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Schematic map of Ischia Island showing the location of sampling sites. The structural setting is from Vezzoli (1988). Groundwater samples are distinguished according to the location of sampling sites: Northern-sector (black diamonds; sample numbers from 1 to 30), North–North-Western-sector (white triangles; sample numbers from 31 to 44), Easternsector (white circles; sample numbers from 45 to 68), Western-sector (white squares; sample numbers from 69 to 90) and Southern-sector (grey triangles; sample numbers from 91 to 120). Sampling sites of fumarolic gas discharges are represented by grey circles. Grey shaded areas represent the major active hydrothermal alteration areas individuated by Sbrana (2007), whilst the grey line (A–A'), linking Casamicciola village (in the North) and Maronti bay (in the South), is the trace of the section of Fig. 12.
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Ischia volcano, in Central Italy, has long been known for its copious surface hydrothermal manifestations, signs of a pervasive circulation of hot fluids in the subsurface. Because of the significant chemical heterogeneity of fumarolic gas discharges and hot spring discharges, evidences of a complex hydrothermal setting, a definite model of fluid c...
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... Island, located in the NW sector of the Gulf of Naples ( Fig. 1), is the emergent portion of a wide volcanic complex that, together with the Phlegrean Field and Somma-Vesuvio, forms the active Campanian Magmatic Province (CMP). The composite geology of the island, with outcropping formations including volcanic rocks (alkali-trachytes to subordinate latites, trachytes and phonolites, belonging to the ...
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... numbers from 91 to 120). Sampling sites of fumarolic gas discharges are represented by grey circles. Grey shaded areas represent the major active hydrothermal alteration areas individuated by Sbrana (2007), whilst the grey line (A-A'), linking Casamicciola village (in the North) and Maronti bay (in the South), is the trace of the section of Fig. 12. Table 1 Chemical and isotopic compositions of Ischia groundwaters from springs (s) and drilled wells (d.w. Latitude and longitude of collected samples are on WGS 84 datum. The depths of wells are expressed in meters below ground level. Temperature (T, ± 0.1 °C), pH (± 0.05 pH units), conductivity (Cond, in mS at 20 °C; ±2%) and redox ...
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... water samples were collected at 120 sites during several hydrogeochemical surveys from 2002 to 2007. Most samples ( Fig. 1), collected from wells and springs exploited in spas and hotels, are concentrated along the coastal sectors of the island. The well depths range from 3 to 247 m below ground surface (Table 1), and as such capture the main shallow aquifer system of Celico et al. ...
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... atmospheric to CO 2 -rich (and N 2 -O 2 -poor). In Fig. 4a, several groundwater samples cluster near the representative point of air-saturated groundwaters (a.s.w.), supporting an atmospheric derivation for their Fig. 5. Spatial distribution maps of (a) CO 2 and (b) He contents ( vol.%) in Ischia groundwaters. Samples are distinguished as in Fig. 1. dissolved gas phase. In this context, the apparent N 2 enrichments (or oxygen depletions) of several samples (relative to a.s.w.) are likely due to oxygen-consuming redox reactions taking place in the hydrothermal environment (Stumm and Morgan, 1996). The same Fig. 4a shows however a clear CO 2 -excess compared to air-saturated ...
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... survey period, unsteady behaviour was manifested in samples 40 and 81 (Fig. 8a). In both cases, T-changes were not indicative of modifications in hydrothermal system. Indeed, decrease in temperature since May 2006 (from 98 to ~ 50 °C) in sample 81 (Table 4), a small steaming pool near Citara bay (western sector of the island; Table 1 and Fig. 1), was probably related to modifications of the pool's geometry that caused a partial obstruction of the emission vent, giving rise to water stagnation in the pool and more extensive cooling upon contact with the atmosphere. On the contrary, random fluctuations (from 94 to 48 °C) observed at well 40 (Table 4), were related to various ...
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... A group of these samples plot on the right portion of the diagram (see grey shaded area), and cluster along a mixing line connecting B and C end-members (Serrara thermal endmembers and Citara thermal end-member, respectively). This is in strong support of interconnected hydrothermal circulation in the area between Forìo and Serrara Fontana (Fig. 1), and point to the likely existence of one single water reservoir fed by sea water in the western portion, and by meteoric water in the southern sector. Finally, it can be noted (Fig. 9) that several samples collected in the same part of the island are on the contrary shifted towards Mg and Cl poor compositions, and plot on the ...
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... end-members A, B and C are indicated as dark grey solid line, black dashed line and dotted line, respectively. The chemical composition of samples plotting within the light grey shaded area can be interpreted as the resulting of mixing between end-members C and B, pointing to a single reservoir in the sector between Forìo and Serrara Fontana (see Fig. 1). On the other hand, several water samples from the southern and western sectors, clustering on the dark grey shaded area on the left of the diagram, are likely affected by dilution from steam. This causes heating (temperatures of these samples are higher than 50 °C) and depletion in the dissolved ions (very low Mg and Cl-contents, ...
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... potential deep equilibrium temperatures of these hydrothermal reservoirs were here estimated using classic solute geothermometers from Giggenbach et al. (1983). In the triangular diagrams of Fig. 10, thermal manifestations from the northern sector of Ischia (Casamicciola area; Fig. 10a) cluster along an array from Mg-rich to Mg-poor compositions at relatively constant Na/K ratios. The Mg-rich compositions are likely indicative of fluids circulating in a very shallow environment (note that many samples cluster along the sea water ...
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... potential deep equilibrium temperatures of these hydrothermal reservoirs were here estimated using classic solute geothermometers from Giggenbach et al. (1983). In the triangular diagrams of Fig. 10, thermal manifestations from the northern sector of Ischia (Casamicciola area; Fig. 10a) cluster along an array from Mg-rich to Mg-poor compositions at relatively constant Na/K ratios. The Mg-rich compositions are likely indicative of fluids circulating in a very shallow environment (note that many samples cluster along the sea water compositional field in Fig. 10a); or reflect near-surface reequilibration during ascent ...
