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Contributo para o estudo das emanações gasosas permanentes de CO2 e 222Rn no flanco sul do Vulcão do Fogo (São Miguel, Açores)
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Abstract(s)
Durante as erupções vulcânicas são libertadas normalmente grandes quantidades de voláteis para a atmosfera, contudo, mesmo durante os períodos de dormência, os vulcões podem emitir gases quer de modo visível (e.g., fumarolas), quer de forma impercetível, através da denominada desgaseificação difusa. A desgaseificação difusa através dos solos ocorre, normalmente, associada à presença de zonas mais permeáveis (e.g., falhas), sendo o dióxido de carbono (CO2) e o radão (222Rn) os principais gases emitidos.
Presentemente, o vulcanismo secundário do Vulcão do Fogo (ilha de São Miguel), concentra-se sobretudo no seu flanco norte, sendo caracterizado pela presença de três campos fumarólicos principais (Caldeira Velha, Caldeiras da Ribeira Grande e Pico Vermelho), de nascentes termais e de água fria gasocarbónica, e de áreas de desgaseificação difusa de CO2 (Pico Vermelho, Ribeira Seca e Caldeiras da Ribeira Grande). As emissões submarinas na zona da Ladeira da Velha, junto ao Porto Formoso, também estão associadas ao flanco norte do vulcão.
A realização da cartografia de desgaseificação difusa no flanco sul do Vulcão do Fogo foi efetuada entre novembro de 2022 e dezembro de 2023. Os valores de fluxo de CO2, medidos com recurso ao método da câmara de acumulação, oscilaram entre 0,9 e 1060,5 g m-2 d-1 nas 953 medições realizadas. A aplicação da análise estatística gráfica (GSA) aos dados permitiu reconhecer a presença de duas populações, e permitiu também determinar um ruído de fundo de aproximadamente 32 g m-2 d-1.
A discriminação de diferentes origens para o CO2 emitido foi complementada com o recurso à análise da composição isotópica do 13C de 24 amostras de gás (efluxo, δ13CCO2). Os valores de δ13CCO2 variaram entre -32,46 ‰ e -6,35 ‰ vs. PDB (Pee Dee Belemnite), confirmando a existência de origens biogénica e vulcânica/hidrotermal para os fluxos de CO2 medidos nas diferentes áreas amostradas. Esta análise permitiu observar variações na composição isotópica do 13C dos fluxos biogénicos, que poderão estar eventualmente associadas à distinta tipologia de plantas (C3 ou C4), para além de associar alguns fluxos de CO2 mais elevados (> 50 g m-2 d-1) a uma origem biogénica, o que não teria sido detetado apenas com a GSA. Assim, identificaram-se cinco zonas com valores de fluxo de CO2 acima do ruído de fundo definido (∼ 32 g m-2 d-1). Destas, apenas para a Praia da Pedreira a análise isotópica permitiu confirmar a anomalia detetada como indubitavelmente associada a uma origem profunda, e foi possível correlacioná-la com as estruturas tectónicas de direção geral N-S cartografadas previamente na zona.
A análise espacial dos dados de fluxo de CO2 foi efetuada através quer da simulação sequencial Gaussiana (sGs) para a Praia da Pedreira, quer do método de interpolação determinístico IDW (Inverse Distance Weight) para as restantes áreas. O fluxo total de CO2 emitido pela Praia da Pedreira (área de ∼ 4350 m2) foi de 0,23 t d-1, equivalente a uma taxa de emissão de 53 t km-2 d-1. Considerando o valor de limite biogénico de 32 g m-2 d-1, a emissão hidrotermal na Praia da Pedreira foi estimada em 0,079 t d-1 (cerca de 34 % da emissão total), correspondendo a uma área de cerca de 1437 m2 (∼ 55 t km-2 d-1). O fluxo total estimado no presente estudo para o flanco sul do Vulcão do Fogo (área ∼ 700000 m2), incluindo a Praia da Pedreira (0,23 t d-1) e restantes áreas (11,5 t d-1), foi de aproximadamente 11,7 t d-1 (∼ 18 t km-2 d-1).
