Jan-Hendrik Körber

• cold seeps
• marine oil- and gas-emissions
• remote sensing in the marine environment
• application of GIS for the analysis of marine data
• marine biogeography

Analysis and Inventory of Natural Hydrocarbon Seepage in the Marine Environment and its Impact on the Global Methane Cycle using Remote Sensing Techniques

Background

Emissions of liquid and gaseous hydrocarbons at the seafloor are widespread phenomena and known to occur at many places around the world. Innumerable sites of hydrocarbon seepage have been reported and locations were such seeps might be discovered can be identified as processes fuelling these systems become more understood. However, it is still debated how much of these marine hydrocarbon emissions, especially the strong greenhouse gas methane, are reaching the atmosphere.
Where emissions occur in very shallow waters gas bubbles can be seen right at the sea surface. This does not apply for gas coming from the deep sea and continental margins. On their long way up to the sea surface gas bubble plumes become deflected by currents and a major fraction of the gas dissolves in the water column. Single bubbles which anyway might reach the sea surface are almost impossible to detect. In general the influence of deep sea gas emissions are considered to be negligible in the atmospheric gas budget.
The situation is changing when oil joins the emitted gas bubbles. Oil droplets which are migrating upward from deep sources together with gas can form thin coatings around gas bubbles when being released to the water column. These coatings might prevent the complete dissolution of the gas during the ascent to the sea surface and thus enhance the transport of methane to the atmosphere. While elevated methane concentrations in the lowest level of the atmosphere can not, yet, be mapped in large scale and high resolutions (sample point distances of few meters and global coverage), it is possible to detect thin oil films on the water surface in high spatial and temporal resolution and coverage by different satellite sensors. Better knowledge on the distribution and temporal variability and the ratios of oil and gas emitted at deep sea seeps with combined oil and gas release might help to determine a first order-of-magnitude estimate of methane and other climate relevant gases to the atmosphere.

Remote Sensing of gas and oil emissions

Remote sensing comprises all techniques which allow investigating certain features or parameters without physical contact. In my project I am using different ship and satellite based remote sensing tools and techniques to access information on hydrocarbon emissions.

Dissolved vs. gaseous methane fluxes – what’s more important?

One part of my project deals with the quantification of the methane budget of a deep sea cold seep site off shore Pakistan to gain better knowledge on the importance of dissolved and gaseous fluxes from the sediments to the water column and their impact on the water column chemistry in this area. To achieve this areal flux quantification I am using video and geophysical data acquired with a remotely operated vehicle (ROV) just a few meters above the actual seep site. The video data allows producing video mosaics (image maps) of the seep area and certain chemosynthetic communities which populate this seep site. These communities comprise different clam, tubeworm and bacteria species which rely on sulphide and methane fluxes from below as energy source. As all of these different species can only thrive in a very narrow range of sulphide/methane fluxes they can be used as indicators for gas fluxes in this area. Thus the visual mapping of abundances and densities of different species serves as a proxy to quantify the areal fluxes of gases as, e.g. sulphide which is known from point measurements done by colleagues. These areal flux quantifications refer to dissolved gas fluxes, which can be compared to the emission rates of free gas bubbles in this area which have been determined in another study.
The visual mapping of the seep site is supported by geophysical mapping of the site using sonar systems which are capable of imaging different seafloor features as morphology and density of the shallow sediments. Both features are strongly altered by processes related to the seepage of methane and the chemosynthetic communities themselves. The geophysical approach allows to map a larger area than the visual mapping, but not to distinguish between different faunal species or different sediment properties in such detail as the video mosaicing. The combination of both techniques however gives a comprehensive picture of the entire seep structure.

Smooth areas in rough waters – an indicator for methane emissions from the deep?

As described above, sites are known where oily gas bubbles are emitted at the seafloor from natural seeps. When these bubbles reach the sea surface the gas is released to the atmosphere and becomes hard to detect. The oil however remains as thin films on the sea surface and forms natural oil slicks. Oil slicks have an impact on the surface tension of the water by dampening small capillary waves. The result is a surface which is smoother than the surrounding oil-free water. These smooth areas can be mapped by active radar sensors as, e.g. the Advanced Synthetic Aperture Radar sensor onboard the ENVISAT satellite. This sensor actively emits microwaves and captures the signal which is scattered back from the earth’ surface. As rougher the surface is, the more of the radar signal is reflected back to the satellite, whereas from very smooth surfaces most of the signal is reflected away from the sensor. This allows the detection of areas on the sea surface which are significantly less rough than their surrounding. The use of many images (provided by ESA) covering the same area enables us to separate natural oil slicks from man-made slicks or other surfactants which might cause similar backscatter patterns as oil.
In my project I focus on the identification of natural oil seep sites and the analysis of their spatial and temporal variability. The satellite derived informations are verified during ship based ground-truthing experiments which comprise the localization of the seep site on the seafloor using hydro-acoustic techniques, sampling of gas emissions above the emission site and sampling of the oil on the sea surface. In a first step the region of the Black Sea is investigated. Findings from these regional studies will then be applied to other areas. Creating a large database on distributions and emission intensities of natural oil/gas seeps shall contribute to the better understanding of the impact these sources have on global atmospheric methane budgets.