Space-time evolution of a large field of pockmarks in the Bay of Concarneau (NW Brittany)

Abstract

About a decade ago, a large field of pockmarks, covering an overall area of 36 km(2) was discovered in water depths of < 30m in the central part of the Bay of Concarneau (Southern Brittany, France). This field, composed of features from 5 m to 35 m in diameter and < 1 m in depth, is characterized by unusual high densities of pockmarks, up to 5840 per square kilometre. Geophysical data correlated with sedimentary samples acquired in 2005 and 2009 show that pockmarks and their immediate surroundings are associated with dense tubes benches cover, built by a filter-feeding amphipod: Haploops nirae, forming original benthic communities. Two complementary surveys were carried out in April 2011 (Pock and Ploops) and April 2014 (Pock and Tide), on the Oceanographic Vedette (O/V) Haliotis (Ifremer/Genavir), to map the limit of the pockmarks and Haploops fields with the maximum accuracy. The link between the presence of the Haploops nirae communities and the occurrence of pockmarks / gas was then established and the proposed hypothesis was that tidal cycles may provide a good candidate for a short-term (monthly) triggering mechanism of fluid expulsion (Baltzer A, Ehrhold A, Rigolet C, Souron A, Cordier C, Clouet H. 2014. Geophysical exploration of an active pockmark field in the Bay of Concarneau, southern Brittany, and implications for resident suspension feeders. 34,[215][216][217][218][219][220][221][222][223][224][225][226][227][228][229][230]. Due to the high-level precision (50 cm) of the positioning system (Magellan Aquarius Ixsea Hydrins) coupled with the RTK attitude system, these new bathymetric and imagery maps together with the sub-bottom Chirp profiles, allow us to compare the data sets from April 2011 and April 2014. The superimposition of the two data sets shows that the distribution of the 2011 (Pock and Ploops) et Avril 2014 (Pock and Tide), sur la vedette océnographique Haliotis (Ifremer/Génavir), afin de cartographier très précisément la limite des champs de Haploops et des pockmarks. Le lien constaté (mais non élucidé) de la présence des colonies de Haploops et de ces pockmarks implique une activité répétée des pockmarks, et la marée comme facteur déclenchant à court terme constitue l'hypothèse la plus robuste (Baltzer A, Ehrhold A, Rigolet C, Souron A, Cordier C, Clouet H. 2014. Geophysical exploration of an active pockmark field in the Bay of Concarneau, southern Brittany, and implications for resident suspension feeders. Geo-Marine Letters, 34,[215][216][217][218][219][220][221][222][223][224][225][226][227][228][229][230]. Grâce au système de positionnement RTK, les données d'imagerie, de bathymétrie et de l'échosondeur à sédiments (chirp) ont pu être parfaitement superposées et donc comparées entre 2011 et 2014. Les premiers résultats montrent que la répartition des pockmarks reste similaire entre ces 2 dates pour le groupe de grands pockmarks dispersés, enracinés plus profondément dans le remplissage sédimentaire holocène des paléo-vallées et le groupe de pockmarks plus petits, plus denses, alig...

Methane Bubble Escape From Gas Horizon in Muddy Aquatic Sediment Under Periodic Wave Loading

“…The buoyancy-driven ascent of CH 4 mature bubbles in muddy sediment-from their place of nucleation and growth toward the sediment-water interface-was found to be controlled by the bubble solute exchange with ambient pore waters (Tarboush Sirhan et al, 2019). Field-observed CH 4 bubble ascent is usually associated with deep microbial (Baltzer et al, 2017;DelSontro et al, 2011;Wik et al, 2013) or thermogenic (Feldens et al, 2016;León et al, 2014) CH 4 production, with places of dissociation of gas hydrates (Berndt et al, 2014;Skarke et al, 2014;Westbrook et al, 2009), with degrading permafrost (Shakhova et al, 2014), and even with the presence of deep coal deposits (Karnaukh et al, 2016).…”

“…Destabilization of shallow gas deposits on the southern Brazilian margin and released CH 4 (Portilho-Ramos et al, 2018) were associated with the combination of sea level lowstand during the last glacial period, vigorous bottom currents, and warmer bottom waters. Finally, the reactivation of pockmarks at shallow water depths (<30 m) in the central part of the Bay of Concarneau (Southern Brittany, France) was associated with intensive spring tides with high tidal coefficients, whereas new pockmarks were suggested to form only during major storms or earthquake events (Baltzer et al, 2017).…”

Characterization of gas‐bearing sediments in the coastal environment using geophysical and geotechnical data

“…The Bay of Concarneau is characterized by a large and dense field of pockmarks (Ehrhold, Hamon and Guillaumont 2006;Baltzer et al 2014) whose diameters are up to 30 m, forming 2 m deep depressions, formed by fluid escape from gas-saturated sediment. This field is present in water depths <30 m and covers an area of 36 km² (Baltzer et al 2017). The pockmarks density unusually reaches ~6000 per km² in places (Baltzer et al 2017; Figure 1).…”

“…This field is present in water depths <30 m and covers an area of 36 km² (Baltzer et al 2017). The pockmarks density unusually reaches ~6000 per km² in places (Baltzer et al 2017; Figure 1). The gas is concentrated within the geological unit U5 (Figure 2a) which corresponds to tidal flat deposits and coarse lag deposits within tidal channels.…”