Deciphering “temperature” and “salinity” from biogenic phosphates: the δ18O 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 δ18O values of phosphate from coexisting fishes and mammals of the Middle Miocene sea of western France. Teeth of different species of teleosteans and selachians, and rib fragments of Metaxytherium medium (related to modern Dugongs) were sampled in the Langhian-Seravallian calcareous sediments of eastern Brittany and Touraine. δ18O values of fishes range from 21.4 to 22.7‰ while those of mammals range from 20.1 to 21.7‰. The variability in δ18O values of Miocene fishes is not related either to sampling localities or taxa. Oxygen isotope analyses were also carried out on living Dugong dugon from Djibouti (δ18O = 19.6−20.3‰) and on Hydrodamalis gigas which lived two hundred years ago in the low salinity waters of the Bering Strait (δ18O = 17.3‰). The results show that the slope of the oxygen isotope fractionation curve between the dugongids and water is probably close to 1. δ18O variations within Miocene fish and mammal populations are similar and close to or slightly higher than 1‰. This variation may be attributed mainly to a δ18O change in the ambient seawater composition. The Miocene shallow water masses in western France were thus characterized by varying δ18O values (as well as average salinities) equal to or higher than the contemporaneous open world ocean. Calculated average temperatures of 20±2°C are compatible with a sub-tropical climate. Local and global causes for this variation in the oxygen isotope composition of Miocene seawater are discussed