# CO2 flux emissions from the Earth's most actively degassing volcanoes, 2005-2015

Alessandro Aiuppa, Tobias P. Fischer, Terry Plank, Philipson Bani

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Alessandro Aiuppa, Tobias P. Fischer, Terry Plank, Philipson Bani. CO2 flux emissions from the Earth's most actively degassing volcanoes, 2005-2015. Scientific Reports, 2019, 9, 289, p. 75-89. 10.1038/s41598-019-41901-y. insu-03708956

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Submitted on 30 Jun 2022

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# CO₂ flux emissions from the Earth's most actively degassing volcanoes, 2005–2015

Alessandro Aiuppa<fnref n="1" />, Tobias P. Fischer², Terry Plank³ & Philipson Bani⁴

Received: 11 November 2018

Accepted: 20 March 2019

Published online: 01 April 2019

The global carbon dioxide (CO₂) flux from subaerial volcanoes remains poorly quantified, limiting our understanding of the deep carbon cycle during geologic time and in modern Earth. Past attempts to extrapolate the global volcanic CO₂ flux have been biased by observations being available for a relatively small number of accessible volcanoes. Here, we propose that the strong, but yet unmeasured, CO₂ emissions from several remote degassing volcanoes worldwide can be predicted using regional/global relationships between the CO₂/S<sub>T</sub> ratio of volcanic gases and whole-rock trace element compositions (e.g., Ba/La). From these globally linked gas/rock compositions, we predict the CO₂/S<sub>T</sub> gas ratio of 34 top-degassing remote volcanoes with no available gas measurements. By scaling to volcanic SO₂ fluxes from a global catalogue, we estimate a cumulative "unmeasured" CO₂ output of 11.4 ± 1.1 Mt/yr (or 0.26 ± 0.02-10¹² mol/yr). In combination with the measured CO₂ output of 27.4 ± 3.6 Mt/yr (or 0.62 ± 0.08-10¹² mol/yr), our results constrain the time-averaged (2005–2015) cumulative CO₂ flux from the Earth's 91 most actively degassing subaerial volcanoes at 38.7 ± 2.9 Mt/yr (or 0.88 ± 0.06-10¹² mol/yr).

Volcanism is the primary mechanism through which carbon (C) stored in the deep Earth<fnref n="1" />,² is transferred to surface environments to feed C exchanges in the atmosphere-ocean-biosphere system³. Over geological time, volcanic CO₂ emissions have been a key control on atmospheric-oceanic CO₂ levels⁴⁻⁸, ultimately regulating evolution of climate and life on our planet⁹,¹⁰.

The global volcanic CO₂ flux in modern Earth remains inadequately known¹¹,¹² and, ironically, is less constrained for subaerial volcanoes than for the less-accessible mid-ocean ridges, for which the ³He flux¹³ or the CO₂/Ba ratio¹⁴ proxies have successfully been applied. Direct volcanic CO₂ observations at subaerial volcanoes are technically challenging from both ground¹¹,¹⁵ and space¹⁶ due to the large atmospheric CO₂ burden, and thus remain limited in number¹⁷,¹⁸. The volcanic CO₂ flux can be quantified indirectly by combining simultaneous acquisitions of UV-sensed sulphur dioxide (SO₂) fluxes¹¹,¹⁵,¹⁹,²⁰ and gas compositions (CO₂/SO₂ ratios), but gas observational networks are still in a developing stage²¹,²², resulting in sparse and incomplete gas catalogues²³,²⁴. CO₂ flux data have so far been obtained¹¹,¹⁵ for only <60 of the several hundred currently degassing Holocene volcanoes²⁵. CO₂ flux records are continuous enough only for a few (<10) volcanoes where permanent instrumentation is operating²⁶⁻²⁹, while sparse results (one or a few campaign-style measurements at most) are available for the remaining ~50. In addition, scarce or even no information exists for several top-ranking degassing volcanoes³⁰ in remote regions of the world (e.g., Vanuatu³¹, Papua New Guinea, the Solomon arc, and the Sunda-Banda arc in Indonesia³²,³³). Attempts to extrapolate available measurements to all the subaerial degassing volcanoes have been problematic¹¹,²³ and require use of questionable statistical approaches³⁴,³⁵. Estimates of the global volcanic CO₂ flux thus vary widely, from 66 to 540 Mt/yr¹¹,²³.

Ideally, refining the volcanic CO₂ inventory would require a comprehensive record comprising simultaneous composition/emission measurements for all the currently active strong volcanic gas emitters globally. The top-degassing volcanic targets during 2005–2015 (Table 1) have recently been identified³⁰ from satellite-based observations of the SO₂ flux using the Ozone Mapping Instrument (OMI). Carn *et al.* (ref. ³⁰) identified 91 volcanoes, listed in Table 1, releasing SO₂ at rates above the OMI detection limit of 16 tons/day. Gas CO₂/S<sub>T</sub> ratios (where S<sub>T</sub> is Total Sulfur, corresponding to SO₂ in these strongly degassing magmatic-volatile emitting volcanoes) are available for 57 out of these 91 volcanic sources³⁶, from which SO₂ fluxes can straightforwardly be converted

