# Geochemical characterisation of shallow pre-eruptive magma degassing at Marapi Volcano, Sumatra (Indonesia)

Charlotte Barrington, Hilma Alfianti, Ugan Boyson Saing, Patrick Allard, Philipson Bani, Fidel Costa, Benoit Taisne

► To cite this version:

Charlotte Barrington, Hilma Alfianti, Ugan Boyson Saing, Patrick Allard, Philipson Bani, et al.. Geochemical characterisation of shallow pre-eruptive magma degassing at Marapi Volcano, Sumatra (Indonesia). Bulletin of Volcanology, 2026, 88 (3), pp.24. 10.1007/s00445-026-01942-4. hal-05514917

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# Geochemical characterisation of shallow pre-eruptive magma degassing at Marapi Volcano, Sumatra (Indonesia)

Charlotte Barrington<fnref n="1" /> Ⓓ · Hilma Alfianti<fnref n="2" /> · Ugan Boyson Saing<fnref n="2" /> · Patrick Allard<fnref n="1" />,<fnref n="3" /> Ⓓ · Philipson Bani<fnref n="4" /> ·
Fidel Costa<fnref n="1" />,<fnref n="3" />,<fnref n="5" /> Ⓓ · Benoit Taisne<fnref n="1" />,<fnref n="5" /> Ⓓ

Received: 1 September 2025 / Accepted: 17 January 2026

© The Author(s) 2026

## Abstract

Marapi, Sumatra's most persistently active volcano, exhibits frequent short-lived explosive activity and sporadic prolonged eruptions, often occurring with little to no detectable geophysical precursory signals. Despite advances in monitoring, our understanding of Marapi's eruptive dynamics remains limited by a lack of information on its gas emissions. Here we present the first chemical dataset of Marapi's volcanic gases measured using a Multi-component Gas Analyser System (Multi-GAS) from the rim of the active Verbeek crater during inter-eruptive activity in 2014 and 2018. Both datasets reveal gas compositions typical of magma-derived arc gases and a puffing degassing regime, with a periodicity of ~26 min in 2014 that decreased to 8–10 min in 2018. Notably, our 2018 measurements—performed just one month before two explosive outbursts on 27 April and 2 May 2018—show comparable SO₂/H₂S molar ratios (3.3 and 2.5) but a marked increase in total sulphur relative to CO₂, resulting in a CO₂/SO₂ ratio more than three times lower than in 2014. This chemical shift is consistent with shallow magma degassing preceding the April and May 2018 explosive activity. Our findings underscore the importance of regular, ideally continuous, gas monitoring at Marapi to better resolve magmatic processes that drive its explosive activity. We also report the first CO₂/S<sub>T</sub> gas ratio measured for a Sumatran volcano, providing further evidence for sulphur-limited degassing along the Sunda Arc compared with other subduction zones.

**Keywords** Marapi volcano · Sumatra · Pre-eruptive degassing · Volcanic gas monitoring · Multi-GAS measurements · Sunda Arc · Arc volcanism

## Abstract

Gunung Marapi, gunung api paling aktif di Sumatra, dicirikan oleh letusan-letusan singkat yang sering terjadi dan sesekali mengalami erupsi yang berlangsung dalam jangka waktu panjang, sering kali tanpa pendahulu geofisika yang jelas. Meskipun

Editorial responsibility: Editorial responsibility: M. Edmonds

<figure><img src="image_2.png" /></figure>

<footnote>Charlotte Barrington
charlotte.barrington@ntu.edu.sg</footnote>

<footnote n="1">¹ Earth Observatory of Singapore, Nanyang Technological University, Singapore, Singapore</footnote>

<footnote n="2">² Center for Volcanology and Geological Hazard Mitigation, Geological Agency, Ministry of Energy and Mineral Resources, Jalan Diponegoro No. 57, Bandung, West Java 40122, Indonesia</footnote>

<footnote n="3">³ Institut de Physique du Globe de Paris, Université Paris Cité, CNRS, Paris, France</footnote>

<footnote n="4">⁴ Laboratoire Magmas et Volcans, Université Blaise Pascal-CNRS-IRD, OPGC, Aubière, France</footnote>

<footnote n="5">⁵ Asian School of the Environment, Nanyang Technological University, Singapore, Singapore</footnote>
pemantauan telah mengalami kemajuan, pemahaman kita terhadap dinamika erupsi Marapi masih terbatas karena kurangnya data emisi gas vulkanik. Dalam studi ini, kami menyajikan dataset kimia pertama dari gas vulkanik Gunung Marapi selama periode inter-eruptif, yang dikumpulkan menggunakan Multi-component Gas Analyser System (Multi-GAS) dari tepi kawah Verbeek yang aktif pada tahun 2014 dan 2018. Kedua dataset menunjukkan komposisi gas yang khas dari magma busur vulkanik dan pola pelepasan gas berupa hembusan kecil dan berulang, dalam periode waktu ~ 26 menit pada tahun 2014, yang kemudian menjadi lebih singkat, yaitu 8–10 menit pada tahun 2018. Secara khusus, pengukuran tahun 2018, dilakukan satu bulan sebelum dua letusan eksplosif pada 27 April dan 2 Mei—menunjukkan rasio molar SO₂/H₂S yang serupa (3,3 dan 2,5), namun terjadi peningkatan signifikan jumlah total belerang relatif terhadap CO₂, sehingga menghasilkan rasio CO₂/SO₂ tiga kali lebih kecil dari tahun 2014. Perubahan ini konsisten dengan proses degassing magma yang lebih dangkal menjelang letusan April dan Mei 2018. Temuan ini menegaskan pentingnya pemantauan gas yang rutin dan idealnya kontinu di Gunung Marapi untuk memperdalam pemahaman tentang proses magmatik yang memicu aktivitas eksplosif. Selain itu, kami juga melaporkan rasio CO₂/ST pertama yang pernah diukur untuk gunung api di Sumatra, memberikan bukti tambahan terkait pola pelepasan gas belerang yang terbatas di sepanjang Busur Sunda dibandingkan dengan zona subduksi lain.

