Gas seepage from a carbonate-cemented sandstone reef on the Kattegat coast of Denmark

Gas seepage rates were measured over a two year period at an intertidal/shallow subtidal methane-derived carbonate-cemented sandstone reef off north-east Denmark. The positions of some of the gas outlets at the intertidal site changed with time but the overall gas flux of the reef area remained approximately constant, with a mean flow-rate of 59.4 ml h −1 m −2 or over 100 l h −1 for the seepage area. In the shallows the flow-rates of seeps varied with the depth of the overlying water. The methane from the seeps had a radiocarbon age of > 39 000 years bp and was depleted in heavy isotopes, δ 13C CH 4 varied from −68.6 to −75.3‰ and δD from −240 to −249‰, suggesting a bacterial origin. The most 13C-depleted methane came from the most active of the seeps tested.