The Earth has experienced hyperthermal events in the past, characterized by maximum durations of hundreds thousand years, significant magnitude, global extent, and drivers associated with increases in greenhouse gas concentrations, therefore making them potential analogues for current climate change. The Middle Eocene Climatic Optimum (MECO) that occurred 40 Ma ago, is marked by a CO2-driven global warming of +4 to +6 degrees C, affecting global temperatures. Here, we present a detailed reconstruction of seasonal fluctuations in seawater temperatures during this warming event in littoral environment, based on geochemical analyses (delta 18O and Delta 47) of shallow-marine mollusks from the Paris Basin. Our data show a stability in mean winter temperatures compared to pre-MECO conditions, but a marked warming of +10 degrees C in maximum estuarine water temperatures, with a seasonal temperature range increasing from 12 degrees C before the MECO to 22 degrees C at the climax of the event. We demonstrate that at mid-latitudes, annual maximum shallow-water temperatures increased from 30 +/- 2 degrees C before the event to a maximum of 41 +/- 4 degrees C at the warming peak. This pattern is associated with a seasonal regime characterized by dry summers and wet winters, implying that the Paris Basin experienced a super-hot summer Mediterranean climate during the MECO.

Accurate and precise mass spectrometric determination of ppm-ppb quantities of mass 47–49 clumped isotopologues of carbonate-derived CO2, expressed as Δ47–Δ49 values, requires advanced processing schemes. Here, we introduce D4Xgui, a user-friendly processing tool that allows correction of mass-spectrometric raw intensities for a pressure baseline artifact, before standardization is carried out using D47crunch. D4Xgui enables rapid processing of multi-session data under consideration of full error-propagation, interactive visualization of results including tools for data quality assurance, calculation of carbonate crystallization temperature from finally processed data, and rapid re-evaluation of datasets with revised processing parameters. Though the primary focus of D4Xgui is on carbonates it can also be applied to the correction of mass spectrometric raw data obtained on CO2 from other sources.

Antarctic ice cores and ocean-sediment records preserve evidence for an increase in the amplitude of glacial-interglacial cycles at around 430 ka, known as the Mid-Brunhes Transition (MBT). However, similar evidence from non-polar terrestrial environments is rare, casting some doubt on the global extent of this transition. Here we present a multi-proxy speleothem record from Corchia Cave (Alpi Apuane, Italy) that spans the MBT. It comprises a stacked d18O and d13C time series from multiple stalagmites anchored in time by U-Th and U-Pb ages; and trace element, 87Sr/86Sr, and d18O and d13C profiles from a subaqueous calcite deposit (CD3) that has grown continuously from 970 ka to the present. We anchored the CD3 record to the chronology of a stalagmite stack by synchronisation of their respective d18O and d13C profiles. CD3 is well suited to this study because it yields a suite of proxies from just a single specimen that covers multiple glacial-interglacial cycles either side of the MBT. In particular, its d13C profile provides a reference for comparing the amplitude of glacial-interglacial temperature changes at Corchia to globally integrated ice-volume (LR04 benthic 18O/16O stack) and greenhouse gas (ice-core CO2 and CH4)time series. The CD3 temperature record builds on a previous trace element study, which revealed that the Mg/Ca in this speleothem is strongly influenced by mineralisation temperature (a proxy for external air temperature at the cave site). This is supported by subsequent clumped-isotope palaeothermometry. We thus developed a continuous palaeotemperature time series for CD3 extending to ~650 ka via a Mg-D47 transfer function. The temperature profile reveals compelling evidence for a shift in glacial-interglacial amplitude across the MBT. Temperatures during the interglacials of MIS15e, 15a and 13a are lower in Corchia compared to those of MIS11c, 9e, 5e and the Holocene; temperatures during MIS7e and 7c are the exception, only reaching the levels of the pre-MBT interglacials. Minimum glacial temperatures for MIS16 and 14 are warmer in Corchia than those of the subsequent glacial maxima, and the MIS12 and 6 glacials are the coldest of the last 650 kyr. All of these patterns are consistent with existing global ice-volume and greenhouse gas records but provide a rare and important terrestrial perspective. This finding confirms previous assessments that the MBT was global in extent.