Os valores de concentração de 222Rn no solo oscilaram entre 1775 e 51350 Bq m-3 nas 27 medições efetuadas ao longo de perfis perpendiculares a algumas estruturas tectónicas previamente identificadas na área de estudo. A aplicação da análise estatística gráfica (GSA) aos dados log-transformados permitiu identificar um valor de ruído de fundo de 8000 Bq m-3 e reconhecer a presença de duas populações, indiciando a presença de diferentes tipos de transporte para o 222Rn e/ou de permeabilidades distintas. A população com valores de concentração de 222Rn no solo mais baixos foi associada à libertação deste gás de rochas e/ou do solo essencialmente por difusão. A população com valores mais elevados foi relacionada com o provável transporte deste gás por advecção, associado à presença de zonas de maior permeabilidade como, por exemplo, estruturas tectónicas. A temperatura do solo a cerca de 15 cm de profundidade foi medida concomitantemente com as campanhas de desgaseificação de CO2 e de 222Rn. Os valores de temperatura no solo nos 980 pontos amostrados variaram entre os 10 °C e os 37,5 °C. A temperatura do ar, por sua vez, oscilou entre os 12, 8 °C e os 33 °C. A seleção de um valor de temperatura do solo que permita identificar eventuais anomalias térmicas revelou-se desafiante, uma vez que, as campanhas de amostragem foram realizadas a diferentes cotas, em diferentes épocas do ano (verão e inverno), e em zonas com exposição solar e cobertura vegetal distintas. Assim, não foi possível identificar de forma clara a presença de anomalias térmicas em nenhuma das áreas amostradas.
Este trabalho permitiu, ainda, a identificação de quatro áreas com emissões submarinas localizadas no chamado “Mar Morto”, na Ponto do Rossio Branco em Água de Alto, e a caracterização, pela primeira vez, da composição química de uma destas emissões. No que se refere à composição dos gases emitidos, o CO2 é o gás dominante (> 98,5 %) e os restantes gases detetados foram o Ar, o O2, o N2, o CH4 e o He. A localização destas emissões foi correlacionada com as áreas de desgaseificação difusa subaéreas identificadas na Praia da Pedreira, bem como com as falhas normais de orientação geral N-S previamente cartografadas.
A realização deste trabalho permitiu reconhecer a presença de fenómenos de desgaseificação no flanco sul do Vulcão do Fogo, e associá-los a algumas das possíveis estruturas tectónicas previamente cartografadas. A identificação e caracterização das áreas subaéreas com emissões gasosas permanentes de CO2 e de 222Rn do solo, bem como das emissões submarinas, são fundamentais para a monitorização sismovulcânica deste vulcão ativo, assim como para a cartografia de riscos geológicos.
ABSTRACT: During volcanic eruptions, large quantities of volatiles are normally released into the atmosphere. However, even during quiescent periods, volcanoes can emit gases either visibly (e.g., fumaroles) or imperceptibly, through the so-known diffuse degassing. Soil diffuse degassing is usually associated with permeable zones (e.g., faults), being carbon dioxide (CO2) and radon (222Rn) the main gases emitted. Nowadays, the secondary manifestations of volcanism at Fogo Volcano (São Miguel Island) are mainly concentrated on its northern flank, and comprise three main fumarolic fields (Caldeira Velha, Caldeiras da Ribeira Grande, and Pico Vermelho), thermal and cold CO2-rich springs, as well as CO2 diffuse degassing areas (Pico Vermelho, Ribeira Seca and Caldeiras da Ribeira Grande sites). Submarine emissions in the Ladeira da Velha area, near Porto Formoso, are also associated with the volcano's northern flank. Diffuse degassing surveys carried out on the southern flank of Fogo Volcano between November 2022 and December 2023 comprised 953 measurements. Soil CO2 flux values, measured using the accumulation chamber method, ranged between 0.9 and 1060.5 g m-2 d-1. Based on a Graphical Statistical Approach (GSA) two populations were identified, and a background value of approximately 32 g m-2 d-1 was determined. The discrimination of different origins for the CO2 released was complemented with the carbon isotopic composition of 24 soil CO2 effluxes (δ13CCO2). The δ13CCO2 values varied between -32.46 ‰ and -6.35 ‰ vs. PDB (Pee Dee Belemnite), confirming the existence of biogenic and volcanic/hydrothermal sources for the soil CO2 fluxes measured in the different sampled areas. Variations in the carbon isotopic composition associated with biogenic CO2 fluxes were potentially attributed to the different types of plants (C3 or C4). Carbon isotopes also showed a biogenic origin for high soil CO2 fluxes (> 50 g m-2 d-1), which would not have been detected using only the GSA. Five zones were identified with soil CO2 flux values above the background ( 32 g m-2 d-1). Of these, only for the Praia da Pedreira (Pedreira Beach), a