# The carbon isotopic signature of the upper mantle is heterogeneous

Yves Moussallam, Kenneth T. Koga, Cyril Aubaud, Hyun Joo Lee, Guillaume Georgeais

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Yves Moussallam, Kenneth T. Koga, Cyril Aubaud, Hyun Joo Lee, Guillaume Georgeais. The carbon isotopic signature of the upper mantle is heterogeneous. Communications Earth & Environment, 2025, 6 (1), pp.6. 10.1038/s43247-024-01973-9. insu-04991729

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# The carbon isotopic signature of the upper mantle is heterogeneous

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Yves Moussallam©<fnref n="1" />, Kenneth T. Koga², Estelle F. Rose-Koga©², Cyril Aubaud©³, Hyun Joo Lee©<fnref n="1" /> & Guillaume Georgeais<fnref n="1" />

Knowledge of the carbon isotopic (δ¹³C) signature of the mantle source region of mid-ocean ridge basalts, is key to understanding carbon cycling on a planetary scale. This value has long been held at -4‰ from the study of popping rocks or -5‰ from the study of peridotitic diamonds, with little evidence of heterogeneity due to scarce direct measurements. Here we report δ¹³C measurements on a series of CO₂-undersaturated melts inclusions from the south-west Indian mid ocean Ridge. These rare magmas never degassed CO₂ hence retaining a δ¹³C signature in direct equilibrium to their source mantle. We found normally distributed δ¹³C values averaging at -7.5‰ (two standard error of ±1.4‰) and uncorrelated with major, trace and volatile element abundances nor degree of partial melting. Our data shows that the upper mantle can be heterogeneous in its carbon isotopic signature, potentially recording variable influence from subducted carbon.

The carbon isotopic signature of the present-day upper mantle that feeds mid-ocean ridge basalts (MORB) is a crucial value for understanding planetary-scale processes, such as (i) the origin of Earth's carbon<fnref n="1" />,², (ii) the efficiency of carbon recycling at subduction zones³,⁴ and (iii) the sources of volatiles emitted to the atmosphere by volcanoes⁵⁻⁷. However, direct measurements of MORB mantle δ¹³C values are lacking. Our most reliable estimate is based on carbon isotope measurements of CO₂ gas trapped in vesicles from two 'popping rock' samples (2πD43 and OT03-09) collected at 14°N and 34°N in the Mid-Atlantic Ridge⁸⁻¹⁰. These rare samples have been interpreted as closed systems with vesicles retaining all CO₂ degassed from the melt during magma ascent and both indicate an initial MORB mantle δ¹³C of -4‰ (all δ¹³C values given relative to Pee Dee Belemnite). A second estimate of the MORB mantle carbon isotope signature comes from studies of peridotitic diamonds. These samples are not from mid-ocean ridges but are collected from kimberlites and lamproites around the world. As such, they are more representative of the upper mantle beneath cratons and provide samples ~1 to 3 Ga old¹¹, much older than today's MORBs. Nevertheless, δ¹³C measurements on peridotitic diamonds are widely used to characterise the carbon isotopic signature of the present-day upper mantle indicating a mantle δ¹³C value of -5‰¹²⁻¹⁴. A third estimate of the MORB mantle carbon isotope signature comes from studies of volcanic gases emitted by volcanoes in the Afar Depression, a region where the Aden and Red Sea Ridges meet the East African Rift. Gas samples collected during the 1978 Asal Rift eruption¹⁵ and at Erta Ale volcano in the seventies¹⁶ show δ¹³C values around -6.5‰. These values however, although sometime cited

as such, are not directly representative of the mantle δ¹³C because they represent surface degassing and the isotopic fractionation between gas and melt during magmatic degassing is significant¹⁷⁻²⁰.

Here we present a novel, more direct approach to determining the isotopic signature of the MORB mantle by studying a series of CO₂-undersaturated MOR melts that preserve direct isotopic equilibrium with their mantle source. We take advantage of the recent development of high precision δ¹³C SIMS analyses in basaltic glasses²¹ and present the first such measurements on a series of melt inclusions (a few δ¹³C analyses of melt inclusions have been reported by previous workers²² but with very high uncertainties around ±5‰). We investigate olivine-hosted melt inclusions from the glassy rims of a pillow basalt erupted at >2,650 m depth from the South West Indian Ridge (SWIR; 37°51'48"S, 49°20'12"E). These samples have previously been characterised for major, minor, trace and volatile elements²³ and have been shown to record one of the few occurrences of CO₂ undersaturated melts, magmas that never reached saturation with a CO₂ gas phase²³⁻²⁶. All melt inclusions analysed were large (87–440 µm diameter), four out of ten contained shrinkage bubbles, which were always less than 1% of the MI volume, and none had evidence of decrepitation or visible cracks prior to polishing (Fig. 1 and Supplementary Fig. 1).

