BUBBLING REEFS IN THE KATTEGAT - SUBMARINE LANDSCAPES OF CARBONATE-CEMENTED ROCKS SUPPORT A DIVERSE ECOSYSTEM AT METHANE SEEPS

Methane seeps in shallow waters in the northern Kattegat off the Danish coast form spectacular submarine landscapes - the 'bubbling reefs' - due to carbonate-cemented sandstone structures which are colonized by brightly coloured animals and plants. These structures may be 100 m2 in area and consist of pavements, complex formations of overlying slab-type layers, and pillars up to 4 m high. The carbonate cement (high-magnesium calcite, dolomite or aragonite) is C-13-depleted, indicating that it originated as a result of microbial methane oxidation. It is believed that the cementation occurred in the subsurface and that the rocks were exposed by subsequent erosion of the surrounding unconsolidated sediment. The formations are interspersed with gas vents that intermittently release gas, primarily methane, at up to 25 l h-1. The methane most likely originated from the microbial decomposition of plant material deposited during the Eemian and early Weichselian periods, i.e. 100 000 to 125 000 years B.P. Aerobic methane oxidation in the sediment was restricted to the upper 4 cm in muddy sand and to the upper 13 cm in coarse sand, Maximum aerobic methane oxidation rates ranged from 4.8 to 45.6-mu-mol dm-3 d-1. The rock surfaces and epifauna around the seeps were also sites of methane-oxidizing activity. Integrated sulphate reduction rates for the upper 10 cm of muddy sand gave 4.2 to 26.6 mmol SO42- m-1 d-1. These rates are higher than those previously reported from similar water depths in the Kattegat but did not relate to the sediment methane content. Since gas venting occurs over several km2 of the sea floor in the Kattegat it is likely to make a significant local contribution to the cycling of elements in the sediment and the water column. The rocks support a diverse ecosystem ranging from bacteria to macroalgae and anthozoans. Many animals live within the rocks in holes bored by sponges, polychaetes and bivalves. Stable carbon isotope composition (delta-C-13) of tissues of invertebrates from the rocks were in the range -17 to -24 parts per thousand, indicating that methane-derived carbon makes little direct contribution to their nutrition. Within the sediments surrounding the seeps there is a poor metazoan fauna, in terms of abundance, diversity and biomass. This may be a result of toxicity due to hydrogen sulphide input from the gas.