Biogeochemistry of three different shallow gas systems in continental shelf sediments of the South-Eastern Baltic Sea (Gulf of Gdańsk): Carbon cycling, origin of methane and microbial community composition

Abstract

The objective of the study was to describe the biogeochemistry of three different shallow gas systems, with particular emphasis on the content of carbon forms, origin of gas and microbial community composition of sediments. Surface sediments (1–1.2 m) in the areas above geophysically different shallow gas structures located in south-eastern Baltic Sea (Gulf of Gdańsk) were investigated: MET1-MP – an active deepwater pockmark with gas seepage and upward infiltration of freshwater, MET2 – shallow gas regular accumulation with lower availability of sulphates and high input of terrestrial organic matter, and MET3 – shallow gas elongated sub-bottom furrow with sulphate presence in the whole sediment profile. For comparison purposes, a reference site (ZGG) without any gas structures in sediments was included in the study. At each station, the following parameters were examined: CH4, CORG and CINORG in sediments; DIC, DOC, H2S, SO42−, NH4+, Cl− in pore and near-bottom waters; stable isotopes in DIC (δ13C-DIC), in gases (δ13C-CH4, δ13C-CO2, δ2H-CH4), and microbial communities in sediments (16S rRNA). In addition, general physico-chemical conditions in bottom waters (dO2, T, S, pH and Eh) as well as molecular composition of gas emitted from MET1-MP pockmark were determined. In order to be able to better compare the geological background of these shallow gas systems, the sediments were investigated by non-invasive acoustic methods: multi-beam echosounder and sub-bottom profiler. Moreover, relative AOM rates were estimated. The first system (MET1-MP) was characterised by a dramatic decrease in the availability of sulphate down the sediment, acetoclastic methanogenesis taking place in surface sediments, the AOM limited to a very thin topmost sediment layer, average stable isotope values of δ13C-CH4 = −58.9‰, δ2H-CH4 = −310‰, δ13C-CO2 = −6.9‰, carbonate precipitation and both methanogenic as well as methanotrophic Archaea present in the whole sediment profile without clear zonation. In the shallow gas regular accumulation, in a close proximity to the coast (MET2), methane was virtually trapped in subsurface sediment layers, even despite not-high sulphate concentrations resulting from lower salinity (~7.5 PSU). In that area, the bacterial community was, unlike typical marine sediments, dominated by Firmicutes, and average stable isotope values were: δ13C-CH4 = −67.0‰, δ2H-CH4 = −243‰, δ13C-CO2 = −7.9‰. The third shallow gas system (MET3) was characterised by most efficient AOM, mostly due to sulphate presence in a relatively thick sediment layer, even several dozen cm below the SMT. The average stable isotope ratios for gases were as follows: δ13C-CH4 = −75.8‰, δ2H-CH4 = −241‰ and δ13C-CO2 = −11.9‰. Despite high concentrations of methane, the methanogenic Archaea were virtually absent in the sediment profile, while the methanotrophic ones showed clear zonation, with ANME-2a-2b present in shallower sediments and ANME-1 in deeper layers. From the environmental point of view, active pockmark shallow-gas systems can be considered as the least efficient in preventing methane from migration to surface sediment layers and the water column, especially when freshwater infiltration occurs, which additionally dilutes the pool of sulphates available for AOM. In our study, the MET1-MP active pockmark was the most “leaky” in terms of methane release to the water column. In contrast, the shallow gas systems being under influence of additional sources of oxidants (in our case – MET2; influence of riverine discharge introducing Fe(III), together with high concentrations of oxygen in bottom waters) or having sulphate present in a relatively thick sediment layer (e.g. due to sulphide reoxidation below the SMT; in our case – the MET3 system) can be considered efficient in stopping methane within sediments, by its almost complete oxidation before reaching the water column.