Gregory A. Brennecka

Rapid expansion of oceanic anoxia immediately before the end-Permian mass extinction

Periods of oceanic anoxia have had a major influence on the evolutionary history of Earth and are often contemporaneous with mass extinction events. Changes in global (as opposed to local) redox conditions can be potentially evaluated using U system proxies. The intensity and timing of oceanic redox changes associated with the end-Permian extinction horizon (EH) were assessed from variations in 238U/235U (δ238U) and Th/U ratios in a carbonate section at Dawen in southern China. The EH is characterized by shifts toward lower δ238U values (from -0.37‰ to -0.65‰), indicative of an expansion of oceanic anoxia, and higher Th/U ratios (from 0.06 to 0.42), indicative of drawdown of U concentrations in seawater. Using a mass balance model, we estimate that this isotopic shift represents a sixfold increase in the flux of U to anoxic facies, implying a corresponding increase in the extent of oceanic anoxia. The intensification of oceanic anoxia coincided with, or slightly preceded, the EH and persisted for an interval of at least 40,000 to 50,000 y following the EH. These findings challenge previous hypotheses of an extended period of whole-ocean anoxia prior to the end-Permian extinction.

238U/235U Variations in Meteorites: Extant 247Cm and Implications for Pb-Pb Dating

How to Get a Date Radiometric dating relies on measuring the abundance of a radioactive isotope and/or its decay products. By knowing a decay rate and an isotopic starting abundance—both assumed to be constant—an age is determined. Using high-resolution mass spectrometry, Brennecka et al. (p. 449, published online 31 December; see the Perspective by Connelly) show that the known starting abundance of 238U and 235U isotopes in meteorites, which decay into 206Pb and 207Pb, respectively, is actually quite variable. Trace amounts of 247Cm in the early solar system may have unexpectedly contributed additional 235U, skewing the ratio. Pb-Pb dating, the method commonly used to date early solar system materials, may thus need a correction of up to 5 million years. Variable abundances of meteorite isotopes may require correcting the lead-based age of the solar system by 5 million years. The 238U/235U isotope ratio has long been considered invariant in meteoritic materials (equal to 137.88). This assumption is a cornerstone of the high-precision lead-lead dates that define the absolute age of the solar system. Calcium-aluminum–rich inclusions (CAIs) of the Allende meteorite display variable 238U/235U ratios, ranging between 137.409 ± 0.039 and 137.885 ± 0.009. This range implies substantial uncertainties in the ages that were previously determined by lead-lead dating of CAIs, which may be overestimated by several million years. The correlation of uranium isotope ratios with proxies for curium/uranium (that is, thorium/uranium and neodymium/uranium) provides strong evidence that the observed variations of 238U/235U in CAIs were produced by the decay of extant curium-247 to uranium-235 in the early solar system, with an initial 247Cm/235U ratio of approximately 1.1 × 10−4 to 2.4 × 10−4.

Uranium Isotope Fractionation during Adsorption to Mn-Oxyhydroxides

Previous work has shown uranium (U) isotope fractionation between natural ferromanganese crusts and seawater. Understanding the mechanism that causes (238)U/(235)U fractionation during adsorption to ferromanganese oxides is a critical step in the utilization of (238)U/(235)U as a tracer of U adsorption reactions in groundwater as well as a potential marine paleoredox proxy. We conducted U adsorption experiments using synthetic K-birnessite and U-bearing solutions. These experiments revealed a fractionation matching that observed between seawater and natural ferromanganese sediments: adsorbed U is isotopically lighter by ∼0.2‰ (δ(238/235)U) than dissolved U. As the redox state of U does not change during adsorption, a difference in the coordination environment between dissolved and adsorbed U is likely responsible for this effect. To test this hypothesis, we analyzed U adsorbed to K-birnessite in our experimental study using extended X-ray absorption fine structure (EXAFS) spectroscopy, to obtain information about U coordination in the adsorbed complex. Comparison of our EXAFS spectra with those for aqueous U species reveals subtle, but important, differences in the U-O coordination shell between dissolved and adsorbed U. We hypothesize that these differences are responsible for the fractionation observed in our experiments as well as for some U isotope variations in nature.

Uranium isotope compositions of the basaltic angrite meteorites and the chronological implications for the early Solar System.

