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A direct comparison of single-grain and multi-grain aliquot luminescence dating of feldspars from colluvial deposits in KwaZulu-Natal, South Africa
No full textInternational audienceThe erosional landscape of the Jojosi dongas in KwaZulu-Natal, South Africa, exposes accretionary slope deposits that preserve important geological and archaeological information. This landscape was occupied by modern humans during the Stone Age for many thousands of years, as evidenced by the presence of numerous stone artefacts on the surface and interbedded within at least three phases of gully cut-and-fill deposits. A contextualisation of the artefacts and their role for human evolution in southern Africa, along with developing an understanding of the environmental conditions that shaped this inhabited landscape, is only made possible by establishing a robust chronological framework. Here, we use luminescence dating of feldspars to constrain the geochronological framework for the sequence of accretionary hillslope deposition at Jojosi at three sampling locations. Measurements of feldspar single grains show low luminescence sensitivity of the individual grains and a variable proportion of grains in saturation. Summing the luminescence signal of individual grains and creating synthetic aliquots enables us to study the effect of signal averaging on the luminescence sensitivity, signal saturation, and dose distributions. We then compare the results from individual grain measurements and synthetic aliquots to true multi-grain aliquots. To allow a quantification of the results, we apply four different dose models, including the Central Age Model (CAM), the Average Dose Model (ADM), BayLum, and a standardised growth curve (SGC) approach, using an averaged Ln/Tn value interpolated onto the SGC. Doses calculated for the different samples range from ∼ 80 to ∼ 800 Gy and contain 6 % to 67 % saturated grains. We evaluate the performance of the different dose models over this range in equivalent doses and relative number of saturated grains. On average, we find good agreement between the results obtained using the different dose models but observe that samples with a large number of saturated grains impact the consistency of the result. Overall, all dose models and data sets give consistent results below a saturated grain threshold of ∼ 15 %, corresponding to a dose of ∼ 120 Gy in this study. Finally, we favour BayLum for age calculations of the single-grain and multi-grain aliquot data sets, representing the opportunity to refine the chronology by including stratigraphic information in the age calculations. We are able to establish a chronology for the three sampled sections within the Jojosi dongas, constraining erosional and depositional processes from ∼ 100 to ∼ 700 ka, and human occupation of the area spanning from early marine isotope stage (MIS) 5 to late MIS 6
The MAGPI Survey: radial trends in star formation across different cosmological simulations in comparison with observations at <i>z</i> ~ 0.3
No full textInternational audienceWe investigate the internal and external mechanisms that regulate and quench star formation (SF) in galaxies at z ∼ 0 . 3 using MAGPI observations and the EAGLE , MAGNETICUM , and ILLUSTRISTNG cosmological simulations. Using SIMSPIN to generate mock observations of simulated galaxies, we match detection/resolution limits in star formation rates and stellar mass, along with MAGPI observational details including the average point spread function and pixel scale. While we find a good agreement in the slope of the global star-forming main sequence (SFMS) between MAGPI observations and all three simulations, the slope of the resolved SFMS does not agree within 1-2 σ . Furthermore, in radial SF trends, good agreement between observations and simulations exists only for galaxies far below the SFMS, where we capture evidence for inside-out quenching. The simulations o v erall agree with each other between ∼ 1 . 5 -4 R e but sho w v arying central suppression within R ∼ 1 . 5 R e for galaxies on and below the SFMS, attributable to different AGN feedback prescriptions. All three simulations show similar dependencies of SF radial trends with environment. Central galaxies are subject to both internal and external mechanisms, showing increased SF suppression in the centre with increasing halo mass, indicating AGN feedback. Satellite galaxies display increasing suppression in the outskirts as halo mass increases, indicative of environmental processes. These results demonstrate the power of spatially resolved studies of galaxies; while global properties align, radial profiles reveal discrepancies between observations and simulations and their underlying physics.</div
The infrared counterpart and proper motion of magnetar SGR0501+4516
