations ([O2]) around threshold levels for metazoan survival in the upper ocean. We present an extensive compilation of iodine to calcium ratios (I/Ca) in marine carbonates. Our record supports a major rise in atmospheric pO2 at ~400 million years ago (Ma), and reveals a step-change in the oxygenation of the upper ocean to relatively sustainable near-modern conditions at ~200 Ma. An Earth system model demonstrates that a shift in organic matter remineralization to greater depths, which may have been due to increasing size and biomineralization of eukaryotic plankton, likely drove the I/Ca signals at ~200 Ma 

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