Abstract:
Relative depth ecologies are inferred for the first time for exquisitely preserved planktic foraminifer species that occur across the Aptian-Albian boundary at a middle bathyal depth site in the subtropical North Atlantic (ODP Site 1049). The interval of interest spans from the upper Globigerinelloides algerianus Zone (mid-Aptian) through the Hedbergella rischi Zone (early Albian). The suggested depth order interpretations assume year-round stability of the thermocline and minimal seasonal variations in light intensity or the carbon isotopic gradient of dissolved CO (sub 2) . As occurs in the modern ocean, planktic species with the most negative delta (super 18) O values and most positive delta (super 13) C values relative to co-occurring benthic species are assumed to have lived at the shallowest depths in a thermally stratified water column. Among the late Aptian assemblages, Ticinella bejaouaensis, Hedbergella trocoidea, H. infracretacea, and H. occulta yield stable isotopic profiles that suggest a mixed surface layer habitat, with T. bejaouaensis and H. trocoidea living at slightly shallower levels than co-occurring H. infracretacea and H. occulta. Planomalina cheniourensis, Globigerinelloides algerianus, G. aptiense, G. ferreolensis, and Gubkinella graysonensis, yield slightly more negative delta (super 18) O and significantly more positive delta (super 13) C values than co-occurring benthic species, suggesting that they lived within or near the thermocline. The planispiral species G. ferreolensis has traditionally been distinguished from G. aptiense by having more chambers in the final whorl and a more weakly lobate periphery. However, we found that, apart from chamber number, there are no morphologic criteria that can be consistently applied to distinguish these forms, and the stable isotope values are essentially indistinguishable from each other in all samples that have been run, suggesting these taxa are synonymous. An abrupt turnover of planktic foraminifera, including extinction of large-sized species T. bejaouaensis and H. trocoidea and disappearance of species bearing well developed pore mound wall textures, exactly corresponds with a 2 per mil negative delta (super 13) C shift at the Aptian/Albian boundary and the onset of Oceanic Anoxic Event 1b. The simultaneous change in planktic foraminifer assemblages and stable isotope values directly implicates an paleoceanographic cause for the turnover event.