## Results and discussion

Ten melt inclusions were analysed, one of which was large enough for three replicate analyses (Table 1). The error on repeat analyses in this inclusion (1σ = ± 0.8‰; n = 3) is comparable to the analytical error of each individual

<footnote n="1">¹Lamont-Doherty Earth Observatory, Columbia University, New York, NY, USA. ²ISTO, UMR 7327, Université d'Orléans-CNRS-BRGM, 1A rue de la Férollerie, 45071 Orléans, cedex 2, France. ³Université Paris Cité, Institut de Physique du Globe de Paris, CNRS, Laboratoire de Géochimie des Isotopes Stables, 1 rue Jussieu, F-75005 Paris, France. e-mail: yves.moussallam@ldeo.columbia.edu</footnote>
<figure><img src="image_4.png" /><figcaption>**Fig. 1** | SWIR CO₂-undersaturated melt inclusions δ¹³C signature. CO₂ content versus δ¹³C (A), CO₂/Ba versus δ¹³C (B) and Rb/Nb versus δ¹³C (C) for SWIR melt inclusions. Inclusions containing a shrinkage bubble are marked with a white dot in their centre. Repeat analyses performed on the same inclusion are marked with a</figcaption></figure>

black cross. Lower right inset is a transmitted light photomicrograph of a typical olivine crystal with three large glassy melt inclusions. Error bars are one standard deviation.
<table><caption>**Table 1 | δ¹³C measurements in SWIR melt inclusions by SIMS (see method section)**</caption><thead><tr><th>Melt inclusion</th><th>δ¹³C (‰)</th><th>error (1σ)</th><th>CO₂ (ppm)</th><th>CO₂/Ba</th><th>Rb/Nb</th><th>Ba/Nb</th><th>Nb/U</th><th>Ce/Pb</th><th>MI max length (μm)</th><th>Bubble size (μm)</th></tr></thead><tbody><tr><td>DR75_E31b</td><td>-9.5</td><td>2.0</td><td>794</td><td></td><td></td><td></td><td></td><td></td><td>201</td><td>39</td></tr><tr><td>DR75_E31c</td><td>-8.6</td><td>1.4</td><td>667</td><td></td><td></td><td></td><td></td><td></td><td>130</td><td>na</td></tr><tr><td>DR75_C29</td><td>-4.7</td><td>1.5</td><td>1138</td><td>109</td><td>0.86</td><td>10</td><td>25</td><td>13</td><td>150</td><td>25</td></tr><tr><td>DR75_E13a@1</td><td>-6.4</td><td>1.4</td><td>909</td><td>124</td><td>0.89</td><td>10</td><td>29</td><td>16</td><td>442</td><td>na</td></tr><tr><td>DR75_E13a@2</td><td>-8.0</td><td>1.1</td><td>909</td><td>124</td><td>0.89</td><td>10</td><td>29</td><td>16</td><td>442</td><td>na</td></tr><tr><td>DR75_E13a@3</td><td>-6.8</td><td>1.4</td><td>909</td><td>124</td><td>0.89</td><td>10</td><td>29</td><td>16</td><td>442</td><td>na</td></tr><tr><td>DR75_E65a</td><td>-11.7</td><td>1.6</td><td>726</td><td>113</td><td>0.93</td><td>11</td><td>34</td><td>15</td><td>204</td><td>na</td></tr><tr><td>DR75_E8a</td><td>-9.2</td><td>1.3</td><td>969</td><td>147</td><td>0.81</td><td>8</td><td>36</td><td>17</td><td>296</td><td>na</td></tr><tr><td>DR75_E5b</td><td>-4.3</td><td>1.2</td><td>1185</td><td></td><td></td><td></td><td></td><td></td><td>87</td><td>na</td></tr><tr><td>DR75_E47</td><td>-7.9</td><td>1.6</td><td>832</td><td>108</td><td>0.71</td><td>8</td><td>35</td><td>18</td><td>119</td><td>22</td></tr><tr><td>DR75_E19a</td><td>-9.6</td><td>1.7</td><td>816</td><td>83</td><td>0.86</td><td>9</td><td>36</td><td>15</td><td>246</td><td>na</td></tr><tr><td>DR75_E52a</td><td>-5.3</td><td>1.6</td><td>292</td><td>96</td><td>0.69</td><td>9</td><td>34</td><td>16</td><td>164</td><td>17</td></tr></tbody></table>

