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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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The Steptoean Positive Isotopic Carbon Excursion (SPICE) is a prominent +4--5%o 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 813C 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. [ABSTRACT FROM AUTHOR]

The Orosirian Period (2050–1800 Ma) of the Paleoproterozoic Era represents an interval between the Great Oxidation Event and Lomagundi-Jatuli Excursion, and the apparent environmental stability of the late Paleoproterozoic to mid-Mesoproterozoic (1800–1300 Ma). Here, we present stratigraphic data, carbon isotope chemostratigraphy, element abundances, and U–Pb zircon depositional ages from the Belcher Group in subarctic Canada, providing an unparallelled global reference section for the Orosirian Period. U–Pb dates of 2018.5 ± 1.0 Ma and 2015.4 ± 1.8 Ma for two tuffs in the Kasegalik Formation (lowest Belcher Group) provide the first robust age constraints for Earth's earliest unambiguous cyanobacterial fossil, Eoentophysalis belcherensis. A tuffaceous shale ∼4 km stratigraphically higher, at the contact of the Flaherty and Omarolluk formations (1854.2 ± 1.6 Ma), provides an age for foreland basin development associated with the collision of the Superior and Hearne cratons. Carbonate carbon isotope data show smooth fluctuations ranging from −2.3 to +3.6‰, reaching a maximum near ca. 1.88 Ga. This trend toward heavier carbon isotope values is interpreted as the result of global orogenesis during Nuna assembly that elevated nutrient supply and sedimentation rates, leading to an increase in the proportion of organic carbon burial relative to carbonate carbon burial. • New depositional ages constrain Belcher Group deposition between 2018.5–1854.2 Ma. • Systematic δ 13 C shift from 0–3.6‰ upward through Belcher Group. • Data suggests a strong tectonic control on Orosirian carbon cycle. • Oldest cyanobacterial microfossils, Eoentophysalis belcherensis , are 2016.5 Ma. [ABSTRACT FROM AUTHOR]

Well-preserved strata of the late Tonian Chuar Group exposed in the Grand Canyon host fossil evidence for the development of eukaryotic predation, the presence of unique biomarkers, and large changes in C, S, and Mo isotope chemostratigraphy. Despite the importance of this critical succession, few radioisotopic age constraints are available to place these records into a global context. Here, we couple high-precision U-Pb chemical abrasion-isotope dilution-thermal ionization mass spectrometry (CA-ID-TIMS) on zircon crystals with the rhenium-osmium (Re-Os) sedimentary and sulfide geochronometer to refine the temporal framework of this pivotal interval of Earth history. Zircons recovered from a tuff within the uppermost Walcott Member of the Kwagunt Formation yield a weighted mean 206Pb-238U age of 729.0 ± 0.9 Ma (mean square of weighted deviates [MSWD] = 0.86), differing significantly from the previous air-abrasion upper-intercept age of 742 ± 6 Ma on zircons from this same horizon. Organic-rich carbonates from the Carbon Canyon Member of the Galeros Formation yield a model 1 Re-Os age of 757.0 ± 6.8 Ma (MSWD = 0.47, n = 8), and an initial Os isotope (187Os/188Os [Osi]) composition of 1.13 ± 0.02. The radiogenic Osi value from this horizon suggests that the basin was restricted from the open ocean during de-position of the Carbon Canyon Member, in agreement with sedimentological and stratigraphic data. The Re-Os geochronology of marcasite (FeS2) nodules from the Awatubi Member of the Kwagunt Formation yield a model 1 age of 751.0 ± 7.6 Ma (MSWD = 0.37, n = 5), with an Osi of 0.44 ± 0.01. This Re-Os date is interpreted to constrain the growth of the marcasite nodules in the Awatubi Member during deposition. The formation of sulfides and the less radiogenic Osi value are consistent with an influx of sulfate-laden seawater to the basin during depOsition of the Kwagunt Formation. Attempts to apply the Re-Os geochronometer to the Walcott and Tanner Members of the Chuar Group failed to yield meaningful ages, despite elevated Re enrichments (>20 ng/g). The Re-Os data from these units yielded negative Osi values, which suggest disturbance to the Re-Os system. The low Os abundances (typically <100 pg/g) relative to the amount expected based on the elevated Re abundances suggest leaching of Os due to oxidative weathering on geologically recent time scales. Finally, the Carbon Canyon Member provides a useful case study for quantifying how input uncertainties in the Re-Os geochronometer propagate into the resulting age uncertainty, and we discuss the protocols that will yield the best improvement in age precision for future studies. The U-Pb and Re-Os geochronology presented here illustrates the power of coupling these systems and the importance of recent improvements in both methods. Our analysis suggests that for our data, the most efficient way of reducing uncertainties in the presented Re-Os dates is through improved precision of measured Os values. [ABSTRACT FROM AUTHOR]

The Yanchang Formation was deposited during the Middle Triassic in a vast lacustrine basin in the modern Ordos Basin and is a main target for hydrocarbon exploration in Central China. It is divided, based on sedimentary cycles and lithology, into the Chang 10 (the oldest) to Chang 1 (the youngest) members. During the deposition of the Chang 7 Member, the Ordos lake system reached its maximum depth and large volumes of organic-rich sediments were deposited. The evolution of the Ordos Basin sedimentary system during this phase is, however, not completely understood, and uncertainty still exists as for the chronostratigraphy of Chang 7 Member. We acquired palynological markers and palynofacies and a high-resolution δ13C org record through the entire Chang 7 Member, and a ID-TIMS 206Pb/238U date of 240.95 ± 0.033 Ma from a volcanic ash bed in the middle of this Member. These imply that the maximum deepening phase of the lacustrine system was during the earliest Ladinian. Evidence of marine influence in the Ordos Basin at that time and comparison to the sea-level oscillations observed in Western Tethys suggest that a global eustatic rise and highstand may have played a role in determining lake-level variations. [Display omitted] • We report a zircon age of 240.950 ± 0.033 Ma from the middle portion of Chang 7 Member that indicates an early Ladinian. • The identified palynological assemblage also supports a late Middle Triassic age for Chang 7 Member. • The maximum expansion of the Ordos lake was probably influenced by the early Ladinian maximum in eustatic sea level. [ABSTRACT FROM AUTHOR]