Potential of Chromium Isotopes as a Tracer of Past Ocean Oxygenation

Levels of atmospheric oxygen (O2) have increased from 53Cr analysis in low concentration samples has been developed. In addition, the Cr isotopic composition of modern and Phanerozoic carbonates, and seawater, have been determined to provide new constraints of the environmental behaviour of Cr isotopes. Modern seawater is characterised by heavy δ53Cr values (+0.55-1.55%) relative to the continental crust (-0.12%), which indicates that Cr isotopes are fractionated during oxidative weathering. Importantly, the Cr isotopic composition of modern carbonates, precipitated in shallow water, is within the range of seawater. This suggests that fractionation of Cr isotopes during carbonate precipitation is minimal, and carbonates should provide a record of seawater δ53Cr. Carbonate samples of Phanerozoic age also have high δ53Cr values (0.737-1.994%), consistent with oxidative weathering on the continents at this time. Most of the Neoproterozoic carbonates have significantly lower δ53Cr values than the modern and Phanerozoic carbonates. However, oolitic limestones deposited just prior to the first Cryogenian glaciations, have higher δ53Cr (+0.571-1.004%). Together, the δ53Cr and REE data suggest that the operation of the Cr cycle was significantly different in the Neoproterozoic. Changes in δ53Cr reflect either a shift in the relative importance of riverine and hydrothermal sources, and/or changes in levels of atmospheric O2 . Modelling indicates that shallow waters were either dysoxic or suboxic during the Cryogenian, and levels of atmospheric O2 must have been less than 30-40% PAL

Mass dependent fractionation of stable chromium isotopes in mare basalts: implications for the formation and differentiation of the Moon

We present the first stable chromium isotopic data from mare basalts in order to investigate the similarity between the Moon and the Earth’s mantle. A double spike technique coupled with MC-ICP-MS measurements was used to analyse 19 mare basalts, comprising high-Ti, low-Ti and KREEP-rich varieties. Chromium isotope ratios (δ53Cr) for mare basalts are positively correlated with indices of magmatic differentiation such as Mg# and Cr concentration which suggests that Cr isotopes were fractionated during magmatic differentiation. Modelling of the results provides evidence that spinel and pyroxene are the main phases controlling the Cr isotopic composition during fractional crystallisation. The most evolved samples have the lightest isotopic compositions, complemented by cumulates that are isotopically heavy. Two hypotheses are proposed to explain this fractionation: (i) equilibrium fractionation where heavy isotopes are preferentially incorporated into the spinel lattice and (ii) a difference in isotopic composition between Cr2+ and Cr3+ in the melt. However, both processes require magmatic temperatures below 1200 °C for appreciable Cr3+ to be present at the low oxygen fugacities found in the Moon (IW −1 to −2 log units). There is no isotopic difference between the most primitive high-Ti, low-Ti and KREEP basalts, which suggest that the sources of these basalts were homogeneous in terms of stable Cr isotopes. The least differentiated sample in our sample set is the low-Ti basalt 12016, characterised by a Cr isotopic composition of −0.222 ± 0.025‰, which is within error of the current BSE value (−0.124 ± 0.101‰). The similarity between the mantles of the Moon and Earth is consistent with a terrestrial origin for a major fraction of the lunar Cr. This similarity also suggests that Cr isotopes were not fractionated by core formation on the Moon

Multi-messenger observations of a binary neutron star merger

On 2017 August 17 a binary neutron star coalescence candidate (later designated GW170817) with merger time 12:41:04 UTC was observed through gravitational waves by the Advanced LIGO and Advanced Virgo detectors. The Fermi Gamma-ray Burst Monitor independently detected a gamma-ray burst (GRB 170817A) with a time delay of ~1.7 s with respect to the merger time. From the gravitational-wave signal, the source was initially localized to a sky region of 31 deg2 at a luminosity distance of 40+8-8 Mpc and with component masses consistent with neutron stars. The component masses were later measured to be in the range 0.86 to 2.26 Mo. An extensive observing campaign was launched across the electromagnetic spectrum leading to the discovery of a bright optical transient (SSS17a, now with the IAU identification of AT 2017gfo) in NGC 4993 (at ~40 Mpc) less than 11 hours after the merger by the One- Meter, Two Hemisphere (1M2H) team using the 1 m Swope Telescope. The optical transient was independently detected by multiple teams within an hour. Subsequent observations targeted the object and its environment. Early ultraviolet observations revealed a blue transient that faded within 48 hours. Optical and infrared observations showed a redward evolution over ~10 days. Following early non-detections, X-ray and radio emission were discovered at the transient’s position ~9 and ~16 days, respectively, after the merger. Both the X-ray and radio emission likely arise from a physical process that is distinct from the one that generates the UV/optical/near-infrared emission. No ultra-high-energy gamma-rays and no neutrino candidates consistent with the source were found in follow-up searches. These observations support the hypothesis that GW170817 was produced by the merger of two neutron stars in NGC4993 followed by a short gamma-ray burst (GRB 170817A) and a kilonova/macronova powered by the radioactive decay of r-process nuclei synthesized in the ejecta