CO₂ content, trace element ratios and MI sizes are from ref. 23.

**Fig. 2 | Trace element systematics of global and SWIR MORBs.** Nb/U versus Ce/Pb diagram showing the distribution of MORB melt inclusions²⁵,⁴⁷⁻⁵⁰, glasses²⁹ and whole rock²⁸. Vertical blue line marks the furthest extent of the “back-arc-like MORBs”²⁸ trend to minimum value of Nb/U (≈ 2). Yellow line shows modelled mixing between the “normal” MORB and a subduction-fluid metamor- somatized mantle endmember passing through the average composition of this study and previously published DR75 melt inclusions²³, indicated as black points with error bars (1 standard deviation).

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analysis (average $1\sigma = \pm 1.3\%$) indicating no intra-inclusion heterogeneity within analytical error. We found no correlation between the CO₂ content and the δ¹³C signature of the melt inclusions, as expected for undersaturated melts that have not undergone CO₂ degassing and therefore not experienced carbon isotope fractionation due to CO₂ degassing (Fig. 1). Supplementary Fig. 2 illustrates the predicted relationship between δ¹³C and CO₂ for both closed and open system styles of degassing, indicating that neither model aligns with our observations. Furthermore, degassing is incompatible with the uniform CO₂/Ba ratio observed across all our melt inclusions. Note that while ten melt inclusions were analysed for δ¹³C only seven were analysed for trace elements by LA-ICPMS (analyses from ref. 23). We found no significant correlation between the MI δ¹³C and any major, minor, trace or volatile element concentrations. Since the MI trace elements are generated by varying degrees of mantle partial melting²³ this suggests that δ¹³C does not fractionate during partial melting, confirming a long-standing but rarely stated assumption. Finally, we found no correlation between the δ¹³C of the MIs and their oxidation state (Supplementary Fig. 3). The close correspondence between the analytical error on each analysis ($\pm 1.5\%$ on average) and the standard deviation across the entire population ($\pm 2.2\%$), together with the lack of correlation with other variables, serves as evidence

in favour of treating our measurements as multiple analyses of a singular unknown. Taken as a group, our MIs δ¹³C analyses define a normal distribution (kurtosis=-0.6; skewness = -0.04) with a mean δ¹³C signature of -7.7‰, a median of -8.0‰, a weighted mean of -7.5‰ and a 95% confidence level of ±1.4‰. Inclusions containing a shrinkage bubble are evenly distributed in the population in terms of both their δ¹³C value and CO₂ contents, excluding them would only slightly shift the population mean to lower values (-8.0‰, median of -8.3‰, weighted mean of -7.8‰). Since these inclusions have the same CO₂/Ba ratio as those without bubbles (Fig. 1), it is reasonable to assume that they have not lost any CO₂ to their shrinking bubble.

Taken together, our measurements hence indicate a SWIR mantle source δ¹³C at -7.5 ± 1.4‰. This is different from the -4‰ mantle source determined from the Mid Atlantic Ridge (MAR) popping rocks⁸,⁹. The statistical significance of this difference given our measurement population can be tested, giving a Z-score of 1.65, which translates into a 5% probability that the true value is equal to or greater than -4‰ (using the population mean and standard deviation). In other words, we have 95% confidence in the fact that the SWIR mantle source is lighter in carbon isotopes than the previously accepted modern MORB mantle (89% confidence when
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**Fig. 3** | The influence of subducted carbon on MORB mantle δ¹³C signature. Nb/U versus δ¹³C showing modelled mixing lines between the “normal” MORB mantle and three possible subduction-fluid metasomatized mantles endmembers. Error bars on the SWIR average (large circle symbol) show 95% confidence intervals based on 31 analyses for Nb/U from ref. 23 and 9 analyses for δ¹³C, this study. Error bars on individual analyses are one standard deviation analytical errors and their propagation for the Nb/U ratio. Horizontal dashes show the 25, 50 and 75% of mixing along each mixing lines. The SWIR melt mantle δ¹³C can be explained by ~80% mixing between the “normal” MORB mantle and a metasomatized mantles component with a δ¹³C = -10‰ signature.

compared to the -5‰ value of peridotitic diamonds). We therefore argue that the modern-day upper mantle must be heterogeneous in terms of its carbon isotopic signature.

