An approach to estimate Lower Jurassic seawater oxygen-isotope composition using delta 18O and Mg/Ca ratios of belemnite calcites (Early Pliensbachian, northern Spain)

• Published in: Terra nova (Oxford, England) England), 2013-12, Vol.25 (6), p.439-445
• Main Authors: Armendariz, Maider, Rosales, Idoia, Badenas, Beatriz, Pinuela, Laura, Aurell, Marc, Garcia-Ramos, Jose Carlos
• Format: Article
• Language: English
• Subjects: Marine
• ISSN: 0954-4879; 1365-3121
• DOI: 10.1111/ter.12054

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 delta 18O 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 delta 18Ow composition of about -0.1ppt for the Jamesoni-Ibex zones. This value may have been increased by about 0.6ppt during the Davoei Zone due to the effect of waters with a different delta 18Ow composition. These findings illustrate the inaccuracy of using a globally homogeneous ice-free value of delta 18Ow = -1ppt for delta 18Ocarb-based palaeotemperature reconstructions. Our data suggest that previous palaeotemperatures calculated in the region from delta 18O values of belemnites may have been underestimated as the seawater oxygen isotopic composition could have been higher.