Can oxygen and carbon isotope ratios of Jurassic foraminifera be used in palaeoenvironmental reconstructions?

Abstract

Oxygen and carbon isotope ratios of calcareous tests of foraminifera can serve as a tool for reconstructing seawater temperatures and bioproductivity. The foraminiferal isotope studies are, however, largely limited to Neogene, Paleogene and Cretaceous strata. In the current study are presented δ18O and δ13C values of well-preserved Jurassic representatives of different groups of benthic foraminifera: low-Mg calcite Lenticulina (Lagenata), high-Mg calcite Paleomiliolina (Miliolida, Tubothalamea) and aragonite Epistomina (Robertinida, Globothalamea) derived from clay deposits of Poland. Moreover, it provides the detailed comparison of isotopic data of Jurassic foraminifera with those of various co-occurring macrofossils (belemnite rostra, ammonite and bivalve shells). Our study and comparative data substantiate the existence of the offset of Lenticulina's δ18O values from isotope equilibrium of ca. 1‰, which seems to be relatively constant over geological time. This value may be used for calculation of ancient water temperatures from oxygen isotope ratios of well-preserved specimens of this very common and widespread Jurassic foraminiferal taxon. δ18O values of Epistomina and δ13C values of all studied foraminifera are significantly biased from isotope equilibrium.