Why is the SWIR mantle lighter in δ¹³C and how much heterogeneity in mantle δ¹³C should we expect? One feature of the mantle beneath the South West Indian Ridge is that it has been recognised as having been modified by a subduction component²⁷,²⁸, which is not the case for the section of the Mid-Atlantic Ridge from which the popping rocks originated²⁸. A reasonable hypothesis, therefore, would be that regions of the mantle which are most affected by subducting slab components would be expected to show greater variability in their δ¹³C signature. Yang et al.²⁸, proposed a trace element systematic to identify the influence of subduction fluids on MORB mantle sources. Following their approach, most MORB show little influence of a subduction fluid-contaminated component in their source mantle, with Ce/Pb vs. Nb/U ratios close to the “canonical” values (of ~25 and 47 respectively, Fig. 2) so we would expect their source δ¹³C values to be close to those of the MAR popping rocks (at ~-4‰). Our SWIR samples however, fall along the trend of “back-arc-like MORBs”²⁸, MORBs whose source has been influenced by subduction fluids. We can therefore develop a two-component mixing model between two mantle sources, one highly metasomatized by subduction fluids and one without subduction influence. We use the global trend to identify the subduction fluid metasomatized endmember (Fig. 2). Assuming a fluid-immobile character of Nb, the global minimum value of Nb/U (≈ 2) is expected to indicate the most likely value of the subduction fluid metasomatized mantle endmember. The “normal” MORB average (with Ce/Pb and Nb/U ratios of ~25 and 47) is taken as the other endmember, and the mixing line constrained by the average SWIR DR 75 melt inclusions, is shown in Fig. 2 (see supplementary methods for modelling detailed). The corresponding concentrations of magma derived from the metasomatized mantle source are, Pb = 0.020 ppm, and Ce = 2.46. The δ¹³C values of the metasomatized mantle endmember can be derived from our mixing model using a δ¹³C value of -7.5 ± 1.4‰ and a CO₂/Nb ratio of 1134 ± 415²³ for SWIR and using a δ¹³C value of -4‰ and a CO₂/Nb

ratio of 557 ± 79²⁵ for the “normal” MORB source mantle. This would give a δ¹³C value of -10‰ for the metasomatized endmember (Fig. 3).

We expect that other ridge systems influenced by subduction zone fluids, such as the Chile Rise, the Gakkel Ridge and the southernmost part of the Mid-Atlantic ridge near the Bouvet Triple Junction to sample mantle with a δ¹³C signature distinct from -4‰. However, as the bulk δ¹³C of subducting slabs can vary widely, depending on the relative proportions of organic carbon (at ~-25‰), altered oceanic crust (at ~-5‰) and marine carbonate (at ~0‰) components²⁹, a simple relationship between tracers of slab influence and mantle δ¹³C is not necessarily expected (Fig. 3). Therefore, the metasomatized mantle component we infer here (with a ~10‰ δ¹³C signature) is unlikely to be globally widespread; instead, we envision the existence of several such components with specific carbon isotopic signatures, mixing with the normal MORB mantle (with Fig. 3 depicting the possible ranges). These metasomatized mantle components should also get episodically sampled by mantle plumes and result in a variety of mantle δ¹³C signatures at hotspot volcanoes. Indeed while studies at the Kilauea³⁰, Réunion³¹ and Society³² hotspots all point to source δ¹³C values around -3 to -4‰, CO₂ rich (> 3000 ppm) vesicles found in submarine basalts from the Pitcairn hotspots clearly show a distinct source δ¹³C values around -6‰³³ while minerals hosted fluid inclusions in mantle xenoliths from the Canary Hotspot (El Hierro) show initial δ¹³C values around 0‰³⁴.