## Introduction

Changes in the emission rate and composition of volcanic gases can track the progressive depressurisation of ascending magma, often allowing the detection of unrest before magma reaches the surface (e.g., Edmonds & Wallace 2017; Giggenbach 1997; Wallace et al. 2015). As such, gas monitoring is a critical component of eruption forecasting, particularly at volcanoes that exhibit limited seismic or geodetic precursors (e.g., Chaussard et al. 2013; Shreve et al. 2022; Smittarello et al. 2022; De Moor et al. 2016). When integrated with geophysical monitoring, volcanic gas measurements enhance our understanding of subsurface processes and the nature of volcanic unrest (e.g., Asensio-Ramos et al. 2025; Battaglia et al. 2019; Gardine et al. 2011; Hidalgo et al. 2018; Surono et al. 2012; Young et al. 2003).

The interpretation of gas monitoring signals is fundamentally based on volatile exsolution from the melt to form gas bubbles as magma ascends towards the surface and decompresses. These bubbles migrate and evolve chemically in response to the contrasting pressure-dependent solubilities of different volatile species. Among the dominant magmatic volatiles, carbon dioxide (CO₂) begins exsolving at much greater depths than H₂O and sulphur species (SO₂, H₂S), whereas chlorine (HCl) and fluorine (HF) exsolve at shallower depths (e.g., Baker & Alletti 2012; Botcharnikov et al. 2005; Carroll & Holloway 1994; Carroll and Holloway 1994; Spilliaert et al. 2006 and references therein). Consequently, CO₂ is a key tracer of deep magmatic degassing, whereas S, Cl, and F degassing reflect shallower degassing processes. Thus, monitoring temporal changes in the ratios of volatiles with contrasting solubilities can provide valuable information on the depth and evolution of magma degassing prior to eruptions (e.g., Agusto et al. 2023; Aiuppa et al. 2021; De Moor et al. 2019; Spilliaert et al. 2006; Surono et al. 2012). However, magmatic gases can undergo further modification upon ascent through interaction with a shallow hydrothermal system (e.g., Allard et al. 2014; Giggenbach 1997; Symonds

et al. 2001). This interaction particularly affects SO₂, HCl, and HF, which are highly soluble in liquid water. SO₂ scrubbing is a common process at many dormant volcanoes (e.g., De Moor et al. 2019; Edmonds et al. 2003; Symonds et al. 2001). Consequently, the SO₂/H₂S ratio of volcanic gases is strongly dependent on the pressure-temperature-redox conditions of the magmatic system (Giggenbach 1987) and is also sensitive to interaction with a hydrothermal system prior to gas exit at the surface (Aiuppa et al. 2006). These complexities underscore the need of interpreting volcanic gas compositions within the context of subsurface processes, as observed gas signatures may reflect both magmatic and hydrothermal contributions.

Indonesia is home to 126 active volcanoes, making it one of the most volcanically active regions in the world. However, to date, compositions have been characterised at only a limited number of Indonesian volcanoes (Aiuppa et al. 2015; Allard 1986; Allard et al. 1989; Bani et al. 2017, 2018, 2022; Nadeau et al. 2023). In this study, we present the first chemical dataset of gas emissions from a volcano in Sumatra, Marapi, which is the most active volcano on this main island of the Sunda Arc. Our results provide novel insights into shallow magma degassing at Marapi and contribute to broader understanding on the geochemistry of volcanic gas emissions along the Sunda Arc.

## Marapi volcano

Situated in the Padang Highlands of central-western Sumatra, Marapi is a 2.8-km-high stratovolcano that has grown along the ENE-WSW-trending Sumatran strike-slip fault. It is topped by a relatively flat summit region, locally known as *Puncak Merpati* (2757 m), which hosts several overlapping craters aligned along the regional fault direction. The earliest documented eruption dates back to 1770 (Global Volcanism Program 2013). Since then, more than 50 eruptions of small to moderate explosive intensity—reaching a maximum
Volcanic Explosivity Index (VEI) of 2—have been recorded (Global Volcanism Program 2013). Over time, eruptive activity has migrated from the eastern summit craters (Tuo and Bungo) towards the west, with eruptions in the twentieth and twenty-first centuries primarily focused at the Verbeek crater, also referred to as *Kapundan Tenga* (Fig. 1; Global Volcanism Program 2013). These eruptions have produced tephra columns, ash plumes, ballistic ejecta, lava flows, and, occasionally, pyroclastic flows. The erupted magmatic products predominantly range in composition from basaltic andesite to evolved andesite with ~60 wt% SiO₂ and a rhyolitic matrix (Nurfiani et al. 2021).

