DECIPHERING TEMPERATURE AND SALINITY FROM BIOGENIC PHOSPHATES - THE DELTA-O-18 OF COEXISTING FISHES AND MAMMALS OF THE MIDDLE MIOCENE SEA OF WESTERN FRANCE

This study examines the possibility of estimating the oxygen isotopic composition and temperature of seawater by using a combination of delt a(18)O values of phosphate from coexisting fishes and mammals of the M iddle Miocene sea of western France. Teeth of different species of tel eosteans and selachians, and rib fragments of Metaxytherium medium (re lated to modern Dugongs) were sampled in the Langhian-Seravallian calc areous sediments of eastern Brittany and Touraine. delta(18)O values o f fishes range from 21.4 to 22.7 parts per thousand while those of mam mals range from 20.1 to 21.7 parts per thousand. The variability in de lta(18)O values of Miocene fishes is not related either to sampling lo calities or taxa. Oxygen isotope analyses were also carried out on liv ing Dugong dugon from Djibouti (delta(18)O=19.6-20.3 parts per thousan d) and on Hydrodamalis gigas which lived two hundred years ago in the low salinity waters of the Bering Strait (delta(18)O=17.3 parts per th ousand). The results show that the slope of the oxygen isotope fractio nation curve between the dugongids and water is probably close to 1. d elta(18)O variations within Miocene fsh and mammal populations are sim ilar and close to or slightly higher than 1 parts per thousand. This v ariation may be attributed mainly to a delta(18)O change in the ambien t seawater composition. The Miocene shallow water masses in western Fr ance were thus characterized by varying delta(18)O values (as well as average salinities) equal to or higher than the contemporaneous open w orld ocean. Calculated average temperatures of 20+/-2 degrees C are co mpatible with a sub-tropical climate. Local and global causes for this variation in the oxygen isotope composition of Miocene seawater are d iscussed.