Modest volcanic SO2 emissions from the Indonesian archipelago

Abstract

Indonesia hosts the largest number of active volcanoes, several of which are renowned for climate-changing historical eruptions. This pedigree might suggest a substantial fraction of global volcanic sulfur emissions from Indonesia and are intrinsically driven by sulfur-rich magmas. However, a paucity of observations has hampered evaluation of these points-many volcanoes have hitherto not been subject to emissions measurements. Here we report new gas measurements from Indonesian volcanoes. The combined SO₂ output amounts to 1.15 ± 0.48 Tg/yr. We estimate an additional time-averaged SO₂ yield of 0.12-0.54 Tg/yr for explosive eruptions, indicating a total SO₂ inventory of 1.27-1.69 Tg/yr for Indonesian. This is comparatively modest-individual volcanoes such as Etna have sustained higher fluxes. To understand this paradox, we compare the geodynamic, petrologic, magma dynamical and shallow magmatic-hydrothermal processes that influence the sulfur transfer to the atmosphere. Results reinforce the idea that sulfur-rich eruptions reflect long-term accumulation of volatiles in the reservoirs.

Fig. 1. Indonesian active volcanoes.

The distribution of the 126 active volcanoes across the archipelago of Indonesia, including 120 aerial and six known submarine edifices (not shown on the map). 77 are classified as Type-A (red triangles), 29 as type-B (yellow squares) and 20 as type-C (green circles) The volcanoes visited in this work are highlighted in red-bold-italic.

Fig. 2. The main volcanic degassing points of Indonesia.

The SO₂ emission rates across the four volcanic arcs of Indonesia highlight the Sunda arc as the largest SO₂ contributor and Dukono is the strongest individual source. The question marks (?) denote the unmeasured sources and the error bars correspond to standard deviation.

Fig. 3. The explosive SO₂ released per volcano over the period of 2010–2020.

The names of the volcanoes that erupted over the decade are grouped by arc. The SO₂ mass per volcano obtained from satellite data are displayed on the left column whilst the right column shows the SO₂ amount obtained from the VEIs. The 0 Tg correspond to undetected eruptive emission by satellite sensors. The color code differentiates the years of observation and the height corresponds to the amount of SO₂ released per year. The number of eruptions per volcano is provided above each SO₂ mass value on the left column. Note that Dukono exhibits a continuous eruptive manifestation but only the largest event with ash fall on the nearby cities are considered.

Fig. 4. The new SO₂ flux results compared to other estimates.

A Estimates of the global volcanic SO₂ inventory that include contributions from Indonesian volcanoes, highlighted by gray square with the corresponding values. B The Indonesian SO₂ emission budget compared with other arcs (data from ref. ). The annual SO₂ emission per km of each arc are shown for comparison. C The SO₂ emission budgets from the four Indonesian arcs. D Strength of passive SO₂ emissions by altitude (in 500-m bins) from the observations reported in Table 3.

Fig. 5. Sulfur content in fluids and melt inclusions versus SO₂ fluxes.

A Variation of the sulfur content of volcanic fluids (XS_Total = XSO₂ + XH₂S) with the measured SO₂ flux along the Indonesian arc. References for fluid compositions are from Allard et al. 1981 (Krakatau); Poorter al. 1989 (Lowotolo); Giggenbach et al. 2001 (Merapi, Tangkuban Parahu, Papandayan); Clor et al. 2005 (Soputan); Aiuppa et al. 2015 (Bromo); Gunawan et al. 2017 (Kawah Ijen); Bani et al. 2017 (Sirung); Bani et al. 2017 (Dukono); Saing et al. 2020 (Gamkonora); Kunrat et al. 2020 (Gamalama); Bani et al. 2020 (Awu). B The record of sulfur in melt inclusions along the Indonesian arc in respect to the main degassing sources. C Relationships between the maximum sulfur content analysed in melt inclusions (MI) and the measured SO₂ flux. Note, we did not find melt inclusion values for data points with zero SO₂ flux and conversely. References for MI are: Vidal et al., 2016 (Rinjani); Mandeville et al., 1996 (Krakatau); Bani et al., 2017 (Dukono); Preece et al., 2014 (Merapi); de Hoog et al. 2001 (Guntur, Ili Boleng); Vigouroux et al. 2012 (Kawah Ijen, Galunggung, Tambora); Self and King, 1996 (Agung); Self et al. 2004 (Tambora), Kunrat, 2017 (Soputan).