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The bubbling reefs scientific explanation
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 is up to 500 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 13C-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 1 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.
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 (S13C) of tissues of invertebrates from the rocks were in the range -17 to -24 0/oo, 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.
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Illustration: Chr. Würgler Hansen
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Scientific explanation