Events occurring within the first 10 million years of the Solar System’s approximately 4.5 billion-year history, such as formation of the first solids, accretion, and differentiation of protoplanetary bodies, have determined the evolutionary course of our Solar System and the planetary bodies within it. The application of high-resolution chronometers based on short-lived radionuclides is critical to our understanding of the temporal sequence of these critical events. However, to map the relative ages from such chronometers onto the absolute time scale, they must be “anchored” to absolute ages of appropriate meteoritic materials using the high-precision lead–lead (Pb–Pb) chronometer. Previously reported Pb–Pb dates of the basaltic angrite meteorites, some of which have been used extensively as time anchors, assumed a constant 238U/235U ratio (= 137.88). In this work, we report measurements of 238U/235U ratios in several angrites that are distinct from the previously assumed value, resulting in corrections to the Pb–Pb ages of ≥1 million years. There is no resolvable variation in the 238U/235U ratio among the angrite bulk samples or mineral separates, suggesting homogeneity in the U isotopic composition of the angrite parent body. Based on these measurements, we recalculated the Pb–Pb age for the commonly used anchor, the D’Orbigny angrite, to be 4563.37 ± 0.25 Ma. An adjustment to the Pb–Pb age of a time anchor (such as D’Orbigny) requires a corresponding correction to the “model ages” of all materials dated using that anchor and a short-lived chronometer. This, in turn, has consequences for accurately defining the absolute timeline of early Solar System events.

A nucleosynthetic origin for the Earth’s anomalous 142Nd composition

A long-standing paradigm assumes that the chemical and isotopic compositions of many elements in the bulk silicate Earth are the same as in chondrites. However, the accessible Earth has a greater 142Nd/144Nd ratio than do chondrites. Because 142Nd is the decay product of the now-extinct 146Sm (which has a half-life of 103 million years), this 142Nd difference seems to require a higher-than-chondritic Sm/Nd ratio for the accessible Earth. This must have been acquired during global silicate differentiation within the first 30 million years of Solar System formation and implies the formation of a complementary 142Nd-depleted reservoir that either is hidden in the deep Earth, or lost to space by impact erosion. Whether this complementary reservoir existed, and whether or not it has been lost from Earth, is a matter of debate, and has implications for determining the bulk composition of Earth, its heat content and structure, as well as for constraining the modes and timescales of its geodynamical evolution. Here we show that, compared with chondrites, Earth’s precursor bodies were enriched in neodymium that was produced by the slow neutron capture process (s-process) of nucleosynthesis. This s-process excess leads to higher 142Nd/144Nd ratios; after correction for this effect, the 142Nd/144Nd ratios of chondrites and the accessible Earth are indistinguishable within five parts per million. The 142Nd offset between the accessible silicate Earth and chondrites therefore reflects a higher proportion of s-process neodymium in the Earth, and not early differentiation processes. As such, our results obviate the need for hidden-reservoir or super-chondritic Earth models and imply a chondritic Sm/Nd ratio for the bulk Earth. Although chondrites formed at greater heliocentric distances and contain a different mix of presolar components than Earth, they nevertheless are suitable proxies for Earth’s bulk chemical composition.

Evidence for supernova injection into the solar nebula and the decoupling of r-process nucleosynthesis

Significance We present isotopic evidence that supports the injection of supernova material into the early Solar System within a small window of time around the formation of the first solids in the protoplanetary disk. This injected material was not incorporated into the first solids, but subsequently changed the elemental and isotopic composition of the nascent Solar System. The isotopic differences between the first solids and subsequently formed matter in the Solar System require the existence of multiple sources for r-process nucleosynthesis. The isotopic composition of our Solar System reflects the blending of materials derived from numerous past nucleosynthetic events, each characterized by a distinct isotopic signature. We show that the isotopic compositions of elements spanning a large mass range in the earliest formed solids in our Solar System, calcium–aluminum-rich inclusions (CAIs), are uniform, and yet distinct from the average Solar System composition. Relative to younger objects in the Solar System, CAIs contain positive r-process anomalies in isotopes A < 140 and negative r-process anomalies in isotopes A > 140. This fundamental difference in the isotopic character of CAIs around mass 140 necessitates (i) the existence of multiple sources for r-process nucleosynthesis and (ii) the injection of supernova material into a reservoir untapped by CAIs. A scenario of late supernova injection into the protoplanetary disk is consistent with formation of our Solar System in an active star-forming region of the galaxy.