Deep carbon emissions from historically inactive volcanoes, hydrothermal, and tectonic structures are among the greatest unknowns in the long‐term (∼Myr) carbon cycle. Recent estimates of diffuse CO2 ...flux from the Eastern Rift of the East African Rift System (EARS) suggest this could equal emissions from the entire mid‐ocean ridge system. We report new CO2 surveys from the Main Ethiopian Rift (MER, northernmost EARS), and reassess the rift‐related CO2 flux. Since degassing in the MER is concentrated in discrete areas of volcanic and off‐edifice activity, characterization of such areas is important for extrapolation to a rift‐scale budget. Locations of hot springs and fumaroles along the rift show numerous geothermal areas away from volcanic edifices. With these new data, we estimate total CO2 emissions from the central and northern MER as 0.52–4.36 Mt yr−1. Our extrapolated flux from the Eastern Rift is 3.9–32.7 Mt yr−1 CO2, overlapping with lower end of the range presented in recent estimates. By scaling, we suggest that 6–18 Mt yr−1 CO2 flux can be accounted for by magmatic extension, which implies an important role for volatile‐enriched lithosphere, crustal assimilation, and/or additional magmatic intrusion to account for the upper range of flux estimates. Our results also have implications for the nature of volcanism in the MER. Many geothermal areas are found >10 km from the nearest volcanic center, suggesting ongoing hazards associated with regional volcanism.
Plain Language Summary
The amount of carbon dioxide seeping out of the Earth's surface is poorly understood. Magma carries dissolved carbon dioxide from the deep earth toward the surface, where it is released and travels along fractures in the crust. This study attempts to quantify this phenomenon in central Ethiopia, where a continental rift is splitting one tectonic plate in two. In Kenya and Tanzania, a similar study suggested that the flow of carbon dioxide through the East African Rift was much larger than previously thought. We undertook new surveys and found that it varies greatly, which makes estimating a total flow through the rift very difficult. The distribution of hot springs and volcanic vents provides clues concerning where heat and carbon dioxide come to the surface—by compiling the locations of these features we were able to extrapolate from our surveys for a new estimate. Our results suggest that the East African Rift releases less carbon dioxide than was thought from Kenya and Tanzania, but still a substantial amount. If the rift does emit as much carbon dioxide as suggested, either more carbon is below the crust in East Africa than we thought or more magma is involved.
Key Points
New gas surveys show that diffuse CO2 degassing in the Main Ethiopian Rift is highly variable, concentrated along faults in geothermal areas
Locations of hot springs and fumaroles indicate numerous (∼14–20) geothermal areas some distance from major volcanic edifices
Extrapolation from our data suggests a total diffuse CO2 flux from the Main Ethiopian Rift of 0.5–4.4 Mt yr−1 (whole Eastern Rift: 4–33 Mt yr−1)
The ∼3.48 billion-year-old Dresser Formation, Pilbara Craton, Western Australia, is a key geological unit for the study of Earth's earliest life and the habitats it occupied. Here, we describe a new ...suite of spheroidal to lenticular microstructures that morphologically resemble some previously reported Archean microfossils. Correlative microscopy shows that these objects have a size distribution, wall ultrastructure, and chemistry that are incompatible with a microfossil origin and instead are interpreted as pyritized and silicified fragments of vesicular volcanic glass. Organic kerogenous material is associated with much of the altered volcanic glass; variable quantities of organic carbon line or fill the insides of some individual vesicles, while relatively large, tufted organic-rich laminae envelop multiple vesicles. The microstructures reported herein constitute a new type of abiogenic artifact (pseudo-fossil) that must be considered when evaluating potential signs of early life on Earth or elsewhere. In the sample studied here, where hundreds of these microstructures are present, the combined evidence permits a relatively straightforward interpretation as vesicular volcanic glass. However, reworked, isolated, and silicified microstructures of this type may prove particularly problematic in early or extraterrestrial life studies since they adsorb carbon onto their surfaces and are readily pyritized, mimicking a common preservation mechanism for bona fide microfossils. In those cases, nanoscale analysis of wall ultrastructure would be required to firmly exclude a biological origin. Key Words: Microfossils-Pseudo-fossils-Volcanic vesicles-Archean life-Pilbara Craton-Dresser Formation. Astrobiology 18, 539-555.
Several lines of evidence have previously been used to suggest that ice retreat after the last glacial maximum (LGM) resulted in regionally-increased levels of volcanic activity. It has been proposed ...that this increase in volcanism was globally significant, forming a substantial component of the post-glacial rise in atmospheric CO2, and thereby contributing to climatic warming. However, as yet there has been no detailed investigation of activity in glaciated volcanic arcs following the LGM. Arc volcanism accounts for 90% of present-day subaerial volcanic eruptions. It is therefore important to constrain the impact of deglaciation on arc volcanoes, to understand fully the nature and magnitude of global-scale relationships between volcanism and glaciation.