The phenomenon of polar amplification causes high-latitude warming to exceed the global mean and enhances seasonal variations in temperature and precipitation. As such, it complicates the understanding of both modern and past global warming impacts in polar regions. Characterizing the climate change in polar regions is further hampered by limited data coverage and persistent challenges in interpreting local paleoenvironmental archives.To help address these knowledge gaps, we present bulk and seasonally resolved stable oxygen (δ¹⁸O) and clumped isotope (Δ₄₇) data from exceptionally well-preserved bivalves from the Early Jurassic North Pole (eastern Siberia), spanning the late Pliensbachian icehouse-Toarcian hothouse transition. Upper Pliensbachian Harpax specimens show pronounced seasonal δ¹⁸O variability of 1.5-6 ‰, corresponding to apparent temperature ranges of 6-26 °C assuming invariant seawater δ¹⁸O. In contrast, seasonally resolved clumped isotope data do not yield a statistically significant seasonal temperature difference (1.4 ± 1 °C). Together with a low mean Δ₄₇ temperature of ~3.5 ± 1 °C, this discrepancy implies that the large δ¹⁸O amplitude reflects strong seasonal variability in seawater δ¹⁸O, likely driven by enhanced precipitation, and/or meltwater input in a relatively proximal sedimentary setting with near-freezing mean annual temperatures.By contrast, Toarcian Dacryomya jacutica specimens show reduced δ¹⁸O variability of 1-2 ‰ (~4-8 °C) and a larger, statistically significant seasonal Δ₄₇ temperature difference of 5.9 ± 1.3 °C, with a mean Δ₄₇ temperature of ~10.3 °C. The agreement between δ¹⁸O and Δ₄₇-derived seasonality indicates a limited contribution of seawater δ¹⁸O variability during the Toarcian and points to a shift toward more distal sedimentary conditions. Collectively, these results provide one of the first quantitative constraints on Arctic temperature seasonality under greenhouse climate conditions in deep time.

The early Toarcian was characterized by a massive release of carbon and a major hyperthermal event, marking the acme of a series of rapid and profound environmental perturbations that punctuated the late Pliensbachian–Toarcian period (185–175 Ma). It has been suggested that some of these environmental perturbations were accompanied by the waxing and waning of cryosphere reservoirs (ice caps and permafrost) in high-latitude regions. These Early Jurassic environmental perturbations, however, have been reported almost exclusively in open-marine sediments deposited at mid to low paleolatitudes, so that their global impact and underlying causes remain uncertain. Here we report new litho-, bio-, and chemo-stratigraphic data from key Pliensbachian–Toarcian siliciclastic successions in the Vilyui River Basin, which were deposited in a proximal position along the Anabar Shield near the Early Jurassic North Pole. The new stable carbon, sulfur, and radiogenic strontium isotope data combined with robust biostratigraphy allow to identify the Toarcian hyperthermal event confidently. A major increase in sedimentation rates and a shift from cold, nearshore conditions (with high glendonite and sand contents) to a warmer and deeper setting occurred during that time. Rock-Eval pyrolysis data reveal a thermally immature kerogen dominated by terrestrial material, with a higher contribution and/or better preservation of marine organic matter at the very beginning of the Toarcian hyperthermal event, probably due to a decrease in marine oxygenation. Our results broadly support scenarios involving extreme changes in high-latitude environmental conditions across the Toarcian hyperthermal and set the stage for future evaluations of climate changes in polar seas during this key interval of Earth's history.

Reconstructions of past sea surface temperatures (SSTs) are essential for understanding long-term climate variability, yet different proxy methods can yield divergent results. In this study, we compare Mg/Ca-derived SSTs from Globigerinoides ruber sensu stricto and Trilobatus sacculifer with clumped isotope (X47) SSTs measured on G. ruber s.s. from the same core, MD96-2048 (Indian Ocean), covering the last 1.25 million years (Ma). Using the same species and samples allows minimizing ecological and environmental biases. We find that X47-derived SSTs are systematically colder than Mg/Ca-SSTs prior to 0.4 Ma, while both proxies agree well after this point. This offset is not explained by diagenetic alteration (as assessed via SEM), nor by corrections for seawater salinity, pH, or Mg/Ca composition. The Mg/Ca-SSTs from T. sacculifer are more consistent with SSTs in the older interval, but do not fully resolve the discrepancy. We found that the apparent X47-based cooling before 0.4 Ma is not supported by seawater 518O estimates or other climate indicators. Our results suggest that X47-derived SSTs may be affected by an unknown bias in older intervals, although a combination of multiple factors explored in this study could also contribute to the observed offset.