deep-seated origin for the CO2 was undoubtedly attributed. This anomaly was correlated with the tectonic structures with a general N-S direction previously mapped in the area. Spatial analysis of the CO2 flux data was carried out using both sequential Gaussian simulation (sGs), for Praia da Pedreira, and the deterministic interpolation IDW (Inverse Distance Weight) for the other areas. The total CO2 flux emitted by Praia da Pedreira (area 4350 m2) was 0.23 t d-1, equivalent to an emission rate of 53 t km-2 d-1. Considering the biogenic threshold of 32 g m-2 d-1, the hydrothermal emission at Praia da Pedreira was estimated as 0.079 t d-1 (around 34 % of the total emission), corresponding to an area of around 1437 m2 ( 55 t km-2 d-1). The total CO2 output estimated in this study for the southern flank of Fogo Volcano (area 700000 m2), including Praia da Pedreira (0.23 t d-1) and the remaining areas (11.5 t d-1), was approximately 11.7 t d-1 ( 18 t km-2 d-1). Soil 222Rn concentrations ranged from 1775 to 51350 Bq m-3 in the 27 measurements taken along profiles perpendicular to some tectonic structures previously identified in the study area. By applying the Graphical Statistical Approach (GSA) to the log-transformed data, it was possible to identify a background value of 8000 Bq m-3 and recognize the presence of two populations, indicating the presence of different types of transport for 222Rn and/or different permeabilities. The population with lower soil 222Rn concentration values was associated with the diffusion of this gas from rocks and/or soils. The population with higher radon concentrations was related to the possible transport of this gas by advection, associated with the presence of zones with greater permeability, such as tectonic structures. The soil temperature at a depth of around 15 cm was measured at the same time as the CO2 and 222Rn degassing campaigns. Soil temperature values at the 980 sampled points ranged from 10 °C to 37.5 °C. On the other hand, the air temperature ranged from 12.8 °C to 33 °C. Selecting a soil temperature value as threshold for the identification of thermal anomalies was challenging, since the sampling campaigns were carried out at different elevations, at different times of the year (summer and winter), and in areas with different sun exposure and vegetation cover. It was therefore not possible to identify the presence of thermal anomalies in any of the sampled areas. This work also identified four areas with submarine gas emissions located in the so-called "Dead Sea", at Ponto do Rossio Branco in Água de Alto parish. Chemical composition of one of these emissions was obtained for the first time. Regarding the composition of the gases emitted, CO2 is the dominant gas (> 98.5 %) and the other gases detected were Ar, O2, N2, CH4 and He. The location of these emissions was correlated with the areas of diffuse subaerial degassing identified at Praia da Pedreira, as well as with the previously mapped normal faults with a general N-S orientation. This study allowed to recognize the presence of degassing phenomena on the southern flank of the Fogo Volcano and to associate them with some of the possible tectonic structures previously mapped. The identification and characterization of subaerial areas with permanent soil gas emissions for CO2 and 222Rn, as well as submarine emissions, are fundamental for seismovolcanic monitoring of this active volcano, as well as for mapping geological risks.
ABSTRACT: During volcanic eruptions, large quantities of volatiles are normally released into the atmosphere. However, even during quiescent periods, volcanoes can emit gases either visibly (e.g., fumaroles) or imperceptibly, through the so-known diffuse degassing. Soil diffuse degassing is usually associated with permeable zones (e.g., faults), being carbon dioxide (CO2) and radon (222Rn) the main gases emitted. Nowadays, the secondary manifestations of volcanism at Fogo Volcano (São Miguel Island) are mainly concentrated on its northern flank, and comprise three main fumarolic fields (Caldeira Velha, Caldeiras da Ribeira Grande, and Pico Vermelho), thermal and cold CO2-rich springs, as well as CO2 diffuse degassing areas (Pico Vermelho, Ribeira Seca and Caldeiras da Ribeira Grande sites). Submarine emissions in the Ladeira da Velha area, near Porto Formoso, are also associated with the volcano's northern flank. Diffuse degassing surveys carried out on the southern flank of Fogo Volcano between November 2022 and December 2023 comprised 953 measurements. Soil CO2 flux values, measured using the accumulation chamber method, ranged between 0.9 and 1060.5 g m-2 d-1. Based on a Graphical Statistical Approach (GSA) two populations were identified, and a background value of approximately 32 g m-2 d-1 was determined. The discrimination of different