Modeling apparent Pb loss in zircon U–Pb geochronology

oai:publications.copernicus.org:gchron110471The loss of radiogenic Pb from zircon is known to be a major factor that can cause inaccuracy in the U–Pb geochronological system; hence, there is a need to better characterize the distribution of Pb loss in natural samples. Treatment of zircon by chemical abrasion (CA) has become standard practice in isotope dilution–thermal ionization mass spectrometry (ID-TIMS), but CA is much less commonly employed prior to in situ analysis via laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) or secondary ionization mass spectrometry (SIMS). Differentiating the effects of low levels of Pb loss in Phanerozoic zircon with relatively low-precision in situ U–Pb dates, where the degree of Pb loss is insufficient to cause discernible discordance, is challenging. We show that U–Pb isotopic ratios that have been perturbed by Pb loss may be modeled by convolving a Gaussian distribution that represents random variations from the true isotopic value stemming from analytical uncertainty with a distribution that characterizes Pb loss. We apply this mathematical framework to model the distribution of apparent Pb loss in 10 igneous samples that have both non-CA LA-ICP-MS or SIMS U–Pb dates and an estimate of the crystallization age, either through CA U–Pb or 40Ar/39Ar geochronology. All but one sample showed negative age offsets that were unlikely to have been drawn from an unperturbed U–Pb date distribution. Modeling apparent Pb loss using the logit–normal distribution produced good fits with all 10 samples and showed two contrasting patterns in apparent Pb loss; samples where most zircon U–Pb dates undergo a bulk shift and samples where most zircon U–Pb dates exhibited a low age offset but fewer dates had more significant offset. Our modeling framework allows comparison of relative degrees of apparent Pb loss between samples of different age, with the first and second Wasserstein distances providing useful estimates of the total magnitude of apparent Pb loss. Given that the large majority of in situ U–Pb dates are acquired without the CA treatment, this study highlights a pressing need for improved characterization of apparent Pb-loss distributions in natural samples to aid in interpreting non-CA in situ U–Pb data and to guide future data collection strategies.</p

Geoeconomic variations in epidemiology, ventilation management, and outcomes in invasively ventilated intensive care unit patients without acute respiratory distress syndrome: a pooled analysis of four observational studies

Background: Geoeconomic variations in epidemiology, the practice of ventilation, and outcome in invasively ventilated intensive care unit (ICU) patients without acute respiratory distress syndrome (ARDS) remain unexplored. In this analysis we aim to address these gaps using individual patient data of four large observational studies. Methods: In this pooled analysis we harmonised individual patient data from the ERICC, LUNG SAFE, PRoVENT, and PRoVENT-iMiC prospective observational studies, which were conducted from June, 2011, to December, 2018, in 534 ICUs in 54 countries. We used the 2016 World Bank classification to define two geoeconomic regions: middle-income countries (MICs) and high-income countries (HICs). ARDS was defined according to the Berlin criteria. Descriptive statistics were used to compare patients in MICs versus HICs. The primary outcome was the use of low tidal volume ventilation (LTVV) for the first 3 days of mechanical ventilation. Secondary outcomes were key ventilation parameters (tidal volume size, positive end-expiratory pressure, fraction of inspired oxygen, peak pressure, plateau pressure, driving pressure, and respiratory rate), patient characteristics, the risk for and actual development of acute respiratory distress syndrome after the first day of ventilation, duration of ventilation, ICU length of stay, and ICU mortality. Findings: Of the 7608 patients included in the original studies, this analysis included 3852 patients without ARDS, of whom 2345 were from MICs and 1507 were from HICs. Patients in MICs were younger, shorter and with a slightly lower body-mass index, more often had diabetes and active cancer, but less often chronic obstructive pulmonary disease and heart failure than patients from HICs. Sequential organ failure assessment scores were similar in MICs and HICs. Use of LTVV in MICs and HICs was comparable (42\ub74% vs 44\ub72%; absolute difference \u20131\ub769 [\u20139\ub758 to 6\ub711] p=0\ub767; data available in 3174 [82%] of 3852 patients). The median applied positive end expiratory pressure was lower in MICs than in HICs (5 [IQR 5\u20138] vs 6 [5\u20138] cm H2O; p=0\ub70011). ICU mortality was higher in MICs than in HICs (30\ub75% vs 19\ub79%; p=0\ub70004; adjusted effect 16\ub741% [95% CI 9\ub752\u201323\ub752]; p&lt;0\ub70001) and was inversely associated with gross domestic product (adjusted odds ratio for a US$10 000 increase per capita 0\ub780 [95% CI 0\ub775\u20130\ub786]; p&lt;0\ub70001). Interpretation: Despite similar disease severity and ventilation management, ICU mortality in patients without ARDS is higher in MICs than in HICs, with a strong association with country-level economic status. Funding: No funding

