Over long periods of time (~Ma), we may consider the oceans, atmosphere and biosphere as a single exospheric reservoir for CO2. The geological carbon cycle describes the inputs to this exosphere from mantle degassing, metamorphism of subducted carbonates and outputs from weathering of aluminosilicate rocks (Walker et al. 1981). A feedback mechanism relates the weathering rate with the amount of CO2 in the atmosphere via the greenhouse effect (e.g., Wang et al. 1976). An increase in atmospheric CO2 concentrations induces higher temperatures, leading to higher rates of weathering, which draw down atmospheric CO2 concentrations (Berner 1991). Atmospheric CO2 concentrations are therefore stabilized over long timescales by this feedback mechanism (Zeebe and Caldeira 2008). This process may have played a role (Feulner et al. 2012) in stabilizing temperatures on Earth while solar radiation steadily increased due to stellar evolution (Bahcall et al. 2001). In this context the role of CO2 degassing from the Earth is clearly fundamental to the stability of the climate, and therefore to life on Earth. Notwithstanding this importance, the flux of CO2 from the Earth is poorly constrained. The uncertainty in our knowledge of this critical input into the geological carbon cycle led Berner and Lagasa (1989) to state that it is the most vexing problem facing us in understanding that cycle.

Notwithstanding the uncertainties in our understanding of CO2 degassing from Earth, it is clear that these natural emissions were recently dwarfed by anthropogenic emissions, which have rapidly increased since industrialization began on a large scale in the 18th century, leading to a rapid increase in atmospheric CO2 concentrations. While atmospheric CO2 concentrations have varied between 190–280 ppm for the last 400,000 years (Zeebe and Caldeira 2008), human activity has produced a remarkable increase …

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Deep Carbon Emissions from Volcanoes

Sr and Pb isotopes exhibit global trends with the concentrations of major elements (SiO 2 , TiO 2 , FeO, Al 2 O 3 and K 2 O) and major elements ratios (CaO/Al 2 O 3 and K 2 O/TiO 2 ) in the shield-stage lavas from 18 oceanic hotspots (including Hawaii, Iceland, Galapagos, Cook-Australs, St. Helena, Cape Verde, Cameroon, Canary, Madeira, Comoros, Azores, Samoa, Society, Marquesas, Mascarene, Kerguelen, Pitcairn, and Selvagen). Based on the relationships between major elements and isotopes in ocean island basalts (OIBs), we find that the lavas derived from the mantle end members, HIMU (or high ‘μ’ =  238 U/ 204 Pb), EM1 (enriched mantle 1), EM2 (enriched mantle 2), and DMM (depleted MORB [mid-ocean ridge basalt] mantle) exhibit distinct major element characteristics: When compared to oceanic hotspots globally, the hotspots with a HIMU (radiogenic Pb-isotopes and low 87 Sr/ 86 Sr) component, such as St. Helena and Cook-Australs, exhibit high CaO/Al 2 O 3 , FeO T , and TiO 2 and low SiO 2 and Al 2 O 3 . EM1 (enriched mantle 1; intermediate 87 Sr/ 86 Sr and low 206 Pb/ 204 Pb; sampled by hotspots like Pitcairn and Kerguelen) and EM2 (enriched mantle 2; high 87 Sr/ 86 Sr and intermediate 206 Pb/ 204 Pb; sampled by hotspots like Samoa and Societies) exhibit higher K 2 O concentrations and K 2 O/TiO 2 weight ratios than HIMU lavas. EM1 lavas exhibit the lowest CaO/Al 2 O 3 in the OIB dataset, and this sets EM1 apart from EM2. A plot of CaO/Al 2 O 3 vs K 2 O/TiO 2 perfectly resolves the four mantle end member lavas. Melting processes (pressure, temperature and degree of melting) fail to provide an explanation for the full spectrum of major element concentrations in OIBs. Such processes also fail to explain the correlations between major elements and radiogenic isotopes. Instead, a long, time integrated history of various parent–daughter elements appears to be coupled to major element and/or volatile heterogeneity in the mantle source. End member lava compositions are compared with experimental partial melt compositions to place constraints on the lithological characteristics of the mantle end members.

Compositions of HIMU, EM1, and EM2 from Global Trends between Radiogenic Isotopes and Major Elements in Ocean Island Basalts

Transfer of CO2 from Earth’s interior to the atmosphere happens largely by volcanic degassing. Measurements of CO2 emissions from faults in the East African Rift system imply that tectonic degassing is also important for deep carbon release. Carbon from Earth’s interior is thought to be released to the atmosphere mostly via degassing of CO2 from active volcanoes1,2,3,4. CO2 can also escape along faults away from active volcanic centres, but such tectonic degassing is poorly constrained1. Here we use measurements of diffuse soil CO2, combined with carbon isotopic analyses to quantify the flux of CO2 through fault systems away from active volcanoes in the East African Rift system. We find that about 4 Mt yr−1 of mantle-derived CO2 is released in the Magadi–Natron Basin, at the border between Kenya and Tanzania. Seismicity at depths of 15–30 km implies that extensional faults in this region may penetrate the lower crust. We therefore suggest that CO2 is transferred from upper-mantle or lower-crustal magma bodies along these deep faults. Extrapolation of our measurements to the entire Eastern rift of the rift system implies a CO2 flux on the order of tens of megatonnes per year, comparable to emissions from the entire mid-ocean ridge system2,3 of 53–97 Mt yr−1. We conclude that widespread continental rifting and super-continent breakup could produce massive, long-term CO2 emissions and contribute to prolonged greenhouse conditions like those of the Cretaceous.

