Biogeosciences www.biogeosciences.net 1726-4170 1726-4189 6 11 2009 10.5194/bg-6-2549-2009 http://www.biogeosciences.net/6/2549/2009/ http://www.biogeosciences.net/6/2549/2009/bg-6-2549-2009.html http://www.biogeosciences.net/6/2549/2009/bg-6-2549-2009.pdf 2549 2560 2009-11-13 Interpretation of benthic foraminiferal stable isotopes in subtidal estuarine environments P. Diz paula.diz@ineti.pt F. J. Jorissen G. J. Reichart C. Poulain F. Dehairs E. Leorri Y.-M. Paulet Laboratoire des Bio-Indicateurs Actuels et Fossiles (BIAF), UPRES EA 2644, Université d'Angers, 2, Boulevard Lavoisier, 49045 Angers Cedex, and LEBIM, Ker Chalon, 85350 Ile D'Yeu, France Faculty of Geosciences, Organic Geochemistry, Utrecht University, Budapestlaan 4, Utrecht, 3584 CD, The Netherlands Alfred Wegener Institute for marine and polar research, Bremerhaven, Germany LEMAR, CNRS/UBO/IRD 6539, Institut Universitaire Européen de la Mer, Place Nicolas Copernic, 29 280 Plouzané, France Faculty of Sciences, Vrije Universiteit, Pleinlaan 2, 1050, Brussels Centro de Geologia, Faculdade de Ciências da Universidade de Lisboa, Campo Grande, 1749-016 Lisboa, Portugal Unidade de Geologia Marinha, Laboratório Nacional de Energia e Geologia, Estrada da Portela, Zambujal, Amadora 2721-866, Portugal Department of Geological Sciences, East Carolina University, Graham Building 103B, Greenville NC, 27858, USA Here we present a novel approach for the interpretation of stable isotope signatures recorded in benthic foraminifera from subtidal estuarine environments. The stable isotopic composition (&delta;¹⁸O and &delta;¹³C) of living <i>Ammonia tepida</i> and <i>Haynesina germanica</i> is examined at four stations in the Auray River estuary (Gulf of Morbihan, France) sampled in two contrasting seasons, spring 2006 and winter 2007. Comparing benthic foraminiferal &delta;¹⁸O measurements with theoretical oxygen isotopic equilibrium values, calculated on the basis of water temperature and salinity measurements in the upper and lower estuary, i.e., T-S-&delta;¹⁸O_<i>eq.</i> diagrams, strongly suggests that foraminiferal faunas sampled at the four stations calcified during different periods of the year. This interpretation can be refined by using the benthic foraminiferal &delta;¹³C, which is mainly determined by the mixing of sea and river water. In the upper estuary foraminifera mainly calcified in early spring and winter, whereas in the lower estuary calcification mainly took place in spring, summer and autumn. 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