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... from the northern sector of Ischia (Casamicciola area; Fig. 10a) cluster along an array from Mg-rich to Mg-poor compositions at relatively constant Na/K ratios. The Mg-rich compositions are likely indicative of fluids circulating in a very shallow environment (note that many samples cluster along the sea water compositional field in Fig. 10a); or reflect near-surface reequilibration during ascent of deep-reservoir fluids, giving rise to the modifications of the original Na-Mg and K-Mg ratios (Giggenbach, 1988). On the other hand, Mg-poor samples plot near the fullequilibrium curve (Fig. 10a), and thus have compositions likely representative of deep-reservoir T conditions, ...
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... shallow environment (note that many samples cluster along the sea water compositional field in Fig. 10a); or reflect near-surface reequilibration during ascent of deep-reservoir fluids, giving rise to the modifications of the original Na-Mg and K-Mg ratios (Giggenbach, 1988). On the other hand, Mg-poor samples plot near the fullequilibrium curve (Fig. 10a), and thus have compositions likely representative of deep-reservoir T conditions, which we evaluate at 190-270 °C using the Na/K geothermometer of Giggenbach et al. (1983). Notably, the compositional end-member "A" (sample 26; see Fig. 9 and Table 1) consistently belongs to this second group, and allow deriving an equilibrium T of 254 ...
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... geothermometer of Giggenbach et al. (1983). Notably, the compositional end-member "A" (sample 26; see Fig. 9 and Table 1) consistently belongs to this second group, and allow deriving an equilibrium T of 254 °C. A similar range of temperatures (190-260 °C) is evaluated for fluids feeding thermal manifestations on the NW-sector (Lacco Ameno area; Fig. ...
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... waters from the southern sector (Fig. 10b) also plot close to the equilibrium line, but have higher Na/K ratios, and consequently significantly lower estimated equilibrium temperatures (110-180 °C), than the N and NW sectors. The estimated equilibrium temperatures are in good agreement with bottom-well temperatures measured in explorative drillings located in this sector of the ...
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... this sector of the island: 140 °C for IFV2 and 150 °C for IFV1 wells (Ippolito, 1942;Penta 1949Penta , 1954Penta and Conforto, 1951a,b), and 200 °C for IS6 well ( Barbier et al., 1998). A similar range of deep equilibrium temperatures (ranging from about 130 to 210 °C) has been estimated from compositions of fluids collected on the western sector (Fig. 10c). Again, the estimated equilibrium temperatures are consistent with the range of bottom-well temperatures measured during explorative geothermal drillings of the 50s: ~150 °C in well ICV1 (depth ~290 m) (Ippolito, 1942;Penta 1949Penta , 1954Penta and Conforto, 1951a,b), and ~225 °C in well IS3 (~ 1 051 m of the depth) ( Barbier et al., ...
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... deep temperatures calculated by Na-K-Mg geothermometers. estimated for sample 81 (end-member C; see Fig. 9 and Table 1) is fairly consistent with the IS3 bottom well temperature, supporting the hypothesis of thermal end-member compositions being indeed representative of deep-reservoir conditions. Finally, samples collected on the eastern sector (Fig. 10d) cluster in proximity of the Mg corner and below the "partial equilibrium curve" (Giggenbach, 1988), implying their compositions only marginally preserved high-T hydrothermal interaction (if any are occurring). Only sample 55 (Table 1), the deepest well in this area (~ 200 m below the surface) and closest to the Mt. Epomeo horst, has a ...
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... above calculated deep-temperatures also put the basis for a first tentative derivation of the depths of hydrothermal reservoirs in the different sector of the island. At this aim, Fig. 11a plots, for each of the thermal waters of the southern thermal area, the equilibrium temperatures (derived from the Na-K geothermometer) against the relative sampling depths (depths of the wells below the surface). In the same diagram, bottom-well temperature measurements carried out during explorative geothermal drillings of the 50s ...
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... propose that the best-fit regression curve to measured bottomwell temperatures of Ippolito (1942), Penta (1949Penta ( , 1954 and Penta and Conforto (1951a,b) offers a good guess on the local geothermal gradient (black line in Fig. 11a). If this is correct, the range of estimated equilibrium temperatures (110-180 °C) of thermal waters sampled in this study corresponds to depths ranging from ~ 100 to ~750 m. However, we recognize that equilibrium temperatures for most manifestations form a cluster at 130-150 °C, which seems to support the existence of a shallow ...
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... to depths ranging from ~ 100 to ~750 m. However, we recognize that equilibrium temperatures for most manifestations form a cluster at 130-150 °C, which seems to support the existence of a shallow (~150-300 m deep) reservoir in the southern sector of the volcano. Such a suggestion is indeed consistent with data from IFV1 and IFV2 drillings (Fig. 11a), which both indicate the existence of relatively stable temperatures (typical of hydrothermal reservoirs) from depths as shallow as 140-150 m in this area (Ippolito, 1942;Penta, 1949Penta, , 1954Penta and Conforto, 1951a,b). The few thermal manifestations with equilibrium temperatures N160 °C are unlikely to be representative of fluids ...
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... (Ippolito, 1942;Penta, 1949Penta, , 1954Penta and Conforto, 1951a,b). The few thermal manifestations with equilibrium temperatures N160 °C are unlikely to be representative of fluids deriving from the same reservoir (IFV2, IFV1 and IS2 wells indicate indeed that this shallow reservoir maintains relatively constant T b 150 °C to as deep as 850 m; Fig. 11a). As such, these higher-T manifestations may suggest the existence of a second (deeper and hotter) reservoir. This is indeed consistent with the sharp increase (to 200 °C) in bottom-well temperatures measured in well IS6 at depths N1100 ...