Between 2000 and 2025, Marapi experienced five VEI 2 eruptions—in 2000, 2001, 2004, 2018, and 2023–2025—lasting from several days to several months. These were interspersed with frequent short-lived VEI 1 explosive events, phreatic to phreatomagmatic in nature, occurring almost annually and sometimes twice within a year. Such events have produced ash plumes rising as high as 6 km above the summit, resulting in localised ashfall up to 17 km from the volcano (Global Volcanism Program 2013). The 2023–2025 VEI 2 eruption, ongoing at the time of writing, began abruptly on 3 December 2023, with a major explosion that generated pyroclastic flows extending up to 3 km down the northern flank. Tragically, this event resulted in the deaths of 24 hikers and severe injuries to many of the 52 survivors who had entered the 3-km exclusion zone (Global Volcanism Program 2025). The dispersal of ash-laden columns has led to repeated closures of Minangkabau International Airport, located 113 km away, and thick deposits of ash now blanket much of the volcano's slopes. On 11 May 2024, intense rainfall triggered lahars and mudslides, resulting in a further 67 fatalities, with 20 individuals reported missing. The region surrounding Marapi is densely populated, with nearly one million residents living within a 30-km radius (Global Volcanism Program 2013). While Marapi's typical eruptive style involves short-lived moderate events, geological evidence indicates the potential for more extensive hazards, with pyroclastic flows from past eruptions having reached as far as Bukittinggi, the third-largest city in West Sumatra, located just 13 km from the summit (Fig. 1; Kusnadi et al. 2015; Nurfiani et al. 2021).

Eruptions at Marapi frequently occur without detectable precursory seismic signals (Fig. 2; Global Volcanism Program 2013). Therefore, the sudden, discrete explosions—though often confined to the summit area—pose serious risks to hikers, local communities, and scientists. In response, the Centre for Volcanology and Geological Hazard Mitigation (CVGHM) has maintained an alert level of 2 (on a scale of 1–4) for most of the past two decades, restricting access within a 3-km radius of the active Verbeek crater. Following the onset of the 2023–2025 eruption, the alert

level was raised to 3 in January 2024, with the exclusion zone extended to 4.5 km.

Despite the significant hazards posed by Marapi, very little is known about its intra- and inter-eruptive degassing processes. Data collected between 2005 and 2015 by the Ozone Monitoring Instrument (OMI) aboard NASA's Aura satellite indicate a very low time-averaged SO₂ emission rate of 35 ± 35 t/d—effectively at the instrument's detection limit (Carn et al. 2017). This low output was corroborated by summit ground-based traverses using a UV spectrometer on 25 October 2014, which revealed a very low SO₂ emission rate of 2.6 ± 1.5 t/d (Bani et al. 2022). Among the 37 volcanoes along the Sunda Arc that have displayed increased activity since 1600 (Badan Geologi 2011), Marapi thus appears to belong to a subset of frequently erupting volcanoes with low inter-eruptive SO₂ flux (e.g., Kerinci, Merapi, Semeru, Sangeang Api) (Bani et al. 2022; Carn et al. 2017; Global Volcanism Program 2013). However, until now, the chemistry of Marapi's volcanic gases has remained essentially undocumented.

Here, we present the first dataset on the chemical composition of volcanic gases emitted from Marapi, acquired at the rim of the Verbeek (*Kapundan Tenga*) summit crater during two field campaigns in 2014 and 2018. The latter campaign, conducted on 24 March 2018, took place just one month prior to two explosive outbursts on 27 April and 2 May (Fig. 2), following a nearly year-long period of quiescence.

## Methods

To measure the concentrations of key volcanic gas species at Marapi volcano, we deployed Multi-component Gas Analyser System (Multi-GAS) instruments (Aiuppa et al. 2005; Shinohara 2005). Two distinct instruments were used during our field campaigns. The first, a compact and portable Multi-GAS developed by the Istituto Nazionale di Geofisica e Vulcanologia (INGV)—hereafter referred to as the *INGV-type Multi-GAS*—was operated on 25 October 2014. The second, a fully automated *USGS-type Multi-GAS*, originally designed for permanent installations, was temporarily deployed for this study on 24 March 2018. Both instruments were positioned on the southern rim of Verbeek crater (Fig. 1b) to measure concentrations and ratios of carbon dioxide (CO₂), sulphur dioxide (SO₂), and hydrogen sulphide (H₂S), along with hydrogen (H₂; INGV-type) and water vapour (H₂O; USGS-type), in the diluted gas plume issuing from the crater.

Both instruments integrate a compact infrared spectrometer for CO₂—additionally for H₂O in the case of the USGS-type—as well as electrochemical sensors for SO₂ and H₂S (and for H₂ in the INGV-type). The INGV instrument is equipped with a Gascard infrared spectrometer
<figure><img src="image_3.png" /></figure>

<figure><img src="image_4.png" /></figure>
►Fig. 1 a Location of Marapi volcano in Indonesia, West Sumatra in relation to other selected volcanoes in Indonesia. b Google Earth image of Marapi showing the summit craters, including the currently most active Verbeek crater. The cyan circle indicates the approximate location of our Multi-GAS measurements in 2014 and 2018. The red line, drawn at an angle, represents the distance across the flat summit area known as Puncak Merpati

(model EDI030102NG), which measures CO₂ over a range of 0–3000 ppmv with a precision of ±3% at 2σ, while the USGS-type uses a LI-COR spectrometer (model LI-840A) to measure CO₂ and H₂O within ranges of 0–20,000 ppm and 0–60 mmol/mol, respectively. According to the manufacturer, the LI-840A achieves an accuracy better than 1.5% of the reading for both species.

For SO₂ and H₂S, both instruments employ electrochemical sensors manufactured by City Technology. The INGV-type is equipped with a SO₂ sensor (model 3ST/F, 0–200 ppm), a H₂S sensor (model EZ3H, 0–200 ppm), and a H₂ sensor (model EZT3HYT “Easy Cal”, 0–200 ppm). The USGS-type includes a SO₂ sensor (model TD2F-1A, 0–100 ppm) and an H₂S sensor (model TC4F-1A, 0–100 ppm). These sensors are reported to have a repeatability of ±1%.

