Using Copernicus global ocean reanalysis data to estimate the evolution of the gas hydrate stability zone

Although climate change is mainly observed on the Earth's surface, it is known that ocean circulation is also changing and the seabed is subject to temperature fluctuations. Seafloor sediments are often permeated by a methane hydrate phase whose stability depends primarily on temperature and pressure fields. Any change in the temperature of the seabed can alter the stability state of the methane hydrate. The dissociation of methane hydrate, a consequence of its unstable state, could release large amounts of methane into the water column. Methane, which could then impact submarine geologic hazards such as submarine landslides, and the eventual reaching of the atmosphere by methane would exacerbate ongoing climate change.

In this work, we computed the depth of the gas hydrate stability zone (GHSZ) at the global scale using data from the EU Copernicus Marine Service (CMS), and its changes in the period from 1993 to 2023 were analyzed at 5-year intervals. The aim was to investigate the impact of climate change on the methane hydrate stability zone.

We used oceanographic temperature and salinity data from the Global Ocean Physics Reanalysis dataset (GLORYS12V1), which was produced as part of CMS, depth data from GEBCO - The General Bathymetric Chart of the Oceans, and geothermal gradient data, derived from the heat flow data reported by Lucazeau (2019, Geochemistry, Geophysics, Geosystems, 20: 4001-4024, https://doi.org/10.1029/2019GC008389).

The depth of the gas hydrate stability zone was calculated from monthly data, which were then averaged over the 12 months of each year considered to obtain annual average values of GHSZ depth. The salinity and temperature data extracted from GLORYS12V1 have a resolution of 1/12 of a degree in longitude and latitude, resulting in a decomposition of the sea surface into approximately 9 million cells and referenced to 50 different depth levels. The availability of salinity and temperature data for the entire water column was essential for a more accurate calculation of seafloor pressure. The contribution to the seafloor pressure of each of the 50 layers into which the water column was divided was calculated using Stevino's law, rather than using the more commonly used dbar-meter approximation. It was also analyzed how the reanalysis data uncertainty obtained from the quality information document provided by CMS affected the final result of the gas hydrate stability zone depth estimate.

The high resolution and completeness of the data made it possible to obtain a relevant result on a global scale, in agreement with literature, showing that over the period considered, the number of model cells subject to GHSZ thinning is much greater than the number of cells subject to GHSZ thickening, particularly in the Southern Hemisphere.