Decades of geochemical studies of MORBs have shown that the upper mantle is heterogeneous in isotope and trace element systematics (such as Hf and Nd³⁵, Fe and Mg³⁶⁻³⁸ and Ca³⁹,⁴⁰ among many others⁴¹⁻⁴³), our data show that this heterogeneity also applies to the upper mantle carbon isotopic signature. Precise estimate of carbon fluxes in and out of the mantle and of the size of the mantle carbon reservoir through geological time are difficult to obtain. Assuming an influx of carbon from subduction to the mantle in the order of ~10¹³ g of C/yr⁴⁴, a outflux of carbon from the mantle of similar magnitude and a reservoir size of ~10²³ g of C⁴⁴ and further assuming that these fluxes are representative of ~3.5 billion years of subduction on earth one would predict that less than 0.1% of the carbon in the Earth's current mantle is recycled from subduction. The fact that we identify a heterogeneity in the mantle carbon isotopic composition suggests either that the current flux and/or reservoir size estimates are incorrect, or that subducted carbon is not easily mixed with the rest of the mantle. The latter view is consistent with the subduction recycling model, in which distinct subduction lithologies are preserved and preferentially contribute to chemical heterogeneities of mantle derived magmas⁴⁵. An alternative explanation would be that the earth's mantle is heterogeneous in its primordial carbon isotopic composition.

## Methods

We used olivine-hosted melt inclusions, doubly polished down to 0.25 µm with corundum mats prepared in ref. 23. These samples were mounted in indium such that the MI side exposed to the beam would be the one on which no previous SIMS nor EMPA analyses were obtained and were no previous metallic coating had been applied.

Carbon isotopes were measured using a CAMECA irms-1270 at CRPG (Nancy, France). The area for analysis is chosen away from any cracks or gaps on the surface of the sample. The samples were sputtered with a Cs primary beam with presets ranging from 1 nA to 3 nA and the extracted ions were accelerated by a 10 kV potential. Depending on the intensity of the signals on the standards and samples, the primary intensity was adjusted to maintain the signal of ¹³C in the electron multiplier detector (i.e. <500 000 cts/s). For the samples the primary intensity was held at 3 ± 0.5 nA while for the standards (given higher variability in their CO₂ content) the primary intensity varied from 0.8 to 3.6 nA. As shown in Fig. S4B the instrument mass fractionation (as determined from the glass standards) is constant over this beam intensity range. A pre-sputtering of 120 s with a 15 µm grid was set to avoid any surface contamination. A 10 µm beam, centred on the gridded area, was then used for analysis. Contamination of the carbon isotopic signal from instrument background can be assessed using the
basaltic volcanoes: First insights from Piton de la Fournaise volcano (La Réunion Island). *Geochimica et. Cosmochimica Acta* **235**, 376-401 (2018).

32. Aubaud, C., Pineau, F., Hékinian, R. & Javoy, M. Degassing of CO2 and H2O in submarine lavas from the Society hotspot. *Earth Planet. Sci. Lett.* **235**, 511–527 (2005).

33. Aubaud, C., Pineau, F., Hékinian, R. & Javoy, M. Carbon and hydrogen isotope constraints on degassing of CO₂ and H₂O in submarine lavas from the Pitcairn hotspot (South Pacific). *Geophys. Res. Lett.* **33**, https://agupubs.onlinelibrary.wiley.com/doi/epdf/10.1029/2005GL024907 (2006).

34. Sandoval-Velasquez, A. et al. Recycled crustal carbon in the depleted mantle source of El Hierro volcano, Canary Islands. *Lithos* **400–401**, 106414 (2021).

35. Liu, B. & Liang, Y. The prevalence of kilometer-scale heterogeneity in the source region of MORB upper mantle. *Sci. Adv.* **3**, e1701872 (2017).

36. Sun, P. et al. Large iron isotope variation in the eastern Pacific mantle as a consequence of ancient low-degree melt metasomatism. *Geochimica et. Cosmochimica Acta* **286**, 269–288 (2020).

37. Zhong, Y., Zhang, G.-L., Lv, W.-X. & Huang, F. Iron isotope constraints on the lithological heterogeneity of the upper mantle in the South China Sea. *J. Asian Earth Sci.* **220**, 104934 (2021).

38. Guo, P. et al. Low-degree melt metasomatic origin of heavy Fe isotope enrichment in the MORB mantle. *Earth Planet. Sci. Lett.* **601**, 117892 (2023).