Both instruments also record ambient air pressure and relative humidity, with the INGV-type additionally logging air temperature, which enabled pressure correction of CO₂ readings using the following equation:

$$ \text{CorrectedCO2} = \text{measuredCO2} \times \left( \frac{P}{1013.25} \right) \quad (1) $$

where P is the measured pressure in hPa. The LI-840A infrared spectrometer in the USGS-type instrument applies automatic pressure correction, ensuring accurate readings despite ambient pressure fluctuations.

The instruments were operated at sampling rates of 0.5 Hz (INGV-type) and 0.3 Hz (USGS-type). Deployment times were necessarily brief due to the hazardous environment of Verbeek crater—Marapi was under Alert Level 2 (on a scale of 1–4) during both campaigns, with public access prohibited within 3 km of the active crater—and because of relatively poor weather conditions. Measurements made during persistent low-level clouds and/or drizzle, which are common at tropical volcanoes such as Marapi, can be affected by high humidity, as water droplets may partially scavenge soluble gases in the plume; consequently, data were collected only during periods of favourable weather.

The INGV-type Multi-GAS was calibrated in Palermo (Italy) prior to deployment. Although no real-time correction was applied for the known interference of SO₂ on the H₂S electrochemical sensor, this effect was accounted for during post-processing to avoid overestimating H₂S concentrations (Roberts et al. 2014). Specifically, we subtracted an interference-adjusted SO₂ contribution from the raw H₂S

signal, following established methods (Aiuppa et al. 2005; Roberts et al. 2012; Tamburello 2015; Witt et al. 2008). This approach relies on the robustness of the SO₂ sensor, which is not significantly affected by cross-sensitivity to other gases (Roberts et al. 2012, 2014; Tamburello 2015). The corrected H₂S concentration was calculated as:

$$ \text{CorrectedH2S} = \text{measuredH2S} - (\text{SO2} \times \text{FSO2}/\text{H2S}) \quad (2) $$

where SO₂ is the concentration of sulphur dioxide measured by the SO₂ electrochemical sensor and FSO₂/H₂S is the interference factor representing the degree to which SO₂ inflates the H₂S signal. This factor has been estimated through laboratory testing to range between 7 and 25% (Battaglia et al. 2019; Roberts et al. 2014). With regards to H₂, our results are uncorrected for possible cross-sensitivity with other unmeasured reduced gas species, such as carbon monoxide (CO).

As the INGV-type instrument does not allow direct spectrometric H₂O quantification, the water vapour partial pressure (pH₂O) was derived from the recorded relative humidity (RH, ±0.1%) and temperature (T in °C, ±0.1 °C) (Buck 1981):

$$ \text{pH2O} = \text{RH} \times 10^{\left(\frac{8.07131}{\frac{1730.63}{T+233.426}}\right)} \quad (3) $$

The USGS-type instrument was calibrated with standard gases at the Earth Observatory of Singapore prior to the 2018 field campaign. Sensor cross-sensitivities were assessed during calibration and corrected accordingly. After the campaign, zero baseline measurements were conducted to confirm the sensor stability. Data acquisition was performed using a script provided by Peter Kelly (USGS), which includes correction for differing sensor response times. Time-series data from both campaigns were examined for co-varying gas species. Minor baseline corrections were applied using a constant offset based on the median signal during air-background (non-plume) periods. All data processing was carried out in MATLAB© software, and all gas ratios (e.g., CO₂/SO₂, H₂S/SO₂) are expressed as molar ratios.

During the 2018 campaign, we attempted to measure the SO₂ emission rate from Verbeek crater using a Palermo-built SO₂ camera, similar in design and operating principles to the instrument described by Delle Donne et al. (2019). The camera was positioned on the western rim of Bongsu crater, providing a clear line of sight to the volcanic plume (Fig. 1b). The detection limit of the camera depends on several factors, such as atmospheric conditions, camera settings and distance to the plume. Under typical conditions, the instrument can reliably detect fluxes of approximately 10–20 t d⁻¹. However, during our campaign, SO₂ concentrations in the plume were too low to
<figure><img src="image_5.png" /><figcaption>**Fig. 2** Chronogram of eruptive activity at Marapi volcano from 1970 to 2025, showing major VEI 1–2 events and the summit craters from which they originated. The timing of our 2014 and 2018 gas sampling campaigns is indicated by inverted cyan triangles. Eruption phases that exhibited seismic precursors are marked by asterisks. Data were</figcaption></figure>

compiled from the Global Volcanism Program (GVP) and PVMBG databases and do not include the date uncertainties reported in the GVP. A minimum eruption duration of 30 days was imposed for visual clarity. For the complete eruption history of Marapi included in this plot, see Supplementary Material 1 (S1)

be quantitatively detected in the camera's images, thereby preventing any flux computation.

