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Titel |
Diffuse CO2 fluxes from Santiago and Congro volcanic lakes (São Miguel, Azores archipelago) |
VerfasserIn |
César Andrade, José Cruz, Fátima Viveiros, Rafael Branco |
Konferenz |
EGU General Assembly 2017
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Medientyp |
Artikel
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Sprache |
en
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Digitales Dokument |
PDF |
Erschienen |
In: GRA - Volume 19 (2017) |
Datensatznummer |
250141156
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Publikation (Nr.) |
EGU/EGU2017-4632.pdf |
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Zusammenfassung |
Diffuse CO2 degassing occurring in Santiago and Congro lakes, both located in depressions
associated to maars from São Miguel Island (Azores, Portugal), was studied through detailed
flux measurements. Four sampling campaigns were developed between 2013 and 2016 in
each water body, split by the cold and wet seasons.
São Miguel has an area of 744.6 km2, being the largest island of the archipelago. The
geology of the island is dominated by three quiescent central volcanoes (Sete Cidades, Fogo
and Furnas), linked by volcanic fissural zones (Picos and Congro Fissural Volcanic
systems). The oldest volcanic systems of the island are located in its eastern part
(Povoação-Nordeste).
Santiago lake, with a surface area of 0.26 km2 and a depth of 30.5 m, is located inside a maar
crater in the Sete Cidades volcano at an altitude of 355 m. The watershed of the lake has an
area of 0.97 km2 and a surface flow estimated as 1.54x10 m3/a.
A total of 1612 CO2 flux measurements using the accumulation chamber method were made
at Santiago lake, 253 in the first campaign (November 2013), and 462, 475 and 422 in the
three other campaigns, respectively, in April 2014, September 2016 and December 2016. The
total CO2 flux estimated for this lake varies between 0.4 t d−1 and 0.59 t d−1, for the surveys
performed, respectively, in November 2013 and September 2016; higher CO2 outputs of 1.57
and 5.87 t d−1 were calculated for the surveys carried out in April 2014 and December 2016.
These higher CO2 emissions were associated with a period without water column
stratification.
Similarly to Santiago lake, Congro lake is located inside a maar, in the Congro
Fissural Volcanic system, and has a surface area of 0.04 km2 with 18.5 m depth and a
storage of about 2.4x105 m3/a. The lake, located at an altitude of 420 m, is fed
by a watershed with an area of 0.33 km2 and a runoff estimated as about 8x104
m3/a.
In Congro lake a total of 713 CO2 flux measurements were performed during four surveys
from November 2013 to February 2016. The CO2 flux output was estimated as ranging
between 0.06 t d−1 and 0.31 t d−1; the lower CO2 emission occurred in July 2015 and should
reflect the stratification of the water column that prevents the CO2 flux release at the lake
surface.
Considering both volcanic lakes, the mean CO2 emissions, standardized per area, in the cold
season were ∼14.9 t km−2 d−1 and ∼7.1 t km−2 d−1, respectively, for Santiago and Congro
lakes. During summer period, CO2 emissions were lower in both lakes (∼1.9 t km−2 d−1 and
∼4.1 t km−2 d−1 for Santiago and Congro, correspondingly), what is explained by the lake
stratification. Due to the organic processes that occur in the lakes, the CO2 emission is
mostly associated to a biogenic origin, but a volcanic influence cannot be excluded
and further research using carbon isotopic data is crucial to discriminate the CO2
sources.
Key words: volcanic lakes, CO2 flux, maars, São Miguel Island |
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