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

摘要

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)Ow 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)Ow composition. These findings illustrate the inaccuracy of using a globally homogeneous ice-free value of δ~(18)Ow = -1‰ for δ~(18)Ocarb-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.