eEarth Discussions www.electronic-earth-discuss.net 1815-3836 1815-3844 1 1 2006 10.5194/eed-1-1-2006 http://www.electronic-earth-discuss.net/1/1/2006/ http://www.electronic-earth-discuss.net/1/1/2006/eed-1-1-2006.html http://www.electronic-earth-discuss.net/1/1/2006/eed-1-1-2006.pdf 1 19 2006-05-15 Stable isotope compositions of a late Jurassic ammonite shell: a record of seasonal surface water temperatures in the southern hemisphere? C. Lécuyer H. Bucher Laboratory “Paléoenvironnements & Paléobiosphère", UMR CNRS 5125 , B&acirc;timent “Geode", Université Claude Bernard Lyon I et Institut Universitaire de France, 27–43 Boulevard du 11 Novembre 1918, 69622 Villeurbanne, France Paläontologisches Institut und Museum, Universität Zürich, Karl Schmid-Strasse 4, 8006 Zürich, Switzerland Exceptional preservation of aragonite secreted by ammonites offers an opportunity to determine the seasonal temperature variations of Mesozoic surface waters. Ontogenetic profiles of carbon and oxygen isotope compositions have been obtained from the nacreous layer of a well-preserved Late Jurassic (Oxfordian) ammonite (<I>Perisphinctes</I>) from Madagascar. A similar range of oxygen isotope compositions was also obtained from an associated benthic bivalve (<I>Astarte</I>) which suggests the absence of sampling bias. Late Jurassic seasonal variations in the southern Hemisphere were close to 2.5&deg;C and relatively weak when compared to the 2.5&ndash;6.5&deg;C temperature range prevailing in the present-day Indian ocean at a paleolatitude of 40&plusmn;1&deg; S. According to the hypothesis of an ice cap-free Late Jurassic Earth, average sea surface temperatures may have been up to 7&deg;C higher than now.