Evidence of climatic cooling at the Early/Late Maastrichtian boundary from inoceramid distribution and isotopes: Sopelana sections, Basque Country, Spain

Abstract

The sections of Sopelana I and II are mainly characterised by couplets of alternating marly limestones and marls that were deposited in a deep-sea basin environment of Maastrichtian age in the Basque Arc Domain. Both are characterised by a great abundance of inoceramids, which disappear below the Lower/Upper Maastrichtian boundary. Petrographic and isotopic evidence suggests that the carbonate sediment and inoceramid shells have undergone the diagenetic effects of burial processes. Nevertheless, the trends in palaeoenvironmental signals are still recognised on the isotopic curves plotted, albeit somewhat attenuated. Three stages have been defined and named E1 to E3, from oldest to youngest. Stage E1 is characterised by an abundance and diversity of inoceramids in a Transgressive Systems Tract (TST), as well as by a close parallelism between δ 18O and δ 13C curves in both the inoceramid shells and the matrix. Stage E2 comprises the final part of the TST and the Highstand Systems Tract (HST), showing a positive excursion of δ 18O values. However, there are some fluctuations, and this excursion is more clearly seen in the inoceramid shells (>3‰) than in the matrix (>1.5‰) of the Sopelana I section. Following a decline in abundance and diversity, inoceramid extinction is observed in the Basque Arc Domain. Finally, stage E3 is at the base of the Upper Maastrichtian, and characterised by a drastic lithologic change to red marls and a depletion in δ 13C values, an isotopic pattern typical of lowstand stages (LST). The record of the boreal inoceramid Spyridoceramus tegulatus, which is coincident with the positive δ 18O excursion in stage E2, suggests the entry of deep, cold, oxygenated waters arriving from the North Atlantic. These observations confirm the modification of the thermocline and the onset of climatic cooling close to the Early/Late Maastrichtian boundary, which might have been one of the main causes of the inoceramid disappearance in the Basque Arc Domain.