Lithospheric delamination involves short-lived crustal and surface responses, alkaline magmatism, high heat flow and extension. In the Western Mediterranean, delamination is hypothesized to have triggered uplift at the origin of the Messinian Salinity Crisis (MSC). But delamination as the primary cause of uplift is questioned due to the insufficient temporal resolution. We report new U-Pb ages and clumped isotope analyses from calcite veins formed in an eastern Betic intramontane basin. They reveal a brief fluid event from 8.5 to 5 Ma linked to extension and retreating delamination. After extension, shortening and uplift began at 4.5-3 Ma across the boundary between the Cabo de Gata arc basement and the Iberia margin. We show that the MSC occurred before shortening and during delamination. Slab detachment caused the demise of the MSC, the formation of a new plate boundary fault and tectonic escape between Africa and Iberia around 5 Ma.

Joint measurements of the 18O/16O and 17O/16O ratios of carbonate minerals and waters are increasingly used to investigate various geochemical, physical and biological processes. Diverse analytical methods, each of them technically challenging in one way or another, have been developed or refined in recent years to measure oxygen-17 anomalies (Δ17O) with instrumental precisions of 10 ppm or better. A critical underpinning of all these methods is how the international carbonate reference materials currently anchoring the VPDB 18O/16O scale are linked to the primary VSMOW-SLAP scale in (18O/16O, 17O/16O) space. For now, however, substantial systematic discrepancies persist between different groups and methods, even after all measurements are nominally standardized to VSMOW-SLAP. Here we take advantage of VCOF-CRDS, a novel spectroscopic method combining the ease and simplicity of near-infra-red absorption measurements in pure CO2 with metrological performance competitive with state-of-the-art IRMS techniques, to precisely characterize, based on previously reported equilibrium fractionation factors between water and CO2, the relative triple oxygen isotope compositions of international water standards (VSMOW2, SLAP2, GRESP) and CO2 produced by phosphoric acid reaction of carbonate standards (NBS18, NBS19, IAEA603, IAEA610, IAEA611, IAEA612). The robustness of our results derives from the demonstrated linearity of our measurements (RMSE ≈ 1 ppm), but also from the fact that, when equilibrated with or converted to CO2, all of these reference materials yield analytes with closely comparable oxygen-18 compositions. In light of these observations, we revisit potential causes of the large inter-laboratory discrepancies reported so far. Collectively reconciling the different types of measurements constraining the relative 17O/16O ratios of the two standards most often used to normalize carbonate analyses (NBS18, IAEA603) is a matter of high priority.

Kinetic isotopes effect (KIEs) describes a very common phenomenon related to change in chemical reaction rate due isotopic substitution. If in biological sciences, KIEs has received a lot of interest with the aim at understanding reaction mechanisms, their control on the isotopic composition of biogenic carbonate has long been overlooked. However, the initial assumption that isotopic fractionation primarily reflects a thermodynamic equilibrium process in the H2O-DIC-CaCO3 system is challenged by a growing number of observations. Not accounting for these disequilibrium effects leads to inaccurate estimates of carbonate growing temperature. In this scientific context, Triple oxygen isotopes systematic can help constraining kinetics isotopes fractionation associated with metabolic reactions implicated in biocarbonates formation. We thus took advantage from recent development in spectroscopic technique (VCOF-CRDS) to measured O17 isotopic anomalies in CO2 produced by carbonate acid reaction [1]. These samples were also analyzed for their δ13C, δ18O and Δ47 composition using a more classical mass spectroscopy technic. In this contribution we investigated cold-water corals (CWC) known to display strong isotopic disequilibria. For this 1st application, we selected four modern CWC species for which calcification conditions (T, S, pH, δ18Owater, Δ17Owater and δ13CDIC) are independently constrained. The measured isotopic signatures were compared to their respective expected values based on environmental constrains, assuming “pseudo-equilibrium” carbonate precipitation. In particular, corals Δ17O signatures were compared to the newly established equilibrium for O17 fractionation between calcite-water based on slow growing carbonates from Laghetto Basso and Devils Hole, measured using the same VCOF-CRDS technic [2]. We finally compared our experimental data with theoretical predictions for KIEs on DIC isotopic composition [3]. Interestingly, the correlation slope among Δ47 - Δ17O disequilibrium differ from the previous one derived from dual clumped (Δ47 - Δ48) isotopic measurements of the same species [4]. This founding suggesting that other biological parameter(s) should be taken into account to resolve CWC isotopic disequilibria. [1] Chaillot. J., Daëron. M., Casado, M., Landais. A., Pesnin. M., Clauzel. T., Kassi. S. (in prep) Triple oxygen analyses of carbon dioxide, water and carbonates using VCOF-CRDS. [2] Clauzel, T., Chaillot, J., Pesnin, M., Jautzy, J., Kessy, S., Daëron, M. (in prep) Advancing triple oxygen isotope analysis of carbonate and water using V-shaped Cavity Optical Feedback Cavity Ring-Down Spectroscopy (VCOF-CRDS): Calibration and implications for paleoclimate reconstruction. [3] Guo. W. (2020). Kinetic clumped isotope fractionation in the DIC-H2O-CO2 system: Patterns, controls, and implications. Geochimica et Cosmochimica Acta, 268, 230-257. [4] Davies. A. J., Guo. W., Bernecker. M., Tagliavento. M., Raddatz. J., Gischler. E., Floter. S., Fiebig. J. (2022). Dual clumped isotope thermometry of coral carbonate. Geochimica et Cosmochimica Acta, 338, 66-78.