<footnote n="1">¹Dipartimento DiSTeM, Università di Palermo, Palermo, Italy. ²Department of Earth and Planetary Sciences, New Mexico University, Albuquerque, USA. ³Lamont-Doherty Earth Observatory, Columbia University, New York, USA. ⁴Laboratoire Magmas et Volcans, Université Blaise Pascal - CNRS -IRD, OPGC, Aubière, France. Correspondence and requests for materials should be addressed to A.A. (email: alessandro.aiuppa@unipa.it)</footnote>
<table><thead><tr><th colspan="12">Measured volcanoes</th></tr><tr><th>A</th><th>B</th><th>B</th><th>D</th><th>E</th><th>F</th><th>G</th><th>H</th><th>K</th><th>I</th><th>L=F×H</th><th>M=F×K</th><th>N</th><th>O</th></tr><tr><th>Group</th><th>Volcano</th><th>Country</th><th>Lat</th><th>Long</th><th>Measured SO<sub>2</sub> flux (tons/day)</th><th>SD</th><th>Measured CO<sub>2</sub>/SO<sub>2</sub> (molar)</th><th>Predicted CO<sub>2</sub>/SO<sub>2</sub> (molar)</th><th>SD</th><th>Measured CO<sub>2</sub> flux (tons/day)</th><th>Predicted CO<sub>2</sub> flux</th><th>SD</th><th>Notes/Data Sources</th></tr></thead><tbody><tr><td>1</td><td>Ambrym</td><td>Vanuatu</td><td>-16.25</td><td>168.12</td><td>7356</td><td>3168</td><td>1.5</td><td>—</td><td>0.4</td><td>7586</td><td>—</td><td>3843</td><td></td></tr><tr><td>1</td><td>Asama</td><td>Japan</td><td>36.40</td><td>138.53</td><td>449</td><td>430</td><td>0.8</td><td>—</td><td>0.2</td><td>247</td><td>—</td><td>247</td><td></td></tr><tr><td>1</td><td>Aso</td><td>Japan</td><td>32.88</td><td>131.11</td><td>628</td><td>492</td><td>1.8</td><td>—</td><td>0.5</td><td>777</td><td>—</td><td>650</td><td></td></tr><tr><td>1</td><td>Augustine</td><td>USA</td><td>59.35</td><td>-153.45</td><td>73</td><td>140</td><td>1.5</td><td>—</td><td>0.7</td><td>75</td><td>—</td><td>148</td><td></td></tr><tr><td>1</td><td>Avachinsky</td><td>Russia</td><td>53.25</td><td>158.83</td><td>707</td><td>619</td><td>1.2*</td><td>—</td><td>0.3</td><td>584</td><td>—</td><td>531</td><td>*Ref.<sup>110</sup></td></tr><tr><td>1</td><td>Chikurachki + Ebeko<sup>5</sup></td><td>Russia</td><td>50.33</td><td>155.46</td><td>496</td><td>469</td><td>0.9*</td><td>—</td><td>0.3</td><td>320</td><td>—</td><td>317</td><td>*Ref.<sup>111</sup></td></tr><tr><td>1</td><td>Cleveland</td><td>USA- AK</td><td>52.83</td><td>-169.77</td><td>152</td><td>142</td><td>1.0*</td><td>—</td><td>0.3</td><td>105</td><td>—</td><td>102</td><td>*Upper limit from ref.<sup>111</sup></td></tr><tr><td>1</td><td>Copahue</td><td>Argentina</td><td>-37.86</td><td>-71.16</td><td>341</td><td>425</td><td>0.9</td><td>—</td><td>0.3</td><td>211</td><td>—</td><td>272</td><td></td></tr><tr><td>1</td><td>Dukono</td><td>Indonesia</td><td>1.68</td><td>127.88</td><td>1726</td><td>611</td><td>0.4</td><td>—</td><td>0.1</td><td>475</td><td>—</td><td>206</td><td></td></tr><tr><td>1</td><td>Gareloi</td><td>USA- AK</td><td>51.79</td><td>-178.79</td><td>52</td><td>47</td><td>0.5</td><td>—</td><td>0.1</td><td>18</td><td>—</td><td>17</td><td></td></tr><tr><td>1</td><td>Isluga</td><td>Chile</td><td>-19.15</td><td>-68.83</td><td>78</td><td>107</td><td>1.0</td><td>—</td><td>0.0</td><td>51</td><td>—</td><td>70</td><td></td></tr><tr><td>1</td><td>Kliuchevskoi + Bezymianny<sup>5</sup></td><td>Russia</td><td>56.06</td><td>160.64</td><td>580</td><td>461</td><td>1.3*</td><td>—</td><td>0.4</td><td>519</td><td>—</td><td>442</td><td>*Assumes equal flux for the 2 volcanoes</td></tr><tr><td>1</td><td>Krakatau</td><td>Indonesia</td><td>-6.11</td><td>105.42</td><td>303</td><td>252</td><td>0.4</td><td>—</td><td>0.1</td><td>83</td><td>—</td><td>73</td><td></td></tr><tr><td>1</td><td>Kudriavy</td><td>Russia</td><td>45.39</td><td>148.84</td><td>187</td><td>103</td><td>0.9</td><td>—</td><td>0.3</td><td>116</td><td>—</td><td>72</td><td></td></tr><tr><td>1</td><td>Lastarria</td><td>Argentina</td><td>-25.17</td><td>-68.50</td><td>248</td><td>62</td><td>1.6</td><td>—</td><td>0.4</td><td>273</td><td>—</td><td>96</td><td></td></tr><tr><td>1</td><td>Miyake-jima</td><td>Japan</td><td>34.08</td><td>139.53</td><td>1018</td><td>934</td><td>0.7</td><td>—</td><td>0.2</td><td>490</td><td>—</td><td>471</td><td></td></tr><tr><td>1</td><td>Mutnovsky + Gorely<sup>5</sup></td><td>Russia</td><td>52.45</td><td>158.20</td><td>753</td><td>690</td><td>1.7*</td><td>—</td><td>0.5</td><td>880</td><td>—</td><td>847</td><td>*Assumes equal flux for the 2 volcanoes</td></tr><tr><td>1</td><td>Pagan</td><td>Marianas</td><td>18.14</td><td>145.79</td><td>583</td><td>547</td><td>0.8</td><td>—</td><td>0.2</td><td>321</td><td>—</td><td>311</td><td></td></tr><tr><td>1</td><td>Redoubt</td><td>USA</td><td>60.49</td><td>-152.75</td><td>368</td><td>1051</td><td>1.0</td><td>—</td><td>0.2</td><td>253</td><td>—</td><td>724</td><td></td></tr><tr><td>1</td><td>Sabancaya</td><td>Peru</td><td>-15.80</td><td>-71.86</td><td>87</td><td>158</td><td>1.5</td><td>—</td><td>0.1</td><td>90</td><td>—</td><td>163</td><td></td></tr><tr><td>1</td><td>Sakura-jima</td><td>Japan</td><td>31.59</td><td>130.66</td><td>1056</td><td>757</td><td>0.9</td><td>—</td><td>0.3</td><td>653</td><td>—</td><td>516</td><td></td></tr><tr><td>1</td><td>San Miguel</td><td>El Salvador</td><td>13.42</td><td>-88.47</td><td>88</td><td>134</td><td>1.5</td><td>—</td><td>0.2</td><td>91</td><td>—</td><td>139</td><td></td></tr><tr><td>1</td><td>Santa Ana</td><td>El Salvador</td><td>13.85</td><td>-89.63</td><td>97</td><td>180</td><td>1.0</td><td>—</td><td>0.3</td><td>66</td><td>—</td><td>125</td><td></td></tr><tr><td>1</td><td>Satsuma-Iojima</td><td>Japan</td><td>30.79</td><td>130.31</td><td>585</td><td>190</td><td>0.4</td><td>—</td><td>0.1</td><td>161</td><td>—</td><td>70</td><td></td></tr><tr><td>1</td><td>Shishaldin</td><td>USA- AK</td><td>54.76</td><td>-163.97</td><td>347</td><td>278</td><td>1.4*</td><td>—</td><td>0.4</td><td>334</td><td>—</td><td>284</td><td>*ref.<sup>112</sup></td></tr><tr><td>1</td><td>Shiveluch</td><td>Russia</td><td>56.64</td><td>161.34</td><td>530</td><td>284</td><td>1.3*</td><td>—</td><td>0.4</td><td>473</td><td>—</td><td>289</td><td>*gas data for Klyucheskovoy are used</td></tr><tr><td>1</td><td>Spurr</td><td>USA</td><td>61.30</td><td>-152.25</td><td>106</td><td>106</td><td>1.1</td><td>—</td><td>0.3</td><td>80</td><td>—</td><td>83</td><td></td></tr><tr><td>1</td><td>Suwanose-jima</td><td>Japan</td><td>29.64</td><td>129.72</td><td>863</td><td>314</td><td>1.0</td><td>—</td><td>0.3</td><td>593</td><td>—</td><td>280</td><td></td></tr><tr><td>1</td><td>Tokachi</td><td>Japan</td><td>43.42</td><td>142.69</td><td>135</td><td>98</td><td>0.4</td><td>—</td><td>0.1</td><td>37</td><td>—</td><td>29</td><td></td></tr><tr><td>1</td><td>Turrialba + Poas</td><td>Costa Rica</td><td>10.03</td><td>-83.77</td><td>751</td><td>681</td><td>1.0 (3.4*)</td><td>—</td><td>0.8</td><td>1756</td><td>—</td><td>1644</td><td>*Mean (2002–2017) Turrialba composition from de Moor et al., Pers. Comm..</td></tr><tr><td>1</td><td>Villarrica</td><td>Chile</td><td>-39.42</td><td>-71.93</td><td>281</td><td>160</td><td>1.0</td><td>—</td><td>0.3</td><td>193</td><td>—</td><td>124</td><td></td></tr><tr><td>1</td><td>Yasur</td><td>Vanuatu</td><td>-19.53</td><td>169.44</td><td>1408</td><td>563</td><td>1.6</td><td>—</td><td>0.4</td><td>1549</td><td>—</td><td>730</td><td></td></tr><tr><td>2</td><td>Galeras</td><td>Colombia</td><td>1.20</td><td>-77.39</td><td>218</td><td>317</td><td>3.3</td><td>—</td><td>0.5</td><td>495</td><td>—</td><td>723</td><td></td></tr><tr><td>2</td><td>Lokon-Empung</td><td>Sulawesi</td><td>1.36</td><td>124.79</td><td>204</td><td>154</td><td>3.2*</td><td>—</td><td>1.0</td><td>449</td><td>—</td><td>366</td><td>*This study</td></tr><tr><td>2</td><td>Masaya</td><td>Nicaragua</td><td>11.98</td><td>-86.16</td><td>867</td><td>364</td><td>2.7</td><td>—</td><td>0.7</td><td>1610</td><td>—</td><td>794</td><td></td></tr><tr><td>2</td><td>Mayon</td><td>Philippines</td><td>13.26</td><td>123.69</td><td>453</td><td>274</td><td>2.4</td><td>—</td><td>0.7</td><td>747</td><td>—</td><td>501</td><td>*ref.<sup>113</sup></td></tr><tr><td>2</td><td>Nevado del Huila</td><td>Colombia</td><td>2.93</td><td>-76.03</td><td>627</td><td>665</td><td>2.0</td><td>—</td><td>0.6</td><td>862</td><td>—</td><td>947</td><td></td></tr><tr><td>2</td><td>Nevado del Ruiz</td><td>Colombia</td><td>4.90</td><td>-75.32</td><td>1074</td><td>1376</td><td>3.0</td><td>—</td><td>0.5</td><td>2215</td><td>—</td><td>2862</td><td></td></tr><tr><td>2</td><td>Raung + Ijen</td><td>East Java</td><td>-8.06</td><td>114.24</td><td>631</td><td>238</td><td>2.6*</td><td>—</td><td>0.5</td><td>1111</td><td>—</td><td>472</td><td>*Uses composition of Ijen only (ref.<sup>114</sup>)</td></tr><tr><td>2</td><td>San Cristobal + Telica</td><td>Nicaragua</td><td>12.70</td><td>-87.00</td><td>621</td><td>283</td><td>3.5*</td><td>—</td><td>2.0</td><td>1494</td><td>—</td><td>1092</td><td>*Assumes equal flux for the 2 volcanoes</td></tr><tr><td>2</td><td>Sirung</td><td>Pantar</td><td>-8.51</td><td>124.13</td><td>373</td><td>162</td><td>3.2*</td><td>—</td><td>2.0</td><td>820</td><td>—</td><td>624</td><td>*ref.<sup>115</sup></td></tr><tr><td>2</td><td>Soufriere Hills</td><td>Montserrat</td><td>16.72</td><td>-62.18</td><td>1296</td><td>761</td><td>3.0</td><td>—</td><td>1.1</td><td>2672</td><td>—</td><td>1851</td><td></td></tr><tr><td>2</td><td>Ubinas</td><td>Peru</td><td>-16.34</td><td>-70.90</td><td>222</td><td>252</td><td>2.4</td><td>—</td><td>0.5</td><td>367</td><td>—</td><td>423</td><td></td></tr><tr><td>2</td><td>White Island</td><td>New Zealand</td><td>-37.52</td><td>177.18</td><td>254</td><td>107</td><td>4.0</td><td>—</td><td>1.2</td><td>699</td><td>—</td><td>362</td><td></td></tr><tr><td>3</td><td>Bromo + Semeru</td><td>Java</td><td>-7.94</td><td>112.95</td><td>775</td><td>298</td><td>4.1*</td><td>—</td><td>0.7</td><td>2184</td><td>—</td><td>920</td><td>*Uses Bromo gas composition</td></tr><tr><td>3</td><td>Etna</td><td>Italy</td><td>37.73</td><td>15.00</td><td>2032</td><td>517</td><td>6.5</td><td>—</td><td>2.2</td><td>9083</td><td>—</td><td>3844</td><td></td></tr><tr><td>3</td><td>Merapi</td><td>Java</td><td>-7.56</td><td>110.44</td><td>32</td><td>51</td><td>4.7</td><td>—</td><td>0.5</td><td>104</td><td>—</td><td>165</td><td></td></tr></tbody></table>