The first part of this paper examines the post-glacial explosive eruption history of the Andean southern volcanic zone (SVZ), a typical arc system, with additional data from the Kamchatka and Cascade arcs. In all cases, eruption rates in the early post-glacial period do not exceed those at later times at a statistically significant level. In part, the recognition and quantification of what may be small (i.e. less than a factor of two) increases in eruption rate is hindered by the size of our datasets. These datasets are limited to eruptions larger than 0.1km3, because deviations from power-law magnitude–frequency relationships indicate strong relative under-sampling at smaller eruption volumes. In the southern SVZ, where ice unloading was greatest, eruption frequency in the early post-glacial period is approximately twice that of the mid post-glacial period (although frequency increases again in the late post-glacial). A comparable pattern occurs in Kamchatka, but is not observed in the Cascade arc. The early post-glacial period also coincides with a small number of very large explosive eruptions from the most active volcanoes in the southern and central SVZ, consistent with enhanced ponding of magma during glaciation and release upon deglaciation.
In comparison to non-arc settings, evidence of post-glacial increases in rates of arc volcanism is weak, and there is no need to invoke significantly increased melt production upon ice unloading, as occurred in areas such as Iceland. Non-arc volcanoes may therefore account for a relatively higher proportion of global volcanic emissions in the early post-glacial period than is suggested by the relative contributions of arc and non-arc settings at the present day.
The second part of this paper critically examines global eruption records, in an effort to constrain global-scale changes in volcanic output since the LGM. Accurate interpretation of these records relies on correcting both temporal and spatial variability in eruption recording. In particular, very low recording rates, which also vary spatially by over two orders of magnitude, prevent precise, and possibly even accurate, quantitative analysis. For example, if we assume record completeness for the past century, the number of known eruptions (volcanic explosivity index≥2) from some low-latitude regions, such as Indonesia, is approximately 1 in 20,000 (0.005%) for the period 5–20ka. There is a need for more regional-scale studies of past volcanism in such regions, where current data are extremely sparse. We attempt to correct for recording biases, and suggest a maximum two-fold (but potentially much less) increase in global eruption rates, relative to the present day, between 13 and 7ka. Although volcanism may have been an important source of CO2 in the early Holocene, it is unlikely to have been a dominant control on changes in atmospheric CO2 after the LGM.
Mercury is a highly volatile, bioaccumulating toxic trace metal with a long (∼1
yr) atmospheric residence time. Hg is strongly enriched in volcanic emanations, and volcanoes are the only natural ...sources of direct Hg emission to the free troposphere and stratosphere. However, there is considerable uncertainty over the annual emission rate of mercury from volcanoes. Previous estimates, based on limited measurements from volcanic plumes, span three orders of magnitude (∼10
0–10
3
Mg
Hg/yr), or from <1% to ∼50% of total natural Hg emissions.
Here we critically evaluate published data from volcanic plumes, and combine this with information from natural archives to show unequivocally the significance of volcanoes for the global biogeochemical mercury cycle. ‘Low’ global volcanic flux estimates (<∼50
Mg/yr) are based on the inappropriate extrapolation of data from low-temperature fumarolic degassing at non-erupting volcanoes to the high-temperature emissions from active volcanoes. Based on data from active volcanoes, we estimate that the time-averaged volcanic Hg emission is ∼700
Mg/yr, or 20–40% of total natural emissions. Continuous degassing accounts for only ∼10% of this flux, while 75% of volcanic Hg is released during ‘smaller’ sporadic eruptions (<10–10
2
Mg/event). Rare, large (>10
3
Mg) explosive eruptions overwhelm the total atmospheric burden several times per century, and account for ∼15% of total volcanic Hg emissions.
Restless silicic calderas present major geological hazards, and yet many also host significant untapped geothermal resources. In East Africa, this poses a major challenge, although the calderas are ...largely unmonitored their geothermal resources could provide substantial economic benefits to the region. Understanding what causes unrest at these volcanoes is vital for weighing up the opportunities against the potential risks. Here we bring together new field and remote sensing observations to evaluate causes of ground deformation at Aluto, a restless silicic volcano located in the Main Ethiopian Rift (MER). Interferometric Synthetic Aperture Radar (InSAR) data reveal the temporal and spatial characteristics of a ground deformation episode that took place between 2008 and 2010. Deformation time series reveal pulses of accelerating uplift that transition to gradual long‐term subsidence, and analytical models support inflation source depths of ∼5 km. Gases escaping along the major fault zone of Aluto show high CO2 flux, and a clear magmatic carbon signature (CO2‐δ13C of −4.2‰ to −4.5‰). This provides compelling evidence that the magmatic and hydrothermal reservoirs of the complex are physically connected. We suggest that a coupled magmatic‐hydrothermal system can explain the uplift‐subsidence signals. We hypothesize that magmatic fluid injection and/or intrusion in the cap of the magmatic reservoir drives edifice‐wide inflation while subsequent deflation is related to magmatic degassing and depressurization of the hydrothermal system. These new constraints on the plumbing of Aluto yield important insights into the behavior of rift volcanic systems and will be crucial for interpreting future patterns of unrest.
Key Points
New satellite radar interferometry identifies accelerating uplift pulses at Aluto volcano, Ethiopia
Deformation source models suggest inflation events are linked to magmatic or fluid intrusion at 5 km
CO2‐δ13C reveals connected magmatic‐hydrothermal system that could account for subsequent deflation
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