Massive and prolonged deep carbon emissions associated with continental rifting

Volcano surveillance using infrared cameras

Review on biomass feedstocks, pyrolysis mechanism and physicochemical properties of biochar: State-of-the-art framework to speed up vision of circular bioeconomy

The global CO 2 discharge from subaerial volcanism has been estimated at ~300 Mt yr –1 . However, estimates of CO 2 emissions from volcanic lakes have not been considered. In order to improve this information, extensive research on CO 2 emissions of volcanic lakes worldwide has been performed. The observed normalized average CO 2 emission rates increase from alkaline (5.5 t km –2 d –1 ), to neutral (201.2 t km –2 d –1 ), to acid (614.2 t km –2 d –1 ) in volcanic lakes. Taking into account (1) normalized CO 2 emission rates, (2) the number of volcanic lakes in the world (~769), and (3) the fraction and average areas of the investigated alkaline, neutral, and acid volcanic lakes, the estimated global CO 2 emission from volcanic lakes is 117 ± 19 Mt yr –1 , with 94 ± 17 Mt yr –1 as deep-seated CO 2 . This study highlights the importance of a revision of the actual global CO 2 discharge from subaerial volcanism.

Global CO2 emission from volcanic lakes

Diffuse CO2 emission rate from Pululahua and the lake-filled Cuicocha calderas, Ecuador

Significant increases in helium emissions from the soil and 3 He/ 4 He ratios in groundwater on El Hierro Island (Canary Islands, Spain) were observed prior to the 2011–2012 submarine eruption off the coast of the island. The changes of diffusive helium emissions rate were observed one month prior to the submarine eruption onset (12 October 2011) and the major increase preceded increases in seismic energy release during the volcanic unrest. Measured 3 He/ 4 He ratios in groundwaters from a well in El Hierro Island increased from 2–3 R A to 7.2 R A (R A = 3 He/ 4 He ratio in air) 1 month prior to the eruption onset, and reached a peak of 8.2 R A , indicating a dominant magmatic contribution to the dissolved gases in ground waters. 3 He/ 4 He values and diffusive helium emission studies have been extremely important for forecasting the onset of the volcanic unrest and subsequent volcanic eruption. An aseismic exsolution of magmatic gases from magma bodies beneath El Hierro Island through fractures and vertical permeability structures increased the diffusive helium emission rate prior to episodes of seismic energy release associated with the volcanic unrest.

Diffusive helium emissions as a precursory sign of volcanic unrest

Global CO2 Emission from Volcanic Lakes

We report the first detailed study of spatial variations on the diffuse emission of carbon dioxide (CO2) and hydrogen sulfide (H2S) from Hengill volcanic system, Iceland. Soil CO2 and H2S efflux measurements were performed at 752 sampling sites and ranged from nondetectable to 17,666 and 722 g m−2 day−1, respectively. The soil temperature was measured at each sampling site and used to evaluate the heat flow. The chemical composition of soil gases sampled at selected sampling sites during this study shows they result from a mixing process between deep volcanic/hydrothermal component and air. Most of the diffuse CO2 degassing is observed close to areas where active thermal manifestations occur, northeast flank of the Hengill central volcano close to the Nesjavellir power plant, suggesting a diffuse degassing structure with a SSW–NNE trend, overlapping main fissure zone and indicating a structural control of the degassing process. On the other hand, H2S efflux values are in general very low or negligible along the study area, except those observed at the northeast flank of the Hengill central volcano, where anomalously high CO2 efflux and soil temperatures were also measured. The total diffuse CO2 emission estimated for this volcanic system was about 1,526 ± 160 t day−1 of which 453 t day−1 (29.7 %) are of volcanic/hydrothermal origin. To calculate the steam discharge associated with the volcanic/hydrothermal CO2 output, we used the average H2O/CO2 mass ratio from 12 fumarole samples equal to 88.6 (range, 9.4–240.2) as a representative value of the H2O/CO2 mass ratios for Hengill fumarole steam. The resulting estimate of the steam flow associated with the gas flux is equal to 40,154 t day−1. The condensation of this steam results in thermal energy release for Helgill volcanic system of 1.07 × 1014 J day−1 or to a total heat flow of 1,237 MWt.

Eleazar Padrón

Papers

Global CO2 emission from volcanic lakes

Diffuse CO2 emission rate from Pululahua and the lake-filled Cuicocha calderas, Ecuador

Diffusive helium emissions as a precursory sign of volcanic unrest

Global CO2 Emission from Volcanic Lakes

Diffuse volcanic degassing and thermal energy release from Hengill volcanic system, Iceland