DOI 10.17882/85836 Understanding the dynamics and fate of methane (CH4) release from oceanic seepages on margins and shelves into the water column, and quantifying the budget of its total discharge at different spatial and temporal scales, currently represents a major scientific undertaking. Previous works on the fate of methane escaping from the seafloor underlined the challenge in both, estimating its concentration distribution and identifying gradients.
In April 2019, the Envri Methane Cruise has been conducted onboard the R/V Mare Nigrum in the Western Black Sea to investigate two shallow methane seep sites at ∼120 m and ∼55 m water depth. Dissolved CH4 measurements were conducted with two continuous in-situ sensors: a membrane inlet laser spectrometer (MILS) and a commercial methane sensor (METS) from Franatech GmbH. Additionally, discrete water samples were collected from CTD-Rosette deployment and standard laboratory methane analysis was performed by gas chromatography coupled with either purge-and-trap or headspace techniques.
The resulting vertical profiles (from both in situ and discrete water sample measurements) of dissolved methane concentration follow an expected exponential dissolution function at both sites. At the deeper site, high dissolved methane concentrations are detected up to ∼45 m from the seabed, while at the sea surface dissolved methane was in equilibrium with the atmospheric concentration. At the shallower site, sea surface CH4 concentrations were four times higher than the expected equilibrium value. Our results seem to support that methane may be transferred from the sea to the atmosphere, depending on local water depths.
In accordance with previous studies, the shallower the water, the more likely is a sea-to-atmosphere transport of methane. High spatial resolution surface data also support this hypothesis. Well localized methane enriched waters were found near the surface at both sites, but their locations appear to be decoupled with the ones of the seafloor seepages. This highlights the need of better understanding the processes responsible for the transport and transformation of the dissolved methane in the water column, especially in stratified water masses like in the Black Sea.
© DOI: 10.3389/feart.2021.626372
Chemical oceanography
dissolved gas, methane, black sea, in situ measurements, gas seepages, instrumental inter-comparison
44.26N, 44.04S, 30.77E, 29.47W
A fast response membrane inlet laser spectrometer (MILS) prototype (t90 < 30 s, Grilli et al., 2018) and a commercial Franatech METS sensor were used. Data from laboratory analysis by Purgeand-trap (PT) and head-space (HS) technique on water samples are also reported.
| File | Size | Format | Processing | Access | |
|---|---|---|---|---|---|
Data from the MILS and MET sensor during the atmopsheric near-surface profile HP01 | 5 Mo | ASC | Processed data | ||
Data from the MILS sensor during the atmopsheric near-surface profile HP03 | 1 Mo | ASC | Processed data | ||
Data from the MILS sensor during the horizontal near-surface profile HP04 | 197 Ko | ASC | Processed data | ||
Data from the MILS sensor during the horizontal near-surface profile HP05 | 323 Ko | ASC | Processed data | ||
Data from the MILS sensor during the horizontal near-surface profile HP06 | 4 Mo | ASC | Processed data | ||
Data from the MILS sensor during the vertical profile VP01 | 163 Ko | ASC | Processed data | ||
Data from the MILS sensor during the vertical profile VP02 | 1 Mo | ASC | Processed data | ||
Data from the MILS sensor during the vertical profile VP03 | 260 Ko | ASC | Processed data | ||
Data from the MILS sensor during the vertical profile VP04 | 312 Ko | ASC | Processed data | ||
Data from the MILS sensor during the vertical profile VP05 | 750 Ko | ASC | Processed data | ||
Data from the discrete measurements using PT and HS techniques on the hydrocast HY02 | 475 octets | ASC | Processed data | ||
Data from the discrete measurements using PT and HS techniques on the hydrocast HY04 | 424 octets | ASC | Processed data | ||
Data from the discrete measurements using PT and HS techniques on the hydrocast HY05 | 450 octets | ASC | Processed data |
Copy this text
