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Figure Z8
o.soo � �� ===i- communities (GIONGO et al., 2016). In This intersection of the BSR reflector
A the chemosynthetic communities, micro- with the seafloor has been observed in

organisms that live externally attached only a few other regions of the world (e.g.,
(epibionts) to tubes of the Escarpia worm Mauritania slope, DAVIES et al., 2015). The
have been described (Figure Z7). This is a rarity of such features is puzzling, as the
group of methanotrophic bacteria, which thinning of the gas hydrate stability zone re-
Outcrop
indicates that these life forms are adapt- lated to decreasing pressure (water depth)
ed to the presence of methane, located and increasing bottom water temperature
near the gas seeps (MEDINA-SILVA et al., should naturally lead to BSR outcrops.
2018 a and b). The position of the edge of the GHSZ on
In addition to gas exudations asso- the RGC seafloor, calculated from current
ciated with the presence of faults and groundwater temperature measurements,
“pockmarks”, they also occur in the por- is more distal to that of the BSR outcrop,
tions where the stability zone thins in indicating a thermodynamic imbalance be-
the proximal region of the RGC. Ketzer tween the hydrate system and the ground-
et al. (2022) identify the presence of a water. (KETZER et al., 2020). Bottom water
singular phenomenon, where the GHSZ warming to depths >500 m in the South
thins towards the seafloor until outcrops, Atlantic, which can be traced back at least
characterizing an upwelling zone of the to the 1970s (SCHMIDTKO and JOHNSON,
BSR. This phenomenon occurs between 2012), provides strong evidence that this
the isobaths of 515 and 520 m (KETZER imbalance may be linked to contempo-
o.soo B et al., 2020 – Figure Z8) where 394 gas rary climate change (KETZER). et al., 2019;
Gas plumes
I up to 50 m in the water column. Margins transfer from sediments to the ocean were
S15-540m plumes were detected reaching heights of KETZER et al., 2020). The rates of methane
estimated by Ketzer et al. (2020) between
with well-defined BSR outcrops constitute
unique sites to investigate interactions 31.3 and 144 MgCH /year (megagrams of
4
between marine gas hydrate systems and methane per year) in this region.
climate change, as the edge of the GHSZ Numerical modeling carried out for
is the site most susceptible to gas hydrate this area involving a scale of hundreds of
dissociation driven by warming waters at years confirmed the sensitivity of the hy-
the ocean bottom (Ruppel, 2011). drates to variations in background tem-
perature, causing seasonality at the limit of
Figure Z8: (A) Seismic profile showing the GHSZ, which can significantly retreat
the BSR outcrop zone in the Rio Grande down the slope, depending on the rate of
Cone; (B) Detail of (A) showing the BSR increase in temperatures. This would result
outcrop on the seafloor at 515-520 m in complete dissociation of the hydrates
water depth, the range of occurrence and significant releases of methane to the
of gas plumes and the current range of ocean. Another phenomenon related to re-
the gas hydrate stability zone (GHSZ). leases of gas to the ocean is the gravity-in-

Source: Modified from Ketzer et al., (2020) duced collapse of depocenters down the

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