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The geologically rapid appearance of fossils of modern animal phyla within Cambrian strata is a defining characteristic of the history of life on Earth. However, temporal calibration of the base of the Cambrian Period remains uncertain within millions of years, which has resulted in mounting challenges to the concept of a discrete Cambrian explosion. We present precise zircon U–Pb dates for the lower Wood Canyon Formation, Nevada. These data demonstrate the base of the Cambrian Period, as defined by both ichnofossil biostratigraphy and carbon isotope chemostratigraphy, was younger than 533 Mya, at least 6 My later than currently recognized. This new geochronology condenses previous age models for the Nemakit–Daldynian (early Cambrian) and, integrated with global records, demonstrates an explosive tempo to the early radiation of modern animal phyla. 

The end-Triassic extinction (ETE) event represents one of the ‘big five’ episodes of mass extinction. The leading hypothesis for the cause of the ETE is the intrusion of voluminous magmas of the Central Atlantic Magmatic Province (CAMP) into carbon-rich sediments of two South American sedimentary basins, around 201.5 Ma. The timing of dikes and sills emplacement, however, must be considered in light of age models from CAMP rocks occurring in North America. In this work, we present new high-precision ages for critical samples in NE Brazil (201.579 ± 0.057 Ma) and Canada (201.464 ± 0.017 Ma), in order to evaluate how the South and North American magmatic events compare at the 100-ka level, and to the ETE timing. We also discuss inter-laboratory reproducibility of high-precision CAMP ages, including the²³⁰Th disequilibrium corrections that are made to zircon U–Pb dates. Our findings in this newly discovered extension of the CAMP large igneous province in NE Brazil support the hypothesis that the CAMP may be responsible for the ETE through the triggering of greenhouse gas release from magma-evaporite interactions (contact metamorphism) in the South American basins.

 

The Wilkins Peak Member (WPM) of the Green River Formation in Wyoming, USA, comprises alternating lacustrine and alluvial strata that preserve a record of terrestrial climate during the early Eocene climatic optimum. We use a Bayesian framework to develop age-depth models for three sites, based on new 40Ar/39Ar sanidine and 206Pb/238U zircon ages from seven tuffs. The new models provide two- to ten-fold increases in temporal resolution compared to previous radioisotopic age models, confirming eccentricity-scale pacing of WPM facies, and permitting their direct comparison to astronomical solutions. Starting at ca. 51 Ma, the median ages for basin-wide flooding surfaces atop six successive alluvial marker beds coincide with short eccentricity maxima in the astronomical solutions. These eccentricity maxima have been associated with hyperthermal events recorded in marine strata during the early Eocene. WPM strata older than ca. 51 Ma do not exhibit a clear relationship to the eccentricity solutions, but accumulated 31%−35% more rapidly, suggesting that the influence of astronomical forcing on sedimentation was modulated by basin tectonics. Additional high-precision radioisotopic ages are needed to reduce the uncertainty of the Bayesian model, but this approach shows promise for unambiguous evaluation of the phase relationship between alluvial marker beds and theoretical eccentricity solutions. 

Abstract The Steptoean Positive Isotopic Carbon Excursion (SPICE) is a prominent +4–5‰ shift in the Cambrian δ13C record used for global chronostratigraphic correlation. The onset of this excursion is traditionally placed at the base of the Pterocephaliid trilobite biomere (base of the Furongian Series). Recent studies have documented local controls on the expression of the SPICE and emphasize the need for chronostratigraphic standards for these complex biogeochemical signals. We build upon prior work in western Laurentia by integrating δ13C and biostratigraphy with high-precision isotope dilution U-Pb detrital zircon maximum depositional ages that are coincident with the onset, peak, and falling limb of the SPICE. Our study provides the first useful numerical age constraint for the onset of the SPICE and the Laurentian trilobite biozones and requires revision of the late Cambrian geologic time scale boundaries by several million years. 

Volcanic highlands in the tropics increased global weatherability ~0.9–1.6 Ma before onset of Cryogenian Snowball Earth. 

