## 5 Discussion
### Along-Arc and Inter-Arc Volcanic Gas CO\({}_{2}\)/S\({}_{\rm T}\) Variations  
Recent global volcanic gas compilations have revealed systematic inter-arc and along-arc variations in gas CO\({}_{2}\)/S\({}_{\rm T}\) ratios [15; 21] which are thought to be mainly governed by variations in mineralogy and chemistry of sediments (especially their C load) subducted at the corresponding trenches [6; 7]. A better understanding of these global correlations between arc inputs (sediments) and outputs (volcanic gases) requires extending volcanic gas measurements to remote, poorly studied arc regions, such as Melanesia. Indeed, as noted by [PERSON] et al. [15; 21] several of the top volcanic arc emitters have yet to be measured for their CO\({}_{2}\)/S\({}_{\rm T}\) ratios, due to their remote location and/or high level of volcanic activity. The results we report here add new data to this global dataset to allow a more complete characterization of the source, fate and cycling of volatiles, especially C, in subduction zones. In this section, we focus the discussion of our results within the context of recently collected gas data for other Melanesian volcanoes. As two of the most active volcanoes in the world, volcanic gas emissions at Yasur and Ambrym (Vanuatu; see Figure 1) have been studied extensively in recent years. First reports on volcanic plume chemistry at Ambrym were presented by [PERSON] et al. [33; 34], while [PERSON] et al. [30], [PERSON] et al. [31] and [PERSON] et al. [32] reported on gas compositions at Yasur volcano. These studies inferred the plume volcanic gas CO\({}_{2}\)/S\({}_{\rm T}\) ratios at \(\sim\)1.5 [15; 33; 34] and \(\sim\)1.6 [31] at Ambrym and Yasur, respectively, within the range of CO\({}_{2}\)-poor (Group 1) volcanic gases (CO\({}_{2}\)/S\({}_{\rm T}\)\(<\) 2) [15; 21]. [PERSON] et al. [15; 21] noted that this CO\({}_{2}\)-poor volcanic gas composition for the Vanuatu arc, as inferred from observations at Ambrym and Yasur, is somewhat at odds with the relatively high C content of sediment in the subducting slab (bulk sediments in the Vanuatu trench contain up to 4 wt.% CO\({}_{2}\)) [6]. In other arc sectors where C is subducted via sediments (e.g., Central and northern Southern America) [7], volcanic gases are typically more C-rich, and display characteristics of those volcanoes in the Group 2 category (2 \(<\) CO\({}_{2}\)/S\({}_{\rm T}\)\(<\) 4) [15; 21; 50; 58]. In contrast, the Vanuatu gas compositions measured thus far resemble more closely those of subduction zones of the NW Pacific (the Kuril-Kamchatka and Marianas-Japan arc segments) [15], all of which exhibit low C content in subducting sediments [6]. Our results here provide a first insight into the CO\({}_{2}\)/S\({}_{\rm T}\) signature of volcanic gases in the northern part of the Vanuatu arc. Our average volcanic gas CO\({}_{2}\)/S\({}_{\rm T}\) estimate at Gaua is \(\sim\)0.9, for measurements performed in very dense plume conditions (up to \(>\)50 ppmv SO\({}_{2}\)). The 2018 Gaua gas composition also plots at the low-CO\({}_{2}\) end of the population of volcanic arc gases worldwide (Figure 5). Notably, a similarly C-poor gas measurement was recently obtained further to the north at Manam volcano, in Papua New Guinea, for which [PERSON] et al. [37] quoted a CO\({}_{2}\)/S\({}_{\rm T}\) ratio of \(\sim\)1. The relatively high C content of sediments at the Vanuatu trench led [PERSON] et al., [21] to predict a CO\({}_{2}\)/S\({}_{\rm T}\) ratio of \(\sim\)2.5 for Gaua, well in excess of our 2018 measured ratio of \(\sim\)0.9. Thus, the C-rich nature of the subducted sedimentary input in the region does not appear to translate into a C-rich volcanic gas output, as seen elsewhere (e.g., central and northern Southern America). The possibility that the sedimentary C input into the subduction zone is accreted to the accretionary wedge (rather than being subducted) is inconsistent with the results of [PERSON] and [PERSON] [59], who classified the Vanuatu arc as a non-accreting margin, with effectively no accretionary prism present.