Pore-water in marine sediments associated to gas hydrate dissociation 1 offshore

19 Gas hydrate occurrences along the Chilean margin has been documented, but the 20 processes associated to fluid escapes originated by gas hydrate dissociation yet are 21 unknown. Here, we report morphologies growing related to fluid migration in the 22 continental shelf offshore western Lebu (37 °S) by analysing mainly geochemical 23 features. In this study oxygen and deuterium stable water isotopes in pore water 24 were measured. Knowledge was completed by analysing bathymetric data, 25 biological and sedimentological data. From bathymetric interpretation a positive 26 relief at 127 m below sea level was recognised; it is oriented N55°E and 27 characterised by five peaks. Moreover, enrichment values for δO (from 0.0 to 28 1.8‰) and δD (from 0.0 to 5.6‰) were obtained. These are typical values related to 29 hydrate melting during coring and post-sampling. The evident orientation of positive 30 relief could be associated with faults and fractures reported by others authors, in 31 which these structures constitute pathways for fluid migration from deep to shallow 32 zones. Finally, benthic foraminifera observed in the core sample can be associated 33 to seep areas. On the basis of theoretical modelling, we conclude that the positive 34 relief correspond to mud growing processes related to gas hydrates dissociation and 35 represent a key area to investigate fluid migration processes. 36