Continued
<table><thead><tr><th colspan="12">Measured volcanoes</th></tr><tr><th>A</th><th>B</th><th>B</th><th>D</th><th>E</th><th>F</th><th>G</th><th>H</th><th>K</th><th>I</th><th>L=F×H</th><th>M=F×K</th><th>N</th><th>O</th></tr><tr><th>Group</th><th>Volcano</th><th>Country</th><th>Lat</th><th>Long</th><th>Measured SO<sub>2</sub> flux (tons/day)</th><th>SD</th><th>Measured CO<sub>2</sub>/SO<sub>2</sub> (molar)</th><th>Predicted CO<sub>2</sub>/SO<sub>2</sub> (molar)</th><th>SD</th><th>Measured CO<sub>2</sub> flux (tons/day)</th><th>Predicted CO<sub>2</sub> flux</th><th>SD</th><th>Notes/Data Sources</th></tr></thead><tbody><tr><td>3</td><td>Popocatepétl</td><td>Mexico</td><td>19.02</td><td>-98.62</td><td>1658</td><td>893</td><td>8.2</td><td>—</td><td>7.0</td><td>9345</td><td>—</td><td>9434</td><td></td></tr><tr><td>3</td><td>Stromboli</td><td>Italy</td><td>38.79</td><td>15.21</td><td>181</td><td>82</td><td>7.2</td><td>—</td><td>2.8</td><td>894</td><td>—</td><td>535</td><td></td></tr><tr><td>4</td><td>Alu-Dalafilla + Erta Ale</td><td>Ethiopia</td><td>13.60</td><td>40.67</td><td>64</td><td>24</td><td>2.3*</td><td>—</td><td>0.9</td><td>99</td><td>—</td><td>56</td><td>*Uses Erta Ale comp.; ref.<sup>116</sup></td></tr><tr><td>4</td><td>Erebus</td><td>Antarctica</td><td>-77.53</td><td>167.17</td><td>52</td><td>31</td><td>27.6*</td><td>—</td><td>4.9</td><td>983</td><td>—</td><td>612</td><td>*Ref.<sup>117</sup></td></tr><tr><td>4</td><td>Kilauea</td><td>USA</td><td>19.42</td><td>-155.29</td><td>5019</td><td>2275</td><td>0.9*</td><td>—</td><td>0.2</td><td>2933</td><td>—</td><td>1578</td><td>*Refs<sup>28,118</sup></td></tr><tr><td>4</td><td>Nyiragongo + Nyamuragira<sup>8</sup></td><td>DR Congo</td><td>-1.41</td><td>29.20</td><td>3533</td><td>2408</td><td>6.5*</td><td>—</td><td>1.2</td><td>15790</td><td>—</td><td>11149</td><td>*Refs<sup>119-121</sup></td></tr><tr><td>4</td><td>Piton de la Fournaise</td><td>Reunion, France</td><td>-21.23</td><td>55.71</td><td>134</td><td>162</td><td>0.3*</td><td>—</td><td>0.1</td><td>28</td><td>—</td><td>34</td><td>*Refs<sup>122</sup>; Di Muro, pers. comm.</td></tr><tr><td>N.D.</td><td>Marapi</td><td>Sumatra</td><td>-0.39</td><td>100.46</td><td>34</td><td>34</td><td>20.5*</td><td>—</td><td>1.1</td><td>480</td><td>—</td><td>485</td><td>*This study</td></tr><tr><td colspan="12">"Unmeasured" volcanoes: those for which no CO<sub>2</sub> gas data exist.</td></tr><tr><th>A</th><th>B</th><th>B</th><th>D</th><th>E</th><th>F</th><th>G</th><th>H</th><th>K</th><th>I</th><th>L=F×H</th><th>M×F×K</th><th>N</th><th>O</th></tr><tr><th>Group</th><th>Volcano</th><th>Country</th><th>Lat</th><th>Long</th><th>Measured SO<sub>2</sub> flux (tons/day)</th><th>SD</th><th>Measured CO<sub>2</sub>/SO<sub>2</sub> (molar)</th><th>Predicted CO<sub>2</sub>/SO<sub>2</sub> (molar)</th><th>SD</th><th>Measured CO<sub>2</sub> flux</th><th>Predicted CO<sub>2</sub> flux (tons/day)</th><th>SD</th><th>Notes/Data Sources</th></tr><tr><td>1</td><td>Anatahan</td><td>Northern Mariana Islands</td><td>16.35</td><td>145.67</td><td>1335</td><td>1867</td><td>—</td><td>1.2</td><td>0.5</td><td>—</td><td>1102</td><td>1607</td><td></td></tr><tr><td>2</td><td>Aoba</td><td>Vanuatu</td><td>-15.40</td><td>167.83</td><td>2870</td><td>1229</td><td>—</td><td>2.5</td><td>0.7</td><td>—</td><td>4933</td><td>2524</td><td></td></tr><tr><td>2</td><td>Bagana</td><td>Papua New Guinea</td><td>-6.09</td><td>155.23</td><td>3779</td><td>886</td><td>—</td><td>2.4</td><td>0.7</td><td>—</td><td>6245</td><td>2335</td><td></td></tr><tr><td>2</td><td>Barren Island</td><td>India</td><td>12.28</td><td>93.86</td><td>243</td><td>341</td><td>—</td><td>2.2*</td><td>1.3</td><td>—</td><td>372</td><td>566</td><td>*From the Sunda-Banda gas-rock association; Table S1c; Fig. 2g</td></tr><tr><td>2</td><td>Batu Tara + Lewotolo</td><td>Indonesia</td><td>-8.27</td><td>123.51</td><td>632</td><td>177</td><td>—</td><td>2.4*</td><td>0.7</td><td>—</td><td>1043</td><td>420</td><td>*From the Sunda-Banda gas-rock association; Table S1c; Fig. 2g</td></tr><tr><td>1</td><td>Bulusan</td><td>Philippines</td><td>12.77</td><td>124.05</td><td>206</td><td>199</td><td>—</td><td>1.2</td><td>0.5</td><td>—</td><td>170</td><td>179</td><td></td></tr><tr><td>1</td><td>Chiginagak</td><td>USA-AK</td><td>57.14</td><td>-156.99</td><td>138</td><td>127</td><td>—</td><td>1.2</td><td>0.5</td><td>—</td><td>114</td><td>115</td><td></td></tr><tr><td>2</td><td>Ebulobo</td><td>Indonesia/ Nusa</td><td>-8.82</td><td>121.18</td><td>86</td><td>63</td><td>—</td><td>2.6*</td><td>1.3</td><td>—</td><td>153</td><td>137</td><td>*From the Sunda-Banda gas-rock association; Table S1c; Fig. 2g</td></tr><tr><td>1</td><td>Fuego + Pacaya<sup>5</sup></td><td>Guatemala</td><td>14.47</td><td>-90.88</td><td>252</td><td>46</td><td>—</td><td>1.6*</td><td>0.8</td><td>—</td><td>269</td><td>139</td><td>*From the CAVA gas-rock association; Table S1a; Fig. 2a</td></tr><tr><td>2</td><td>Gaua</td><td>Vanuatu</td><td>-14.27</td><td>167.50</td><td>434</td><td>382</td><td>—</td><td>2.5*</td><td>0.7</td><td>—</td><td>745</td><td>688</td><td>*From the Group 2 global gas-rock association; Table S1d; Fig. 3a</td></tr><tr><td>4</td><td>Jebel-at-Tair</td><td>Yemen</td><td>15.55</td><td>41.83</td><td>103</td><td>295</td><td>—</td><td>6.2*</td><td>1.8</td><td>—</td><td>445</td><td>1527</td><td>*Average on non-arc volcanoes</td></tr><tr><td>1</td><td>Kanlaon</td><td>Philippines</td><td>10.41</td><td>123.13</td><td>70</td><td>182</td><td>—</td><td>1.2</td><td>0.5</td><td>—</td><td>57</td><td>152</td><td></td></tr><tr><td>3</td><td>Karangetang</td><td>Indonesia/ Sulawesi</td><td>2.78</td><td>125.40</td><td>313</td><td>85</td><td>—</td><td>5.0*</td><td>1.3</td><td>—</td><td>1069</td><td>403</td><td>*From the Sunda-Banda gas-rock association; Table S1c; Fig. 2g</td></tr><tr><td>1</td><td>Karymsky</td><td>Russia</td><td>54.05</td><td>159.45</td><td>912</td><td>250</td><td>—</td><td>1.2</td><td>0.5</td><td>—</td><td>752</td><td>375</td><td></td></tr><tr><td>2</td><td>Kerinci</td><td>Indonesia/ Sumatra</td><td>-1.70</td><td>101.26</td><td>294</td><td>99</td><td>—</td><td>2.6*</td><td>0.8</td><td>—</td><td>525</td><td>233</td><td>*From the Sunda-Banda gas-rock association; Table S1c; Fig. 2g</td></tr><tr><td>1</td><td>Ketoi</td><td>Russia</td><td>47.34</td><td>152.48</td><td>139</td><td>151</td><td>—</td><td>1.2</td><td>0.5</td><td>—</td><td>114</td><td>133</td><td></td></tr><tr><td>1</td><td>Kizimen</td><td>Russia</td><td>55.12</td><td>160.36</td><td>711</td><td>1544</td><td>—</td><td>1.2</td><td>0.5</td><td>—</td><td>587</td><td>1297</td><td></td></tr><tr><td>1</td><td>Korovin</td><td>USA-AK</td><td>52.38</td><td>-174.15</td><td>198</td><td>160</td><td>—</td><td>1.2</td><td>0.5</td><td>—</td><td>163</td><td>148</td><td></td></tr><tr><td>2</td><td>Langila</td><td>Papua New Guinea</td><td>-5.53</td><td>148.42</td><td>629</td><td>527</td><td>—</td><td>2.3*</td><td>0.7</td><td>—</td><td>994</td><td>886</td><td>*From the Group 2 global gas-rock association; Table S1d; Fig. 3a</td></tr><tr><td>2</td><td>Manam</td><td>Papua New Guinea</td><td>-4.08</td><td>145.04</td><td>1484</td><td>753</td><td>—</td><td>2.7*</td><td>0.7</td><td>—</td><td>2755</td><td>1570</td><td>*From the Group 2 global gas-rock association; Table S1d; Fig. 3a</td></tr></tbody></table>

