Melissa Pfeffer

Gradual caldera collapse at Bárdarbunga volcano, Iceland, regulated by lateral magma outflow

Driven to collapse Volcanic eruptions occur frequently, but only rarely are they large enough to cause the top of the mountain to collapse and form a caldera. Gudmundsson et al. used a variety of geophysical tools to monitor the caldera formation that accompanied the 2014 Bárdarbunga volcanic eruption in Iceland. The volcanic edifice became unstable as magma from beneath Bárdarbunga spilled out into the nearby Holuhraun lava field. The timing of the gradual collapse revealed that it is the eruption that drives caldera formation and not the other way around. Science, this issue p. 262 Magma flow from under the Bárdarbunga volcano drove caldera collapse during the 2014 eruption. INTRODUCTION The Bárdarbunga caldera volcano in central Iceland collapsed from August 2014 to February 2015 during the largest eruption in Europe since 1784. An ice-filled subsidence bowl, 110 square kilometers (km2) in area and up to 65 meters (m) deep developed, while magma drained laterally for 48 km along a subterranean path and erupted as a major lava flow northeast of the volcano. Our data provide unprecedented insight into the workings of a collapsing caldera. RATIONALE Collapses of caldera volcanoes are, fortunately, not very frequent, because they are often associated with very large volcanic eruptions. On the other hand, the rarity of caldera collapses limits insight into this major geological hazard. Since the formation of Katmai caldera in 1912, during the 20th century’s largest eruption, only five caldera collapses are known to have occurred before that at Bárdarbunga. We used aircraft-based altimetry, satellite photogrammetry, radar interferometry, ground-based GPS, evolution of seismicity, radio-echo soundings of ice thickness, ice flow modeling, and geobarometry to describe and analyze the evolving subsidence geometry, its underlying cause, the amount of magma erupted, the geometry of the subsurface caldera ring faults, and the moment tensor solutions of the collapse-related earthquakes. RESULTS After initial lateral withdrawal of magma for some days though a magma-filled fracture propagating through Earth’s upper crust, preexisting ring faults under the volcano were reactivated over the period 20 to 24 August, marking the onset of collapse. On 31 August, the eruption started, and it terminated when the collapse stopped, having produced 1.5 km of basaltic lava. The subsidence of the caldera declined with time in a near-exponential manner, in phase with the lava flow rate. The volume of the subsidence bowl was about 1.8 km3. Using radio-echo soundings, we find that the subglacial bedrock surface after the collapse is down-sagged, with no indications of steep fault escarpments. Using geobarometry, we determined the depth of magma reservoir to be ~12 km, and modeling of geodetic observations gives a similar result. High-precision earthquake locations and moment tensor analysis of the remarkable magnitude M5 earthquake series are consistent with steeply dipping ring faults. Statistical analysis of seismicity reveals communication over tens of kilometers between the caldera and the dike. CONCLUSION We conclude that interaction between the pressure exerted by the subsiding reservoir roof and the physical properties of the subsurface flow path explain the gradual near-exponential decline of both the collapse rate and the intensity of the 180-day-long eruption. By combining our various data sets, we show that the onset of collapse was caused by outflow of magma from underneath the caldera when 12 to 20% of the total magma intruded and erupted had flowed from the magma reservoir. However, the continued subsidence was driven by a feedback between the pressure of the piston-like block overlying the reservoir and the 48-km-long magma outflow path. Our data provide better constraints on caldera mechanisms than previously available, demonstrating what caused the onset and how both the roof overburden and the flow path properties regulate the collapse. The Bárdarbunga caldera and the lateral magma flow path to the Holuhraun eruption site. (A) Aerial view of the ice-filled Bárdarbunga caldera on 24 October 2014, view from the north. (B) The effusive eruption in Holuhraun, about 40 km to the northeast of the caldera

Cirrus cloud seeding has potential to cool climate

] Cirrus clouds, thin ice clouds in the upper troposphere,have a net warming effect on Earth’s climate. Consequently,a reduction in cirrus cloud amount or optical thicknesswould cool the climate. Recent research indicates that byseeding cirrus clouds with particles that promote icenucleation, their lifetimes and coverage could be reduced.We have tested this hypothesis in a global climate modelwith a state-of-the-art representation of cirrus clouds and findthat cirrus cloud seeding has the potential to cancel the entirewarming caused by human activity from pre-industrial timesto present day. However, the desired effect is only obtainedfor seeding particle concentrations that lie within an optimalrange. With lower than optimal particle concentrations, aseeding exercise would have no effect. Moreover, a higherthan optimal concentration results in an over-seeding thatcould have the deleterious effect of prolonging cirrus lifetimeand contributing to global warming.

