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An approach to estimate Lower Jurassic seawater oxygen‐isotope composition using δ 18 O and Mg/Ca ratios of belemnite calcites (Early Pliensbachian, northern Spain)
Author(s) -
Armendáriz Maider,
Rosales Idoia,
Bádenas Beatriz,
Piñuela Laura,
Aurell Marc,
GarcíaRamos José Carlos
Publication year - 2013
Publication title -
terra nova
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.353
H-Index - 89
eISSN - 1365-3121
pISSN - 0954-4879
DOI - 10.1111/ter.12054
Subject(s) - seawater , isotopes of oxygen , geology , oxygen , δ18o , isotope , composition (language) , stable isotope ratio , mineralogy , oxygen 18 , geochemistry , oceanography , chemistry , linguistics , physics , philosophy , organic chemistry , quantum mechanics
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.