Continued
<table><caption>**Table 1.** Gas composition and fluxes for the 91 strongest SO₂ volcanic gas sources in 2005–2015 worldwide (from Carn *et al.*, 2017). The quoted SO₂ fluxes (column F) are 2005–2015 averages (and standard deviation, SD) taken from the compilation of ref. 30. The “measured volcanoes” list includes those volcanoes for which SO₂ flux and gas composition molar CO₂/SO₂ ratios have both been measured. Each volcano is assigned to a given Group (1–4) (column A) based on the original categorization of ref. 36 (non-arc volcanoes are assigned to Group 4). Unless indicated (see references in column O), the measured CO₂/SO₂ ratios (column H) are from ref. 36. For these strongly degassing volcanoes, we assume total S (S<sub>T</sub>; quoted in 36) equals to SO₂; SO₂ satellite detection for all these volcanoes implies high-emission temperatures and limited or no interaction with hydrothermal system (and thus trivial reduced S species, such as H₂S). Marapi volcano in Sumatra is an exception because of its hydrothermal signature (high CO₂/SO₂, high H₂S) and is not assigned to any specific group (N.D. = not determined). In cases where combined emissions from two volcanoes are listed in the original dataset 30 (see volcanoes labelled with superscripts & and $ in column B), due to insufficient spatial OMI resolution, a weighted average was calculated from the available volcanic gas information for the 2 where possible. Otherwise, equal$</caption><thead><tr><th colspan="11">Measured volcanoes</th></tr><tr><th>A</th><th>B</th><th>B</th><th>D</th><th>E</th><th>F</th><th>G</th><th>H</th><th>K</th><th>I</th><th>L=F×H</th><th>M=F×K</th><th>N</th><th>O</th></tr><tr><th>Group</th><th>Volcano</th><th>Country</th><th>Lat</th><th>Long</th><th>Measured SO<sub>2</sub> flux (tons/day)</th><th>SD</th><th>Measured CO<sub>2</sub>/SO<sub>2</sub> (molar)</th><th>Predicted CO<sub>2</sub>/SO<sub>2</sub> (molar)</th><th>SD</th><th>Measured CO<sub>2</sub> flux (tons/day)</th><th>Predicted CO<sub>2</sub> flux</th><th>SD</th><th>Notes/Data Sources</th></tr></thead><tbody><tr><td>1</td><td>Michael</td><td>South Sandwich Isl. (UK)</td><td>-57.80</td><td>-26.49</td><td>263</td><td>63</td><td>—</td><td>1.2</td><td>0.5</td><td>—</td><td>217</td><td>104</td><td></td><td></td></tr><tr><td>1</td><td>Montagu</td><td>South Sandwich Isl. (UK)</td><td>-58.42</td><td>-26.33</td><td>142</td><td>179</td><td>—</td><td>1.2</td><td>0.5</td><td>—</td><td>117</td><td>155</td><td></td><td></td></tr><tr><td>2</td><td>Paluweh</td><td>Indonesia/ Nusa</td><td>-8.32</td><td>121.71</td><td>60</td><td>65</td><td>—</td><td>2.6*</td><td>1.3</td><td>—</td><td>108</td><td>130</td><td></td><td>*From the Sunda-Banda gas-rock association; Table S1c; Fig. 2g</td></tr><tr><td>2</td><td>Reventador</td><td>Ecuador</td><td>-0.08</td><td>-77.66</td><td>206</td><td>187</td><td>—</td><td>2.2*</td><td>0.8</td><td>—</td><td>304</td><td>298</td><td></td><td>*From the SA gas-rock association; Table S1b; Fig. 2d</td></tr><tr><td>3</td><td>Rinjani</td><td>Indonesia/ Lombok</td><td>-8.42</td><td>116.47</td><td>74</td><td>131</td><td>—</td><td>4.3*</td><td>1.3</td><td>—</td><td>219</td><td>392</td><td></td><td>*From the Sunda-Banda gas-rock association; Table S1c; Fig. 2g</td></tr><tr><td>3</td><td>Sangeang Api</td><td>Indonesia/ Nusa</td><td>-8.21</td><td>119.07</td><td>71</td><td>150</td><td>—</td><td>4.9*</td><td>1.3</td><td>—</td><td>239</td><td>508</td><td></td><td>*From the Sunda-Banda gas-rock association; Table S1c; Fig. 2g</td></tr><tr><td>1</td><td>Santiaguito</td><td>Guatemala</td><td>14.76</td><td>-91.55</td><td>247</td><td>119</td><td>—</td><td>1.6*</td><td>0.8</td><td>—</td><td>271</td><td>182</td><td></td><td>*From the CAVA gas-rock association; Table S1a; Fig. 2a</td></tr><tr><td>1</td><td>Sarychev</td><td>Russia</td><td>48.08</td><td>153.21</td><td>260</td><td>324</td><td>—</td><td>1.2</td><td>0.5</td><td>—</td><td>214</td><td>282</td><td></td><td></td></tr><tr><td>2</td><td>Sinabung</td><td>Indonesia/ Sumatra</td><td>3.17</td><td>98.39</td><td>327</td><td>595</td><td>—</td><td>2.4*</td><td>1.3</td><td>—</td><td>550</td><td>1043</td><td></td><td>*From the Sunda-Banda gas-rock association; Table S1c; Fig. 2g</td></tr><tr><td>2</td><td>Slamet</td><td>Indonesia/ Java</td><td>-7.24</td><td>109.21</td><td>206</td><td>132</td><td>—</td><td>2.2*</td><td>1.3</td><td>—</td><td>311</td><td>272</td><td></td><td>*From the Sunda-Banda gas-rock association; Table S1c; Fig. 2g</td></tr><tr><td>2</td><td>Tavurvur</td><td>Papua New Guinea</td><td>-4.24</td><td>152.21</td><td>1729</td><td>2535</td><td>—</td><td>2.6*</td><td>0.7</td><td>—</td><td>3091</td><td>4607</td><td></td><td>*From the Group 2 global gas-rock association; Table S1d; Fig. 3a</td></tr><tr><td>2</td><td>Tinakula</td><td>Solomon back-arc</td><td>-10.38</td><td>165.80</td><td>256</td><td>276</td><td>—</td><td>2.1*</td><td>0.7</td><td>—</td><td>370</td><td>417</td><td></td><td>*From the Group 2 global gas-rock association; Table S1d; Fig. 3a</td></tr><tr><td>1</td><td>Tofua</td><td>Tonga Islands</td><td>-19.75</td><td>-175.07</td><td>284</td><td>89</td><td>—</td><td>1.2</td><td>0.5</td><td>—</td><td>235</td><td>122</td><td></td><td></td></tr><tr><td></td><td>Tungurahua</td><td>Ecuador</td><td>-1.47</td><td>-78.44</td><td>342</td><td>235</td><td>—</td><td>2.5*</td><td>0.8</td><td>—</td><td>588</td><td>445</td><td></td><td>*From the SA gas-rock association; Table S1b; Fig. 2d</td></tr><tr><td>2</td><td>Ulawun</td><td>Papua New Guinea</td><td>-5.05</td><td>151.33</td><td>630</td><td>581</td><td>—</td><td>2.4*</td><td>0.7</td><td>—</td><td>1040</td><td>1005</td><td></td><td>*From the Group 2 global gas-rock association; Table S1d; Fig. 3a</td></tr><tr><td>1</td><td>Veniaminof</td><td>USA-AK</td><td>56.17</td><td>-159.38</td><td>255</td><td>214</td><td>—</td><td>1.2</td><td>0.5</td><td>—</td><td>211</td><td>197</td><td></td><td></td></tr><tr><td></td><td></td><td></td><td></td><td></td><td>Measured SO<sub>2</sub> flux</td><td>SD</td><td></td><td></td><td></td><td>Measured CO<sub>2</sub> flux</td><td>Predicted CO<sub>2</sub> flux</td><td>Total CO<sub>2</sub> flux</td><td></td><td></td></tr><tr><td></td><td>GRAND TOTAL (Mt/yr, 10<sup>9</sup>kg/yr)</td><td></td><td></td><td></td><td>23</td><td>15</td><td></td><td></td><td></td><td>27.4 ± 3.6</td><td>11.4 ± 1.4</td><td>38.7 ± 2.9</td><td></td><td></td></tr><tr><td></td><td>GRAND TOTAL (10<sup>12</sup>mol/yr)</td><td></td><td></td><td></td><td>0.36</td><td>0.23</td><td></td><td></td><td></td><td>0.62 ± 0.08</td><td>0.26 ± 0.02</td><td>0.88 ± 0.06</td><td></td><td></td></tr></tbody></table>
gas contribution was assumed for the two volcanoes. The measured CO₂ flux (column L) is calculated from the product of F by H (the quoted standard deviations in column N are based on propagation of the respective errors). The “unmeasured volcanoes” list includes volcanoes for which gas CO₂/S_T data are unavailable. Thirteen of such “unmeasured” (for gas) volcanoes are sited in arc segments with no subducted carbonate-rich lithologies at the respective trenches, and are therefore assigned to Group 1 (e.g., they are assigned the mean CO₂/S_T ratio of 1.2 ± 0.5 of Group 1 volcanoes; see Table S1). For the remaining volcanoes, we predict the time-averaged CO₂/S_T (here considered as equivalent to CO₂/SO₂; column K) from the averaged (mean) trace-element composition of the corresponding volcanic rocks (Table S1) and the individual arc/global arc CO₂/S_T vs. Ba/La associations (see Figs 2 and 3). Uncertainty in the predicted CO₂/S_T ratios (column I) is the confidence interval calculated from the regression line and one standard deviation about the regression, and incorporates uncertainty/variability in “measured” gas CO₂/S_T ratios (average uncertainty at 1σ, ~26%) and whole-rock Ba/La ratios (average uncertainty at 1σ, ~16%) (see Table S1). Column O references the supplementary table (Table S1) detailing the specific CO₂/S_T vs. Ba/La relation used. The SO₂ flux GRAND TOTAL of 23 ± 15 Mt/yr is from ref.³². The GRAND TOTAL for measured, predicted and total CO₂ flux is obtained by applying Monte Carlo method to the CO₂ datasets of columns L, M and L + M, respectively. For each of the three datasets, 100 simulations are considered. In each simulation, the CO₂ flux for each volcano is left to vary randomly within its mean ± SD value, and the resulting CO₂ fluxes are summed up. The procedure is repeated 100 times, yielding 100 random-generated sums. The GRANDTOTAL values quoted in the tables are ranges (mean ± 1 SD) of 70% of the three populations of random-generated sums (e.g., the 15% outliers on each end of the populations are omitted). With this procedure, the global volcanic CO₂ flux is assessed at 38.7 ± 2.9 Mt/yr, ~11.4 ± 1.4 Mt/yr of which is estimated for the 34 “unmeasured” volcanoes (those with no measured gas data available).

into CO₂ fluxes (Table 1). The remaining 34 volcanoes, however, have so far been impossible to characterise for gas composition, owing to their remoteness and/or poor accessibility, leaving their CO₂ fluxes unconstrained.

Here, we explore an alternative approach of indirectly inferring the CO₂/S_T ratio signature of these “unmeasured” volcanoes, and ultimately their CO₂ flux, based on the (far more commonly measured) trace element compositions of their erupted volcanic rocks. Volcanic arc gas CO₂/S_T ratios and whole-rock trace element ratios (e.g., Ba/La or Sr/Nd ratios) are globally linked³⁶, as both volatiles and fluid/melt-mobile elements (e.g., Ba and Sr) are sourced from fluids delivered from dehydration/melting of subducting slab sediments and altered ocean crust³⁷⁻⁴². Based on their gas vs. whole-rock associations, arc volcanoes cluster into three Groups³⁶. Group 1, which includes C-poor arc volcanoes (gas CO₂/S_T ratios ≤2), are thought³⁶ to be sourced by a mantle wedge source contaminated by C-poor slab fluids (derived from either terrigenous sediments or altered oceanic crust). Group 2 volcanoes are assumed to inherit their C-richer (2≤ CO₂/S_T ratios ≤4) gas composition from incorporation into the mantle wedge of slab fluids derived from melting of carbonated sediments. Group 3 (CO₂/S_T ratios >4), finally, includes C-rich arc gases, supporting the involvement of an additional crustal C contribution (de-carbonation/assimilation of upper crustal limestones⁴³,⁴⁴).

We here establish systematic gas vs. rock relationships at the scale of individual arc segments and/or groups of volcanoes. These relationships, once set, allow us to predict the CO₂/S_T ratio for any volcano for which trace-element whole-rock information (but not gas composition) is available. Ultimately, using these predicted CO₂/S_T ratios in tandem with available SO₂ flux information³⁰, we derive CO₂ fluxes for all current top-degassing volcanoes and, by summation, a refined inventory of decadal (2005–2015) global CO₂ emissions from subaerial volcanism.

## Results

**CO₂ fluxes for the Earth's best-studied volcanoes.** Roughly ~62% of the 91 strongest volcanic SO₂ sources globally³⁰ have been characterised for both SO₂ flux and (episodically) for volcanic gas compositions (Table 1). CO₂ fluxes are thus obtained (see “Methods”) by pairing the OMI-based time-averaged 2005–2015 SO₂ fluxes³⁰ with the characteristic (mean) CO₂/SO₂ ratios in the corresponding high-temperature magmatic gases (data from ref.³⁶ unless otherwise noted). The so-derived CO₂ fluxes (Table 1) range from 28 to 15,800 tons/day, and are in reasonable agreement (typically within a factor ≤40%) with the CO₂ fluxes estimated using ground-based SO₂ flux measurements¹¹,¹⁵. We estimate the cumulative CO₂ flux from the 57 volcanic sources with “measured” gas compositions by applying a Monte Carlo method (see Table 1) to the dataset. The obtained cumulative “measured” flux is 27.4 ± 3.6 Mt/yr (or 0.62 ± 0.08·10¹² mol/yr).

**Matching gas and whole-rock trace element compositions.** Thirty-four top-ranking volcanic SO₂ sources do not have gas compositional records (Table 1). We hereafter refer to such volcanoes without CO₂/S_T information as “unmeasured” volcanoes.

We thus explore a methodology to predict the characteristic volcanic gas CO₂/S_T ratio of each of these 34 “unmeasured” volcanoes using their averaged trace-element volcanic rock composition (Table S1). Gas CO₂/S_T ratios in arc volcanoes exhibit systematic global relationships with slab fluid trace-element proxies (e.g., Ba/La or Sr/Nd ratios) in the corresponding whole-rocks, which are interpreted³⁶ as resulting from a common CO₂-Ba-Sr derivation from melting of subducted sediments in the slab⁴⁰ (variably enriched in CO₂; ref.⁴²). These relationships, once set at the scale of individual arc segments (Figs 1 and 2) or volcano Groups (Fig. 3), can now be used to infer the representative volcanic gas CO₂/S_T ratio signature of the 34 “unmeasured” volcanoes (Tables 1 and S1).