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... similar scenario can be proposed for the western sector (Fig. 11b), yet less constrained because of the limited number of bottom-well measurements in this area. This notwithstanding, the range of estimated equilibrium temperatures for the western thermal waters (130 to 210 °C) is similar to that of the S-sector; whilst comparison with bottom-well measurements in ICV1 well (Fig. 11b) suggests that the ...
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... for the western sector (Fig. 11b), yet less constrained because of the limited number of bottom-well measurements in this area. This notwithstanding, the range of estimated equilibrium temperatures for the western thermal waters (130 to 210 °C) is similar to that of the S-sector; whilst comparison with bottom-well measurements in ICV1 well (Fig. 11b) suggests that the existence of a relatively shallow (from depths of ~200-300 m) and cold (T ~ 150-160 °C) reservoir in the western Ischia sector is extremely likely. As in Fig. 11a, however, thermal manifestations with higher equilibrium temperatures (Na-K temperature max ~ 210 °C) also indicate the potential presence of a deeper and ...
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... temperatures for the western thermal waters (130 to 210 °C) is similar to that of the S-sector; whilst comparison with bottom-well measurements in ICV1 well (Fig. 11b) suggests that the existence of a relatively shallow (from depths of ~200-300 m) and cold (T ~ 150-160 °C) reservoir in the western Ischia sector is extremely likely. As in Fig. 11a, however, thermal manifestations with higher equilibrium temperatures (Na-K temperature max ~ 210 °C) also indicate the potential presence of a deeper and hotter reservoir (as earlier suggested by Chiodini et al., ...
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... thermal measurements are unavailable for the N-sector (Casamicciola area), making evaluation of depths of the reservoirs highly hypothetic. However, the high (190 to 270 °C) estimated equilibrium temperatures ( Fig. 10a) make the eventuality unlikely that a shallow and cold hydrothermal reservoir exists in the northern sector, as hypothesised above for the S and W sectors. Indeed, a very deep (N1000 m) hydrothermal reservoir would be assessed for the northern sector using the same geothermal gradient as in Fig. 11a. We recognize however that a ...
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... to 270 °C) estimated equilibrium temperatures ( Fig. 10a) make the eventuality unlikely that a shallow and cold hydrothermal reservoir exists in the northern sector, as hypothesised above for the S and W sectors. Indeed, a very deep (N1000 m) hydrothermal reservoir would be assessed for the northern sector using the same geothermal gradient as in Fig. 11a. We recognize however that a different hydrothermal regime (e.g., a higher geothermal gradient and heat flux) may characterize the N sector of the volcano, also considering its peculiar stratigraphic and tectonic setting ( Molin et al., 2003). Interestingly, it is manifest in Fig. 11c that thermal wells from the N sector plot in ...
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... the northern sector using the same geothermal gradient as in Fig. 11a. We recognize however that a different hydrothermal regime (e.g., a higher geothermal gradient and heat flux) may characterize the N sector of the volcano, also considering its peculiar stratigraphic and tectonic setting ( Molin et al., 2003). Interestingly, it is manifest in Fig. 11c that thermal wells from the N sector plot in between the boiling curves of pure water and 5% NaCl brine, which might be taken as an evidence of liquid-vapour coexistence at depth in this sector of the volcano. If this is true, the highest deep-temperature estimated for hydrothermal-end member A (sample 26) (~254 °C) would correspond to ...
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... the boiling curves of pure water and 5% NaCl brine, which might be taken as an evidence of liquid-vapour coexistence at depth in this sector of the volcano. If this is true, the highest deep-temperature estimated for hydrothermal-end member A (sample 26) (~254 °C) would correspond to a hypothetic reservoir depth of ~ 300 to 400 m below surface (Fig. 11c). Whatever the case, and in spite additional geological and geophysical information are highly required in the area, thermal manifestations from the N sectors seem to offer a viable tool to explore deeper and hotter portions of Ischia's hydrothermal ...
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... compositional time series shown in this work (cfr. 3.4) have confirmed steady conditions (in terms of both T and composition) for Fig. 11. Deep equilibrium temperatures (calculated from the Na-K geothermometer) for the sampled waters, contrasted against the corresponding water level depths below ground level (springs are drawn at 0 m of the depth); (a) S-sector, (b) W-sector and (c) N-sector. Boiling curves (for pure water and 5% Na-Cl brine) are drawn for reference. In ...
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... and 5% Na-Cl brine) are drawn for reference. In plots (a) and (b), SAFEN's data from explorative wells drilled on the southern and western sectors are also reported, based on measured bottom-well temperature profiles (data from Ippolito, 1942;Penta, 1949Penta, , 1954Penta and Conforto, 1951a,b;Barbier et al., 1998). See text for explanation. Fig. 12. Geochemical conceptual model of Ischia geothermal system in a N-S section (from Casamicciola village (A) to Maronti bay (A'); see Fig. 1). See text for discussion. the shallow groundwater system. In addition, the steady compositions of steam and fumarolic gas discharges over the island (Tedesco, 1996;Inguaggiato et al., 2000;Chiodini ...
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... sectors are also reported, based on measured bottom-well temperature profiles (data from Ippolito, 1942;Penta, 1949Penta, , 1954Penta and Conforto, 1951a,b;Barbier et al., 1998). See text for explanation. Fig. 12. Geochemical conceptual model of Ischia geothermal system in a N-S section (from Casamicciola village (A) to Maronti bay (A'); see Fig. 1). See text for discussion. the shallow groundwater system. In addition, the steady compositions of steam and fumarolic gas discharges over the island (Tedesco, 1996;Inguaggiato et al., 2000;Chiodini et al., 2004) suggest constant T-P conditions for the deep-seated reservoirs. In such steady-state conditions, it is reasonable to ...