## Results

During the 2014 and 2018 field campaigns, gas emissions were recorded from the deep Verbeek crater under similar visual conditions. In 2018, weak puffing degassing was continuously observed at Verbeek Crater (Fig. 3). Time series of volcanic gas concentrations measured with the Multi-GAS

<figure><img src="image_6.png" /><figcaption>**Fig. 3** Degassing activity at Verbeek Crater during the 24 March 2018 field campaign. View from the Multi-GAS measurement site showing a “puff” of volcanic gases during weak degassing</figcaption></figure>
during the 2014 and 2018 campaigns are shown in Figs. 4 and 5. These data reveal well-correlated, fluctuating peaks in SO₂ and other species, including CO₂ (Fig. 4), consistent with the puffing degassing regime observed in 2018. Some variability may arise from changes in plume transport caused by winds. However, our instruments were positioned to sample the vertically ascending gases immediately as they emerged from Verbeek Crater. Thus, most of the variability shown in Figs. 4 and 5 likely reflects the intrinsic puffing degassing regime rather than plume advection by wind. To quantify the periodicity of these fluctuations, a Fast Fourier Transform (FFT) was applied to the SO₂ time series (Fig. 6). The FFT analysis reveals oscillatory degassing patterns, with the dominant timescale of approximately 26 min (~0.001 Hz). Analysis of shorter 30–40-min segments from the 2018 dataset indicates a dominant timescale of 8–10 min (~0.002 Hz). However, the limited segment duration reduces the ability to resolve longer-period fluctuations. To determine whether similar periodicities occurred in the 2014 dataset, additional segmented FFT analyses were performed, showing consistent short-term variability across intervals (see Supplementary Material 3).

Molar gas ratios among the main volatile species were directly obtained from linear regressions of each gas concentration versus purely volcanic SO₂ (Fig. 7). A strong correlation was observed between CO₂ and SO₂ in both

2014 (R²=0.80) and 2018 (R²=0.73), indicating a common magmatic source of these species. However, the correlation slopes of the CO₂/SO₂ ratio (Fig. 7) is much higher in 2014 (17.6±8.5) than in 2018 (5.2±4.4). Notably, the measured atmospheric CO₂ background, given by the y-axis intercept of the regression lines (i.e., corresponding to zero volcanic SO₂), was lower in 2014 (280 ppmv) than in 2018 (370 ppmv). This difference likely results from the lower ambient pressure recorded in 2014 (~680 hPa) compared to 2018 (730 hPa) at the 2.8 km altitude of Verbeek crater, which could imply a ~30% underestimation of the pressure-corrected volcanic CO₂ concentrations in 2014. However, this difference cannot account for the roughly threefold higher CO₂/SO₂ ratio in 2014 than in 2018.

The SO₂ and H₂S concentrations were also well correlated over time during both field measurements (Fig. 7). In contrast to CO₂/SO₂, the H₂S/SO₂ molar ratio displays remarkably similar values in 2014 (0.3±0.16) and in 2018 (0.4±0.23), showing a persistent predominance of sulphur dioxide over H₂S in the Marapi volcanic gases. In comparison, the variations of H₂O and H₂ with SO₂ are less clear, and mask any significant temporal correlation between these species. In particular, the H₂ signal displayed a large variability at or below its detection limit. The H₂O/SO₂ ratio inferred from relative humidity measurements in 2014 (850; Table 1) is higher than the

<figure><img src="image_7.png" /><figcaption>**Fig. 4** Temporal evolution of CO₂ and SO₂ concentrations measured with Multi-GAS at Verbeek Crater in (a) October 2014 and (b) March 2018, showing co-oscillations of the two species. Reported</figcaption></figure>

CO₂ concentrations are pressure-corrected; values for 2014 were likely underestimated, as discussed in the main text
<figure><img src="image_8.png" /><figcaption>**Fig. 5** Time series of volcanic gas concentrations measured using Multi-GAS at Verbeek Crater during the October 2014 (a, b) and March 2018 (c, d) campaigns. Panels (a, c) show CO₂ (grey squares) and H₂ or H₂O (blue circles); panels (b, d) show SO₂ (red squares) and H₂S (green circles). X-axes indicate elapsed time from the start of sampling. The 2018 data were collected from 12:00 to 13:30 UTC (10:45–12:30 local time). Error bars represent manufacturer-reported</figcaption></figure>

precision (±3% at 2σ for CO₂ in 2014), accuracy (±1.5% for CO₂ and H₂O in 2018), or repeatability (±1%) for electrochemical sensors. After the baseline correction, the H₂ time series exhibited a slow drift over the ~2 h measurement interval. This low-frequency trend was removed using a 5-min moving-median filter to preserve short-term plume variability. The Multi-GAS measurements presented in this figure are provided in Supplementary Material 2 (S2)

value estimated for 2018 (274 ± 870), although the latter is associated with large uncertainty.

Given the predominance of SO₂ over H₂S and the consistency of SO₂/H₂S ratio in both 2014 and 2018, the ratio of CO₂ to total sulphur (S_T) mirrors the trend observed for CO₂/SO₂ and is about a factor of four lower in 2018 (3.6 ± 0.5) compared to 2014 (14.0 ± 0.51). Table 1 summarizes all the measured gas ratios and the calculated molar composition (%) of Marapi volcanic gas for both campaigns.

# Discussion

## A pulsed degassing regime

Marapi displayed a consistent pattern of weak degassing during both the 2014 and 2018 field measurements, with minor plume emissions issuing from the Verbeek crater during these non-eruptive periods. This observation is consistent with the very low SO₂ flux of 2.6 ± 1.5 t/d measured via ground-based traverses with UV spectrometer in 2014 (Bani et al. 2022), as well as with the low emission rate of 35 ± 35 t/d (at detection limit) inferred from NASA's OMI satellite between 2005 and 2015 (Carn et al. 2017). Moreover, in March 2018, using a SO₂ camera, SO₂
<figure><img src="image_9.png" /><figcaption>**Fig. 6** a SO₂ time series measured at Verbeek Crater and (b) corresponding Fourier Transform (FFT) analysis. For the 2018 dataset, the time series was split into two 30–40-min segments to avoid padding</figcaption></figure>

data gaps. Segmented FFT analyses of the 2014 dataset are provided in Supplementary Material 3

was undetectable above the Verbeek crater rim, further supporting that Marapi's SO₂ emission is generally weak between eruptive events.