Extensive bryozoan fossil records date back to the early Ordovician, forming an important part of sedimentary archives, yet the applicability of classical 18‑oxygen thermometry to bryozoan carbonate is still a matter of debate, with mineralogical and biological issues still hindering paleoclimate reconstructions. Complementing other methods (i.e., δ18O, Mg/Ca), clumped-isotope thermometry (Δ47) could provide more accurate paleotemperature estimates and help identify potential biotic factors influencing the isotopic composition of bryozoans. Here we report on the first investigation of clumped-isotope thermometry applied to bryozoan carbonate, spanning a broad range of modern species living in different environments from two localities (Atlantic Ocean and Mediterranean Sea). We confirm that bryozoan δ13C and δ18O records are affected by biotic and abiotic factors susceptible to bias in growth condition estimates. Our Atlantic bryozoans yield Δ47 derived temperatures (T47) consistent with spring/fall seawater temperature (but only after correcting for minor mineralogical effects), reflecting seasonal growth bias or, more likely, moderate isotopic disequilibrium. By contrast, Mediterranean samples display large positive offsets from Δ47 equilibrium values. We propose that this stronger disequilibrium is related to the higher salinity at this site, decreasing Carbonic Anhydrase activity, favoring CO2 hydroxylation over hydration, and slowing down DIC (dissolved inorganic carbon) equilibration reactions at the precipitation site. Our findings highlight how “vital effects” in bryozoans is not only species-specific as often assumed for other biocalcifiers, but could also depend on mineralogy and environmental factors.

In situ measurements of water vapour isotopic composition in polar regions has provided needed constrains of post-deposition processes involved in the archiving of the climatic signal in ice core records. During polar winter, the temperatures, and thus the specific humidity, are so low that current commercial techniques are not able to measure the vapour isotopic composition with enough precision. Here, we make use of new developments in infrared spectroscopy and combine an optical-feedback frequency-stabilised laser source (OFFS technique) using a V-shaped cavity optical feedback (VCOF) cavity and a high-finesse cavity ring-down spectroscopy (CRDS) cavity to increase the signal-to-noise ratio while measuring absorption transitions of water isotopes. We present a laboratory infrared spectrometer leveraging all these techniques dedicated to measure water vapour isotopic composition at low humidity levels. At 400 ppmv, the instrument demonstrates a precision of 0.01 ‰ and 0.1 ‰ in δ18O and d-excess, respectively, for an integration time of 2 min. This set-up yields an isotopic composition precision below 1 ‰ at water mixing ratios down to 4 ppmv, which suggests an extrapolated precision in δ18O of 1.5 ‰ at 1 ppmv. Indeed, thanks to the stabilisation of the laser by the VCOF, the instrument exhibits extremely low drift and very high signal-to-noise ratio. The instrument is not hindered by a strong isotope–humidity response which at low humidity can create extensive biases on commercial instruments.

Abstract. In situ measurements of water vapour isotopic composition in Polar Regions has provided needed constrains of post-deposition processes involved in the archiving of the climatic signal in ice core records. During polar winter, the temperatures are so low that current commercial techniques are not able to measure the vapour isotopic composition with enough precision. Here, we make use of new developments in infrared spectroscopy and combine an optical feedback frequency stabilised laser source (OFFS technique) using a V-shaped optical cavity (VCOF) and a high-finesse cavity ring down cavity (CRDS) which yield sufficient precision to measure isotopic composition at water mixing ratios down to 1 ppmv. Indeed, thanks to the stabilisation of the laser by the VCOF, the instrument suffers extremely low drift and very high signal to noise ratio. Using new constrains on the fitting technique, the instrument is additionally not hindered by a large isotope-humidity response which at low humidity can create extensive biases on commercial instruments.

