Atmosphere injection of sea salts during large explosive submarine volcanic eruptions

Abstract

The 15 January 2022 submarine eruption at Hunga volcano was the most explosive volcanic eruption in 140 years. It involved exceptional magma and seawater interaction throughout the entire submarine caldera collapse. The submarine volcanic jet breached the sea surface and formed a subaerial eruptive plume that transported volcanic ash, gas, sea salts and seawater up to ~ 57 km, reaching into the mesosphere. We document high concentrations of sea salts in tephra (volcanic ash) collected shortly after deposition. We also discuss the potential climatic consequences of large-scale injection of salts into the upper atmosphere during submarine eruptions. Sodium chloride in these volcanic plumes can reach extreme concentrations, and dehalogenation of chlorides and bromides poses the risk of long-term atmospheric and weather impact. Salt content in rapidly collected tephra samples may also be used as a proxy to estimate the water:magma ratio during eruption, with implications for quantification of fragmentation efficiency in submarine breaching events. The balance between salt loading into the atmosphere versus deposition in ash aggregates is a key factor in understanding the atmospheric and climatic consequences of submarine eruptions.

Figure 1

(a) Map showing the sampling location. (b) Average monthly rainfall in Tonga, for the first four-month period of 2022, modified from. The solid line represents the cumulative average rainfall, shadowed area represents the 25^th to 75^th percentile bands. The date on which samples were taken is indicated so the effects of rain on the deposits can be assessed against the results.

Figure 2

SEM analysis combining BSE images and EDX maps of Group 1 (salt-rich) samples from the 15 January 2022 eruption. (a–c) BSE image, Cl and S maps for the sample HT7 in the grain size 250–355 µm, showing that all particles are partially covered by salt phases. Note that Cl and S show a systematic correlation with Na and Ca maps (not shown here), respectively. (d–h) BSE and EDX maps for Ca-SO₄ (e,f) and Na-Cl (g,h) to illustrate the salt coverage in the fine ash rim of a coated coarse ash particle. (i,j) Ca-sulfate cluster in sample HT6, with sulfates colourized in orange in the image (j). (k,l) Cluster of cubic halite crystals (colourized in green in l) and one Ca-SO₄ crystal (in orange in l) in the fine ash rim of a coarse ash particle from sample HT1.

Figure 3

Signal of the evolved gas analysis (EGA) signal for natural samples (colourized curves) and preparations of pure glass, pure salts and sulfides and mix of these (black curves). (a) SO₂ signal during thermal heating at 30 K/min. (b) H₂O signal with gypsum-related peak at low temperature (< 100 °C) and magmatic water peak from the glass once T_g is crossed. Small signal between these two episodes of water loss may be an artefact or some loss of meteoric (non-magmatic) water from the glass. (c) HCl signal during thermal heating at 30 K/min. *Sample HT2 was wet-sieved before analysis and is therefore salt-free.

Figure 4

Comparison of EGA and leaching results. (a–c) Comparison of signal areas for SO₂, HCl and low temperature H₂O from the EGA analysis. (d) Results from ion chromatography with the Cl and SO₄ concentrations. The blue line corresponds to a seawater-ash mixing line. (e,f) Molar concentrations showing the 1:1 stoichiometric relationship (black dashed line) between Na-Cl and Ca-SO₄. Blue circles and red diamonds are Group 1 and Group 2 samples, respectively. The black triangle corresponds to sample HT2 that was wet-sieved before analysis and is hence salt-free. The blue square corresponds to the seawater concentrations.

Figure 5

(a) Proportions of ion concentrations in the leachates. Only ions that are highly concentrated in seawater (values shown as blue dashed line) are included here. (b) Ratio of concentrations of the leachates C_l normalised to seawater concentrations C_sw for the Group 1 samples.

Figure 6

Vertical column density of bromine monoxide (BrO) from the Global Ozone Monitoring Experiment-2 (GOME-2) onboard the Meteorological Operational Satellite-C (Metop-C) for January 14th to 17th 2022.