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... et al., 2004) suggest constant T-P conditions for the deep-seated reservoirs. In such steady-state conditions, it is reasonable to hypothesize that the amount of thermal energy dissipated by the shallow groundwater system (which is referred as Q out ) is perfectly balanced by the energy supply from the deep hydrothermal reservoirs (Q res ) (Fig. 12). In line with the widelyaccepted schematic model of volcano-hosted hydrothermal systems (Fournier, 1999), and with recent applications at other dormant volcanoes (e.g., Yellowstone Caldera; Lowenstern and Hurwitz, 2008), we also suppose that heat is supplied to the deep hydrothermal reservoirs by a degassing and cooling magmatic body ...
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... line with the widelyaccepted schematic model of volcano-hosted hydrothermal systems (Fournier, 1999), and with recent applications at other dormant volcanoes (e.g., Yellowstone Caldera; Lowenstern and Hurwitz, 2008), we also suppose that heat is supplied to the deep hydrothermal reservoirs by a degassing and cooling magmatic body at depth (Q in ; Fig. 12). The existence of such a convective magmatic energy (and fluid) release is supported by the presence of magma-derived He and CO 2 in the hydrothermal system (cfr. ...
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... shallow hydrothermal system should also be considered: (i) the amount of heat (Q cd ) lost from the shallow thermal groundwater system by conduction to the overlying rocks, and (ii) the amount of heat (Q st ) dissipated through steam transport and condensation in the very near-surface (in the soil system) of active fumarolic fields (grey areas in Fig. ...
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... proper assessment of Q st would require knowledge of the rates of steam (and consequently heat) release through the main fumarolic zones of the island (Fig. 1). In the Donna Rachele area, a 0.86 km 2 fumarolic field located on the western Monte Epomeo flank, Chiodini et al. (2004) estimated that the thermal energy associated to the diffuse soil degassing is 40.5 MW. In order to extend the above calculations to other active hydrothermal areas on Ischia (grey areas in the western side of Mt. ...
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... 1). In the Donna Rachele area, a 0.86 km 2 fumarolic field located on the western Monte Epomeo flank, Chiodini et al. (2004) estimated that the thermal energy associated to the diffuse soil degassing is 40.5 MW. In order to extend the above calculations to other active hydrothermal areas on Ischia (grey areas in the western side of Mt. Epomeo; Fig. 1), we assume similar regimes of degassing for these unexplored fumarolic fields, and then computed Q st for the entire island by simply scaling the heat released from Donna Rachele (0.86 km 2 ) to the total surface of active hydrothermal alteration zones on Ischia (~2 km 2 ; Sbrana, 2007). So doing, we evaluate Q st at ~108 ...
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... B.C.; Arso eruption, 1302 A.D.) indicate that Ischia's trachytic melts (the resident magmas stored in the shallow Ischia plumbing system during quiescent periods; Civetta et al., 1991;Piochi et al., 1999) have dissolved H 2 O contents ranging Calculations demonstrate that the largest CO 2 transport takes place on the eastern sector of the island (Fig. 1), which is consistently characterized by active extensional tectonics and where the most recent volcanic events have been concentrated. The main thermal areas of Citara-Serrara Fontana (W-S-sector) and Casamicciola-Lacco Ameno (N and NNW-sector) are characterized by lower CO 2 fluxes. a Calculated from the average of TDIC (sum of ...
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... 2), and from the groundwater discharge rates of N, NNW, W, and S sectors of the aquifer ( Celico et al., 1999). b Calculated from the average of TDIC and dissolved He contents in all sampled manifestations (average of the whole dataset in Tables 1 and 2), and from the total groundwater discharge rate of the shallow aquifer ( Celico et al., 1999). Fig. 13. Diagrams summarising the chemical and isotopic variations observed in the 15 monitored thermal waters during periodic sampling surveys, relative to CO 2 -O 2 -N 2 contents (a), δ 13 C isotope compositions of dissolved CO 2 (b), and Mg-Cl dissolved contents (c). For each sample, the average value (symbol) and the standard deviation (as ...
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... the case of a new unrest, samples are expected to migrate towards compositions enriched in CO 2 and with δ 13 C CO2 of − 3.51‰ (see panels (a) and (b)). Black and grey curves in Fig. 13b are drawn as in Fig. 6. Curves in Fig. 13c are the mixing lines of Fig. 9, and are here representative of the probable evolution of each sampled manifestations in response to changes in the mixing proportions between end-members (these being in turn due to variations in structural tectonic setting of the hydrothermal system). In ...
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... the case of a new unrest, samples are expected to migrate towards compositions enriched in CO 2 and with δ 13 C CO2 of − 3.51‰ (see panels (a) and (b)). Black and grey curves in Fig. 13b are drawn as in Fig. 6. Curves in Fig. 13c are the mixing lines of Fig. 9, and are here representative of the probable evolution of each sampled manifestations in response to changes in the mixing proportions between end-members (these being in turn due to variations in structural tectonic setting of the hydrothermal system). In general, the three diagrams support the idea of a ...
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... rates. The total groundwaters CO 2 flux is evaluated at ~10 4 tons yr − 1 (Table 5). Then, in order to derive the total CO 2 output from Ischia, this groundwater CO 2 flux is added to the CO 2 flux released in diffuse form from soils. If we accept this diffuse CO 2 release to be mainly concentrated in the active hydrothermal alteration zones of Fig. 1, and again assuming that the unexplored areas degas at the same rate as Donna Rachele (3285 tons yr − 1 Chiodini et al., 2004), we estimate a total diffuse CO 2 output of 7640 tons yr − 1 . Finally, the total CO 2 output from Ischia can tentatively be evaluated from the above data at ~1.9 10 4 tons yr − 1 . Interestingly, scaling this ...
