Gas hydrate versus seabed morphology offshore Lebu (Chilean margin)

Iván Vargas-Cordero,Alessandra Rivero,Carolina Cárcamo,Umberta Tinivella,Marion San Juan,Lucía Villar-Muñoz,Joaquim P Bento,Diego López-Acevedo,Francisco Fernandoy

doi:10.1038/s41598-020-78958-z

Abstract

Gas-hydrate occurrences along the Chilean margin have been widely documented, but the processes associated with fluid escapes caused by the dissociation of gas hydrates are still unknown. We report a seabed morphology growth related to fluid migration offshore Lebu associated with mud cones by analysing oxygen and deuterium stable water isotopes in pore water, bathymetric, biological and sedimentological data. A relief was observed at − 127 m water depth with five peaks. Enrichment values of δ18O (0.0–1.8‰) and δD (0.0–5.6‰) evidenced past hydrate melting. The orientation of the relief could be associated with faults and fractures, which constitute pathways for fluid migration. The benthic foraminifera observed can be associated with cold seep areas. We model that the mud cones correspond to mud growing processes related to past gas-hydrate dissociation. The integration of (i) the seismic data analysis performed in the surrounding area, (ii) the orientation of our studied relief, (iii) the infaunal foraminifera observed, (iv) the grain size and (v) the total organic matter and isotope values revealed that this area was formerly characterised by the presence of gas hydrates. Hence, this part of the Chilean margin represents a suitable area for investigating fluid-migration processes.

Highlights

Gas-hydrate occurrences along the Chilean margin have been widely documented, but the processes associated with fluid escapes caused by the dissociation of gas hydrates are still unknown
Fluid escape morphologies related to gas escape include a) seafloor depressions as pockmarks and b) seafloor mud growing as mud volcanoes, mud cones, mud diapirs and mounds
Others indicate that when the extruded sedimentary material is responsible for the seafloor morphology, it can be described as mud volcanoes and mud cones[14], whereas when the fluidised material is not expelled, it can be defined as mud d iapir[1]

Summary

Introduction

Gas-hydrate occurrences along the Chilean margin have been widely documented, but the processes associated with fluid escapes caused by the dissociation of gas hydrates are still unknown. The integration of (i) the seismic data analysis performed in the surrounding area, (ii) the orientation of our studied relief, (iii) the infaunal foraminifera observed, (iv) the grain size and (v) the total organic matter and isotope values revealed that this area was formerly characterised by the presence of gas hydrates. This part of the Chilean margin represents a suitable area for investigating fluid-migration processes. Morphological features associated with fluid escapes such as mud volcanoes, mud mounds, pockmarks, seeps, precipitates—carbonates and hydrates-, piping/rills and brine pools have been reported worldwide (e.g.1–7). The presence of methane, a potent greenhouse gas[34], in zones of shallow fluid escapes could may contribute to (a) an increase in surface temperature in favour of global warming, (b) change in the physic-chemical conditions of seawater, (c) affect the marine microfaunal diversity pattern, and (d) affect the nucleation and the rupture propagation of earthquakes, related to slides and tsunamis[35,36,37]

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Conclusion