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1. Upper Windermere Supergroup and the transition from rifting to continent-margin sedimentation, Nadaleen River area, northern Canadian Cordillera

   https://doi.org/10.1130/B32039.1  

   Moynihan, David P. ; Strauss, Justin V. ; Nelson, Lyle L. ; Padget, Colin D. ( March 2019 , GSA Bulletin)

   Abstract Neoproterozoic–Cambrian rocks of the Windermere Supergroup and overlying units record the breakup of Rodinia and formation of the northwestern Laurentian ancestral continental margin. Understanding the nature and timing of this transition has been hampered by difficulty correlating poorly dated sedimentary successions from contrasting depositional settings across Mesozoic structures. Here we present new litho- and chemo-stratigraphic data from a Cryogenian–lower Cambrian succession in east-central Yukon (Canada), establish correlations between proximal and distal parts of the upper Windermere Supergroup and related strata in the northern Canadian Cordillera, and consider implications for the formation of the northwestern Laurentian margin. The newly defined Nadaleen Formation hosts the first appearance of Ediacaran macrofossils, while the overlying Gametrail Formation features a large negative carbon isotope anomaly with δ13Ccarb values as low as –13‰ that correlates with the globally developed Shuram-Wonoka anomaly. We also define the Rackla Group, which includes the youngest (Ediacaran) portions of the Windermere Supergroup in the northern Cordillera. The top of the Windermere Supergroup is marked by an unconformity above the Risky Formation that passes into a correlative conformity in the Nadaleen River area. This surface has been interpreted to mark the top of the rift-related succession, but we draw attention to evidence for tectonic instability through the early-middle Cambrian and argue that the transition from rifting to post-rift thermal subsidence is marked by a widespread unconformity that underlies upper Cambrian carbonate rocks. This is younger than the interpreted age of the rift to post-rift transition elsewhere along the ancestral western Laurentian continental margin. 

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2. Calibrating the Russøya excursion in Svalbard, Norway, and implications for Neoproterozoic chronology

   https://doi.org/10.1130/G49593.1  

   Millikin, Alexie E.G. ; Strauss, Justin V. ; Halverson, Galen P. ; Bergmann, Kristin D. ; Tosca, Nicholas J. ; Rooney, Alan D. ( February 2022 , Geology)

   Abstract The Tonian–Ediacaran Hecla Hoek succession of Svalbard, Norway, represents one of the most complete and well-preserved Neoproterozoic sedimentary successions worldwide. With diverse fossil assemblages, an extensive carbonate δ13C record, and sedimentary evidence for two distinct Cryogenian glaciations, this succession will continue to yield insights into the Neoproterozoic Earth system; however, at present there are no direct radiometric age constraints for these strata. We present two new Re-Os ages and initial Os isotope data that constrain the timing of Neoproterozoic glaciation in Svalbard, providing further support for two globally synchronous Cryogenian glaciations and insight into pre- and post-snowball global weathering conditions. An age from the Russøya Member (Elbobreen Formation) facilitates correlation of the negative carbon isotope excursion recorded therein with the pre-glacial “Islay” excursion of the Callison Lake Formation of northwestern Canada and the Didikama and Matheos Formations of Ethiopia. We propose that this globally synchronous ca. 735 Ma carbon isotope excursion be referred to as the Russøya excursion with northeastern Svalbard as the type locality. This new age provides an opportunity to construct a time-calibrated geological framework in Svalbard to assess connections between biogeochemical cycling, evolutionary innovations within the eukaryotes, and the most extreme climatic changes in Earth history. 

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3. Uncovering the spatial heterogeneity of Ediacaran carbon cycling

   https://doi.org/10.1111/gbi.12222  

   Li, C. ; Hardisty, D. S. ; Luo, G. ; Huang, J. ; Algeo, T. J. ; Cheng, M. ; Shi, W. ; An, Z. ; Tong, J. ; Xie, S. ; et al ( December 2016 , Geobiology)

   Records of the Ediacaran carbon cycle (635–541 million years ago) include the Shuram excursion (SE), the largest negative carbonate carbon isotope excursion in Earth history (down to −12‰). The nature of this excursion remains enigmatic given the difficulties of interpreting a perceived extreme global decrease in the δ¹³C of seawater dissolved inorganic carbon. Here, we present carbonate and organic carbon isotope (δ¹³C_carband δ¹³C_org) records from the Ediacaran Doushantuo Formation along a proximal‐to‐distal transect across the Yangtze Platform of South China as a test of the spatial variation of theSE. Contrary to expectations, our results show that the magnitude and morphology of this excursion and its relationship with coexisting δ¹³C_orgare highly heterogeneous across the platform. Integrated geochemical, mineralogical, petrographic, and stratigraphic evidence indicates that theSEis a primary marine signature. Data compilations demonstrate that theSEwas also accompanied globally by parallel negative shifts of δ³⁴S of carbonate‐associated sulfate (CAS) and increased⁸⁷Sr/⁸⁶Sr ratio and coastalCASconcentration, suggesting elevated continental weathering and coastal marine sulfate concentration during theSE. In light of these observations, we propose a heterogeneous oxidation model to explain the high spatial heterogeneity of theSEand coexisting δ¹³C_orgrecords of the Doushantuo, with likely relevance to theSEin other regions. In this model, we infer continued marine redox stratification through theSEbut with increased availability of oxidants (e.g., O₂and sulfate) limited to marginal near‐surface marine environments. Oxidation of limited spatiotemporal extent provides a mechanism to drive heterogeneous oxidation of subsurface reduced carbon mostly in shelf areas. Regardless of the mechanism driving theSE, future models must consider the evidence for spatial heterogeneity in δ¹³C presented in this study.

