PLANKTONIC FORAMINIFERAL EXTINCTION AND ASSOCIATED PALEOCEANOGRAPHIC CHANGES ACROSS THE APTIAN-ALBIAN BOUNDARY

Remarkably well preserved late Aptian-early Albian foraminifera from the subtropical North Atlantic (ODP Site 1049) and southern South Atlantic (DSDP Site 511) provide evidence for ~80% extinction among planktonic species at precisely the same level as major shifts in oxygen, carbon, and strontium isotopic ratios. The extinction marks the termination of several large-sized, distinctly ornamented lineages that dominated during the late Aptian. The two species that survived the extinction, one of which is new, are both very small and weakly calcified. These species give rise to several slowly evolving Albian lineages that show a gradual increase in shell size and ornamentation complexity.

Extraordinarily good microfossil preservation at Site 1049 enables acquisition of highly reliable and detailed oxygen and carbon isotopic records spanning from about 3 m.y. below and 2 m.y. above the boundary level. Stable isotopic analyses of bulk carbonate and of single benthic and planktonic species reveal coherent trends, with abrupt negative shifts at the boundary of 2.2‰ for d¹³C and 1‰ for d¹⁸O. Vertical d¹³C and d¹⁸O gradients are quite small, and surface and bottom water temperature estimates are surprisingly low throughout the late Aptian, whereas higher vertical stable isotope gradients and surface water temperatures are associated only with the early Albian OAE 1b black shale.

⁸⁷Sr/⁸⁶Sr values from Site 1049 also show an abrupt shift at the Apt/Alb boundary from average values from 0.707220 in the latest Aptian to 0.707448 in the earliest Albian. The global species extinction and geochemical shifts may have been related to water mass changes associated with opening of the South Atlantic and the environmental consequences of Cretaceous tectonism (e.g., increased outgassing, carbonate chemistry change). The foraminiferal species turnover and geochemical shifts should be considered as marker events for defining the GSSP at the base of the Albian Stage.