Stable isotope compositions of a late Jurassic ammonite shell: a record of seasonal surface water temperatures in the southern hemisphere?

Abstract

Exceptional preservation of aragonite secreted by ammonites offers an opportunity to determine the sea- sonal temperature variations of Mesozoic surface waters. Ontogenetic profiles of carbon and oxygen isotope com- positions have been obtained from the nacreous layer of a well-preserved Late Jurassic (Oxfordian) ammonite (Perisphinctes) from Madagascar. A similar range of oxygen isotope compositions was also obtained from an associated benthic bivalve (Astarte) which suggests the absence of sam- pling bias. Late Jurassic seasonal variations in the southern hemisphere were close to 2.5 C and relatively weak when compared to the 2.5-6.5 C temperature range prevailing in the present-day Indian Ocean at a paleolatitude of 40±1 S. According to the hypothesis of an ice cap-free Late Jurassic Earth, average sea surface temperatures may have been up to 7 C higher than now.