The procedure is illustrated in Fig. 1 and Table 2 for Pacaya volcano as an example (see “Methods”). The initial step involves establishing a CO₂/S_T vs. Ba/La relationship using data for volcanoes for which both gas and
<figure><img src="image_3.png" /><figcaption>**Figure 1.** The proxy approach for estimating the CO₂/Sₜ ratio for “unmeasured” volcanoes (i.e., those for which no gas data exist), based on the averaged trace-element composition of the corresponding volcanic rocks. The procedure is illustrated for Pacaya volcano in Guatemala. Firstly, the association between CO₂/Sₜ ratios in volcanic gases (corresponding to CO₂/SO₂ gas ratios in the high-temperature systems studied here) and whole-rock Ba/La ratios is established at the scale of the Central American Volcanic Arc (CAVA) segments, using data for volcanoes for which both gas and trace element data are available (see Table S1a). Secondly, the gas vs. trace-element trend is fitted via either a linear or logarithm best-fit regression function. Tests made excluding (panel A) or including (panel B) the compositional point of the Depleted Mid-ocean ridge Mantle (DMM; refs¹²³,¹²⁴) in the data-fitting found that the second option systematically led to the best-data fits (see Table 2). Finally, the preferred regression model function (RM3 in the Pacaya example; see panel B and Table 2) is used to calculate a “predicted” gas CO₂/SO₂ from available Ba/La data for Pacaya whole-rocks (uncertainty is estimated from confidence interval at one standard deviation on the regression). Our inferred gas CO₂/Sₜ ratio (1.4 ± 0.75; Table 2) is well within the magmatic gas range (CO₂/SO₂ ratio of 1.1 ± 1.0.) measured during recent plume observations⁴⁶. A similar CO₂/Sₜ ratio (see Table 2) is predicted using the CAVA gas vs. Sr/Nd ratio association (panel C). In this plot, the yellow and green dashed lines are the linear best-fit regression lines for Group 1 and 2 sub-populations, respectively.</figcaption></figure>

trace element data are available (for the specific Pacaya example, we use gas/whole-rock information for Central American volcanoes, see Table S1a and Fig. 1). As in previous work³⁶, the representative CO₂/Sₜ ratios used in Fig. 1 (listed in Table 1 and S1a) are obtained by averaging available results for high-temperature gas samples, in the attempt to reduce the effect of secondary processes (e.g., magmatic S scrubbing during gas-water-rock reactions⁴⁵) that become substantial at hydrothermal (temperature <400 °C) conditions. Secondly, regression analysis is used to fit the gas vs. trace-element association via either a (i) linear or (ii) logarithmic regression model (Fig. 1; see “Methods”). We focus on the two regression models based on the assumption that linear/logarithmic functions best describe depleted mantle (DMM)-slab fluid mixing in a CO₂/Sₜ ratio vs. Ba/La (or Sr/Nd) compositional field³⁶. Finally, the adopted regression function is used to derive a “predicted” gas CO₂/Sₜ from available Ba/La whole rock data (Fig. 1). In the specific Pacaya example (Fig. 1 and Table 2), using a linear regression to fit the volcanic gas and DMM data-points (our RM3 regression model, see “Methods” and Table S1), the “predicted” gas CO₂/SO₂ ratio is 1.4 ± 0.75, well within the magmatic gas range (CO₂/SO₂ ratio of 1.1 ± 1.0) recently determined⁴⁶ from plume measurements (Fig. 1).

**CO₂/Sₜ ratios from individual-arc gas vs. trace-element relationships.** Gas vs. rock (trace element composition) associations are initially analysed at the scale of individual arc segments, in the assumption that, at such regional scales, sources and transport pathways of volatiles and trace elements are relatively uniform. In truth, intra-arc variations in thickness, age, thermal properties and composition of the slab and overlying plate⁴⁷, and in the composition of subducted sediments⁴², are large enough to impact the mechanisms of magma generation, and thus impart regional trends in volatile⁴⁸ and trace element⁴⁹ signatures of erupted magmas. Nonetheless, it is on these individual-arc trends that we rely below. Three arc segments have enough volcanoes measured for both gases and rocks to allow reliable gas vs. rock associations to be established (Fig. 2).

The Central American Volcanic Arc (CAVA) CO₂/Sₜ vs. Ba/La relationship, obtained from results listed in Table S1a, is illustrated in Figs 1 and 2a. The systematic along-arc trace-element patterns in CAVA volcanic rocks⁴⁹ (Fig. 2b,c) originate from changes in geometry, age, thermal regime and extent of serpentinization of the subducting Cocos plate slab⁵⁰. As more recently found³⁶,⁵¹, such trace-element variations correlate with those of CO₂/Sₜ ratios in high-temperature magmatic CAVA gases. These correlations (e.g., Figs 1 and 2a) have been explained³⁶,⁵¹
<figure><img src="image_4.png" /><figcaption>**Figure 2.** Scatter plots of mean Ba/La whole-rock ratios vs. volcanic gas CO₂/Sₜ ratios (panels A, D and G), whole-rock Sr/Nd ratios (panels B, E and H) and whole-rock U/Th ratios (panels C, F and K) for three arc segments (left, Central America; middle, Southern America; right Sunda-Banda arc in Indonesia). Each symbol corresponds to an individual volcano for which gas and trace element information is simultaneously available (see Tables S1a–c for the list of volcanoes, compositions used, and data sources). The gas vs. trace element correlations are explained in terms of mixing between a C-Ba-Sr-U-poor Depleted Mid-ocean ridge Mantle (DMM) and C-Ba-Sr-U-rich slab fluids. C-poor arc volcanoes (Group 1, in green) plot close to the DMM, while Group 2 arc volcanoes (in yellow) are C-enriched to larger slab fluid influx. The even more C-rich signature of Group 3 arc volcanoes (in red) may reflect some addition of crustal carbon³⁶. For each arc segment, panels A, D, and G show the best-fit regression functions used to predict the volcanic gas CO₂/Sₜ ratios for “unmeasured” volcanoes (open symbols; see Tables 1 and S1b–d). The grey lines illustrate (for two “unmeasured” volcano examples) the procedure used to convert whole-rock Ba/La ratios into gas CO₂/Sₜ ratios, using the equations of the best-fit regression lines.</figcaption></figure>

<table><caption>**Table 2.** Comparison between measured⁴⁶ and predicted (this work) volcanic gas CO₂/Sₜ ratios in the Pacaya magmatic gases. At the high-T magmatic gas conditions explored here, total S (Sₜ) corresponds to SO₂. The predicted CO₂/Sₜ ratios are obtained from the mean Ba/La ratio (or Sr/Nd; see RM5) in Pacaya whole-rocks using the regression functions through the CO₂/Sₜ vs. Ba/La (or Sr/Nd; see RM5) association for CAVA volcanoes (dataset listed in Table S1a). Five distinct regression functions are tested, being illustrated (with their corresponding equations and regression coefficients) in Fig. 1. RM1 and RM2 (Fig. 1a) use linear and logarithmic regression models, respectively, and do not include the composition of the Depleted mid-ocean ridge Mantle in the fit. Regression models RM3 and RM4 (Fig. 1b) are, respectively, linear and logarithmic, and the composition of the Depleted mid-ocean ridge Mantle is included in the fit. Regression models RM5 (Fig. 1c) uses linear regression functions through the CO₂/Sₜ vs. Sr/Nd association for Group 1 CAVA volcanoes. The linear regression model RM3 yields the highest regression coefficient (R² = 0.7; see Fig. 1b), and is thus adopted here.</caption><thead><tr><th>Measured CO<sub>2</sub>/SO<sub>2</sub> (molar)</th><th>Predicted CO<sub>2</sub>/S<sub>T</sub> (linear regression model RM1)</th><th>Predicted CO<sub>2</sub>/S<sub>T</sub> (logarithm regression model RM2)</th><th>Predicted CO<sub>2</sub>/S<sub>T</sub> (linear regression model RM3)</th><th>Predicted CO<sub>2</sub>/S<sub>T</sub> (logarithm regression model RM4)</th><th>Predicted CO<sub>2</sub>/S<sub>T</sub> (linear regression model RM5)</th></tr></thead><tbody><tr><td>1.1 ± 0.9</td><td>1.4 ± 0.75</td><td>1.6 ± 0.8</td><td>1.4 ± 0.75</td><td>2.0 ± 0.8</td><td>1.3 ± 0.75</td></tr></tbody></table>

as resulting from the variable addition of C-Ba-Sr-rich fluids issuing from melting of limestone-bearing slab sediments, with the highest slab-fluid influx being observed in Nicaragua⁵², where magmatic gases consistently have C-rich (Group 2) affinity (Fig. 2a). In Costa Rica and El Salvador, magmatic gases are typically C-poorer³⁶,⁵¹ (Group 1), in line with the lower slab affinity (and more depleted mantle-like signature) of trace-element ratios (Fig. 2). All the CAVA volcanic SO₂ emitters of Table 1 have been measured for gas composition (at least for their CO₂/Sₜ ratio), except for Guatemalan volcanoes Fuego and Santa Maria. We use the CAVA CO₂/Sₜ vs. Ba/La association (of Fig. 2a) to fill this gap of knowledge. Using the RM3 regression model in tandem with mean whole-rock Ba/La ratios (Table S1a and Fig. 2a), we infer CO₂/Sₜ ratios of respectively 1.7 ± 0.75 (Fuego) and 1.6 ± 0.75 (Santa Maria).
<figure><img src="image_5.png" /><figcaption>**Figure 3.** Scatter plots of mean Ba/La whole-rock ratios vs. (A) volcanic gas CO₂/Sₜ ratios, (B) whole-rock Sr/Nd ratios and (C) whole-rock U/Th ratios (panels C, F and K) for Group 1 volcanoes (green, see Table S1d) and Group 2 volcanoes (yellow, see Table S1e) globally. Each symbol corresponds to an individual arc volcano for which gas and trace element information is simultaneously available (see Tables S1d-e for the list of volcanoes, compositions used, and data sources). Volcanoes with no gas compositional information are shown as open circles. The Vanuatu arc volcanoes are plotted in light green. The best-fit regression functions through the populations of Group 1 and Group 2 volcanoes are separately illustrated. Group 1 volcanoes exhibit little change in gas CO₂/Sₜ ratios on increasing Ba/La. Their mean CO₂/Sₜ ratio of 1.2 ± 0.5 (see Table S1d) is thus adopted for all the “unmeasured” (for gas) Group 1 volcanoes (Table 1). For the “unmeasured” Group 2 volcanoes, we average the predicted volcanic gas CO₂/Sₜ ratios obtained from regression functions RM3 and RM4 (see Tables 1 and S1e).</figcaption></figure>

Our compilation (Table 1) shows that volcanic gas CO₂/Sₜ data are available for the majority of the volcanic SO₂ emitters in the Northern (NVZ), Central (CVZ) and Southern (SVZ) Volcanic Zones⁵³ of the Andes (Southern America). Very limited gas information is available⁵⁴ for Ecuadorian volcanoes, however, and here we use the CO₂/Sₜ vs. Ba/La association (for South-America: Fig. 2d) to fill this knowledge gap. In the Andes, there is documented evidence in the literature for large along-arc variations in volcanic rock trace-element geochemistry⁵⁵⁻⁵⁸. Our partial whole-rock dataset, based on the subpopulation of Andean volcanoes listed in Table S1b, demonstrates an overall south-to-north increase in trace-element slab-fluid proxies (Ba/La, Sr/Nd and U/Th; Fig. 2e,f), from Copahue volcano in Argentina (SVZ) to Nevado de Ruiz in Colombia (NVZ). Importantly, the along-arc variations in the volcanic gas CO₂/Sₜ ratio scale well with the trace-element variation patterns (Fig. 2d), again suggesting common source processes. The trace-element signature of the three most actively degassing volcanoes today in Ecuador, Tungurahua⁵⁹, El Reventador⁶⁰ and Cotopaxi⁶¹ (the latter not appearing in the 91 list of top degassing volcanoes³⁰), places Ecuadorian magmatism in an intermediate position between Colombian volcanoes in the NVZ (the richest in Ba and Sr, but also CO₂; Fig. 2d) and intermediate C-rich Peruvian volcanoes⁶²
<figure><img src="image_6.png" /></figure>

