**6. Conclusions**

We present new results for the chemical composition and the mass flow rate of fumarolic gases emitted from different vents on top of La Soufrière volcano in 2016–2017, during an ongoing phase of degassing unrest that has developed since 1992. Our results reveal a wide range in gas compositions, reflecting the variable influence of shallow processes (SO2 scrubbing in liquid water and near-surface sulfur precipitation in the volcanic ground), that closely relates to the evolution of the fumarolic activity with respect to the underground circulation of hydrothermal fluids inside the lava dome, as imaged from a recent electrical tomography [29]. Moreover, we find that the spatio-temporal evolution in degassing activity, gas compositions and gas emission rates coherently relate to the temporal deformation pattern (fracture widening/closing) of the lava dome since the onset of the current degassing unrest. When compared to previous data, fumarolic gas fluxes determined with the highest possible accuracy in 2016–2017 verify the persistency of an elevated degassing rate through the central conduits of La Soufrière. However, they also reveal a recent spatial shift in fumarolic degassing intensity towards the eastern and northern sectors of the lava dome where SW-NE fractures linked to the Breislack fault system, as well as the south-oriented Fracture du 30 Août 1976 (Figure 1c), are progressively widening, whereas other fractures tend to close or remain stable. These coupled geochemical and geophysical observations at La Soufrière provide a new framework to better elucidate the actual significance of fumarolic gas changes during the current unrest phase and in the future. At present, the available dataset for gas emissions and geophysical signals at La Soufrière do not support the hypothesis of a shallow magma intrusion as being responsible for the current unrest. Instead, they sugges<sup>t</sup> a possible increase of the magmatic gas and heat supply arising from the crustal magma reservoir emplaced at 6–7 km depth beneath the volcano. If correct, such a mechanism and its evolution over time must be carefully surveyed and quantified. The major phreatic eruptions at La Soufrière in past centuries (1797–1798, 1836–1837, 1976–1977) have often involved laterally directed explosions from pressurized regions of the shallow hydrothermal and associated hazards [6,29]. Our study illustrates the powerful potential of combining geochemical and geophysical investigations to better anticipate such events at La Soufrière, but also to interpret hydrothermal unrest at other active volcanoes in hydrothermal activity worldwide.

**Author Contributions:** Conceptualization, G.T., P.A., J.-C.K. and A.L.F.; Data curation, G.T., S.M., S.V., M.R.-C., S.D., F.B., J.-B.D.C., A.L.M. and M.B.; Formal analysis, G.T., S.M., S.V., M.R.-C., S.D., J.-C.K., F.B., J.-B.D.C. and A.L.M.; Funding acquisition, S.M. and J.-C.K.; Investigation, G.T., S.M., P.A., S.V., V.R., G.-T.K., T.D., J.-C.K., F.B., J.-B.D.C. and C.D.; Methodology, G.T., S.M., P.A., V.R., S.D. and J.-C.K.; Project administration, P.A., J.-C.K. and R.M.; Resources, S.M., P.A., S.D., G.-T.K., T.D., J.-C.K., F.B., A.L.M., C.D. and R.M.; Software, G.T.; Supervision, J.-C.K.; Visualization, G.T. and J.-C.K.; Writing—original draft, G.T.; Writing—review & editing, S.M., P.A., S.V., M.R.-C., J.-C.K., F.B., C.D. and R.M.

**Funding:** This research received no external funding.

**Acknowledgments:** Our study was supported by funding from a CNRS-INSU contract (Aleas-2015, S. Moune), from University of Palermo (ref. G. Tamburello) and from IPGP (equipment gran<sup>t</sup> and postdoctoral contract to G. Tamburello). We thank G. Hammouya (OVSG) and O. Crispi (OVSG) for gas and acid lake sampling and measurements; D. Gibert, F. Nicollin, and participants of the ANR projects DOMOSCAN (2009-2013) and DIAPHANE (ANR-14-ce04-0001; 2014-2018) for their assistance with electrical resistivity measurements and TAR acid lake pH measurements; M. Bitetto and A. Aiuppa (University of Palermo) for their support to the development and testing of the UniPa-type MultiGAS; and Glyn Williams-Jones for help and calibration of the SFU MultiGAS. The sta ff of OSVG-IPGP provided us with invaluable assistance in the field, for maintenance of the stations, and with unpublished data. The Service National d'Observation en Volcanologie (SNOV) of INSU is acknowledged for recurrent funding, and the Swiss National Science Foundation and ANR Diaphane project for funding to M. Rosas-Carbajal. The authors wish to thank the three anonymous reviewers for their useful comments and suggestions upon reviewing the manuscript. Raw gas concentrations retrieved during the surveys are downloadable from the EarthChem Library (https://doi.org/10.1594/IEDA/111184).

**Conflicts of Interest:** The authors declare no conflict of interest.