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... (cfr. 3.4) set a first baseline for the chemical and isotopic temporal variations of Ischia hydrothermal manifestations during the current period of quiescence; and indicate steady temperature and compositions of thermal manifestations (Fig. 8), in support to a relatively-constant deep supply of heat and volatiles to the hydrothermal system (Fig. 13a, ...
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... and geochemical variations. As such, they put some confidence on the fact that systematic and periodic geochemical measurements in hydrothermal manifestations might contribute to identify (and possibly predict) any future change in the state of activity of Ischia. In this context, the budget calculations and the related conceptual model of Fig. 12 allow some considerations to be made on the sort of chemical variations that might occur during or before a possible reactivation of volcanic activity at Ischia. Assuming that renewal of volcanic activity at Ischia requires replenishment of the volatile-poor alkali-trachytic stored magma with volatile-rich basaltic magmas, as from ...
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... the shallow hydrothermal system. Whilst enhanced heat transfer might be captured by geochemical observations as an increase in discharge temperature (note however that random non-volcanic T variations observed at some monitored sites might complicate detection; see Fig. 8a), we identify that a shift towards CO 2 -rich and 13 C-rich compositions ( Fig. 13a and b) as the potentially more sensible parameters to the magma supply process; and thus the signals to consider with more attention for future (hopefully continuous) monitoring of the island. It is also likely that the intrusion at depth of new magma would prompt a renewal of the resurgence phenomenon ( Orsi et al., 1991); with the obvious ...
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... earthquakes (Mercalli, 1881(Mercalli, , 1884aDe Rossi, 1884;Molin et al., 2003). Since the compositions of Ischia surface manifestations results from the subtle equilibrium of fluid contributions from different shallow and deep reservoirs (cfr. 4.1), larger variations in major ion compositions than those observed during present dormancy (Fig. 13c) should be expected, in the eventuality of a new volcanic ...
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... and isotopic compositions of groundwater discharges and gas emissions from Ischia hydrothermal system has been used to draw a model of fluid circulation at depth. These data have allowed refining previous knowledge on hydrothermal circulation, leading to more robust identification of P-T conditions of two superposed hydrothermal reservoirs (Fig. 12): a shallow (150-300 m deep) reservoir, located on the southern and western sectors of the island, having estimated equilibrium temperatures of ~ 150 °C, and being mainly recharged by meteoric fluids on the S and marine fluids on the W; and a deeper (N300 m deep, and possibly N1000 m) and hotter (equilibrium temperatures ~200-270 °C) ...
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... Intensification of gas emission, seismicity, and ground deformation are symptoms continuously monitored to infer changes in the Ischia volcano dynamics 19,31,32 . While the long-term deformation of the island consists of a central resurgence, and the high heat flow suggests magmatic temperatures at crustal levels, evidence for the presence of eruptible magma within the crust, like for most active volcanoes around the world, remains elusive. ...
The destructive (Mw 3.9) earthquake of 21 August 2017 re-opened the question on where magma resides at the Ischia island volcano. The peculiar complexity of the seismic source initiated the debate on the involvement of fluid-related processes, but the magmatic origin of the event remains uncertain. Here we use ground displacement and broadband seismic data to investigate the magmatic system of Ischia volcano, where progressive underplating of mafic material at mid-crustal depth feeds the deep structures, which are characterized by exceptionally high seismic velocity and are connected with a shallow crystal mush. Although no direct evidence of large molten volumes was found, strong anisotropy suggests that the crystal mush is pervaded by magma-intruded dykes. We propose that the 2017 event was due to a negative tensile deformation caused by depressurization of supercritical fluids along a shallow southwest (SW-)-dipping fault defined by receiver functions (RFs) data, which acted as a valve regulating the overpressure of deep magmatic fluids.
... In hydrothermal systems, discharged fluids of deep origin could be significantly contaminated by shallow infiltrations, especially meteoric water (rainfall). Di Napoli et al.'s [39] investigations in 2002-2007 at Ischia island (Italy) evidenced how the rainfall influences the dynamics of the hydrothermal system lowering CO 2 emissions especially during the winter. Rainfall and seismic series from the Soufrière Hills Volcano (Montserrat) were analyzed by Matthews et al. [40] to investigate the exogenous forcing on the volcanic system in 2001-2003. ...
A passive seismic experiment is carried out at the non-volcanic highly degassing site of Mefite d’Ansanto located at the northern tip of the Irpinia region (southern Italy), where the 1980 MS 6.9 destructive earthquake occurred. Between 2020 and 2021, background seismic noise was recorded by deploying a broadband seismic station and a seismic array composed of seven 1 Hz three-component sensors. Using two different array configurations, we were allowed to explore in detail the 1–20 Hz frequency band of the seismic noise wavefield as well as Rayleigh wave phase velocities in the 400–800 m/s range. Spectral analyses and array techniques were applied to one year of data showing that the frequency content of the signal is very stable in time. High frequency peaks are likely linked to the emission source, whereas at low frequencies seismic noise is clearly correlated to meteorological parameters. The results of this study show that small aperture seismic arrays probe the subsurface of tectonic CO2-rich emission areas and contribute to the understanding of the link between fluid circulation and seismogenesis in seismically active regions.
... Polarization parallel to the fault strike has been widely observed both in active (Lewis et al., 2005;Spudich & Olsen, 2001, and references therein) and inactive faults (Rovelli et al., 2002), where damaged rocks with high crack density produce lowvelocity fault zone layers (Ben-Zion & Sammis 2003, and references therein) which act as a waveguide trapping seismic energy as a result of constructive interference of critically reflected phases (Ben-Zion and Aki 1990; Li & Leary 1990;Li et al., 1997). 2) Along the active fault system of the study area, most of the fluids rise from the deep to the superficial water (Di Napoli et al., 2009, and references therein). Crampin and Zatsepin (1997) observed that in the presence of fluid-saturated fractures, a 90°-flip occurs in the seismic anisotropy fast direction, and the anisotropy fast direction becomes orthogonal to cracks. ...