**Figure 4.** Global map illustrating the location of the 91 strongest volcanic CO₂ emitters (data from Table 1). CO₂ flux information for both “measured” (circles with black borders) and “unmeasured” (circles with red borders) volcanoes is shown. Dimension of the symbols is proportional to CO₂ flux, with color fill reflecting the CO₂/S_T ratio (see legend). Trenches are differently colored depending on CO₂ bulk concentration in the trench sediments (data from ref.⁴²). The map shows that the most strongly CO₂ degassing volcanoes are clustered in tropic to sub-tropical regions such as the Vanuatu-Papua New Guinea arc segments, in Central America, Southern American (Northern Volcanic Zone), and in the Lesser Antilles, in addition to Italy (Etna), Congo (Nyrangongo + Nyamuragira) and Hawaii (Kilauea). Volcanic CO₂ fluxes are typically lower in higher latitude volcanic regions such as in the Aleutians-Kamchatka-Kuriles and in the South-Sandwich Islands, where no carbonate-rich lithologies are subducted at the trenches. The map was generated using the open source QGIS software (available at https://www.qgis.org/it/site/) (Copyright © 2019 AIUPPA. Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.3 or any later version published by the Free Software Foundation; with no Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts. A copy of the license is included in the section entitled “GNU Free Documentation License”). The base map is a relief and bathymetry Raster called «Natural Earth II with Shaded Relief and Water» file #NE2_HR_LC_SR_W.tiff (Made with Natural Earth. Free vector and raster map data @ naturalearthdata.com). As for the shaded relief, we use the CleanTOPO2 layer, a modified SRTM30 Plus World Elevation Data also edited by Tom Patterson, US National Park Service. The original source data is from ref. ¹²⁵.

further to the south (in the CVZ). The mean Ba/La ratios, combined with the CO₂/S_T vs. Ba/La linear regression model displayed in Fig. 2d, constrain the CO₂/S_T ratio for Tungurahua and El Reventador at 2.5 ± 0.8 and 2.2 ± 0.8, respectively (see Table 1 and S1b). A consistent CO₂/S_T ratio is inferred for Cotopaxi (2.5 ± 0.8).

The case of Indonesia, which includes the Sunda-Banda and Sangihe-Halmahera arc segments, is particularly problematic (Fig. 2g-k). The large along- and within-arc variations in crustal⁶³ and slab⁶⁴ structures, combined with heterogeneities in the sedimentary slab input⁴² (Fig. 4), make it difficult to characterize regional trends in volatile sources. In the Java sector of the Sunda arc, the respective roles of crust and slab in controlling rock⁶⁵ and gas⁶⁶ geochemistry are widely debated, with some authors stressing the importance of upper plate assimilation⁶⁷,⁶⁸ and others emphasising a slab control⁶⁹⁻⁷¹. The Group 3 signature³⁶ of Merapi and Bromo (Fig. 2g) supports involvement of crustal carbon in Central Java⁷². South-to-north along-arc trends in gas ³He/⁴He (decreasing) and CO₂/³He (increasing) ratios⁶⁶ suggest a crustal volatile contribution is also likely in Sumatra, where the crust is especially thick and limestones widely exposed⁶³,⁶⁷. In contrast, crustal assimilation is supposedly minor (if any) in other sectors, including west and east-Java⁶⁵, Nusa⁶⁹,⁷³ Banda⁷⁴ and Halmahera³³. In these segments of the Sunda-Banda and Sangihe-Halmahera arcs⁷⁵, along-arc variations in He-C isotopes⁶⁶,⁷⁶,⁷⁷, and the sparse high-temperature gas information, suggest variable C delivery from the slab, and thus coexistence of Group 1 and 2 volcanism (Fig. 2g). This is not unexpected, in view of the C heterogeneity in subducted sediments, from terrigenous and C-poor (Sumatra-Java) to pelagic and C-richer (Nusa, east Sunda)⁴² (Fig. 4). The diverse volatile sources that are possibly involved, in addition to the paucity of gas data, create scatter in CO₂/S_T vs. Ba/La (Fig. 2g). Only 9 Indonesian volcanoes have been measured for both whole-rock trace element composition and (high-temperature) magmatic gas composition (Table S1c). These CO₂/S_T vs. Ba/La data can be fitted by either a linear (RM3) or logarithm (RM4) regression model with identical regression coefficients (R² = 0.52; Fig. 2g). We therefore infer the CO₂/S_T ratio signature of the “unmeasured” Indonesian volcanoes (Table 1) by averaging the output of the two regression models (Table S1c). The low regression coefficients (Fig. 2g) imply the inferred CO₂/S_T ratios should be treated with caution, as they require validation/refinement with an improved (more than 9 data-points) gas vs. trace element relationship. We caution, in particular, that the predicted CO₂/S_T ratios (Table 1) may either over-estimate (for Group 1 volcanoes) or under-estimate (for Group 3 volcanoes) by a factor ~1.3 (the max error in Fig. 2g) the real volcanic gas CO₂/S_T ratios of “unmeasured” Indonesian volcanoes.
<figure><img src="image_7.png" /><figcaption>**Figure 5.** Scatter plot exploring the relationship between the SO₂ flux (2005–2015 mean; data from ref. ³³) and the volcanic gas CO₂/Sₜ ratio for the population of “measured” volcanoes in Table 1. For Turrialba + Poas (T), we plot the best-guess estimate for the magmatic gas CO₂/Sₜ ratio for Turrialba volcano (data from 28 top-ranking volcanic point sources of SO₂ (left) and CO₂ (right) during 2005–2015. Data are from Table 1. SO₂ fluxes are 2005–2015 means from ref. ³⁰. The CO₂ fluxes are calculated from SO₂ using measured or predicted CO₂/SO₂ ratios (see Table 1). Different volcano groups are identified by different colours. The global CO₂ budget is dominated by CO₂-rich Group 2–3 arc volcanoes. Two rift volcanoes (Nyiragongo and Nyamuragira) and one within-plate (WP) volcano (Kilauea) appears in the top-10 list of CO₂ emitting volcanoes.</figcaption></figure>

**CO₂/Sₜ ratios from Group-based gas vs. trace element relationships.** Several of the “unmeasured” (for gas) volcanoes in Table 1 are sited in arc segments for which insufficient gas/rock information is currently available to establish individual-arc associations (as those analysed in Fig. 2). In order to derive information on their CO₂/SO₂ ratio gas signature, we use the global relationship between CO₂/Sₜ and Ba/La in Groups 1–2 volcanoes (ref. ³⁶) (Fig. 3).

The majority of the remaining “unmeasured” (for gas) volcanoes in Table 1 are sited in arc segments for which available deep sea drill holes point to the lack of C-rich lithologies (limestones) in the subducted sediment succession ⁴² (Fig. 4). Trench sediments poor in C have been identified in the segment of the Pacific Ring of Fire (Fig. 4) that stretches from Aleutians-Kuril-Kamchatka to the N/NW to Marianas/Japan/Philippines further south (10 “unmeasured” volcanoes in total – see Table 1). Where high-temperature gas information is available, a CO₂-poor (Group 1) signature of volcanic gases³⁶ has typically been observed in such carbonate-poor trenches (Fig. 4), matching well the small sedimentary slab C input⁴². Sediments are similarly C-poor (e.g., prevailingly terrigenous and biosiliceous⁴²) in the Tonga and South Sandwich arcs (3 “unmeasured” volcanoes; Fig. 4). We therefore assign to Group 1 all the “unmeasured” (for gas) arc volcanoes fed by carbonate-poor trenches. Group 1 volcanoes exhibit little change in gas CO₂/Sₜ ratios with increasing Ba/La (Fig. 3a). This implies either (i) limited C delivery from the slab in the absence of carbonated sediments (e.g., that fluids/melts delivered by terrigenous sediments, altered oceanic crust and/or serpentinite are not major C sources³⁶), or (ii) that slab C and S are added to the mantle wedge in 1:1 to 4:1 proportions at most (Group 1 volcanoes typically have CO₂/Sₜ ratios ~3–4 times higher than the DDM). The lack of dependence on Ba/La (Fig. 3a) means that we can prudently use the measured Group 1 CO₂/Sₜ ratio average (1.2 ± 0.5; see Table S1d) for all the “unmeasured” (for gas) Group 1 volcanoes in Table 1.

Group 2 volcanoes are, by definition³⁶, those having CaCO₃-rich sediments in their trenches. These volcanoes typically have more C-rich volcanic gas composition (CO₂/Sₜ ratio >2 but ≤4) and exhibit stronger, steeper correlation between gas CO₂/Sₜ and trace element ratios (Fig. 3a). These Group 2 volcanoes are located in high biological productivity zones close to the tropics, where sediments are increasingly biogenic in nature and/or where seafloor is shallow enough (above the calcite compensation depth, CCD) to support carbonate deposition⁴² (Fig. 4). Of the few remaining “unmeasured” (for gas) volcanoes in Table 1, those in the Papua New Guinea-Solomon-Vanuatu arc segment are thus potential candidates for Group 2. The Papua New Guinea-Solomon arc sectors (Fig. 4) are a particular challenge because no gas samples are available, and no deep sea drill holes have been placed in the seafloor of the Solomon Sea, seaward of their trenches. Likewise, there are few relevant piston cores to provide any seafloor samples. Our inferences are thus based on seafloor depth, assumptions about the regional CCD, and drill sites in other, nearby southwest Pacific marginal seas. At DSDP Site 63, in the East Caroline Basin north of New Britain, carbonate lithologies were encountered throughout the entire section, from the Quaternary to the middle Oligocene basaltic basement⁷⁸. This site, at 4472 m water depth, has thus been above the CCD over its entire history. Similarly, drilling at DSDP 287 (4653 m water depth), in the Coral Sea south of Papua New Guinea and east of the Solomon Islands, intercepted abundant carbonate lithologies through most of the sedimentary section to its lower Eocene basement⁷⁹. Given that the water depths of the Solomon Sea are predominantly <4500 m seaward of the New Britain, Solomon and Northern Vanuatu trenches, we expect this seafloor to have been above the CCD for much of its history as well, and thus to be delivering carbonate-rich sediment to these subduction zones. Based on the above, we consider it very likely that
"unmeasured" volcanoes in the Papua New Guinea- Northern Solomon-Vanuatu arcs belong to Group 2. We use therefore the CO₂/Sₜ ratio vs. Ba/La global association for Group 2 volcanoes (see Fig. 3a) to predict (based on trace element information) CO₂/Sₜ ratios ranging from 2.1 ± 0.7 to 2.7 ± 0.7 for these volcanoes (Tables 1 and S1e). We note that the two "measured" volcanoes in the central and southern Vanuatu arc (Bembow on Ambrym Island, and Yasur on Tanna Island) both exhibit Group 1 gas affinity (CO₂/Sₜ of 1.5–1.6), implying that the predicted C-richer gas signature for northern Vanuatu volcanoes requires validation from measurements.

