An approach to estimate Lower Jurassic seawater oxygen‐isotope composition using δ¹⁸O and Mg/Ca ratios of belemnite calcites (Early Pliensbachian, northern Spain)

Abstract

Abstract Palaeotemperature estimates from the oxygen‐isotope compositions of belemnites have been hampered by not knowing ancient seawater isotope compositions well enough. We have tackled this problem using Mg/Ca as a proxy for temperature and here, we present a ~2 Ma record of paired Mg/Ca and δ 18 O measurements of Jurassic (Early Pliensbachian) belemnites from the Asturian basin as a palaeo‐proxy of seawater oxygen‐isotope composition. From the combined use of the two approaches, we suggest a δ 18 O w composition of about −0.1‰ for the Jamesoni–Ibex zones. This value may have been increased by about 0.6‰ during the Davoei Zone due to the effect of waters with a different δ 18 O w composition. These findings illustrate the inaccuracy of using a globally homogeneous ice‐free value of δ 18 O w = −1‰ for δ 18 O carb ‐based palaeotemperature reconstructions. Our data suggest that previous palaeotemperatures calculated in the region from δ 18 O values of belemnites may have been underestimated as the seawater oxygen isotopic composition could have been higher.