Investigating boron isotopes in a middle Jurassic micritic sequence: Primary vs. diagenetic signal
Résumé
Boron isotope abundance in carbonates provides a powerful proxy for seawater pH reconstructions. Nevertheless, application of this proxy on bulk rocks has never been clearly investigated. Herein, boron isotopes were measured in early Bajocian samples from the Terminilletto section (Italy) to explore the meaning of bulk rock δ11B values in paleo-environmental investigations. In the southwestern Tethys, the early Bajocian is marked by a change in sedimentation from calcareous to radiolaritic lithologies, concomitant with a δ13C positive excursion and a sedimentary hiatus in several calcareous sections. The isotopic compositions of boron extracted from Terminilletto samples range from 10‰ to 12‰ and boron content from 0.75 to 1.45 ppm, both lower than modern ranges. This implies either diagenesis or a lower boron concentration in Jurassic seawater. Boron isotopic compositions are compared to major and trace element analyses to investigate potential alteration of the carbonate primary signal. Fe and Al contents provide useful information on both clay contamination and sedimentation style. The δ11B values measured in carbonates constrain the boron isotopic composition of the precipitating fluid, either seawater or derived from seawater, regarding that both δ11B and δ18O are consistent with marine signatures. Jurassic seawater δ11B might have been lower than the modern one. The δ11B variations, consistent with micropaleontological observations, suggest a seawater alkalinization reflecting the atmospheric CO2 decrease after a probable phase of volcanism intensification prior to the Aalenian–Bajocian boundary. The carbonate crisis itself might also have promoted pH rise through progressive [CO32−] accumulation in seawater.