Discussion

**Validity of whole rock trace element proxy for CO₂/Sₜ.** Our predicted CO₂/Sₜ ratios stand on the assumption that gas compositions are linked to trace element compositions of their source magmas at either regional (Fig. 2) or global (Fig. 3) scale. Implicit in establishing such relationships is that gas (CO₂/Sₜ) and trace-element (Ba/La) whole-rock tracers are inherited by the same processes at their source, and are similarly conserved during magma ascent, decompression and degassing/eruption³⁶. For Ba/La, a link has been made between signatures of arc rocks and subducted sediments at corresponding tranches⁸⁰, so that this and other trace element ratios are commonly used as slab-fluid proxies for charactering the mantle source of magmas⁸¹,⁸². Both elements exhibit incompatible behaviour during magma differentiation, so that the source-inherited ratios are essentially conserved during magma evolution, at least for the mafic to intermediate (andesitic) magma compositions considered here (as outlined in the Method).

The behaviour of volatile components CO₂ and S is obviously complex during the generation and evolution of slab fluids and mantle-derived magmas⁸³. Not only are slab sources and processes only partially understood for C and S¹²,³⁹, but these volatile species will be selectively extracted from melt and partitioned into the vapour phase according to their melt solubilities (that dependent in a complex fashion on magma T-P-X-redox conditions), upon magma decompression and differentiation⁸⁴,⁸⁵. One may thus argue that degassing-related fractionations, for which abundant model⁸⁶⁻⁸⁸, experimental⁸⁹ and observational⁹⁰ evidence exists, act as to render the CO₂/Sₜ ratios in both degassed melt (preserved in melt inclusions in phenocrysts) and exsolved vapour (discharged as volcanic gases) unrepresentative of the mantle source compositions, and thus unlinked⁹¹ to Ba/La or other trace element proxies.

Where sufficient data exists (e.g., Figs 2a,d and 3a), however, the CO₂/Sₜ vs. Ba/La correlations appear systematic and statistically significant, and we consider unlikely that these associations are purely accidental. Our regional/global associations here, thus, implicate that the time-averaged CO₂/Sₜ ratios of volcanic gases ultimately reflect the volatile ratios in the parental (un-degassed) melt, and in the mantle source. To reconcile this with the well-established degassing-driven CO₂ vs. Sₜ fractionations, we observe that, at least at mafic systems, comparison between measured and modelled (from numerical simulation of magma degassing paths using volatile saturation codes⁸⁶⁻⁸⁸) gas CO₂/Sₜ ratios typically imply equilibrium pressures (e.g., pressures of final gas-melt segregation) of 0.1–5 MPa during quiescent degassing activity²⁹,³⁶,⁸⁴,⁸⁵,⁹². Thus, at least during non-eruptive periods, during which the majority of the volcanic gas observations in the literature are taken, observations and models both indicate very shallow (a few hundred meters below the magma-air interface) gas segregation from the convecting feeding magmas⁹³,⁹⁴. If shallow closed-system degassing conditions⁸⁵,⁹⁴ prevail, then the magmatic gas phase released as volcanic gas during open-vent activity does represent an integral of volatiles exsolved from melt during most (P > 5 MPa) of the magma decompression path. This released magmatic gas is thus very similar in composition to the source and parental melt volatile signature, irrespective of its hydrous (for arc volcanoes) or more H₂O-poor (for non-arc systems) nature²⁰,⁹³. The short-lived (days to weeks) pulses of CO₂-rich gas, seen prior to eruption of mafic arc volcanoes²⁷⁻²⁹,⁸⁴,⁹², imply somewhat deeper (typically, ~10–30 MPa) last gas-melt equilibration, but yet suggest closed-system is maintained up to rather shallow levels in a magmatic plumbing system, at least during quiescence. During basaltic explosive activity, deeper gas segregation is implied by gas observations⁹⁵,⁹⁶, but such eruptive degassing contributes only a minor fraction of the total degassing budget, which is dominated by passive emissions⁹³.

The lack of a systematic correlation between volcanic gas CO₂/Sₜ ratios and SO₂ fluxes (Fig. 5) further supports the idea that the former are not significantly affected by variable extents of magma degassing and gas-melt separation depths at various volcanoes. In mafic systems, the SO₂ flux is a proxy for the rates of magma degassing in a volcano's shallow (<3 km) plumbing system⁹³. As such, at least in principle, shallow magma ascent and decompression should be tracked by increasing SO₂ flux and decreasing CO₂/Sₜ ratios in the surface gas output²⁶, a relationship that is not observed in our global dataset (Fig. 5). The SO₂ flux-independent, distinct CO₂/Sₜ distributions of Group 1, 2 and 3 volcanoes (see Fig. 5) suggest, instead, that source signature, rather than degassing, ultimately controls the longer-term, time-averaged volcanic gas compositions. We caution that CO₂/Sₜ ratio volcanic gas compositions may become less source-related in intermediate to silicic systems, where the gas output is often buffered by gas-melt equilibration in crustal, vapour-saturated magma reservoirs⁹⁷⁻¹⁰⁰. It is thus possible that part of the scatter in our gas vs. trace-element associations (Figs 2 and 3) is caused by the intermediate (andesitic) systems included in our dataset. Silicic systems have intentionally been excluded from our compilation.

The good match between our predicted and measured CO₂/SO₂ ratios at Pacaya volcano (Fig. 1) also support, although indirectly, the validity of our gas vs. trace element associations. In addition to Pacaya, recent airborne gas measurements⁵⁴ at Tungurahua and Cotopaxi volcanoes in Ecuador have found CO₂/SO₂ ratios (in the 2 to 2.5 range) fully overlapping our predicted range (2.5 ± 0.8; Table 1). These successful tests provide confidence in the robustness of our predicted CO₂/Sₜ ratios. We caution that, in order to validate our methodology further and reduce the scatter in gas vs. trace element scatter plots (e.g., Fig. 3g), gas observations should be prioritized in remote, unexplored volcanoes in Papua New Guinea, Sandwich Islands, Solomon Islands, Sumatra, east Sunda-Banda, and north-Vanuatu. In some of these arc segments (e.g., Sumatra, Sunda), crustal C may be involved⁶³,⁶⁶,⁶⁷, in which case our predicted CO₂/Sₜ ratios may underestimate the actual magmatic gas ratio (by a factor up to ~1.5–2). We also advise that, since only high-temperature (SO₂-dominated) gas data are used to
<figure><img src="image_8.png" /><figcaption>**Figure 6.** The 28 top-ranking volcanic point sources of SO₂ (left) and CO₂ (right) during 2005–2015. Data are from Table 1. SO₂ fluxes are 2005–2015 means from ref.³⁰. The CO₂ fluxes are calculated from SO₂ using measured or predicted CO₂/SO₂ ratios (see Table 1). Different volcano groups are identified by different colours. The global CO₂ budget is dominated by CO₂-rich Group 2–3 arc volcanoes. Two rift volcanoes (Nyiragongo and Nyamuragira) and one within-plate (WP) volcano (Kilauea) appears in the top-10 list of CO₂ emitting volcanoes¹¹⁰.</figcaption></figure>

establish our gas vs. trace-element associations (Figs 2 and 3), our predicted CO₂/Sₜ ratios are representative of the magmatic gas signature, irrespectively of whether or not hydrothermal processes are acting to alter the actual and total gas volcano emissions. For example, the hydrothermal (H₂S-rich) gas emissions from Marapi volcano in Sumatra have measured CO₂/Sₜ ratios of 20.5 ± 1.1 (Table 1), well distinct from what we would predict (CO₂/Sₜ ratio of ~2.6) using the whole-rock Ba/La (19 ± 3; Table S1c) and the Indonesian gas vs. trace-element relationship (Fig. 2g). As such, discrepancy between measured and predicted CO₂/Sₜ ratio at any other hydrothermal volcano may lead to apportioning the fraction of S lost to (or C produced by) the hydrothermal system. While we believe that hydrothermal processing should be the exception rather than the rule for the satellite-sensed volcanoes here, we ultimately anticipate our predicted CO₂/Sₜ ratios (Table 1) will require revision and upgrading as new high quality gas data become available for newly measured volcanoes.

One important aspect to consider is that our regional/global associations (Figs 2 and 3) are based on averaging trace element information for rocks erupted during decades to millennia of volcanic activity. As such, the CO₂/Sₜ ratios predicted from such associations should be viewed as long-term means over a volcano's lifespan, rather than the instantaneous measurements as obtainable by direct gas observations. These “geologic” gas CO₂/Sₜ ratios may thus serve, when combined with measured S content in mafic glass inclusions, to estimate the initial CO₂ content in parental, un-degassed melts, and eventually in the sub-arc mantle. Both are similarly poorly constrained¹⁰¹,¹⁰² due to pre- and post-entrapment loss to vapour of poorly soluble CO₂.

**A decadal global CO₂ flux budget.** Our predicted CO₂/Sₜ ratios are converted into CO₂ fluxes (Table 1) by assuming Sₜ = SO₂ and scaling to the OMI-based mean SO₂ fluxes for the 2005–2015 period³⁰. We focus on the OMI satellite dataset owing to advantages brought by its global and coincident observations, but yet observe that quantitatively similar results would be obtained using ground-based SO₂ flux observations instead¹⁵. The predicted CO₂ fluxes range from 57 tons/day (Kanlaon volcano in the Philippines) to 6200 tons/day (Bagana volcano in PNG) (Figs 4 and 6). Uncertainty in the derived CO₂ fluxes (see Table 1, column N) is based on propagation of the respective errors on SO₂ flux (column G) and predicted CO₂/Sₜ ratios (column I).

The total cumulative CO₂ emissions from the 34 “unmeasured” volcanoes (those with no measured gas information available) would thus be ~11.4 ± 1.1 Mt/yr (~0.26 ± 0.02-10¹² mol/yr), thus adding an additional ~34% to the cumulative “measured” mean CO₂ emissions in 2005–2015 (27.4 ± 3.6 Mt/yr; Table 1). Finally, our extrapolated (measured + predicted) CO₂ flux budget is 38.7 ± 2.9 Mt/yr (or 0.88 ± 0.06-10¹² mol/yr). It is important to notice that our approach, in which the CO₂/Sₜ ratio signature of each volcano is independently evaluated, leads to far better constrained CO₂ budget (7% uncertainty at 1 SD) that would be possible using any “averaged” volcanic
$CO_2/S_T$ ratio proxy (as has been often attempted in past studies). For example, scaling the mean global $SO_2$ flux (23 ± 15 Mt/yr) to the mean volcanic $CO_2/S_T$ ratio (2.7 ± 3.6) (all data from Table 1) would lead to a global $CO_2$ flux of 62 ± 92 Mt/yr (e.g., 148% uncertainty at 1 SD).

Based on our results, we infer that 6 strongly degassing volcanoes with time-averaged (2005–2015 means) $CO_2$ fluxes of ≥ 5000 tons/day dominate the global $CO_2$ budget (Figs 4 and 6). One of these (Bagana, PNG) is an “unmeasured” volcano and would not have been identified as a top $CO_2$ emitter without the proxy approach developed here. It is interesting to observe that while the $SO_2$ global budget is dominated by the Group 1 volcanoes (accounting for 13 out of the 28 strongest volcanic $SO_2$ sources; Fig. 6), the $CO_2$ global budget is predominantly determined by the $CO_2$-enriched arc volcanoes in Group 2 (13 out of 28) and Group 3 (5 out of 28, with 2 - Popocatepetl and Etna - in the top-5 list) (Fig. 6). Two continental rift volcanoes (Nyiragongo and Nyamuragira) and two within-plate volcanoes (Kilauea and Erebus) also appear in our top-28 list of volcanic $CO_2$ emitters (Fig. 6).