origins for the CO2 released was complemented with the carbon isotopic composition of 24 soil CO2 effluxes (δ13CCO2). The δ13CCO2 values varied between -32.46 ‰ and -6.35 ‰ vs. PDB (Pee Dee Belemnite), confirming the existence of biogenic and volcanic/hydrothermal sources for the soil CO2 fluxes measured in the different sampled areas. Variations in the carbon isotopic composition associated with biogenic CO2 fluxes were potentially attributed to the different types of plants (C3 or C4). Carbon isotopes also showed a biogenic origin for high soil CO2 fluxes (> 50 g m-2 d-1), which would not have been detected using only the GSA. Five zones were identified with soil CO2 flux values above the background ( 32 g m-2 d-1). Of these, only for the Praia da Pedreira (Pedreira Beach), a deep-seated origin for the CO2 was undoubtedly attributed. This anomaly was correlated with the tectonic structures with a general N-S direction previously mapped in the area. Spatial analysis of the CO2 flux data was carried out using both sequential Gaussian simulation (sGs), for Praia da Pedreira, and the deterministic interpolation IDW (Inverse Distance Weight) for the other areas. The total CO2 flux emitted by Praia da Pedreira (area 4350 m2) was 0.23 t d-1, equivalent to an emission rate of 53 t km-2 d-1. Considering the biogenic threshold of 32 g m-2 d-1, the hydrothermal emission at Praia da Pedreira was estimated as 0.079 t d-1 (around 34 % of the total emission), corresponding to an area of around 1437 m2 ( 55 t km-2 d-1). The total CO2 output estimated in this study for the southern flank of Fogo Volcano (area 700000 m2), including Praia da Pedreira (0.23 t d-1) and the remaining areas (11.5 t d-1), was approximately 11.7 t d-1 ( 18 t km-2 d-1). Soil 222Rn concentrations ranged from 1775 to 51350 Bq m-3 in the 27 measurements taken along profiles perpendicular to some tectonic structures previously identified in the study area. By applying the Graphical Statistical Approach (GSA) to the log-transformed data, it was possible to identify a background value of 8000 Bq m-3 and recognize the presence of two populations, indicating the presence of different types of transport for 222Rn and/or different permeabilities. The population with lower soil 222Rn concentration values was associated with the diffusion of this gas from rocks and/or soils. The population with higher radon concentrations was related to the possible transport of this gas by advection, associated with the presence of zones with greater permeability, such as tectonic structures. The soil temperature at a depth of around 15 cm was measured at the same time as the CO2 and 222Rn degassing campaigns. Soil temperature values at the 980 sampled points ranged from 10 °C to 37.5 °C. On the other hand, the air temperature ranged from 12.8 °C to 33 °C. Selecting a soil temperature value as threshold for the identification of thermal anomalies was challenging, since the sampling campaigns were carried out at different elevations, at different times of the year (summer and winter), and in areas with different sun exposure and vegetation cover. It was therefore not possible to identify the presence of thermal anomalies in any of the sampled areas. This work also identified four areas with submarine gas emissions located in the so-called "Dead Sea", at Ponto do Rossio Branco in Água de Alto parish. Chemical composition of one of these emissions was obtained for the first time. Regarding the composition of the gases emitted, CO2 is the dominant gas (> 98.5 %) and the other gases detected were Ar, O2, N2, CH4 and He. The location of these emissions was correlated with the areas of diffuse subaerial degassing identified at Praia da Pedreira, as well as with the previously mapped normal faults with a general N-S orientation. This study allowed to recognize the presence of degassing phenomena on the southern flank of the Fogo Volcano and to associate them with some of the possible tectonic structures previously mapped. The identification and characterization of subaerial areas with permanent soil gas emissions for CO2 and 222Rn, as well as submarine emissions, are fundamental for seismovolcanic monitoring of this active volcano, as well as for mapping geological risks.
Description
Dissertação de Mestrado, Vulcanologia e Riscos Geológicos, 14 de novembro de 2024, Universidade dos Açores.
Keywords
Vulcão do Fogo Gases Vulcânicos Desgaseificação Difusa Dióxido de Carbono Radão Isótopos de Carbono
Citation
Pimentel, João Paulo Pacheco. (2024). "Contributo para o estudo das emanações gasosas permanentes de CO2 e 222Rn no flanco sul do Vulcão do Fogo (São Miguel, Açores)". 174 p. (Dissertação de Mestrado em Vulcanologia e Riscos Geológicos). Ponta Delgada: Universidade dos Açores, 2024. Disponível em http://hdl.handle.net/10400.3/7262