Coastal Response to Global Warming During the Paleocene-Eocene Thermal Maximum

Global warming during the Paleocene-Eocene Thermal Maximum (PETM) is hypothesized to have had a profound effect on the paleohydrologic cycle, including enhanced seasonality and increased water and sediment discharge. Although the PETM may represent the closest geologic analog for future global climate changes, the effects of this event on ancient coastal systems are poorly understood. We examined drill core from two locations in eastern Texas that preserve a record of tidally influenced deltaic sedimentation associated with the paleo-Colorado River that drained up to 2 × 106 km2 of central North America, approximately two-thirds of the area of the modern Mississippi River catchment. In these cores, the development of a regionally extensive sand-rich unit (the Carrizo Formation) at the onset of the PETM is identified from a negative carbon isotope excursion and supported by detrital zircon U—Pb geochronology and pollen occurrence data. The basal Carrizo Formation indicates that the onset of the PETM was characterized by an increase in the delivery of coarser-grained sediments and progradation of the coastline, which occurred despite rising sea level. Using a mass-balance framework for equilibrium deltaic systems, we estimate that sediment delivery to the coastline increased by ca. 46% (2-σ: 13% to 167%). Our findings of enhanced sediment delivery to the coast are consistent with proxy climatic and sedimentologic data indicating heightened precipitation seasonality in the interior of North America during the PETM. Thus, the effects of a regional change in climate forced by greenhouse events were transmitted downstream by large river systems to produce shifts in coastal sediment supply, progradation, and coastline evolution

A chronostratigraphic framework for the upper Stormberg Group: Implications for the Triassic-Jurassic boundary in southern Africa

The upper Stormberg Group (Elliot and Clarens formations) of the main Karoo Basin is well-known for its fossil vertebrate fauna, comprising early branching members of lineages including mammals, dinosaurs, and turtles. Despite 150 years of scientific study, the upper Stormberg Group lacks radioisotopic age constraints and remains coarsely dated via imprecise faunal correlations. Here we synthesise previous litho- and magnetostratigraphic studies, and present a comprehensive biostratigraphic review of the upper Stormberg fauna. We also present the results of the first geochronological assessment of the unit across the basin, using U-Pb dates derived from detrital zircons obtained from tuffaceous sandstones and siltstones, the youngest of which are considered maximum depositional ages. Our results confirm that the Elliot Formation contains the Triassic–Jurassic boundary, making it one of the few fossiliferous continental units that records the effects of the end-Triassic Mass Extinction event. Our work suggests a mid-Norian–Rhaetian age for the lower Elliot Formation and a Hettangian–Sinemurian age for the upper Elliot Formation, although the precise stratigraphic position of the Triassic/Jurassic (Rhaetian/Hettangian) boundary remains somewhat uncertain. A mainly Pliensbachian age is obtained for the Clarens Formation. The new dates allow direct comparison with better-calibrated Triassic-Jurassic faunas of the Western Hemisphere (e.g., Chinle and Los Colorados formations). We show that sauropodomorph, but not ornithischian or theropod, dinosaurs were well-established in the main Karoo Basin ~220 million years ago, and that typical Norian faunas (e.g., aetosaurs, phytosaurs) are either rare or absent in the lower Elliot Formation, which is paucispecific compared to the upper Elliot. While this is unlikely the result of geographic sampling biases, it could be due to historical sampling intensity differences