MARINE CH4 OXIDATION · The role of water column methane oxidation in the global carbon budget

Final Activity and Management Report Summary - MARINE CH4 OXIDATION (The role of water column methane oxidation in the global carbon budget)

At different time points in Earth's history, several million of years ago, major releases of the greenhouse gas methane (CH4) are thought to have caused mass extinctions of marine species and global warming. However, it is not certain if CH4 or its oxidised product, i.e. carbon dioxide (CO2), entered the atmosphere. As CH4 warms the Earth 23 times more than CO2 and thus has a larger greenhouse effect, it is important to know how much of which gas is entering the atmosphere. This Marie Curie project dealt with quantification of CH4 oxidised by bacteria in the ocean, its physical transport in the ocean and the sea and air gas exchange. The Marie Curie fellowship allowed me, Dr Susan Mau, to learn a technique to study oxidation of CH4 in the ocean. I learned the tracer technique at the University of California in Santa Barbara, United States of America. The technique used labelled CH4 as a tracer; either 14C labelled CH4 or tritium labelled CH4 (3H-CH4) could be used. The tracer was injected into a sample and consumed by the bacteria. The products of this microbial oxidation could be quantified by measuring the radioactivity of the products. As the two tracers had not been compared adequately in literature, CH4 oxidation rates were derived using both tracers from a variety of locations. This study revealed that in situ or actual rates were measured when using 3H-CH4, whereas potential rates were measured using 14C-CH4. The potential rates originated by greatly increasing the CH4 concentration in the sample, because the radioactivity per volume of the tracer was smaller in case of 14C-CH4. Hence, the methanotrophic bacteria were artificially fed with 14C-CH4. Depending on what a researcher wanted to study, a tracer should be chosen. As I was interested in the actual rates, 3H-CH4 was used to investigate how much CH4 was microbially oxidised in the ocean. As a first study site, the area down-current of the coal oil point (COP) seep field was chosen. CH4 was naturally bubbling from the seafloor at COP seep field. There were locations where mainly CH4 was escaping from the sediments and other locations where higher chained alkanes, ethane and propane, were also emitted as an additional study revealed. Three kilometres down-current from the seep field, a 198 km2 area was investigated by sampling the entire water column for CH4 concentrations and CH4 oxidation rates. Currents were recorded simultaneously. Based on this data set the amount of CH4 in the ocean, the amount of CH4 oxidised and the amount transferred to the atmosphere was estimated. The results indicated that approximately only 1 % of the dissolved CH4 was oxidised in the study area. Another approximately 1 % of the dissolved CH4 entered the atmosphere via sea and air gas transfer. The rest remained in the ocean and was transported in and out of the studied area. Apparently the physical transport was much faster diluting the CH4 than the methanotrophic bacteria were oxidising it. However, as just a small portion was vented to the atmosphere, the bacteria still appeared to be the main sink for CH4 in the ocean. As the coal oil point seep field was a very special site, we also investigated microbial CH4 oxidation in Storfjorden, a bay located west of Spitsbergen. CH4 was emitted from sediments and produced in the water column in this bay, although in lower quantities that at COP. Measured CH4 consumption by bacteria was found to be similarly fast even though the temperature was approximately 10 °C lower than offshore California. Hence, the project revealed that if CH4 was available in the ocean, microbial CH4 oxidation took place and appeared to be slow but continuous.

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