The SO₂ and H₂S concentrations measured in the dilute volcanic plume issuing from Verbeek crater are generally low; SO₂ peaked at a maximum of ~4 ppmv in 2014 and ~8 ppmv in 2018, while H₂S peaked at ~1 ppmv and ~3 ppmv, respectively. Nevertheless, the strong temporal correlations between SO₂ and H₂S and between CO₂ and SO₂ (Fig. 7a) support a common source derivation of these three gas components. Conversely, the poor temporal correlations of H₂O and H₂ with SO₂ point to variable or non-volcanic contributions. H₂ was close to its detection limits, while the variability in H₂O may reflect atmospheric influence or heterogeneous subsurface sources.

Fourier analysis of SO₂ concentrations suggests periodic fluctuations, with the dominant periodicity appearing to differ between the 2014 (~26 min) and 2018 (8–10 min) datasets (Fig. 6). The 2018 time series were split into two relatively short segments of 30–40 min, limiting the resolution for detecting longer-period cycles. Short-period (8–10 min) oscillations were largely absent or of very low amplitude in the 2014 data (Fig. 6 and Supplementary Material 3). The correspondence between the 2018 oscillations and our field observations of recurrent gas puffing at the bottom of Verbeek crater under low wind conditions (Fig. 1b) suggests that they likely reflect periodic venting from the volcanic conduit rather than environmental modulation. The higher frequency of puffing in the 2018 dataset may indicate increased shallow gas pressure and/or weaker
<figure><img src="image_10.png" /><figcaption>**Fig. 7** Concentration (ppmv) plots of (a) CO₂, (b) H₂S, (c) H₂O, (d) H₂ versus SO₂, and (e) CO₂ versus total sulphur (S_T) at Marapi in 2014 (grey circles) and 2018 (blue diamonds). Molar gas ratios were obtained from linear regressions; R² values are indicated. Mean gas ratios are reported in Table 1</figcaption></figure>

magma-hydrothermal interactions, although this interpretation should be considered preliminary given the limited duration of the measurements.

**Compositional gas changes and magma degassing conditions**

As a general rule, gas ratios provide more robust and meaningful information on volcano degassing processes than absolute gas concentrations, the latter being more susceptible to atmospheric dilution and environmental variability (e.g., Aiuppa, et al. 2005; Allard et al. 2014; De Moor et al. 2019). One key observation from our dataset is the ~three-fold lower CO₂/SO₂ ratio of Marapi volcanic gas in 2018

(5.2 ± 4.4) compared to 2014 (17.6 ± 8.5). This difference, independent of instrumental or pressure correction effects, suggests that degassing conditions at Marapi may have evolved between the 2014 and 2018 observations, the latter which preceded two successive explosive events by approximately one month.

Theoretically, a lower CO₂/SO₂ ratio could reflect either a relative decline in CO₂, increased SO₂ degassing, or reduced SO₂ scrubbing by the hydrothermal system. Because hydrothermal scrubbing affects SO₂ much more extensively than the less soluble H₂S (Symonds et al. 2001), the stability of the SO₂/H₂S ratio during both campaigns argues against hydrothermal scrubbing as the main driver. A relative reduction in CO₂ supply cannot be excluded but is less plausible,
<table><caption>**Table 1** Molar ratios and molar composition (%) of Marapi volcanic gases in 2014 and 2018</caption><thead><tr><th></th><th>25/10/14</th><th>28/3/18</th></tr></thead><tbody><tr><td>CO<sub>2</sub>/SO<sub>2</sub></td><td>17.6±8.5</td><td>5.2±4.4</td></tr><tr><td>CO<sub>2</sub>/S<sub>t</sub></td><td>14.0±0.51</td><td>3.6±0.50</td></tr><tr><td>H<sub>2</sub>S/SO<sub>2</sub></td><td>0.3±0.16</td><td>0.4±0.23</td></tr><tr><td>H<sub>2</sub>O/SO<sub>2</sub></td><td>850*</td><td>270±870</td></tr><tr><td>H<sub>2</sub>/SO<sub>2</sub></td><td>0.23±0.22</td><td>n.d</td></tr><tr><td>H<sub>2</sub>O</td><td>97.8</td><td>98±310</td></tr><tr><td>CO<sub>2</sub></td><td>2.0±0.98</td><td>1.8±1.6</td></tr><tr><td>SO<sub>2</sub></td><td>0.12</td><td>0.36</td></tr><tr><td>H<sub>2</sub>S</td><td>0.03±0.02</td><td>0.16±0.08</td></tr><tr><td>H<sub>2</sub></td><td>0.03±0.02</td><td>n.d</td></tr></tbody></table>

*Values marked with an asterisk were derived from relative humidity measurements

Reported gas ratio errors combine the standard error of the regression slope with the scatter of the data, representing the overall uncertainty in the measured gas ratios. Uncertainties were propagated to estimate errors in the derived molar compositions

given that CO₂ exsolves at much greater depth than sulphur (e.g., Baker & Alletti 2012 and references therein), is less reactive than SO₂, and generally shows sustained emission at dormant active volcanoes. Rather, the combined reduction in both CO₂/SO₂ and CO₂/Sₜ ratios in March 2018 dataset, together with a possibly lower H₂O/SO₂ ratio (Table 1), is consistent with a relatively sulphur-enriched gas composition, and implies that Marapi volcanic gases in 2018 originated from either shallower or enhanced shallow magma degassing.

