Cooper, J.A.G., Smith A.M., and Arnscheidt, J. 2013. Contemporary stromatolite formation in high intertidal rock pools, Giant's Causeway, Northern Ireland: preliminary observations.

Contemporary stromatolites are known from a variety of settings ranging from Antarctic dry valleys to hypersaline tropical lagoons. Their formation requires a source of calcium carbonate, binding by cyanobacteria and the elimination of other competing organisms by extremes of environmental conditions. In this paper we report the contemporary development of stromatolites in intertidal rock pools at the Giant's Causeway, Northern Ireland. The rock pools are high in the tidal frame and located on the landward margin of wide rock platforms that dissipate all but the most energetic waves. They are only infrequently inundated by marine waters but are fed by groundwater that emerges from surrounding high cliffs. Adjacent storm-swash deposits provide a potential source of calcium carbonate. The stromatolites occur in two distinctive settings. In one they are present as a thin covering close to the waterline in rock pools where their precipitation may be aided by wetting and drying of the associated microbial mats. Microscopic examination reveals a close intermeshing of cyanobacteria filaments and precipitated carbonate minerals. In a second setting, they occur on the base of rock pools as a semi-continuous covering. These have a much thicker development with the characteristic stromatolite growth forms forming mounds up to 3mm thick with multiple internal laminae. The occurrence of these structures is superficially similar to recently reported examples from rock pools in South Africa but their limited thickness requires explanation. This is either the result of changing environmental conditions that now favour stromatolite growth, or periodic cycles of formation and destruction driven by quasi cyclic changes in water chemistry.