Abstract
The Barents Sea is a broad, epicontinental Sea in northern Europe. With an area of about 1.4 million km² it extends from Novaya Zemlya (Russia) in the east to the continental slope of the Norwegian-Greenland Sea in the west, and from Svalbard and Franz Josef Land in the north to the northern coast of Norway and Russia in the south. The southwestern part of the Barents Sea is strongly characterized by its geological history with subsidence and uplift periods and several events of glacial erosion. The last glacial maximum (LGM) is one of the most important and best preserved glacial phases in this area. During the last decades the Barents Sea evolved into an oil and gas prospecting area. Several source rocks have been identified and hydrocarbon discoveries have been made. Additionally, indications for hydrocarbon seepage, so called “cold seeps” have been detected. These include extensive pockmark fields, carbonate crusts bearing areas and fault related gas flares. Leaking hydrocarbons, released by cold seeps, gained increasing attention during the last years for two reasons. First because they are potential indicators for underlying hydrocarbon reservoirs in the subsurface and second because emitted hydrocarbons, particularly methane as a greenhouse gas, are known to significantly affect the global climate when released to the atmosphere.
In this thesis samples from different areas located in the Loppa High region in the southwestern Barents Sea were investigated. Two surface manifestations of cold seep systems such as huge pockmark areas and carbonate crust sites were studied in detail, in order to determine the activity, formation and spatial distribution of the different seepage structures as well as the origin and timing of the seeping hydrocarbon fluids. Therefore, samples, collected in three research cruises, were studied. These include sediment cores from pockmarks, reference sites and carbonate crust areas as well as carbonate crust samples. An interdisciplinary approach, applying organic geochemical, biogeochemical and geomicrobiological methods in combination with a geophysical data set was chosen to answer the key questions mentioned above. In order to determine the abundance of seep-associated microorganisms, the microbial activity was investigated by analyzing sulfate reduction rates (SRRs). Furthermore, the assessment of specific biomarkers was used to characterize the seeping fluids. The detection of petroleum related compounds can, for instance, indicate the presence of oil in the sediment. The analysis of biomarkers diagnostic for microorganisms, which perform anaerobic oxidation of methane (AOM) in the presence of methane, can indicate the release of gas from the subsurface. Compound specific carbon isotopic signatures of microbial biomarkers can offer further indications for seeping hydrocarbon gases, since very negative carbon isotope values indicate the utilization of methane as a carbon source by methanotrophs. Furthermore, the precipitation of carbonates often occurs in consequence of AOM.
The presence of carbonate crust patches in the carbonate crust area together with rising gas bubbles from the sediment are first indications for active methane seepage in this area. Furthermore, diagnostic AOM biomarkers were detected in the sediment samples as well as in the corresponding carbonate crusts. The depth profiles of these biomarkers show a distinct interval of higher concentrations, which points towards a shallow AOM zone in this depth interval. This was further supported by very negative compound specific carbon isotopic signatures (δ13C), which suggests the participation of the corresponding organisms in methane consumption processes and, thus, the presence of gas in these study sites.
In the pockmark areas, however, active release of gas from the sediment could not be observed, neither in the data of the gas measurements, nor in the biogeochemical and geomicrobiological data. Throughout the whole depth interval of the sediment cores, both from pockmark and reference cores, unusually low microbial activity was determined. Further, the diagnostic AOM biomarkers were essentially absent. Although the presence of petroleum biomarkers potentially indicates the escape of higher molecular hydrocarbons, it is inferred that their presence is not due to emitted petroleum from the pockmarks. The presence of thermogenic hydrocarbons seems to be ubiquitous in the Loppa High area occurring in pockmark as well as in reference cores. Biomarker depth profiles showed that the mature hydrocarbons show the same variability as the immature background compounds. This implies that the oil-related compounds are derived from mature material which has been eroded, mixed with immature organic matter and distributed over the entire area.
Using geophysical data as well as literature data on the geologic and glacial history of the study area and the data obtained during this study, it is suggested that the present pockmark fields are the result of area wide gas hydrate decomposition as a consequence of the retreat of the ice sheet which covered the Barents Sea during the Weichselian. Due to changing temperature and pressure conditions the destabilization and, thus, the decay of gas hydrates which were accumulated under the ice sheet, occurred. This resulted in a massive release of large amounts of gas associated with the formation of the pockmark craters. This scenario explains well the existence of the large areas of inactive pockmarks which are still preserved on the seabed surface. In contrast, the currently active cold seep structures are claimed to be correlated to fault systems in the subsurface as indicated by seismic data. It is known that faults can act as conduits for migrating hydrocarbons. Since numerous faults are present in the carbonate crust areas, and hydrocarbon reservoirs occur in the vicinity, it is well possible that these faults act as migration pathways for hydrocarbon gases from deeper hydrocarbon sources towards the cold seeps.
It can be concluded, that the southwestern Barents Sea contains at least two cold seep systems. Although these systems seem to be morphologically totally different from each other, it is conceivable that these features were related in the past. The gas hydrates which are claimed to be responsible for the formation of the large pockmark areas may have been fed by the same fault system, which today act as conduits for the active cold seeps. Furthermore, it was shown, that by combining geochemical and geomicrobiological tools a good assessment of the current seeping activity of cold seep structures can be achieved which can, thus, be used as a relatively fast and cheap tool to evaluate cold seep systems.