The climatic effects of modifying cirrus clouds in a climate engineering framework

The climatic effects of climate engineering—or geoengineering—via cirrus cloud thinning are examined. Thinner cirrus clouds can allow more outgoing longwave radiation to escape to space, potentially cooling the climate. The cloud properties and climatic effects due to perturbing the ice crystal fall speed are investigated in a set of hemispheric scale sensitivity experiments with the Community Earth System Model. It is found that increasing the ice crystal fall speed, as an analog to cirrus cloud seeding, depletes high-level clouds and reduces the longwave cloud forcing. Deliberate depletion of cirrus clouds increases outgoing longwave radiation, reduces the upper tropospheric water vapor, and cools the climate. Global cirrus cloud thinning gave a net cloud forcing change of −1.55 W m−2 and a global annual mean temperature change of −0.94 K. Though there is negligible change in the global annual mean precipitation (−0.001 mm/d), the spatially nonhomogeneous forcing induces circulation changes and hence remote climate changes. Climate engineering the Southern Hemisphere only results in a northward shift of the Intertropical Convergence Zone and possible Sahelian drought alleviation, while targeting the Northern Hemisphere alone causes a greater cooling. It was found that targeting cirrus clouds everywhere outside of the tropics results in changes to the circulation and precipitation even in the nonclimate engineered regions, underscoring the risks of remote side effects and indeed the complexity of the climate system.

Major impact of volcanic gases on the chemical composition of precipitation in Iceland during the 2014-15 Holuhraun eruption

The Holuhraun eruption in 2014-15 was the largest in Iceland for more than 200 years. It resulted in emissions of large quantities of volcanic gases into the atmosphere (11 Mt SO2, 0.1 Mt HCl and 0.05 Mt HF). During the eruption the volcanic gases had major effects on F, SO4 and to a lesser extent Cl concentrations in precipitation throughout Iceland, effects not observed in recent decades. The concentrations of F, Cl and SO4 (n = 705) reached values of 444 μM, 12,270 μM and 17,324 μM during the eruption and were on average ~20 times higher for F and SO4 and much lower for Cl compared to pre-eruption times. The concentrations of major cations (Si, Na, K, Ca, Mg, Al and Fe) (n = 151) in the precipitation, were taken as having originated from seawater spray and dissolution of rock dust and aerosol. Based on the mixing model developed here, it is demonstrated that the source of the enrichment of F and SO4 was indeed the volcanic gas emissions with >60-100 mol% of SO4 and F in the precipitation originated from volcanic gas, whereas the Cl originated mostly from seawater spray, making the volcanic gas input of Cl relatively less important than for F and SO4. The results showed that large volcanic eruptions can have major effects on atmospheric chemistry and impact the composition of precipitation.

