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Gas Seeps at the Edge of the Gas Hydrate Stability Zone on Brazil’s Continental Margin

1
Department of Biology and Environmental Science, Linnaeus University, 391-82 Kalmar, Sweden
2
Institute of Petroleum and Natural Resources, PUCRS—Pontifícia Universidade Católica do Rio Grande do Sul, Porto Alegre 90619-900, Brazil
3
LAGEMAR, Department of Geology, Universidade Federal Fluminense (UFF), Niteroi 24210-346, Brazil
4
Géoazur, UMR7329 CNRS, rue Albert Einstein 250, 06560 Valbonne, France
5
Centro de Estudos de Geologia Costeira e Oceânica, Instituto de Geociências, Universidade Federal do Rio Grande do Sul, Porto Alegre 91501-970, Brazil
6
PETROBRAS—E&P Exploration, Rio de Janeiro 20031-170, Brazil
*
Author to whom correspondence should be addressed.
Geosciences 2019, 9(5), 193; https://doi.org/10.3390/geosciences9050193
Received: 6 March 2019 / Revised: 4 April 2019 / Accepted: 8 April 2019 / Published: 28 April 2019
(This article belongs to the Special Issue Gas Hydrate: Environmental and Climate Impacts)
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Abstract

Gas hydrate provinces occur in two sedimentary basins along Brazil’s continental margin: (1) The Rio Grande Cone in the southeast, and (2) the Amazon deep-sea fan in the equatorial region. The occurrence of gas hydrates in these depocenters was first detected geophysically and has recently been proven by seafloor sampling of gas vents, detected as water column acoustic anomalies rising from seafloor depressions (pockmarks) and/or mounds, many associated with seafloor faults formed by the gravitational collapse of both depocenters. The gas vents include typical features of cold seep systems, including shallow sulphate reduction depths (<4 m), authigenic carbonate pavements, and chemosynthetic ecosystems. In both areas, gas sampled in hydrate and in sediments is dominantly formed by biogenic methane. Calculation of the methane hydrate stability zone for water temperatures in the two areas shows that gas vents occur along its feather edge (water depths between 510 and 760 m in the Rio Grande Cone and between 500 and 670 m in the Amazon deep-sea fan), but also in deeper waters within the stability zone. Gas venting along the feather edge of the stability zone could reflect gas hydrate dissociation and release to the oceans, as inferred on other continental margins, or upward fluid flow through the stability zone facilitated by tectonic structures recording the gravitational collapse of both depocenters. The potential quantity of venting gas on the Brazilian margin under different scenarios of natural or anthropogenic change requires further investigation. The studied areas provide natural laboratories where these critical processes can be analyzed and quantified. View Full-Text
Keywords: gas hydrates; gas seeps; ocean acidification gas hydrates; gas seeps; ocean acidification
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Ketzer, M.; Praeg, D.; Pivel, M.A.; Augustin, A.H.; Rodrigues, L.F.; Viana, A.R.; Cupertino, J.A. Gas Seeps at the Edge of the Gas Hydrate Stability Zone on Brazil’s Continental Margin. Geosciences 2019, 9, 193.

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