... Pischiutta et al. (2014 and and Panzera et al. (2017) found that in fault damage zones, ground motion polarization and velocity anisotropy show an orthogonal relation. Therefore, in the area hit by the 2017 earthquake, the presence of recently formed faults and coseismic ruptures affected by relevant hydrothermal fluid circulation (Di Napoli et al., 2009;Falanga et al., 2021) may cause a 90°flip of both ground motion polarization (which, thus, becomes parallel to fracture) and seismic anisotropy fast direction (which, thus, may become orthogonal to fractures). However, further analyses are needed to test this latter hypothesis, implying a study of seismic anisotropy to confirm the occurrence of a 90°flip. ...
In this paper, we investigated ground motion directional amplification and horizontal polarization using ambient noise measurements performed in the northern sector of Ischia Island which suffered damage (VIII EMS) during the 21 August 2017, Md 4.0 earthquake. Over 70 temporary seismic stations were installed by the INGV EMERSITO task force, whose aim is to monitor site effects after damaging earthquakes in Italy. To investigate ground motion directional amplification effects, we have applied three different techniques, testing their performance: the HVSR calculation by rotating the two horizontal components, the covariance matrix analysis, and time–frequency domain polarization analysis. These techniques resulted in coherent outcomes, highlighting the occurrence of directional amplification and polarization effects in two main sectors of the investigated area. Our results suggest an interesting pattern for ground motion polarization, that is mainly controlled by recent fault activity and hydrothermal fluid circulation characterizing the northern sector of the Ischia Island.
... The model of the hydrothermal system of Ischia Island highlights the close relationship between the structural/ geological setting and the fluid circulation. The hydrothermal reservoirs, at least two, ranging in temperature from~150°C to2 70°C and located at depths from~200 m to~1,000 m below the sea level (b.s.l.), are the result of a complex and dynamic interaction between the circulation patterns of meteoric water, sea water, and fluids of deep origin (Di Napoli et al., 2009;Di Napoli et al., 2011). The mixing of these deep-reservoir fluids with the groundwater flow, which occurs in the shallow aquifer system (up to 200 m deep), governs the composition of surface thermal FIGURE 1 | Geological sketch map of Ischia Island (modified after Sbrana et al. (2018) and Della Seta et al., (2021)). ...
Assessing the variations in space and time of groundwater circulation in volcanic islands is of paramount importance to the description of the hydro-geo-thermal system and implementation of hydrogeological, geochemical, and volcanic monitoring systems. In fact, the reliable reconstruction of the groundwater potentiometric surface in such composite volcanic aquifer systems can enable the identification of the most advantageous strategies for both the sustainable use of groundwater resources and the management of volcanic risk. Geographical Information System (GIS) platforms can support the integration and analysis of many spatial and temporal variables derived from monitoring of active volcanoes and the elaboration of spatially continuous data. However, open issues still affect the reliability and general applicability of common spatial interpolation methods in the case of groundwater potentiometric surfaces. This is related to the assessment of the main stratigraphic and volcano-tectonic features affecting the hydraulic head changes. With regard to the dynamically very active Ischia Island (Italy), this study illustrates a GIS-based hydrogeological approach to identify the most accurate interpolation method for mapping the potentiometric surface in complex hydrogeological terrains. The proposed approach has been applied to the existing dataset (1977–2003) stored by Istituto Nazionale di Geofisica e Vulcanologia. Based on a careful geological and hydrogeological survey, a total of 267 wells, from 5 to 250 m in depth, were processed. The data pre-processing involved four meteorological time-series data (1922–1997) and six long records of piezometric water levels (1930–1994). As a result, knowledge of the delineation of rather homogeneous stratigraphic and volcano-tectonic structures at the basin-scale has improved. Thus, new, more reliable potentiometric surfaces of the four main geothermal areas closest to the coast were produced during both dry and wet seasons. The reliability of the processed potentiometric surface was then validated by comparing the spatially continuous data with complementary field data. These findings point toward an optimal interpolation approach for representing the seasonal and areal distribution of main hydrogeological parameters in complex aquifer systems. Finally, insights into variations of hydrological behavior at an active volcanic area will foster an understanding of possible involvement of fresh and thermal waters in triggering phreatic explosions.
... The volcanic island is characterized by an active geothermal system linked to the presence of a shallow magmatic body, whose surface manifestations are hot waters, fumaroles and steaming grounds. Geochemical studies conducted on the waters and fumaroles of the island have highlighted the presence of a complex hydrothermal system resulting from overlapping and interconnected reservoirs fed by meteoric waters, seawater and magma-derived or mixed magmaticcrustal gases (e.g., De Gennaro et al., 1984;Di Napoli et al., 2009;Panichi et al., 1992). ...
The Island of Ischia, one of the Italian active volcanoes, is a famous tourist resort for spa treatments. Spas are supplied by withdrawals from groundwaters which are characterized by a wide range of chemical compositions, salinity and temperature. In natural conditions, the hydrogeological system is recharged by rainfall and by deep fluids; the discharge is towards the sea and the springs. During the peak of the tourist season, when approximately 240 wells are operating simultaneously, a significant additional recharge of the aquifers derives from seawater and from upwelling increase in deep fluids. Although this does not compromise the availability of groundwater, the pumping often determines variation in composition and temperature of groundwater over time. Conversely, the maintenance of a stable quality of thermal waters represents one of the requirements for their therapeutic use in the spas. The study aims to establish game-theoretical modeling of the optimal sustainable exploitation of the groundwater resources of the island by competing users (spas) falling in the same flow tube of the aquifer. In the game the spas are the players, the strategy of a player consists of a fixed pumping rate and daily time durations of pumping, and the player’s utility or payoff is proportional to the total quantity of withdrawn thermal water in a given time period. A special constrained Pareto optimal strategy choice is obtained, considered as a cooperative solution of the game. Pareto optimality means that there is no other strategy choice that makes one player better off without making some other player worse off.