Globally Significant CO2 Emissions From Katla, a Subglacial Volcano in Iceland

Volcanoes are a key natural source of CO2, but global estimates of volcanic CO2 flux are predominantly based on measurements from a fraction of worlds actively degassing volcanoes. We combine high‐precision airborne measurements from 2016 and 2017 with atmospheric dispersion modeling to quantify CO2 emissions from Katla, a major subglacial volcanic caldera in Iceland that last erupted 100 years ago but has been undergoing significant unrest in recent decades. Katlas sustained CO2 flux, 12–24 kt/d, is up to an order of magnitude greater than previous estimates of total CO2 release from Icelands natural sources. Katla is one of the largest volcanic sources of CO2 on the planet, contributing up to 4% of global emissions from nonerupting volcanoes. Further measurements on subglacial volcanoes worldwide are urgently required to establish if Katla is exceptional, or if there is a significant previously unrecognized contribution to global CO2 emissions from natural sources. We combine high‐precision airborne measurements from 2016 and 2017 with atmospheric dispersion modelling to quantify CO2 emissions from Katla, a major subglacial volcanic caldera in Iceland that last erupted 100 years ago but has been undergoing significant unrest in recent decades. Katlas sustained CO2 flux, 12‐24 kt/d, is up to an order of magnitude greater than previous estimates of total CO2 release from Icelands natural sources. Katla is one of the largest volcanic sources of CO2 on the planet, contributing up to 4% of global emissions from non‐erupting volcanoes. Further measurements on subglacial volcanoes world‐wide are urgently required to establish if Katla is exceptional, or if there is a significant previously unrecognized contribution to global CO2 emissions from natural sources.

Sulfur Degassing From Steam-Heated Crater Lakes: El Chichón (Chiapas, Mexico) and Víti (Iceland)

The composition of the gases released by El Chichón (Chiapas, Mexico) and Víti (Askja volcano, Iceland) volcanic lakes is examined by Multi-GAS for the first time. Our results demonstrate that H2S and SO2 are degassed by these pH 2–3 lakes. We find higher CO2/H2S and H2/H2S ratios in the lakes’ emissions (31–5,685 and 0.6–35, respectively) than in the fumarolic gases feeding the lakes (13–33 and 0.08–0.5, respectively), evidencing that only a fraction (0.2–5.4% at El Chichón) of the H2S(g) contributed by the subaquatic fumaroles ultimately reaches the atmosphere. At El Chichón, we estimate a H2S output from the crater lake of 0.02–0.06 t/day. Curiously, SO2 is also detected at trace levels in the gases released from both lakes (0.003–0.3 ppmv). We propose that H2S supplied into the lakes initiates a series of complex oxidation reactions, having sulfite as an intermediate product, and ultimately leading to SO2 production and degassing. Plain Language Summary Volcanic lakes are the site of some of the most unpredictable, and therefore dangerous, volcanic eruptions in nature. Their activity is driven by a feeding volcanic gas phase supplied by the underlying hydrothermal/magmatic system. These volatile species, entering the lake bottom, are absorbed into lake water at different rates/degrees depending on their water solubilities and the lake physical and chemical characteristics. Hyperacidic crater lakes (pH <1) are degassing SO2, a gas that was earlier believed to be totally dissolved into the water. In this study, we investigate for the first time the presence of reactive S gases (SO2 and H2S) in the plumes of less acidic (pH 2–3) lakes El Chichón (Mexico) and Víti (Iceland). Our results demonstrate that H2S, coming from the sublimnic hydrothermal systems is only partially dissolved and oxidized by the lake water. In addition, we discover trace amount of SO2 coming off both lakes. We propose that SO2 is produced into the lake by H2S oxidation, with dissolved sulfite as an intermediate product. Our results thus open new piece of knowledge to our understanding and monitoring the activity of restless volcanic lakes.

Satellite-derived sulphur dioxide (SO 2 ) emissions from the 2014–2015 Holuhraun eruption (Iceland)

Abstract. The six-month-long 2014–2015 Holuhraun eruption was the largest in Iceland for 200 years, emitting huge quantities of sulphur dioxide (SO 2 ) into the troposphere, at times overwhelming European anthropogenic emissions. Weather, terrain and latitude, made continuous ground-based or UV satellite sensor measurements challenging. Infrared Atmospheric Sounding Interferometer (IASI) data, is used to derive the first time-series of daily SO 2 mass and vertical distribution over the eruption period. A new optimal estimation scheme is used to calculate daily SO 2 fluxes and average e-folding time every twelve hours. The algorithm is used to estimate SO 2 fluxes of up to 200 kt per day and a minimum total SO 2 erupted mass of 4.4 ± 0.8 Tg. The average SO 2 e-folding time was 2.4 ± 0.6 days. Where comparisons are possible, these results broadly agree with ground-based near-source measurements, independent remote-sensing data and model simulations of the eruption. The results highlight the importance of high-resolution time-series data to accurately estimate volcanic SO 2 emissions.