No full textInternational audienceSoft gamma repeaters (SGRs) are highly magnetised neutron stars (magnetars) notable for their gamma-ray and X-ray outbursts. In this paper, we use near-infrared (NIR) imaging of SGR 0501+4516 in the days, weeks, and years after its 2008 outburst to characterise the multi-wavelength emission, and to obtain a proper motion from our long temporal baseline observations. Unlike most magnetars, the source has only moderate foreground extinction with minimal crowding. Our observations began only 2 hours after the first activation of SGR 0501+4516 in August 2008, and continued for 4 years, including two epochs of Hubble Space Telescope (HST) imaging. The proper motion constraint is improved by a third HST epoch 10 years later. The near-infrared and X-rays faded slowly during the first week, thereafter following a steeper power-law decay. The behaviour is satisfactorily fit by a broken power-law. Three epochs of HST imaging with a 10-year baseline allow us to determine a quiescent level, and to measure a proper motion of 5.4+/-0.6 mas/yr. This corresponds to a low transverse peculiar velocity of 51+/-14 km/s (at 2 kpc). The magnitude and direction of the proper motion rules out supernova remnant HB9 as the birth-site. We can find no other supernova remnants or groups of massive stars within the region traversed by SGR 0501+4516 during its characteristic lifetime (20 kyr). Our observations of SGR 0501+4516 suggest that some magnetars may be either significantly older than expected, that their progenitors produce low supernova ejecta masses, or alternatively that they can be formed through accretion-induced collapse (AIC) or low-mass neutron star mergers. Although the progenitor of SGR 0501+4516 remains unclear, we propose that SGR 0501+4516 is the best Galactic candidate for a magnetar formed through a mechanism other than massive star core-collapse
Water Reservoirs in the Lower Mantle Beneath Northeastern Asia
No full textInternational audienceWater in the mantle controls mantle convection and chemical transport processes within Earth's interior. However, whether the water in the lower mantle is primordial or recycled remains an open question.Here we report an electrical structure consisting of two stepped high-conductivity anomalies crossing the 660km interface beneath northeastern Asia. The sheet-like first-step conductor locates in the mantle transition zone that can be best interpreted as hydrated stagnant Pacific slab, whereas the funnel-like second-step anomaly occurs in the uppermost lower mantle that is inferred to be a water reservoir replenished by the collapsed stagnant slab. The pivotal mechanism of this water transport process could be the dehydration reaction of dense hydrous magnesium silicates and the rehydration of stishovite and liebermannite in the transition zone. Our findings provide evidence for recycled lower-mantle water reservoirs, and together with seismological and mineralogical studies, illustrate the deep-mantle water plumbing system operated by cold slab subduction.</p
Semi-Supervised Learning for Lensed Quasar Detection
No full textInternational audienceLensed quasars are key to many areas of study in astronomy, offering a unique probe into the intermediate and far universe. However, finding lensed quasars has proved difficult despite significant efforts from large collaborations. These challenges have limited catalogues of confirmed lensed quasars to the hundreds, despite theoretical predictions that they should be many times more numerous. We train machine learning classifiers to discover lensed quasar candidates. By using semi-supervised learning techniques we leverage the large number of potential candidates as unlabelled training data alongside the small number of known objects, greatly improving model performance. We present our two most successful models: (1) a variational autoencoder trained on millions of quasars to reduce the dimensionality of images for input to a gradient boosting classifier that can make accurate predictions and (2) a convolutional neural network trained on a mix of labelled and unlabelled data via virtual adversarial training. These models are both capable of producing high-quality candidates, as evidenced by our discovery of GRALJ140833.73+042229.98. The success of our classifier, which uses only images, is particularly exciting as it can be combined with existing classifiers, which use other data than images, to improve the classifications of both models and discover more lensed quasars
The rise of the galactic empire: luminosity functions at and estimated with the MIDISNGDEEP ultra-deep JWST/NIRCam dataset
No full textInternational audienceWe present a sample of six F200W and three F277W dropout sources identified as galaxy candidates present mass-weighted ages around 30 Myr, and attenuations \mathrm{A(V)}<0.1 mag. Their average stellar mass is , implying a star formation efficiency (stellar-to-baryon mass fraction) around 10%. We find three galaxies with very blue UV spectral slopes () compatible with low metallicity or Pop III and young ( Myr) stars and/or high escape fractions of ionizing photons, the rest presenting slopes similar to samples
Glacio-eustatic forcing on Middle Pleistocene fluvialu2013lacustrine depositional processes: a case history from the Tyrrhenian Sea margin of central Italy