Oxygen-17 excess (Δ17O) in carbonate minerals can provide valuable insights into past continental and marine environments, long-term trends in the temperature and oxygen-isotope composition of ancient oceans, isotopic disequilibrium effects in biogenic and abiotic carbonates, and cryptic diagenesis. Triple oxygen isotope analyses of carbonates and/or CO2 using isotope-ratio mass spectrometers (IRMS) remain challenging, however, because of isobaric interference between 16O13C16O and 16O12C17O. Using spectroscopic methods, the abundance of each CO2 isotopologue may be directly quantified, potentially providing simple, non-destructive measurements of δ13C, δ18O and Δ17O on small samples of CO2.Here we report new data characterizing the application of VCOF-CRDS (V-shaped Cavity Optical Feedback - Cavity Ring Down Spectroscopy [1]) to the analysis of small samples (<40 μmol) of pure CO2, as typically produced by phosphoric acid digestion of carbonate minerals.Instrumental drifts from various sources are observed to bias apparent isotopic abundances by a few tens of ppm, but these drifts are slow enough that they may be precisely monitored and corrected for by repeated analyses of a working gas interspersed between other analyses, requiring only ~8 mn per aliquot and 30 mn between consecutive “unkown” analyses. This approach was tested by analyzing repeated aliquots of another CO2 tank with a different isotopic composition, yielding instrumental repeatabilities of 12 ppm, 13 ppm and 7.4 ppm for δ13C, δ18O and Δ17O, respectively (95 % CL, Nf = 66).The accuracy of our measurements was tested over a wide range of Δ17O values spanning 130 ppm, by analyzing CO2 equilibrated at 25 °C with different waters whose triple oxygen compositions were independently constrained in the SMOW-SLAP scale by IRMS measurements and by simple nonlinear mixing predictions. We find that our Δ17O measurements are well within analytical uncertainties of predicted values (RMSE = 1.2 ppm), with analytical repeatabilities (including isotopic equilibration and gas manipulation) of 8.6 ppm (95 % CL, Nf = 27).We will also report the results of our ongoing investigation regarding the isotopic fractionation and analytical noise associated with different acid digestion protocols at different reaction temperatures, and the triple oxygen composition of various international reference materials already used for δ13C, δ18O, and clumped-isotope measurements.Based on these results, we conclude that VCOF-CRDS offers excellent accuracy, along with state-of-the-art levels of analytical precision/linearity, for straightforward analyses of 17O excess in CO2, water, and carbonate minerals.[1] Stoltmann et al. (2017) 10.1021/acs.analchem.7b02853