The temperature-pressure source conditions of Marapi volcanic gases in 2014 and 2018 can be estimated using our dataset and thermodynamic data (Barin and Knacke 1976) for the following two equilibrium reactions:

$$SO_2 + 3H_2 = H_2S + 2H_2O \quad (4)$$

$$H_2S + 3/2O_2 = SO_2 + H_2O \quad (5)$$

The equilibrium constants of reactions (4) and (5) vary linearly with temperature (T in K) as: Log(K₁) = 11,200/T—3.9315 and Log(K₂) = 18,093/T—2.7361. Ideal gas behaviour is assumed, i.e., the partial pressure of each species equals its molar fraction in the total gas (Table 1), and pH₂O approximates the total gas pressure (~0.98 Pₜot). For these calculations, we used the mean H₂S/SO₂ ratios measured in 2014 and 2018, the average H₂/H₂O ratio from 2014 (4.1×10⁻⁴), assumed constant over time, and the H₂O molar proportion of 0.98 (Table 1). Reaction (4) yields an apparent gas equilibrium temperature of ~540–550 °C (± 15%) for both dates, given our assumption of constant H₂/H₂O ratio. At such temperatures, the H₂/H₂O ratio combined

with thermodynamic data for the equilibrium reaction H₂O = H₂ + ½O₂ (Barin and Knacke 1976), constrains LogfO₂ = -18.70±0.15, corresponding to a relatively high oxidation state of the gas phase at NNO+2.3 (NNO being the Nickel-Nickel Oxide redox buffer; Huebner and Sato, 1970). Reaction (5) then yields an estimated total gas pressure of only 2.8–3.6 atm, implying gas equilibration at very shallow depths in the Verbeek conduit (~30–40 m under hydrostatic conditions, ~11–14 m under lithostatic conditions, assuming a mean upper edifice density of 2650 kg. m⁻³). Uncertainties in the measured gas ratios propagate, yielding ±42 °C for temperature and ±1.2 bars for pressure. Although non-negligible, these uncertainties do not affect the qualitative conclusion regarding shallow versus deep magma degassing.

The lack of H₂ measurements in 2018, compensated by using the 2014 values, prevents identification of any change in T–P–redox source conditions between the two field campaigns, thereby limiting our ability to determine the mechanism responsible for the relatively sulphur-enriched gas composition in 2018. Shallower magma degassing remains the most likely explanation.

Available geophysical and petrological data (Nurfiani et al. 2021) indicate that the magma feeding system of Marapi includes a deep storage zone at between 15 and 20 km depth, connected to a shallower reservoir at about 5 km beneath the summit, filled with more evolved and more crystal-rich andesitic magma (Nurfiani et al. 2021). Degassing from this shallow reservoir through the volcano conduit beneath Verbeek crater likely sustains volcanic gas release at the surface. This suggests that the recurrence of short-lived explosive events at Marapi without significant seismic precursors, and the differences in volcanic gas composition observed in 2014 and 2018, could be due to subtle temporal variations in the crystallization rate of this magma at relatively constant depth, possibly through second boiling processes. The very low apparent equilibrium pressure of Marapi volcanic gases estimated from our thermodynamic computations points to a late, near-surface equilibration of the volcanic gas phase in the upper volcanic conduit during non-eruptive periods. Altogether, these observations indicate a low-flux degassing system in a shallow magmatic environment.

## Implications for monitoring Marapi Volcano

The findings of this study have important implications for monitoring volcanic activity at Marapi. The differences in gas ratios measured in 2014 and 2018, with the latter observations made approximately one month before two explosive eruptions, suggest that shifts in gas composition—particularly when decoupled from seismic activity—could serve as valuable precursory indicators
<figure><img src="image_11.png" /><figcaption>**Fig. 8** Triangular plots of volcanic gas compositions from Indonesian volcanoes: (a) H₂O/10–CO₂–5S_T and (b) 0.5CO₂–3SO₂–5H₂S. Symbols indicate different Indonesian arcs: Banda Arc (circles), Eastern Sunda Arc (squares), Halmahera Arc (upward triangles), Sangihe Arc (diamonds), Sunda Arc (Java) (downward triangles), and Lesser Sunda Arc (six-pointed stars). Gas compositions from Marapi volcano (2014 and 2018), the only data from Sumatra and</figcaption></figure>

newly reported here, are shown as red five-pointed stars. Other data are compiled from the literature, including Aiuppa et al. (2017). Gas ratios were either taken directly from the source or calculated from reported concentrations; conversely, when only ratios were provided, concentrations were estimated accordingly. For a full list of sources, see Supplementary Material 4 (S4)

of volcanic unrest. Tracking temporal variations in gas ratios—especially CO₂/SO₂ and CO₂/S_T, before and after eruptive events, may help identify shallow magmatic processes or changes in degassing style. Monitoring these ratios over longer timescales could also provide insights into less frequent events such as magma recharge, which are thought to occur at intervals of 4–10 years (Nurfiani et al. 2021).

Given the high recurrence rate of brief explosive events at Marapi and the generally low gas flux between them, establishing permanent Multi-GAS monitoring stations presents practical challenges. Low-cost Multi-GAS instruments offer one potential solution, providing critical pre-eruptive information with minimal logistical constraints. Drone-based Multi-GAS monitoring represents another promising approach for capturing gas compositions during periods of unrest. Unoccupied aerial systems (UAS) have been increasingly employed to monitor hazardous or inaccessible volcanic environments, demonstrating the ability to track gas compositions in near-real time with sufficient precision to detect precursory signals (e.g., Aiuppa et al. 2021; Moussallam et al. 2022). The development of miniaturised, low-cost sensors suitable for airborne deployment further enables high-frequency, spatially resolved measurements (De Moor et al. 2019; Liu et al. 2020). These technologies provide effective alternatives to permanent installations, which are often

impractical at low-emission tropical volcanoes such as Marapi.