... The interactions between volcano-tectonic dynamics, ground deformation, and underground fluid circulation have been recognized as resulting from a complex geothermal system. Deep hot fluids rise from distinct superimposed hydrothermal reservoirs, in the northern and southern part of the island (Di Napoli et al., 2009), with temperatures from 150°C to 270°C and depths ranging from 150 to 1000 m b.s.l. In the shallow aquifer, they mix with meteoric and marine waters after which they rise to the surface along highpermeability pathways, e.g., fractures and faults, inducing surface manifestations such as fumarolic activity and thermal springs. ...
... In addition, the seismic signals recorded at both sites are ascribable to the same tremor source related to the fluid flow within the hydrothermal system (Cusano et al., 2020b;Falanga et al., 2021). Past geophysical and geochemical studies have inferred the existence of a deep (>900 m) hydrothermal reservoir feeding a diffuse shallow aquifer (0-200 m of depth) located between the northern coast and the inner part of the island (Di Napoli et al., 2009;Di Giuseppe et al., 2017). Here, meteoric and sea water mix with the ascending deep fluids, causing in turn a pressure build-up in the shallow pipe-like conduits, which originates the hydrothermal seismic tremor. ...
Volcano dynamics results from an interplay between internal and external processes spanning different time scales. Unravelling how such processes interact may provide key insights into the mechanisms that may lead to the destabilization of the volcanoes and eruption, a critical information to forecast hazards. Studies dealing with tidal influence on volcanoes fall within this context, yet the cause-effect relationship between tides and eruptions is still poorly understood. In the present study, we investigate the tidal influence on two nearby volcanoes, Ischia and Campi Flegrei (Italy), characterized by intense hydrothermal activity. We analyze the seismic tremor of hydrothermal origin recorded by four seismic stations between January and June 2020 by using Singular Spectrum Analysis. We detect up to five long term tidal periodicities ranging from ∼5 to ∼29 days. The results indicate that the seismic tremor is modulated by Earth tides at both volcanoes. In addition, differences in phase and amplitude modulation between the response of both hydrothermal systems to tidal forcing reveal specific features related to the tremor source and to properties of the surrounding medium. These phenomena indicate an interplay between solid Earth and the dynamics of these two volcanoes. Similar approaches on hydrothermal systems at volcanoes would contribute to better characterize the hydrothermal circulation and their evolving conditions that may represent a precursor of a new phase of activity.
... The groundwater system of Ischia consists of several permeable aquifers, interbedded with low-permeability levels (Celico et al., 1999). In natural conditions, the Ischia's aquifers are recharged by rainfall with a variable contribution of deep hydrothermal fluids (Di Napoli et al., 2009;Piscopo et al., 2020). Due to this interplay, many thermal springs are present on the island. ...
... Numerous studies have been focused on the hydrothermal system of the island, focusing on thermal fluids composition and origin, providing important information for geothermal energy exploration and volcanic risk assessment. These studies allowed refining knowledge on hydrothermal fluid circulation (De Gennaro et al., 1984;Panichi et al., 1992;Caliro et al., 1999;Celico et al., 1999;Inguaggiato et al., 2000;Lima et al., 2003;Chiodini et al., 2004;Daniele, 2004;Milano et al., 2004;Aiuppa et al., 2006;Morell et al., 2008;Di Napoli et al., 2009Carlino et al., 2012Carlino et al., , 2015. ...
... On the other hand, the groundwater system, presenting a wide range of chemical compositions (from calcium-bicarbonate to alkali-chloride waters), has been previously explained by a mixing process among meteoric water, seawater and deep geothermal fluids (De Gennaro et al., 1984;Panichi et al., 1992;Aiuppa et al., 2006;Di Napoli et al., 2009). Despite these fundamental advances, the hydrogeochemical processes governing the groundwater composition and the magnitude and extension of the saline intrusion process in the island are still unknown. ...
Ischia is a volcanic island located NW of the Gulf of Naples (South Italy). The island of Ischia is a structurally complex hydrothermal active system that hosts a fractured aquifer system whose geometry and hydraulic properties are still partly unknown. The aquifer system of Ischia, composed mainly of Quaternary volcanic deposits and marine sediments, exhibits physically and chemically heterogeneous waters. The intense seismicity and hydrothermal activity are expressed by numerous fumaroles and thermal springs, which have been exploited since ancient times, promoting, and supporting the world-renowned tourist activities that constitute the main economic activity of the island. The aim of this study is to determine the hydrogeochemical processes in the Ischia aquifer system. Also, we calculated the proportion of seawater in the aquifer system of Ischia using historical hydrogeochemical data relative to two sampling campaigns. Sixty-nine groundwater and thermal spring samples collected in July 2000 were analyzed and compared with previously published data to identify the changes in seawater contribution. The sample analysis shows that different physicochemical processes occur in the groundwater of Ischia Island, where recharge water, seawater and deep fluids interact and overlap with different intensity. The calculated saline factor indicates a seawater content of up to 70% in some samples near the coast, suggesting that seawater intrusion is the main process in these areas. Later data show that seawater intrusion increases around the coastline with up to 93% seawater content. Finally, data analysis shows that although a change in chemical composition is observed, no variation in thermal water temperature is recorded over time.
... Remote Sens. 2021, 13, x FOR PEER REVIEW 3 of 13 Md = 4 event, its low frequency character, the low S/P spectral ratio, and the characteristics of the seismic noise also provide evidence of the involvement of pressurized fluids in the source of the Ischia earthquakes [25,26]. The temporal trends of selected geochemical parameters (groundwater discharge temperatures, groundwater Mg/Cl ratios and CO2 partial pressure) do not show significant variations from 1984 to 2007, suggesting, according to [21], a stable system with a nearly constant thermal and mass transport from depth. As concerns the deformation field, results from SAR data [16] and levelling lines collected in the 1987-2010 [11] and 1990-2003 [17] periods, evidence a subsidence trend, with values up to 1.2 cm/yr centered on the Mt. ...