    

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4. Combined Magnetostratigraphy From Three Localities of the Rainstorm Member of the Johnnie Formation in California and Nevada, United States Calibrated by Cyclostratigraphy: A 13 R/Ma Reversal Frequency for the Ediacaran

   https://doi.org/10.3389/feart.2021.764714  

   Kodama, Kenneth P. ( November 2021 , Frontiers in Earth Science)

   A combined magnetostratigraphy for the Rainstorm Member of the Ediacaran Johnnie Formation was constructed using the sediment accumulation rates determined by rock magnetic cyclostratigraphy for three localities of the Rainstorm Member to provide a high resolution, time-calibrated record of geomagnetic field reversal frequency at a critical time period in Earth history. Two previously reported magnetostratigraphy records from Death Valley, California, the Nopah Range and Winters Pass Hills ( Minguez et al., 2015 ), were combined with new paleomagnetic and cyclostratigraphic results from the Desert Range locality of the Rainstorm Member in south central Nevada, United States . The Johnnie oolite marker bed is at the base of each of the three sections and allows their regional correlation. The Nopah Range and Desert Range localities have similar sediment accumulation rates of ∼5 cm/ka, so their stratigraphic sections can be combined directly. The Winters Pass Hills locality has a higher sediment accumulation rate of 8.4 cm/ka, therefore its stratigraphic positions are multiplied by 0.6 to combine with the Desert Range and Nopah Range magnetostratigraphy. The thermal demagnetization results from the Desert Range locality isolates characteristic remanent magnetizations that indicate two nearly antipodal east-west and shallow directions and a mean paleopole (11.7˚N, 348.4˚E) that is consistent with “shallow” Ediacaran directions. The Desert Range also yields a magnetic susceptibility rock magnetic cyclostratigraphy that records short eccentricity, obliquity, and precession astronomically-forced climate cycles in the Ediacaran. The high-resolution combined magnetostratigraphy with nearly meter-scale stratigraphic spacing (nominally 23 ka, based on the Desert Range sediment accumulation rate), indicates 11 polarity intervals in a cyclostratigraphy-calibrated duration of 849 ka, indicating a reversal frequency of 13 R/Ma. The Rainstorm Member records the Shuram carbon isotope excursion, hence its age is ∼574 Ma. Given the recent cyclostratigraphy-calibrated reversal frequency of 20 R/Ma from the Zigan Formation ( Levashova et al., 2021 ) at 547 Ma, our results show that reversal frequency was high but fluctuated during the Ediacaran. 

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5. Integrated carbon, sulfur, and nitrogen isotope chemostratigraphy of the Ediacaran Lantian Formation in South China: Spatial gradient, ocean redox oscillation, and fossil distribution

   https://doi.org/10.1111/gbi.12226  

   Wang, W. ; Guan, C. ; Zhou, C. ; Peng, Y. ; Pratt, L. M. ; Chen, X. ; Chen, L. ; Chen, Z. ; Yuan, X. ; Xiao, S. ( January 2017 , Geobiology)

   The Ediacaran Doushantuo Formation in South China is a prime target for geobiological investigation because it offers opportunities to integrate chemostratigraphic and paleobiological data. Previous studies were mostly focused on successions in shallow‐water shelf facies, but data from deep‐water successions are needed to fully understand basinal redox structures. Here, we report δ¹³C_carb, δ¹³C_org, δ³⁴S_pyr, δ³⁴S_CAS, and δ¹⁵N_seddata from a drill core of the fossiliferous Lantian Formation, which is a deep‐water equivalent of the Doushantuo Formation. Our data confirm a large (>10‰) spatial gradient in δ¹³C_carbin the lower Doushantuo/Lantian formations, but this gradient is probably due to the greater sensitivity of carbonate‐poor deep‐water sediments to isotopic mixing with¹³C‐depleted carbonate cements. A pronounced negative δ¹³C_carbexcursion (EN3) in the upper Doushantuo/Lantian formations, however, is spatially consistent and may be an equivalent of the Shuram excursion. δ³⁴S_pyris more negative in deeper‐water facies than in shallow‐water facies, particularly in the lower Doushantuo/Lantian formations, and this spatial pattern is interpreted as evidence for ocean redox stratification: Pyrite precipitated in euxinic deep waters has lower δ³⁴S_pyrthan that formed within shallow‐water sediments. The Lantian Formation was probably deposited in oscillating oxic and euxinic conditions. Euxinic black shales have higherTOCandTNcontents, but lower δ³⁴S_pyrand δ¹⁵N_sedvalues. In euxinic environments, pyrite was predominantly formed in the water column and organic nitrogen was predominantly derived from nitrogen fixation orNH₄⁺assimilation because of quantitative denitrification, resulting in lower δ³⁴S_pyrand δ¹⁵N_sedvalues. Benthic macroalgae and putative animals occur exclusively in euxinic black shales. If preserved in situ, these organisms must have lived in brief oxic episodes punctuating largely euxinic intervals, only to be decimated and preserved when the local environment switched back to euxinia again. Thus, taphonomy and ecology were the primary factors controlling the stratigraphic distribution of macrofossils in the Lantian Formation.

    

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