## Comparisons with other sunda arc volcanoes

Marapi gas compositions fall within the compositional field defined by other Sunda Arc volcanoes (Fig. 8). The CO₂/SO₂ ratio measured at Marapi in 2018 (5.2 ± 4.4) is comparable to CO₂/SO₂ ratios reported at other Sunda Arc volcanoes, including which Merapi (4.7) and Bromo (4.1) (Aiuppa et al. 2017; Allard 1986; Nadeau et al. 2023). This observation supports the broader pattern that many Indonesian volcanoes exhibit high CO₂/S_T ratios (Bani et al. 2022). Here, we report the first CO₂/S_T data for an active volcano in Sumatra, highlighting Marapi as a low-flux degassing centre with a relatively CO₂-rich signature. Our results thus reinforce the observation that volcanic gas emissions from the Sunda Arc tend to exhibit higher CO₂/S_T ratios than other volcanic arcs, which may reflect both a modest initial sulphur content in the erupted magmas and an enhanced contribution of carbonate-derived CO₂ from either subducted sediments or the crustal basement (Aiuppa et al. 2017).
## Conclusions

We report the first chemical dataset for volcanic gas emissions from Marapi volcano, offering new insights into shallow degassing processes at one of Sumatra's most active yet understudied volcanic systems. Multi-GAS measurements performed in 2014 and 2018 reveal a low-flux, puffing degassing regime, with notable changes in gas composition and degassing periodicity in 2018 relative to 2014. Such a chemical shift, observed just one month before two new explosive events, suggests a transition to shallower magma degassing, consistent with enhanced release of sulphur with respect to carbon dioxide. We therefore highlight the significant potential of geochemical monitoring at Marapi volcano to anticipate shallow-sourced, hazardous explosive events commonly lacking detectable seismic precursors.

Our first gas results from a volcano in Sumatra further extend previous observations of high CO₂/ST ratios in volcanic emissions from other Sunda Arc volcanoes, suggesting a regional signature of CO₂-rich, sulphur-poor degassing systems. Our study also underscores the importance of targeted gas measurements at active but under-monitored volcanic centres. Establishing routine or continuous gas monitoring at Marapi—ideally integrated with geophysical observations—could significantly improve eruption forecasting and hazard mitigation at this volcano. Finally, the baseline gas data presented here for Marapi degassing during non-eruptive stages provide a valuable reference point for future monitoring efforts, which is of special interest in case of the ongoing eruption initiated in December 2023.

**Supplementary Information** The online version contains supplementary material available at https://doi.org/10.1007/s00445-026-01942-4.

**Acknowledgements** This work benefited from collaboration between the Centre for Volcanology and Geological Hazard Mitigation (CVGHM) and the Earth Observatory of Singapore. The standalone Palermo-type Multi-GAS instrument was kindly provided to CVGHM by Alessandro Aiuppa and the University of Palermo, while the USGS-type Multi-GAS instrument and its data acquisition software were supplied by Peter Kelly (USGS).

We extend our gratitude to all individuals involved in field measurements and logistics during both campaigns, including Pak Ahmad Basuki, Bu Mita Marlia, Pak Nur Rokhman, Pak Aziz Yuliawan, Hartanto Prawiro, Suparmo, Ahmad Rifandi, and Teguh Purnomo. We also appreciate the support of the Indonesian Embassy in Singapore.

Special thanks are due to the organisers and participants of the EOS-CVGHM Seismic Workshop held in March 2024 for insightful discussions on Marapi volcano. Finally, we are especially grateful to Christina Widiwijayanti for providing the initial draft of the abstract in Bahasa Indonesia.

We thank Alessandro Aiuppa for guidance regarding overall uncertainties in gas ratio measurements obtained with INGV-type Multi-GAS. We are also grateful to Marie Edmonds and an anonymous reviewer for their thoughtful comments, which improved the quality of this manuscript.

ChatGPT was used to support the writing and editing of this manuscript.

**Author contribution** Data collection for the 2014 field campaign was carried out by HA, UBS, and PB. For the 2018 campaign, data collection was conducted by CB, HA, and PA. CB, PA and HA performed the post-processing and analysis of both datasets, and CB and PA wrote the first and final draft of the manuscript. HA provided the final translation of the abstract. FC and BT contributed to the study's conception and design and, together with PB, provided comments and feedback on subsequent manuscript drafts. All authors reviewed and approved the final manuscript.

**Funding** This research is supported by the Ministry of Education, Singapore, through the projects “Causes, processes, and forecasts of eruptions of open vent volcanoes in SE Asia” and “Integrating Volcano and Earthquake Science and Technology (InVEST) in Southeast Asia.” This work constitutes Earth Observatory of Singapore (EOS) contribution number 662. EOS supported the involvement of PA through a visiting professor grant. The 2014 Marapi field campaign was funded by CVGHM and the Institut de Recherche pour le Développement (IRD) under the JEAI-Commission programme.

**Data availability** All datasets are included as.csv files in the Supplementary Material.

## Declarations

**Ethical approval** Not applicable to this work.

**Conflict of interest** The authors declare no competing interests.

**Open Access** This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/.

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