Timothy G. Ferdelman, Volker Brüchert, Sabine Flury, Henrik Fossing, Bo Barker Jørgensen, Laura Lapham, Nikolay Pimenov, Maja Reinholdsson, and Nguyen M. Thang.

Background and Synopsis of Deliverable:

This deliverable covered the basic depth distributions of methane and other geochemical parameters associated with formation, consumption, transport and, ultimately, the distribution of methane in Baltic Sea sediments.  The basis of the deliverable wasthe delivery of sediment geochemical data to the Pangaea Database.

The deposition of organic material on the sea floor and its burial below the sulfate zone is the basis for a microbiological production of vast amounts of methane – an important greenhouse gas when emitted into the atmosphere. Methane is produced in ocean margin sediments as a consequence of the microbiological degradation of organic matter buried below the zone of sulfate penetration. Continuous methane formation in deep sediments leads to the eventual accumulations of free methane gas, due to methane’s poor solubility in water.

The dominant experimental/field approach to the problem was to study the connection between the seismic signals observed in the sediment (i.e. seismic picture) and ‘in situ’ concentration profiles of methane, sulfate and other pore water constituents. Thus, targeted sediment sampling was performed based on seismic signals along transects reaching from sediments with deep or no ‘methane-reflection’ of the seismic signal (i.e. non-gaseous sediment) to sediments with methane saturation (and thus a sharp reflection, i.e. gaseous sediment) in the (surface) sediments.

Thus a major objective of BONUS Baltic Gas was to map the distribution of key distributions of dissolved oxidants and reductants, organic carbon, and other solid phase chemical species that may play a role in the production and breakdown of methane in Baltic Sea muddy sediments.

Furthermore, these datasets were also obtained with the goal to support methane concentration measurements in the water column (i.e. from CTD casts) by methane concentration measurements in the underlying sediment.

As such, much effort was put into obtaining high resolution porewater profiles of sulfate and methane, as well as a number of corresponding chemical species. Typically sediment obtained in surface cores and was sampled for analyses of CH₄, δ¹³CH₄, density/porosity, CN-content, and pore water (i.e.  SO₄^2-, Cl^-, H₂S, Fe²⁺, PO₄^3-, DIC, metals, and nutrients), and ²¹⁰Pb. Sediment temperature was also usually measured.

Two examples from our sampling areas (Figure 1) are provided below:

An anoxic basin

d. Gotland Deep

Gotland deep (200 meters water depth) could be a depot center for organic-rich sediments, and thus an important source or reservoir of free-gas methane.  We wanted to test this idea by first using seismic techniques to identify free gas in the sediments and then groundtruth this signal by collecting sediment cores to measure for methane.  The acoustic profile shows the deep basin with a thick layer of recent, Holocene mud and a flatter seabed on the flanks (Station 73 in figure d-1).  The basin does not show an acoustic indication of free-gas but we still predicted high methane concentrations based on the thick sediment layer.  Two cores were thus collected, one within the deep basin (core # MSM16-1-73) and one on the flanks (core # MSM16-1-71).  Within these cores, pore-waters were measured for sulfate and methane (figure 2).  Core MSM16-1-73 showed a very steep decrease in sulfate concentrations with sediment depth, indicating a very efficient reduction of sulfate by sulfate reducing bacteria.  Methane concentrations increased quickly and reached near saturation at 1 meter sediment depth.  In contrast, sulfate concentrations in core MSM16-1-71 were similar to overlying water values, 9 mM, indicating no microbial degradation of organic matter through sulfate reduction.  Methane concentrations were very low, suggesting that the methane flux to the sediment water interface was quite low.  A surprising result was that Core MSM16-1-73 showed a high methane flux to the sediment water interface.  There was little evidence of methane oxidation consuming the methane.

Figure 2: Geochemical profiles of sulfate and methane concentrations in Gotland Deep with sediment depth (centimeters below seafloor).

Strange seabed structures

e. Stolpe Fore Delta, Mid-Baltic

The Stolpe Fore Delta was targeted for gravity core and surface core sampling to explore the potential for shallow gas release. “Pockmark-like” structures had been reported as well as possible gas release.

The furrow structures were not associated with gas release from the sediment as also clearly shown in the methane-sulfate profiles from cores taken both within and outside of the furrow structure. Inside the furrow structure there was absolutely no indication for either dissolved nor free methane gas down to 4 meters. The minimal gradient in the sulfate profile suggests that there is also no near-surface methane source.