No full textInternational audienceThe complex stratigraphic setting of alternating volcanic and fluvial–lacustrine sedimentary deposits in the surroundings of Castel Cellesi village (Latium region, central Italy) offers an unprecedented opportunity to examine the influence of glacio-eustatic forcing on the depositional processes during Pleistocene times in the upper catchment of the Tiber River, in the inland sector of the Tyrrhenian Sea margin of Italy. Tight geochronologic constraints on the timing of sediment deposition have been established through precise 40Ar/39Ar dating of intervening volcanic layers. These new age constraints reveal a clear chronological correlation between sediment aggradation phases and sea-level rises, as evidenced by the δ18O record and relative sea-level (RSL) curve. The analysis of pyroclastic-flow deposits within incised paleo-valleys further indicates a relationship between periods of erosion or non-deposition and sea-level lowstands. Specifically, twelve 40Ar/39Ar dates have delineated three successive aggradational fluvial–lacustrine successions deposited during the sea-level rises of Marine Isotope Stages (MIS) 13, 11, and 9, as well as the erosional features associated with the sea-level drops of MIS 12 and 10
Decrease in the Permeability of Microcracked and Macrocracked Granite at Elevated Pressure and Temperature
No full textInternational audienceAbstract Pressure and temperature increase with depth, modifying the microstructure of crustal rocks. The opening or closing of micro‐ and macrocracks resulting from variations in the in‐situ conditions influences the permeability of crustal rocks. While confining pressure is known to close pre‐existing cracks, reducing permeability, the influence of temperature has received less attention. Here, we measured the permeability of micro‐ and macrocracked granite at confining pressures and temperatures up to 50 MPa and 150°C, respectively. We find that the permeability of micro‐ and macrocracked granite decreases with confining pressure. At constant pressure, increasing temperature reduces permeability by up to an order of magnitude, interpreted as the closure of microcracks due to mineral thermal expansion. Using a simplified microstructural model, we show that microcrack aperture is reduced by up to 50% at 150°C compared to room temperature. However, temperature does not appear to influence the permeability of a tortuous macrocrack
Limits on the Ejecta Mass During the Search for Kilonovae Associated with Neutron Star-Black Hole Mergers: A case study of S230518h, GW230529, S230627c and the Low-Significance Candidate S240422ed
No full textInternational audienceNeutron star-black hole (NSBH) mergers, detectable via their gravitational-wave (GW) emission, are expected to produce kilonovae (KNe). Four NSBH candidates have been identified and followed-up by more than fifty instruments since the start of the fourth GW Observing Run (O4), in May 2023, up to July 2024; however, no confirmed associated KN has been detected. This study evaluates ejecta properties from multi-messenger observations to understand the absence of detectable KN: we use GW public information and joint observations taken from 05.2023 to 07.2024 (LVK, ATLAS, DECam, GECKO, GOTO, GRANDMA, SAGUARO, TESS, WINTER, ZTF). First, our analysis on follow-up observation strategies shows that, on average, more than 50% of the simulated KNe associated with NSBH mergers reach their peak luminosity around one day after merger in the - bands, which is not necessarily covered for each NSBH GW candidate. We also analyze the trade-off between observation efficiency and the intrinsic properties of the KN emission, to understand the impact on how these constraints affect our ability to detect the KN, and underlying ejecta properties for each GW candidate. In particular, we can only confirm the kilonova was not missed for 1% of the GW230529 and S230627c sky localization region, given the large sky localization error of GW230529 and the large distance for S230627c and, their respective KN faint luminosities. More constraining, for S230518h, we infer the dynamical ejecta and post-merger disk wind ejecta . Similarly, the non-astrophysical origin of S240422ed is likely further confirmed by the fact that we would have detected even a faint KN at the time and presumed distance of the S240422ed event candidate, within a minimum 45% credible region of the sky area, that can be larger depending on the KN scenario
Assessing the SMAP Level-4 Carbon Product over the Arctic and Subarctic Zones
No full textInternational audienceThe soil moisture active passive (SMAP) satellite mission distributes a product of CO2 flux estimates (SPL4CMDL) derived from a terrestrial carbon flux model, in which SMAP brightness temperatures are assimilated to update soil moisture (SM) and constrain the carbon cyclemodeling. While the SPL4CMDL product has demonstrated promising performance across the continental USA and Australia, a detailed assessment over the arctic and subarctic zones (ASZ) is still missing. In this study, SPL4CMDL net ecosystem exchange (NEE), gross primary production (GPP), and ecosystem respiration (R-E) are evaluated against measurements from 37 eddy covariance towers deployed over the ASZ, spanning from 2015 to 2022. The assessment indicates that the NEE unbiased root-mean-square error falls within the targeted accuracy of 1.6 gC.m(-2).d(-1), as defined for the SPL4CMDL product. However, modeled GPP and R-E are overestimated at the beginning of the growing season over evergreen needleleaf forests and shrublands, while being underestimated over grasslands. Discrepancies are also found in the annual net CO2 budgets. SM appears to have a minimal influence on the GPP and R-E modeling, suggesting that ASZ vegetation is rarely subjected to hydric stress, which contradicts some recent studies. These results highlight the need for further carbon cycle process understanding and model refinements to improve the SPL4CMDL CO2 flux estimatesover the ASZ