Benthic foraminiferal Mg/Ca response across the Aptian-Albian Boundary Interval at DSDP Site 511 (Falkland Plateau)

Abstract

One of the largest extinctions among planktonic foraminifera occurred across the Aptian-Albian Boundary Interval (AABI), 113 million years ago. The cause of this extinction has not been determined and, paradoxically, no taxonomic turnover has been observed among other fossil groups living at the same time. The study presented here is based on core samples from Deep Sea Drilling Project (DSDP) Site 511 (Falkland Plateau), which yields the most complete record of the planktonic foraminiferal extinction record across the AABI. Stable oxygen and carbon isotopic ratios of carbonate in bulk samples both show a 2.5 ‰ negative excursion spanning ∼1 m.y. above and below the AABI. This research uses a multiproxy approach to investigate if there were significant changes in marine temperatures and/or shifts within the carbonate system that could explain the planktonic foraminiferal extinctions. New data include: 1) X-Ray diffraction analyses of surrounding sediments; 2) Mg/Ca analyses of the cross-sections of the test walls of different benthic foraminifera species in samples that span the AABI using in situ techniques; and 3) δ¹⁸O and δ¹³C on shell and spar, of monospecific separates of foraminiferal species analyzed for Mg/Ca. XRD results reveal that all samples are composed of similar mineral phases occurring in similar proportions. The Mg/Ca results show small changes across the interval suggesting a cooling trend. Finally, the new isotope analyses performed separately on the shell and spar show that the large negative excursion within the boundary interval seem more likely to represent post depositional diagenetic overprints than climatic signal.