Journal of Coastal Research, Special Issue No. 65, 2013 Contemporary stromatolite formation 1675 Contemporary stromatolite formation in high intertidal rock pools, Giant’s Causeway, Northern Ireland: preliminary observations. J.A.G. Cooper†‡, A.M. Smith ‡ and J. Arnscheidt † †School of Environmental Science, University of Ulster, Coleraine BT52 1SA, UK jag.cooper@ulster.ac.uk daniel.conley@plymouth.ac.uk ‡ Geological Sciences, , University of KwaZulu-Natal, P. Bag X54001, Durban 4000, South Africa asconsulting@telkomsa.net ‡School of Geological Sciences, University of KwaZulu-Natal, Durban, SA INTRODUCTION Stromatolites have been defined as “organosedimentary structures produced by the trapping, binding, and precipitation of sediments under the influence of microorganisms” (Awramik and Grey, 2005 p.1 ) or “microbial organo-sedimentary deposits with planar to sub-planar laminated internal macro-fabrics of benthic origin” (Jahnert and Colins, 2012, p118). They have been documented from a variety of contemporary environments and are most commonly associated with extreme environmental conditions that limit competition with and/or predation by other organisms. The best known (largest structures and most extensive) contemporary occurrences are from hypersaline coastal lagoons such as Hamelin Pool, Western Australia (Logan et al., 1974) and normal marine salinity fringing reef settings in the Bahamas (Dill et al., 1986). Stromatolites are, however best known from the geological record, and in particular the Archaean when they played an important role in early Earth history (Awramik and Grey, 2005). The form and structure of stromatolites is variable and in modern coastal settings is believed to be influenced by the interaction between energy levels, evaporation rates, sediment supply and underlying topography (Jahnert & Collins, 2012). Modern stromatolites are typically found in the intertidal and subtidal zone, but contemporary occurrences in the supratidal zone are rare. Recently, however, Smith et al., (2011) described calcifying tufa stromatolites from rock pools on a high-intertidal to supratidal rock platform in South Africa that bears comparison to the setting described here. Those occurrences were deemed to be a potential analogue for some Archaean stromatolite occurrences. The aim in this paper is to document the occurrence of stromatolites in supratidal rock pools at the Giant’s Causeway. This occurrence is unique in Ireland and only one other case from Morgan Bay in South Africa (Smith et al., 2011) is known. The environment of deposition and the physical structure of the ABSTRACT 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. In: Conley, D.C., Masselink, G., Russell, P.E. and O’Hare, T.J. (eds.), Proceedings 12 th International Coastal Symposium (Plymouth, England), Journal of Coastal Research, Special Issue No. 65, pp. 1675-1680, ISSN 0749-0208. 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. ADDITIONAL INDEX WORDS: blue green algae, cynaobacteria, groundwater, rock coast, stromatolites.. www.JCRonline.org www.cerf-jcr.org ____________________ DOI: 10.2112/SI65-283.1 received 07 December 2012; accepted 06 March 2013. © Coastal Education & Research Foundation 2013