Ultralight vector dark matter search using data from the KAGRA O3GK run

Among the various candidates for dark matter (DM), ultralight vector DM can be probed by laser interferometric gravitational wave detectors through the measurement of oscillating length changes in the arm cavities. In this context, KAGRA has a unique feature due to differing compositions of its mirrors, enhancing the signal of vector DM in the length change in the auxiliary channels. Here we present the result of a search for U(1)B−L gauge boson DM using the KAGRA data from auxiliary length channels during the first joint observation run together with GEO600. By applying our search pipeline, which takes into account the stochastic nature of ultralight DM, upper bounds on the coupling strength between the U(1)B−L gauge boson and ordinary matter are obtained for a range of DM masses. While our constraints are less stringent than those derived from previous experiments, this study demonstrates the applicability of our method to the lower-mass vector DM search, which is made difficult in this measurement by the short observation time compared to the auto-correlation time scale of DM

Search for gravitational-lensing signatures in the full third observing run of the LIGO-Virgo network

Gravitational lensing by massive objects along the line of sight to the source causes distortions of gravitational wave-signals; such distortions may reveal information about fundamental physics, cosmology and astrophysics. In this work, we have extended the search for lensing signatures to all binary black hole events from the third observing run of the LIGO--Virgo network. We search for repeated signals from strong lensing by 1) performing targeted searches for subthreshold signals, 2) calculating the degree of overlap amongst the intrinsic parameters and sky location of pairs of signals, 3) comparing the similarities of the spectrograms amongst pairs of signals, and 4) performing dual-signal Bayesian analysis that takes into account selection effects and astrophysical knowledge. We also search for distortions to the gravitational waveform caused by 1) frequency-independent phase shifts in strongly lensed images, and 2) frequency-dependent modulation of the amplitude and phase due to point masses. None of these searches yields significant evidence for lensing. Finally, we use the non-detection of gravitational-wave lensing to constrain the lensing rate based on the latest merger-rate estimates and the fraction of dark matter composed of compact objects

Search for eccentric black hole coalescences during the third observing run of LIGO and Virgo

Despite the growing number of confident binary black hole coalescences observed through gravitational waves so far, the astrophysical origin of these binaries remains uncertain. Orbital eccentricity is one of the clearest tracers of binary formation channels. Identifying binary eccentricity, however, remains challenging due to the limited availability of gravitational waveforms that include effects of eccentricity. Here, we present observational results for a waveform-independent search sensitive to eccentric black hole coalescences, covering the third observing run (O3) of the LIGO and Virgo detectors. We identified no new high-significance candidates beyond those that were already identified with searches focusing on quasi-circular binaries. We determine the sensitivity of our search to high-mass (total mass M>70 M⊙) binaries covering eccentricities up to 0.3 at 15 Hz orbital frequency, and use this to compare model predictions to search results. Assuming all detections are indeed quasi-circular, for our fiducial population model, we place an upper limit for the merger rate density of high-mass binaries with eccentricities 0<e≤0.3 at 0.33 Gpc−3 yr−1 at 90\% confidence level

Accurate and precise determination of stable Cr isotope compositions in carbonates by double spike MC-ICP-MS

Techniques for the separation of small quantities of Cr from carbonate material and for the analysis of stable Cr isotopes in carbonates by MC-ICP-MS are presented in this study. In comparison with previously published methods, we have developed a one-step Cr separation procedure that is relatively simple, and has a low blank (0.12–0.20 ng). Moreover, careful optimisation of the desolvating sample introduction system allows a significant increase in the sensitivity of our MC-ICP-MS technique compared to previous studies. Instrumental mass bias effects and fractionation of Cr isotopes during Cr separation are corrected using a carefully optimised 50Cr–54Cr double-spike method. Novel numerical simulations demonstrate that the effects of potential isobaric interferences from Ti, Fe and V are negligible, even if they are isotopically fractionated. Small deviations in the δ53Cr value of the NBS 979 standard between different analytical sessions are due to small deviations from exponential mass fractionation behaviour. The long-term reproducibility of δ53Cr for a spiked NBS 979 Cr isotope reference material is ±0.031‰ (2 S.D., n = 147). Analyses of carbonates reveal that they have δ53Cr values of 0.747 to 1.994‰, distinctly heavier than continental crust and the terrestrial mantle. The carbonates record Cr isotopic fractionation that may be used to understand redox reactions in the oceans. Although this study focuses on carbonate samples, our mass spectrometry technique can be applied to the analysis of any samples with low levels of Cr, including river waters and seawater

Stable cerium isotopes as a tracer of oxidation reactions

International audienceRedox conditions in past oceans have attracted significant interest and many proxies have been used to probe redox changes through time. For example, the redox dependent behaviour of Ce, resulting in negative or positive elemental Ce anomalies, has been widely used. More recently, mass dependent Ce isotopic variations have been proposed as a powerful tool to study Ce oxidation in natural environments. In this study, we demonstrate, for the first time, that Ce isotopes are fractionated during oxidation reaction, confirming the utility of Ce isotopes to study redox reactions. This result suggests that seawater Ce isotopic composition should be fractionated toward heavy values relative to the continental crust. Measured natural rock samples (carbonates, banded iron formations and Mn nodules) have variable Ce isotopic compositions, ranging from −0.055 ± 0.045 ‰ to +0.280 ± 0.045 ‰. The relation between Ce elemental anomalies and Ce isotopic composition in carbonate rocks suggest that mass dependent Ce isotopes can be used to distinguish elemental anomalies produced by oxidation reactions from those produced by other processes. Coupled with La-Ce chronology, mass dependent Ce isotope analysis is a very powerful tool to study redox reactions in past oceans