Our extrapolated global $CO_2$ flux of 38.7 ± 2.9 Mt/yr is lower than previous global volcanic $CO_2$ flux estimates in the literature, ranging from 66 to 540 Mt/yr (see ref. 11 for a review). Several causes can explain this mismatch.

First, and most importantly, our global volcanic $CO_2$ budget here only includes the contribution from the “strongly degassing volcanoes” that emit $SO_2$ in quantities large enough to be detected from space (by OMI in this specific case³⁰). We therefore admittedly do not take into consideration in our estimate the $CO_2$ contribution from mildly degassing “magmatic” volcanoes (those still emitting $SO_2$, but at levels too low to be resolved by satellites) and from “hydrothermal” volcanoes in which $CO_2$ is emitted in combination with $H_2S$ (instead of $SO_2$). Although typically exhibiting weaker surface gas manifestations, compared to the OMI-detected volcanoes characterised here, these magmatic-hydrothermal systems do often exhibit C-rich gas compositions³⁶ (reflecting the extent/ mechanism of gas-water-rock reaction with meteoric-hydrothermal fluids⁴⁵), and do emit $CO_2$ at the ~1000 tons/ day level in the most extreme cases¹⁷, but most typically in the hundreds of tons/day range¹⁵. Considering that several hundreds of volcanoes worldwide are currently undergoing mild magmatic-hydrothermal degassing activity, this emission type could be responsible for the emission of several tens of Mt $CO_2$/yr globally¹¹,¹⁵. Also, we do not account for the $CO_2$ output from volcanic lakes¹⁰³, and diffuse/regional soil $CO_2$ emissions around volcanic systems¹⁰⁴, for which more data and alternative extrapolation approaches would be required. We therefore stress our results are not intended to represent total $CO_2$ emissions from global subaerial volcanism, but rather the magmatic $CO_2$ budget fraction contributed by the most actively degassing volcanoes on Earth.

Secondly, the mismatch in the estimated $CO_2$ fluxes (this work and previous studies) derives (at least partially) from the distinct gas datasets used. We here specifically base our $CO_2$ budget calculations on a consistent set of coincident (satellite-based) $SO_2$ flux measurement, taken during a relatively short (decadal) period and with same retrieval/processing technique. In contrast, previous estimates have been hampered by the combination of sparse observations, taken over several decades, and with diverse observational/retrieval techniques. Even volcanoes which are persistently active alternate periods of elevated degassing with phases of reduced activity, and so non-coincident observations (taken over periods spanning several decades) may lead to biases. For example, by combining measurements taken between 1954 and 2011, a cumulative $CO_2$ flux of 59.7 Mt/yr (from 33 measured volcanic gas plumes) was obtained¹¹, or 2 times more than our mean 2005–2015 flux. We also explicitly use $CO_2/SO_2$ information for high-temperature magmatic gases only, in contrast with previous efforts²³ in which individual arc $CO_2$ emissions have been quantified also considering low-temperature hydrothermal gas samples in which the C-rich composition is not representative of the strongly degassing “magmatic” arc systems. We also cannot rule out that part of the discrepancy is due to our Ba/La approach, which may only represents the sub-Moho magmatic $CO_2$ flux, and not a potentially large⁴⁴ recycled crustal $CO_2$ flux. Finally, our “measured” $CO_2$ dataset is extrapolated to the total number of “unmeasured” strongly degassing volcanoes by predicting, for each of them, the specific $CO_2/SO_2$ ratio gas signature, rather than relying on the assumption that the global $CO_2$ flux population obeys a specific statistical distribution (e.g., the power law distribution¹⁰⁵).

Our results implicate that the arc volcano C flux (~8 ± 0.6 Mt C/yr) corresponds to a significant amount (~50%) of the subducted sedimentary carbonate (15 ± 2 Mt/yr; ref. ¹⁰⁶), but only a relatively small fraction (< 21%) of the total C input at arc trenches (40–114 Mt C/yr; refs¹,¹²). Thus, either the C input is balanced by “dif-fuse” C output forms, such as regional aquifers or soil degassing¹⁰⁷ in the arc crust, or a substantial fraction of the subducted C is ultimately not erupted, but rather stored either in the lithospheric mantle⁸ or in the deep mantle¹,².

## Methods

The $SO_2$ flux compilation³⁰ we rely on in this study includes a list of the 91 top-ranking volcanic $SO_2$ degassing sources in 2005–2015 (Table 1). This set of consistent (identical retrieval/processing technique) and simultaneous (global) measurements has improved upon the shortcomings of previous catalogues¹⁰⁸, which combined $SO_2$ fluxes obtained with diverse techniques and in disparate temporal intervals (often differing by several decades).

These $SO_2$ flux data are converted into $CO_2$ fluxes by using either measured or predicted molar $CO_2/S_T$ ratios. For these strongly degassing volcanoes, $S_T$ is assumed to correspond to $SO_2$ throughout, since $SO_2$ detection by satellites implies limited or no interaction with hydrothermal system (and thus trivial reduced S species, such as $H_2S$).

**Measured volcanoes.** For 57 out of these 91 volcanic $SO_2$ sources, we convert $SO_2$ fluxes into $CO_2$ fluxes, by pairing the former with the characteristic (mean) molar $CO_2/S_T$ ($CO_2/SO_2$) ratios in the corresponding volcanic gases (Table 1). For arc volcanoes, we use the time-averaged molar $CO_2/SO_2$ ratios compiled by (ref. ³⁶), integrated with novel gas information for eight new targets that have only recently been measured for the first time (see Table 1 for data provenance). Arc volcanoes are ranked in Groups (1 to 3) following the original categorization³⁶. For non-arc volcanoes (here referred as Group 4), we average available volcanic gas information in the literature (see Table 1 for data sources). Note that, for both arc and non-arc, in cases where more than one volcano are listed
in the original dataset³⁰ (e.g., Nyiragongo + Nyamuragira) due to insufficient spatial OMI resolution, we averaged the available volcanic gas information for the individual volcanoes, weighting each volcano's CO₂/Sₜ ratio by its ground-based S flux (where available) to obtain a combined CO₂/Sₜ ratio for the pair (see Table 1).

**Unmeasured volcanoes.** Thirty-four out of the 91 top-ranking volcanic SO₂ sources³⁰ have never been characterised for volcanic gas composition (Table 1). These include four of the top-ten ranking volcanic SO₂ emitters³⁰ (Bagana, Rabaul and Manam in Papua New Guinea, and Aoba in the Vanuatu archipelagos; Fig. 4). To indirectly infer the molar CO₂/Sₜ ratio gas signature of each of these 34 volcanoes, we use the averaged (mean) trace-element composition of the corresponding volcanic rocks. To this aim, as in earlier work³⁶, we extract trace-element information (Ba, La, Sr, Nd, U and Th whole-rock concentrations) either from the Earthchem data-portal (http://www.earthchem.org/), or from other sources (for volcanoes that do not appear on Earthchem) (see Table S1). Mafic to intermediate (<55% SiO₂) rocks are only considered, same as in other work¹⁰⁹. From these, we calculate, for each volcano, the mean (±1 SD) of the Ba/La whole-rock ratios (Sr/Nd and U/Th ratios were also calculated; see Table S1). These ratios, in combination with the gas vs. whole-rock relationships illustrated in Figs 1–3, are finally used, to predict the characteristic volcanic gas CO₂/Sₜ ratio signature for each of the 34 “unmeasured” volcanoes.

The procedure is exemplified in Fig. 1 for the Pacaya volcano example. We select Pacaya because the recently obtained gas compositions⁴⁶ can serve as a test of the methodology. The initial step involves establishing the relationship between CO₂/Sₜ gas ratios and whole-rock Ba/La ratios, using data for volcanoes for which both gas and trace element data are available (see Fig. 1; Table S1). The CO₂/SO₂ vs. Ba/La relationship can be established at the scale of individual arc segments (e.g., Figs 1 and 2), or for volcano Groups³⁶ (Groups 1 or 2) (Fig. 3). For the Pacaya example, we rely on gas/whole-rock information for the well-characterised Central American Volcanic Arc (CAVA; Fig. 1). Secondly, we use regression analysis to fit the gas vs. trace-element association via either a (i) linear or (ii) logarithm regression model (Fig. 1). We find that linear regression yields the best data fit in the majority of the cases (see the Pacaya example, Fig. 1a,b), and this regression model is used throughout unless where indicated (see Table S1). We also find that data fitting is systemically optimised when the DMM composition is included in the fitting procedure (compare Fig. 1a,b), and this option is maintained throughout. Note, however, the method output (e.g., the outputted CO₂/Sₜ ratio) is poorly sensitive to this choice (see Table 2). Finally, the adopted regression model function (RM3 in the Pacaya example; Fig. 1 and Table 2) is used to calculate a “predicted” gas CO₂/Sₜ from available Ba/La information (Fig. 1). The confidence interval or delta, calculated from the regression line and one standard deviation about the regression, is taken as a proxy for the uncertainty in the predicted CO₂/Sₜ ratios. Uncertainty on the predicted ratios, as derived, incorporates (although indirectly) uncertainty/variability in “measured” gas CO₂/Sₜ ratios (average uncertainty at 1σ, ~26%) and whole-rock Ba/La ratios (average uncertainty at 1σ, ~16%) (see Table S1). In the specific Pacaya example (Fig. 1 and Table 2), our “predicted” gas CO₂/SO₂ ratio (1.4 ± 0.75) matches well the recently measured⁴⁵ magmatic gas range (CO₂/SO₂ ratio of 1.1 ± 1.0). Our tests show that remarkably similar CO₂/SO₂ ratios (see Table 2) are obtained using other trace-element slab fluid tracers, such as the Sr/Nd ratio (Fig. 1c). We opt in the following for the Ba/La regression model because (i) La is more frequently available than Nd in the Earthchem dataset for the majority of the volcanoes, and (ii) use of the Sr/Nd ratio requires a priori knowledge of volcano affinity for a specific Group (Group 1 and 2 typically exhibit diverse distributions in a CO₂/SO₂ vs. Sr/Nd scatter plot; see Fig. 1c). This latter information is frequently not a priori available (see below). The same procedure is applied to all unmeasured volcanoes (Table S1a), and the “predicted” ratios (assumed to correspond to CO₂/SO₂) are combined with SO₂ flux results to ultimately infer the CO₂ fluxes (Table 1).

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## Acknowledgements

This research was funded by the DECADE research initiative of the Deep Carbon Observatory. We wish to thank E. Liu and anonymous reviewer for helpful comments on an earlier version of the manuscript.

## Author Contributions

A.A. conceived the idea. All of the authors participated in data collection and interpretation. A.A. drafted the manuscript with the help of all co-authors.

## Additional Information

Supplementary information accompanies this paper at https://doi.org/10.1038/s41598-019-41901-y.

**Competing Interests:** The authors declare no competing interests.

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