Main text The pilot study CCQM-P204 was aimed at evaluating the level of compatibility of laboratories' measurement capabilities to value assign isotope ratios in samples of pure CO 2 gas, expressed as isotope delta values relative to the relevant international scale: δ 13 C VPDB and δ 18 O VPDB-CO2 . Pure CO 2 gas samples were prepared by the BIPM in batches of 10 samples of the same gas and circulated to participants for measurement. Each participant received four samples of CO 2 with a different nominal δ 13 C VPDB value: −1 ‰; −9 ‰; −35 ‰; and −42 ‰. The BIPM was also responsible for evaluating the homogeneity and stability of the samples. The co-coordinator IAEA received one sample per batch to confirm the batch-to-batch homogeneity. Within-batch and between-batch inhomogeneity was assessed and found to be negligible in comparison to the spread of results reported by participants. Participants used the analytical technique of their choice to measure the isotope delta values. They were requested to report the measurement results together with detailed information on their traceability, measurement methods and data treatment. Results of the comparison were to be compiled by the BIPM and evaluated jointly by the BIPM and the IAEA. The majority of participants reported results using DI-IRMS, and those that reported results based on laser spectroscopy techniques showed a very similar dispersion of results as for DI-IRMS, although generally with greater uncertainty. A total of nineteen participants reported their measurements, with two of them reporting results with different reference materials to provide more insight into the traceability of the measurements. The results were reported with traceability to three different VPDB scale realizations, notably VPDB, VPDB-LSVEC and VPDB2020, with 8, 7 and 6 results reported respectively for each of these. Participants agreed that results based on VPDB and VPDB2020 scale realizations should, in principle, lead to consistent results, whereas those based on VPDB-LSVEC should show a bias that increased as samples became more depleted in 13 C, with the bias approaching 0.2 ‰ for the most depleted sample. This bias was demonstrated by the participant reporting the most precise measurements based on the VPDB-LSVEC realizations, whereas for 2 participants using VPDB-LSVEC scale realizations other issues dominated the consistency of their results. The 3 laboratories using the NIST (8562,8563, 8564) reference materials (reported as on the VPDB-LSVEC scale), were highly consistent with each other, but the reported bias for the VPDB-LSVEC realization was not evident, with the historical method used for value assignment of the NIST RMs, and their relatively large uncertainty, being identified as possible causes for this. In general, for all results the dispersion was greater than expected based on the measurement uncertainties reported by participants. This dispersion increased as the samples became more depleted in 13 C, so that results that were traceable to realizations of the VPDB scale that could be considered equivalent (VPDB and VPDB2020) did not lead to ensembles that were fully consistent within their stated uncertainties. Either the reduced chi-squared or Birge Ratio provide easily calculated quantities to characterise lack of consistency in a data set, where consistent data would lead to values of unity for either of these, and discrepant data leading to increased values. This is most readily demonstrated by considering results based on DI-IRMS with traceability to the VPDB scale through either VPDB and VPDB 2020 realizations, where the standard deviation of 16 results was 0.043 ‰ and a Birge Ratio of 2.7 calculated for nominally −1‰ for δ 13 C, and the standard deviation was 0.12 ‰ and a Birge Ratio of 2.9 calculated at nominally −9 ‰ for δ 18 O. For the samples where the nominal δ 13 C value was −42 ‰, the standard deviation of 17 results was 0.085 ‰ and a Birge Ratio of 4.5 calculated for δ 13 C, and the standard deviation was 0.24 ‰ and a Birge Ratio of 3.4 calculated for δ 18 O at nominally −36 ‰. The observed magnitude of the standard deviation of results can also be compared to the standard uncertainty of the IAEA-603 materials certified values (0.01 ‰ for δ 13 C and 0.04 ‰ δ 18 O) and the smallest standard uncertainties reported by a participant (0.005 ‰ for δ 13 C, and 0.01 ‰ for δ 18 O). These results indicate an underestimation of uncertainty for reported results, especially for those with the smallest uncertainties. Components of uncertainty that should be reviewed before future comparisons include: the uncertainty contribution from reference materials; the uncertainty associated with the phosphoric acid reaction with carbonate reference materials; corrections and uncertainties related to cross-contamination effects in the IRMS; appropriate methods for combining uncertainties of multiple reference materials and accounting for their correlations. A retreatment of results, which normalizes results to the −1 ‰ and −42 ‰ δ 13 C samples, leads to improvement in the consistency of results as demonstrated for measurements on the nominally −35 ‰ δ 13 C, −30 ‰ δ18O samples for which the standard deviation is reduced to 0.034 ‰ and 0.057 ‰ for δ 13 C and δ 18 O respectively (from 0.072 ‰ and 0.198 ‰ without normalization). The results of the comparison indicate that once issues of non-ideal methods and use of LSVEC are removed, discrepancies in results arise from the challenges in accurately transferring delta values from carbonate reference materials to CO 2 gaseous samples, and that these issues can be reduced by having appropriate gaseous reference standards for calibration when measuring CO 2 gaseous samples. This is consistent with the identical treatment principle that is preferred in the isotope ratio measurement community. An analysis of results is presented in this report, with further consideration of the impact of the measurement method, the scale, and the reference materials. Uncertainties reported by participants are detailed and analysed, highlighting important differences in the uncertainty sources considered. Although CCQM-P204 was a comparison organised within the CCQM/GAWG and IRWG, no reference value was calculated, mainly because not all results appeared to be strictly on the same scale. Instead, a list of recommendations is provided to encourage more harmonised measurement practices and reach better consistency in future comparisons on similar materials. To reach the main text of this paper, click on Final Report . Note that this text is that which appears in Appendix B of the BIPM key comparison database https://www.bipm.org/kcdb/ . The final report has been peer-reviewed and approved for publication by the CCQM, according to the provisions of the CIPM Mutual Recognition Arrangement (CIPM MRA).