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

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.

Gas‐Driven Tensile Fracturing in Shallow Marine Sediments

“…(2011) model, dissociation of 1% hydrate saturation would produce 23.5 L of methane gas per m 3 of sediment. Widespread observations of seafloor methane bubble discharge from discrete vents near the feather edge (Berndt et al., 2014; Ketzer et al., 2019; Sarkar et al., 2012; Skarke et al., 2014) are consistent with our predictions and require only small amounts of hydrate dissociation to appear (e.g., Stranne et al., 2017). However, we caution that venting near the feather edge is not necessarily an indication of hydrate dissociation, since microbial methanogenesis in sediments outside the hydrate stability zone can still produce gas that can cause fracturing and venting (e.g., Naudts et al., 2009; Skarke et al., 2014).…”

Gas hydrate dissociation linked to contemporary ocean warming in the southern hemisphere

“…1b). The presence of carbonate concretions resulting from the anaerobic oxidation of methane 26 implies high gas flux over timescales of at least thousands of years at this location 29 . The striking parallelism of the pockmark field with the adjacent BSR outcrop in water depths of 515-520 m raises the question of whether gas venting could record the response of the feather edge of the GHSZ to changing oceanographic conditions since the last glacial maximum (LGM, ca.…”

“…Seafloor investigations have subsequently yielded samples of gas hydrates, authigenic carbonates, and chemosynthetic ecosystems indicative of gas venting from pockmarks fields in two areas, on the mid-and upper slopes [26][27][28] . Ship-borne multibeam bathymetric imagery show that the upper slope pockmark field lies in water depths of 520-660 m, near the upper limit of the GHSZ, and is elongated parallel to at least 12 km of the slope 26,29 (Fig. 1b).…”

“…On the southern Brazilian margin, in contrast, it has been hypothesised that elevation of the permanent thermocline during sea-level rise resulted in a post-glacial cooling of the upper slope that suppressed hydrate dissociation and methane release 52 . The proposed mechanism was suggested to be relevant to the gas hydrate system of the RGC 52 , where the base of the permanent thermocline lies in depths of 500-700 m and so contains the upper limit of the GHSZ 29 . We note that long-term post-glacial cooling would have favoured gas accumulation and so could account for the presence of a well-developed BSR outcrop, making the southern Brazilian margin a potential modern analogue to LGM margins.…”

Gas Hydrate: Environmental and Climate Impacts

“…In this area the gas hydrate is explored only recently. This Special Issue reported the review of the evidences of venting from gas hydrate provinces along Brazil's continental margin in [39]. In literature, only indirect indications of the presence of gas hydrate were reported analyzing seismic data in two deep-water depocenters: the Rio Grande cone in the Pelotas Basin and the Amazon deep-sea fan in the Foz do Amazonas basin.…”