Spatio-Temporal Relationships between Fumarolic Activity, Hydrothermal Fluid Circulation and Geophysical Signals at an Arc Volcano in Degassing Unrest: La Soufrière of Guadeloupe (French West Indies)
, Séverine Moune 2,3,4, Patrick Allard 2,5, Swetha Venugopal 3,6, Vincent Robert 2,4, Marina Rosas-Carbajal 2, Sébastien Deroussi 2,4, Gaëtan-Thierry Kitou 2,4, Tristan Didier 2,4, Jean-Christophe Komorowski 2, François Beauducel 2,7, Jean-Bernard De Chabalier 2, Arnaud Le Marchand 2, Anne Le Friant 2, Magali Bonifacie 2,4, Céline Dessert 2,4 and Roberto Moretti 2,4
Round 1
Reviewer 1 Report
Review of Spatio-temporal relationships between fumarolic activity, hydrothermal fluid circulation and geophysical signals at an arc volcano in degassing unrest: La Soufriere of Guadeloupe (French West Indies).
This paper presents new multigas results from a recent deployment at La Soufriere, Guadeloupe and places them in the context of previous geophysical studies to create a framework for understanding future monitoring data at this hazardous volcano.
The results of the recent multigas survey are compared to previous surveys with the interpretation that changes are due to both shallow hydrothermal system processes, such as increased SO2 scrubbing, as well as a possible increase in deep gas supply at 6- km. A spatial shift in degassing patterns correlates with widening of main fractures within the dome, suggesting a strong structural/permeability control on where degassing occurs.
Overall the paper is well written, although some of the methods descriptions could be abbreviated to make it of more interest to a non-specialist multigas reader. A lot of the work references Moretti (submitted), so without knowing that work it is difficult to judge the relevance or importance of this reference. In places I suggest more details from the Moretti manuscript are included here for the benefit of the reader.
Can you please comment on if the multigas instruments are calibrated so that they each read the same for a given concentration of gas. As you are using multiple instrument simultaneously it is important to know they are calibrated to a common base line.
The discussion about electrical conductivity interpretation requires some reference to measured liquid or rock conductivities. Without independent measurements of these it is difficult to interpret the tomograms in terms of liquid saturation. See line by line comments for details.
The discussion about magma volumes is a bit contradictory, more details from Moretti are required to understand your conclusions here.
Line by line comments:
Line 68: Do you have a reference for statement that low temp fumaroles contain little SO2 hence not suitable for UV detection methods? For instance at White Island SO2 is routinely measured from low temp discharges using doas UV techniques.
Line 96, delete word significantly. Usually reserve this word for a statistical comparison when something is statistically significant.
Line213: typo previously.
Line 299 : delete “actually”
Line 300: Unclear what “latter” refers to, the locally measured? Not sure you need to mention the video derived velocities if you don’t use them, perhaps delete that sentence about the video.
Line 314: typo mainly.
Line 331: delete “indeed”
Line 388: minimal should be minimum.
Line 419: Can you use the 3 letter abbreviation for these fractures, to make map interpretation easier for the reader.
Line 431: instead of “fits very well”, use “coincides”
Line 435: What is the source of the extension? Can you add a sentence around this or to point to the discussion where it is mentioned.
Line 492: what does “nearly quantitative precipitation” mean? Please reword.
Lines 549 to 555. Some contradiction here about magmatic volumes. In line 549 state that approx. 6.5x10^6 m3 of magma isn’t feasible as it would’ve been detected, but then later state that a model shows that 2.7x10^6 m3 magma was intruded. There isn’t a lot of difference between these 2 numbers, so why do you discount the 6.5x10^6 volume? More details on the Moretti model are needed to explain this.
Line 569: delete “actually”
Lines 603-620. Discussion on electrical conductivity. While electrical conductivity is dependent on porosity/saturation it is more strongly dependent on liquid conductivity. Have you measured liquid conductivity, or can you calculate it from your chemical analyses of hot springs? At low pH there may not be abundant clay minerals and indeed in many rocks from volcanic hydrothermal systems the rock matrix is relatively low conductivity and doesn’t contribute much to the bulk conductivity. Please refer to Ghorbani 2018, JVGR for details on the contributions of rock vs liquid to bulk conductivity. Without independent measurements of liquid or rock conductivity it is generally not possible to unravel the individual contributions of saturation/porosity vs the liquid or rock conductivity. This therefore makes discussion about degree of saturation based on electrical resistivity measurements difficult to back up.
Author Response
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Author Response File: 
Author Response.pdf
Reviewer 2 Report
This well written paper presents a comprehensive account of the results of ongoing gas-chemical analysis of the various hydrothermal manifestations on the Soufriere of Guadeloupe. In particular, the authors present a suite of novel ‘side by side’ measurements of C-O-H-S species in the gas vapour phase, using ‘MultiGAS analysers, and use these data to evaluate fluxes of key species from different centres of degassing, and through time. The authors then evaluate the significance of these measurements for the evolving nature of the subsurface hydrothermal system. The data are of high quality, uncertainties are evaluated carefully, and this is a substantial and valuable paper. There are just a couple of minor comments: 1. On figs 2 and 6 do the seismic energy release curves have any units? 2. Near the end of the paper there is a discussion of the internal 3D structure of the edifice: have any of the recent muon radiography papers on this system all’s helped to illuminate the internal form or changes in the system?Author Response
Please see the attachment.
Author Response File: Author Response.pdf
Reviewer 3 Report
The manuscript entitle “Spatio-temporal relationships between fumarolic activity, hydrothermal fluid circulation and geophysical signals at an arc volcano in degassing unrest: La Soufrière of Guadaloupe (French West Indies)” has made a big effort to monitor the spatial distribution and temporal evolution of hydrothermal manifestations at such volcanoes, in combination with geophysical surveys, crucial to detect and interpret precursors of either non-magmatic explosive activity or magmatic eruptions.
Under my point of view, the manuscript is well written, describe and use a correct methodology, obtained an interested result that can improve our sparse knowledge of this field and finally illustrates the potential of combining geochemical and geophysical investigations to better anticipate such events.
Just some minor text editing:
Line 41. 1.I.ntroduction
Line 407. See section XXX
Line 587. -0.5 not -0,5
Author Response
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Author Response File: Author Response.pdf
