Ambient seismic noise is characterized by randomness incurred by the random position and strength of the noise sources as well as the heterogeneous properties of the medium through which it propagates. Here we use ambient noise data recorded prior to the 1996 Gjálp eruption in Iceland in order to show that a reduction of noise randomness can be a clear short-term precursor to volcanic activity. The eruption was preceded on 29 September 1996 by a Mw ~5.6 earthquake that occurred in the caldera rim of the Bárdarbunga volcano. A significant reduction of randomness started occurring 8 days before the earthquake and 10 days before the onset of the eruption. This reduction was observed even at stations more than 100 km away from the eruption site. Randomness increased to its previous levels 160 minutes after the Bárdarbunga earthquake, during which time aftershocks migrated from the Bárdarbunga caldera to a site near the Gjálp eruption fissure. We attribute this precursory reduction of randomness to the lack of higher frequencies (>1 Hz) in the noise wavefield caused by high absorption losses as hot magma ascended in the upper crust.

The volcanic-hydrothermal geo-diversity of the Basse-Terre Island of Guadeloupe archipelago (Eastern Caribbean, France) is a major asset of the Caribbean bio-geoheritage. In this paper, we use Guadeloupe as a representative of many small island developing states (SIDS), to show that the volcanic-hydrothermal geodiversity is a major resource and strategic thread for resilience and sustainability. These latter are related to the specific richness of Guadeloupe’s volcanic-geothermal diversity, which is de facto inalienable even in the wake of climate change and natural risks that are responsible for this diversity, i.e., volcanic eruptions. We propose the interweaving the specificity of volcanic-geothermal diversity into planning initiatives for resilience and sustainability. Among these initiatives research and development programs focused on the knowledge of geodiversity, biodiversity and related resources and risks are central for the long-term management of the water resource, lato sensu. Such a management should include a comprehensive scientific observatory for the characterization, exploration, and sustainable exploitation of the volcanic-hydrothermal geodiversity alongside planning for and mitigating geophysical risks related to sudden volcanic-induced phenomena and long-term systemic drifts due to climate change. The results of this exercise for Guadeloupe could typify innovative paths for similar SIDS around their own volcanic-hydrothermal geodiversity.

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<p><strong>Abstract</strong></p><p>The hydrothermal system of the andesitic volcano La Soufrière in the Basse-Terre island of Guadeloupe, is an ever evolving and highly dynamic system that is characterized by a variety of surface manifestations such as thermal springs, fumaroles and the Tarissan acid boiling pond (TAS) (<em>Villemant et al.,2014</em>). These produce halogen-rich surface emissions that makes it difficult to interpret subtle perturbations in the magmatic reservoirs using traditional geochemical and geophysical monitoring techniques (<em>Moretti et al., 2020</em>). A challenging situation for monitoring a volcano that has in recent history experienced six phreatic eruptions, the latest being the 1976-1977 eruptive crisis followed by a renewal of unrest in 1992 with the latest accelerated unrest episode occurring in 2018 (<em>Komorowski et al., 2005, Moretti et al.,2020</em>). Concurrently, TAS exhibited reduced Cl/Br ratios from ~1000 to ~300 from 18 January 2018 to 23 November 2020, suggesting marine water as a possible salinity source into the hydrothermal system. Hence, there is a critical need to fully conceptualize and appreciate the sources, full evolution and dynamic response of the hydrothermal system at La Soufrière. The research begins with investigating the potential input of ocean water into the hydrothermal system to aid in developing an exploitable geochemical database for prospective geochemical modelling analysis, leading to possible inferences on the influence of salinity on; 1) the gas-water-rock interactions in the shallow hydrothermal reservoir 2) scrubbing effects and 3) forcing conditions responsible for the measured surface gas emissions. We investigate this by numerically modelling flow transport of NaCl brines (wt.% 5, 25 and 35) using TOUGH2 software. Brines were modelled to represent volcanic- or marine- sourced brines at high temperature and pressure conditions (300°C - 350°C and 195Pa respectively). Steady state solutions of varying mass injections of orders of 1.0E<sup>-4</sup> kg/s/m<sup>2</sup> and 1.0E<sup>-5</sup> kg/s/m<sup>2 </sup>resulted with the brines concentrating at heights of ~1100m a.s.l. at the summit and exiting the system, depending on the adopted permeability values. Thus implying that strong permeability contrasts due to sealing effects promoted by argillic alterations influences the trapping of brines in the upper edifice. </p>