39. Zhu, H. et al. Calcium isotopic compositions of normal mid-ocean Ridge Basalts From the Southern Juan de Fuca Ridge. *J. Geophys. Res.: Solid Earth* **123**, 1303–1313 (2018).

40. Antonelli, M. A. & Simon, J. I. Calcium isotopes in high-temperature terrestrial processes. *Chem. Geol.* **548**, 119651 (2020).

41. Gale, A., Langmuir, C. H. & Dalton, C. A. The global systematics of ocean Ridge Basalts and their Origin. *J. Petrol.* **55**, 1051–1082 (2014).

42. White, W. M. Probing the Earth's Deep Interior through Geochemistry. *Geochem. Perspect.* **4**, 95–96 (2015).

43. Hofmann, A. W. 2.03 - Sampling Mantle Heterogeneity through Oceanic Basalts: Isotopes and Trace Elements. in *Treatise on Geochemistry* (eds. Holland, H. D. & Turekian, K. K.) 1–44 https://doi.org/10.1016/B0-08-043751-6/02123-X. (Pergamon, Oxford, 2007).

44. Dasgupta, R. & Hirschmann, M. M. The deep carbon cycle and melting in Earth's interior. *Earth Planet. Sci. Lett.* **298**, 1–13 (2010).

45. Sobolev, A. V. et al. The amount of recycled crust in sources of Mantle-Derived Melts. *Science* **316**, 412–417 (2007).

46. Maevaray, R. S. Caractérisation des éléments volatils dans une série de basaltes de ride de l'Océan Indien. (Université Paris Diderot, 2017).

47. Shaw, A. M., Behn, M. D., Humphris, S. E., Sohn, R. A. & Gregg, P. M. Deep pooling of low degree melts and volatile fluxes at the 85°E segment of the Gakkel Ridge: Evidence from olivine-hosted melt inclusions and glasses. *Earth Planet. Sci. Lett.* **289**, 311–322 (2010).

48. Wanless, V. D., Behn, M. D., Shaw, A. M. & Plank, T. Variations in melting dynamics and mantle compositions along the Eastern Volcanic Zone of the Gakkel Ridge: insights from olivine-hosted melt inclusions. *Contrib. Miner. Pet.* **167**, 1005 (2014).

49. Wanless, V. D. & Shaw, A. M. Lower crustal crystallization and melt evolution at mid-ocean ridges. *Nat. Geosci.* **5**, 651–655 (2012).

50. Shimizu, K., Saal, A. E., Hauri, E. H., Perfit, M. R. & Hékinian, R. Evaluating the roles of melt-rock interaction and partial degassing on the CO2/Ba ratios of MORB: Implications for the CO2 budget in the

Earth's depleted upper mantle. *Geochimica et. Cosmochimica Acta* **260**, 29–48 (2019).

## Acknowledgements

Samples were provided by the Marine Rock Repository at IPGP (Paris, France, https://lithotheque.ipgp.fr/) and we are most grateful to Catherine Mevel, Javier Escartin and Mathilde Cannat for help in selecting and obtaining them. We thank Nordine Bouden and Johan Villeneuve for their invaluable support during the ion probe analyses at CRPG Nancy. We would like to thank Charles Langmuir, Carrie Soderman and an anonymous reviewer for their very valuable reviews of an earlier version of this manuscript. Y.M acknowledges support from the US National Science Foundation under Award No. EAR-2407264.

## Author contributions

Initial study design: Y.M. Sample preparation: Y.M., G.G. and H.J.L. SIMS analyses: Y.M. and E.R.K. Visualisation, data compilation and modelling: Y.M. and K.K. Writing and interpretation: Y.M., E.R.K., K.K. and C.A. First draft: Y.M.

## Competing interests

The authors declare no competing interests.

## Additional information

**Supplementary information** The online version contains supplementary material available at https://doi.org/10.1038/s43247-024-01973-9.

**Correspondence** and requests for materials should be addressed to Yves Moussallam.

**Peer review information** *Communications Earth & Environment* thanks Sen Hu, Carrie Soderman and the other, anonymous, reviewer(s) for their contribution to the peer review of this work. Primary Handling Editors: Renbiao Tao and Carolina Ortiz Guerrero. [A peer review file is available].

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