Abstract Tandem in situ and isotope dilution U-Pb analysis of zircons from pyroclastic volcanic rocks and both glacial and non-glacial sedimentary strata of the Pocatello Formation (Idaho, northwestern USA) provides new age constraints on Cryogenian glaciation in the North American Cordillera. Two dacitic tuffs sampled within glacigenic strata of the lower diamictite interval of the Scout Mountain Member yield high-precision chemical abrasion isotope dilution U-Pb zircon eruption and depositional ages of 696.43 ± 0.21 and 695.17 ± 0.20 Ma. When supplemented by a new high-precision detrital zircon maximum depositional age of ≤670 Ma for shoreface and offshore sandstones unconformably overlying the lower diamictite, these data are consistent with correlation of the lower diamictite to the early Cryogenian (ca. 717–660 Ma) Sturtian glaciation. These 670–675 Ma zircons persist in beds above the upper diamictite and cap dolostone units, up to and including a purported “reworked fallout tuff,” which we instead conclude provides only a maximum depositional age of ≤673 Ma from epiclastic volcanic detritus. Rare detrital zircons as young as 658 Ma provide a maximum depositional age for the upper diamictite and overlying cap dolostone units. This new geochronological framework supports litho- and chemostratigraphic correlations of the lower and upper diamictite intervals of the Scout Mountain Member of the Pocatello Formation with the Sturtian (716–660 Ma) and Marinoan (≤650–635 Ma) low-latitude glaciations, respectively. The Pocatello Formation thus contains a more complete record of Cryogenian glaciations than previously postulated. 

In situ apatite U-Pb petrochronology and Sr-Nd isotope geochemistry requires well-characterized and matrix-matched references materials (RMs), yet only a few suitable apatite RMs are currently available. To ameliorate this issue, we determined the U-Pb, Sm-Nd, and Sr isotopic and elemental compositions of a suite of prospective apatite RMs using isotope dilution (ID) TIMS and laser ablation (LA) ICP-MS. The two RMs, from Morocco (MRC-1) and Brazil (BRZ-1), are cm-sized and available in significant quantities. The U-Pb ID-TIMS data yield an isochron age of 153.3 ± 0.2 Ma for MRC-1. This age is consistent with laser ablation split stream ICP-MS (LASS) analyses that produce an isochron age of 152.7 ± 0.6 Ma. The weighted mean of ID-TIMS analyses for 143Nd/144Nd analyses is 0.512677 ± 3, for 147Sm/144Nd is 0.10923 ± 9, and for 87Sr/86Sr is 0.707691 ± 2. The range and mean of TIMS Sm-Nd isotopic data are reproducible by LA-ICP-MS, but laser ablation Sr data are consistently offset towards more radiogenic values. For BRZ-1 apatite, ID-TIMS U-Pb analyses are dispersed, but a subset of the data yields a coherent age intercept of 2078 ± 13 Ma. The vast majority of LASS spot transects across the apatite produce an isochron that define a younger age of 2038 ± 14 Ma. We interpret this as incorporation of cryptic, younger altered domains within BRZ-1. Discordant U-Pb spot analyses are associated with chemically distinct cracks, likely a result of fluid infiltration. The weighted means of ID-TIMS analyses of BRZ-1 yield 143Nd/144Nd = 0.510989 ± 5, 147Sm/144Nd = 0.10152 ± 8, and 87Sr/86Sr = 0.709188 ± 3. The distribution of Nd isotopic compositions of this RM measured by LA-MC-ICP-MS analyses are comparable to TIMS analyses. By contrast, 87Sr/86Sr measurements by LA-ICP-MS are inaccurate and exhibit large uncertainties, but this RM can be useful for empirically correcting in situ 87Sr/86Sr measurements. The data indicate that MRC-1 apatite may serve well as a U-Pb, Sm-Nd, and Sr RM, whereas BRZ-1 apatite has the most potential as a Sm-Nd RM. These potential RMs provide new benchmarks for in situ apatite chemical analyses and inter-laboratory calibrations. 

Abstract The numerous biotic, climatic, and tectonic events of the Devonian cannot be correlated and investigated without a well-calibrated time scale. Here, we updated the calibration of the Devonian time scale using a Bayesian age-depth model that incorporates radioisotopic ages and astrochronology durations. We used existing radioisotopic ages collected and harmonized in the last two geologic time scale compilations, as well as new U-Pb zircon ages from Emsian {Hercules I K-bentonite, Wetteldorf, Germany: 394.290 ± 0.097(0.21)[0.47] Ma} and Eifelian K-bentonites {Tioga B and Tioga F K-bentonites, Fayette, New York, USA: 390.82 ± 0.18(0.26)[0.48] Ma and 390.14 ± 0.14(0.23)[0.47] Ma, respectively}. We anchored floating astrochronology stage durations on radioisotopic ages and chained astrochronologic constraints and uncertainty together to extrapolate conditioning age likelihoods up or down the geologic time scale, which is a new method for integrating astrochronology into age-depth modeling. The modeling results in similar ages and durations for Devonian stages regardless of starting biostratigraphic scaling assumptions. We produced a set of rescaled biostratigraphic zonations, and a new numerical calibration of Devonian stage boundary ages with robust uncertainty estimates, which allow us to evaluate future targets for Devonian time scale research. These methods are broadly applicable for time scale work and provide a template for an integrated stratigraphic approach to time scale modeling.