Molecular and Isotopic Composition of Hydrate-Bound, Dissolved and Free Gases in the Amazon Deep-Sea Fan and Slope Sediments, Brazil

Rodrigues, Luiz Frederico; Ketzer, João Marcelo Medina; Oliveira, Rafael Rodrigues de; Santos, Victor Hugo Jacks Mendes dos; Augustin, Adolpho Herbert; Cupertino, José; Viana, Adriano R.; Leonel, Bruno; Dorle, Wilhelm

doi:10.3390/geosciences9020073

Cited by 7 publications

(9 citation statements)

References 65 publications

(118 reference statements)

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“…The latter features include 23 water column gas plumes that rise up to 350 m into the water column from seafloor mounds 10-20 m high [47]. The gas bubble plumes were not sampled, but 24 dissolved and free gas and three gas hydrate samples in piston cores at plume sites revealed a dominantly methane composition (with the absence of heavier hydrocarbons), and a strong depletion in 13 C (δ 13 C from −102.2 to −73.7% , V-PDB), indicating a biogenic origin [55].…”

Section: The Edge Of the Stability Zone And Seafloor Gas Ventsmentioning

confidence: 99%

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

et al. 2019

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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.

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Section: The Edge Of the Stability Zone And Seafloor Gas Ventsmentioning

confidence: 99%

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

et al. 2019

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“…The molecular and isotopic compositions of aqueous‐phase hydrocarbons in groundwater are commonly used to interpret the origin of gas, with considerations given to possible fractionation processes such as microbial oxidation, source mixing (Rodrigues et al., 2019; Sherwood et al., 2016) and, with lesser frequency, solubility differences, and sorption (Chalmers & Bustin, 2008; Li et al., 2017). However, the effect of diffusive transport has been largely neglected to date, despite experimental data pointing to significant fractionations resulting from diffusive transport through rock (Pernaton et al., 1996; Zhang & Krooss, 2001).…”

Section: Resultsmentioning

confidence: 99%

“…

Studies evaluating hydrocarbon gas occurrence in shallow groundwater often include assessment of the isotopic composition of carbon (δ 13 C) and hydrogen (δ 2 H) in dissolved methane, which has been used to postulate the origin of the gas (Rivard et al, 2019;Rodrigues et al, 2019). Generally, gases formed through microbial action tend to be isotopically lighter than thermogenic gases, although there is considerable overlap in δ 13 C values between the two sources (Milkov & Etiope, 2018).

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confidence: 99%

Reactive Transport Modeling of Natural Gas Molecular and Isotopic Evolution During Diffusive Transport in the Subsurface

Loomer

MacQuarrie

2021

Water Resources Research

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Reactive transport modeling was employed to investigate the relative importance of fractionations associated with gas solubility, sorption and diffusive transport on dissolved methane, ethane and propane concentrations and the isotopic composition of carbon in methane (δ13C1) in groundwater. Temperature, pressure and salinity dependencies for the hydrocarbon gases were incorporated. Gas molecular ratios, C1/(C2 + C3), increased with diffusive transport, transitioning from thermogenic values to values typically indicative of biogenic gas sources, >1,000, at the leading edge of the diffusive front. Diffusive isotopic fractionation had a large effect on δ13C1 values, with fractionations ranging from −36‰ to −107‰, the difference being a function of the diffusive fractionation factor (αD0). Larger fractionations resulted from αD0 determined at relatively low pressures, 5–50 atm, and temperatures, 20°C–25°C. Less fractionation occurred with αD0 measured at higher pressures and temperature, 30–89 atm and 90°C. The extremely depleted δ13C1 values indicated from the modeling, less than −110‰, have not been observed in shallow groundwater, suggesting that diffusive fractionation of δ13C1 is offset by other processes such as microbial oxidation. 2k factorial analysis was used to assess the model sensitivity to specific parameters: estimations of hydrocarbon travel distance are most sensitive to porosity and tortuosity, while the molecular ratio was most sensitive to the free‐water diffusion coefficient, and the isotopic fractionation was most sensitive to αD0. The magnitude of diffusive fractionation on the molecular and isotopic composition of transported hydrocarbon gas may be similar to fractionations from microbial oxidation and mixing between different sources.

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“…Reference [40] focused their attention on the Amazon deep-sea fan and adjacent continental slope, investigating the molecular stable isotope compositions of hydrate bound and dissolved gases in sediments. A dominant microbial origin of methane via carbon dioxide reduction was detected; however, a possible mixture of thermogenic and microbial gases are recovered in sites located in the adjacent continental slope.…”

Section: Brazilmentioning

confidence: 99%

Gas Hydrate: Environmental and Climate Impacts

et al. 2019

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This Special Issue reports research spanning from the analysis of indirect data, modelling, laboratory and geological data confirming the intrinsic multidisciplinarity of the gas hydrate studies. The study areas are (1) Arctic, (2) Brazil, (3) Chile and (4) the Mediterranean region. The results furnished an important tessera of the knowledge about the relationship of a gas hydrate system with other complex natural phenomena such as climate change, slope stability and earthquakes, and human activities.

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Molecular and Isotopic Composition of Hydrate-Bound, Dissolved and Free Gases in the Amazon Deep-Sea Fan and Slope Sediments, Brazil

Cited by 7 publications

References 65 publications

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

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

Reactive Transport Modeling of Natural Gas Molecular and Isotopic Evolution During Diffusive Transport in the Subsurface

Gas Hydrate: Environmental and Climate Impacts

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