Methane dynamics in the coastal – Continental shelf transition zone of the Gulf of Cadiz

The concentration of CH4 in water was measured along five transects in the Gulf of Cadiz (Trafalgar, Sancti Petri, Guadalquivir, Tinto - Odiel and Guadiana) during four cruises throughout the months of March, June, September and December 2016. For two of the cruises water overlying three mud volcanoes situated in the Gulf of Cadiz were also sampled (San Petersburgo, 860 m; Pipoca, 460 m; and Anastasya, 500 m. In addition, the stable carbon isotopic compositions of dissolved CH4 (δ13C) were measured in the study area in June and December 2016. The mean CH4 value for the whole water column was 10.5 ± 4.3 nmol L−1, with large spatial and temporal variations. The highest values were found in June 2016 and the lowest in March 2016. In surface waters, the mean dissolved methane concentration was of 9.6 ± 2.6 nmol L−1. In most of the sampling area, high concentrations of CH4 were found in subsurface waters at depths close to the thermocline and at the coastal stations. The highest concentrations were obtained from the bottom waters above the Anastasya mud volcano (125 nmol L−1). The stable carbon isotope compositions ranged between - 29.2 and - 58.4‰, and the least negative values were associated with the highest CH4 concentrations in samples from above the mud volcanoes, related to thermogenic values. The sea-air fluxes of CH4 ranged from 12.4 to 37.7 μmol m−2 d−1, showing that the study area acts as a source of CH4 to the atmosphere. [Display omitted] •There is a decrease of CH4 from the coastal area to open sea.•Higher CH4 concentrations had been measured in the water column above Anastasya MV.•The more negative δ13C values had been observed near coastal area.•The less negative δ13C values had been measured near the mud volcanoes.•The continental shelf of the Gulf of Cadiz acts a source of CH4 to the atmosphere.