Carbon and oxygen stable isotopes in the Maastrichtian of the Basque Country, N. Spain

Abstract

Carbon and oxygen stable isotopes have been analysed from 56 bulk sediment samples taken through an 85 m section across the first occurrence of the planktonic foraminiferan Abathomphalus mayaroensis at Zumaya, and from 10 samples across the same level at Sopelana, Basque Country, Spain. At Zumaya, carbon isotope values lie between +0.82 and +2.38‰ VPBD and correlate well with lithology. Oxygen isotope values lie between −4.00 and −2.72‰ VPBD and correlate weakly with lithology. At Sopelana, carbon isotope values range from 1.48 to 1.83; oxygen isotope values from −3.40 to −2.30‰VPBD. A carbon versus oxygen isotope cross-plot indicates some diagenetic overprinting. Oxygen isotope values imply palaeotemperatures of 27–32 °C, confirming some diagenetic influence. A smoothed carbon isotope curve shows a peak in unit 6, two troughs in unit 7, a peak in unit 8, declining values in unit 9 and another trough in the lower half of unit 10. This pattern correlates well with the limited carbon isotope curve for Sopelana. The two minima adjacent to the first occurrence of A. mayaroensis, the lower of which coincides with the first occurrence of the calcareous nannofossil Lithraphidites quadratus, may prove useful in international correlation of the Lower–Upper Maastrichtian boundary.