... Hydrothermal manifestations are from [20,17]. The depth of the hydrothermal reservoirs in the N-S cross section is from [20,21]. ...
... Hydrothermal manifestations are from [17,20]. The depth of the hydrothermal reservoirs in the N-S cross section is from [20,21]. The survey style network vertices were realized using 3D-type GPS monuments [9,28]. ...
The identification of the mechanisms responsible for the deformation of calderas is of primary importance for our understanding of the dynamics of magmatic systems and the evaluation of volcanic hazards. We analyze twenty years (1997–2018) of geodetic measurements on Ischia Island (Italy), which include the Mt. Epomeo resurgent block, and is affected by hydrothermal manifestations and shallow seismicity. The data from the GPS Network and the leveling route show a constant subsidence with values up to −15 ± 2.0 mm/yr and a centripetal displacement rate with the largest deformations on the southern flank of Mt. Epomeo. The joint inversion of GPS and levelling data is consistent with a 4 km deep source deflating by degassing and magma cooling below the southern flank of Mt. Epomeo. The depth of the source is supported by independent geophysical data. The Ischia deformation field is not related to the instability of the resurgent block or extensive gravity or tectonic processes. The seismicity reflects the dynamics of the shallow hydrothermal system being neither temporally nor spatially related to the deflation.
... The origin of such emissions is considered to be linked to two different sources: one aquifer at around 400 m of depth and with T around 250 • C, and a deeper aquifer at about 900 m depth and at 300 • C. The less deep aquifer is recharged by meteoric water with a component of seawater infiltrating along the shoreline as highlighted by an increase in Total Dissolved Solids (TDS) toward the coastal areas and by the Na/Cl ratio. The source of CO 2 is a deep cooling magmatic body; the CO 2 during its upward movement is modified by crustal input undergoing a δ 13 C differentiation through interaction with the aquifers (Inguaggiato et al., 2000;Chiodini et al., 2004;Di Napoli et al., 2009). A series of submarine vents are located in shallow-water (less than 10 m of depth) along the eastern shore of the island near the Castello Aragones. ...
Submarine fluids emissions in the form of geothermal vents are widespread in a variety of geological settings ranging from volcanic to tectonically active areas. This overview aims to describe representative examples of submarine vents in shallow-water areas around the globe. The areas described include: Iceland, Azores, Mediterranean Sea (Italy and Greece), Caribbean, Baja California, Japan, Papua, New Zealand, Taiwan. Common and divergent characteristics in terms of origin and geochemistry of the emitted fluids and their impact on the indigenous organisms and the surrounding environment have been identified. In the hottest vents seawater concentration is common as well as some water vapor phase separation. Carbon dioxide is the most common gas often associated with compounds of sulfur and methane. In several vents precipitation of minerals can be identified in the surrounding sediments. The analyses of the microbial communities often revealed putative chemoautotrophs, with Campylobacteria abundantly present at many vents where reduced sulfur compounds are available. The techniques that can be used for the detection and quantification of underwater vents are also described, including geophysical and geochemical tools. Finally, the main geobiological effects due to the presence of the hydrothermal activity and the induced changes in water chemistry are assessed.
... The outgassing at fumaroles is prevalently composed of water (steam . 99%) with minor CO 2 , H 2 S, H 2 , N 2 , CO, CH 4 , Ar and He (Chiodini et al. 2004, Di Napoli et al. 2009, Piochi et al. 2019. The outgassing composition indicates the high fraction of separated vapour as well as the energy of the hydrothermal system (Chiodini et al. 2004), which are consistent with the high pressurization of fluids sealed by low-permeability horizons that cause well explosions (Chiodini et al. 2004 and references therein;D'Auria et al. 2018). ...
Geothermal and volcanic systems are prone to gravity-induced slope instability at different scales. Endogenous magmatic, hydrothermal, and seismic forcings can significantly modify mechanical properties and perturb the local stress field and gravitational equilibrium inducing shallow or slope-scale instabilities. The island of Ischia, which is part of the Phlegrean Volcanic District (Italy), is a remarkable example of this kind of complex interacting system. This study focuses on monitoring the hydrothermal system located beneath the ongoing slope-scale deformation, which involves Mt. Nuovo (the western part of Mt. Epomeo) and is a complementary effect of the resurgence of an ancient caldera. Debris and rock avalanches have affected the slopes of this volcanic island, in response to the renewal of volcanic activity and caldera resurgence. Large parts of the corresponding mass-wasting deposits overlay the most active areas of the Ischia hydrothermal system, where ongoing slope-scale gravity-driven deformation due to a mass rock creep (MRC) process is still evolving. To investigate possible relations between the perturbing shallow hydrothermal system and the MRC process, thermal monitoring of selected groups of fumarolic emissions located in several portions of the deforming sector has been carried out since 2008 on a monthly basis by means of direct (thermal probes) and remote sensing (IR-thermography) techniques. Thermal monitoring of specific fumaroles shows a peculiar seasonal trend characterised by a delayed inverse correlation with rainy periods and a short-term pulsating response to dry stages. The fumaroles also appear spatially correlated to the presence of MRC-related structures involving volcanic slopes. According to the measured thermal data, a conceptual model of the thermal interactions within the Mt. Nuovo slope is provided, framing the potential role of thermal actions in accelerating the deformation process. In this view, possible hazard scenarios, due to magmatic or hydrothermal renovation are depicted, delineating the interconnected multi-hazard worst scenario consisting of an accelerating evolution of the MRC process towards paroxysmal collapse.
Supplementary material at https://doi.org/10.6084/m9.figshare.c.5497819
