Diachrony between shallow-water and pelagic crisis: the example of the Bartonian-Priabonian boundary

One of the major goals of the International Commission on Stratigraphy is to define the GSSP (Global Stratotype Section and Point) for each of the basic subdivisions (stages) of the chronostratigraphic scale. In the Paleogene, most of the stages have defined GSSPs, but still Bartonian, Priabonian, and Chattian have not. The base of the Priabonian is especially interesting since it occurred lose to the last greenhouse-icehouse transition (e.g., Zachos et al., 2001). The Eocene-Oligocene boundary was for sure a bigger crisis, nevertheless the Middle-Upper Eocene boundary (=Bartonian-Priabonian) recorded a profound change in the shallow water communities, the extinction of all the large species of nummulites being one of the more conspicuous. In the pelagic realm, the extinction of the muricate planktonic foraminifera and some changes in the calcareous nannoplankton assemblages also mark a generalized biotic turnover. Mass extinctions are defined as differing from the so-called ‘background’ extinctions because of their speed and intensity (Armstrong & Brasier, 2005), but several authors pointed out that they are also poorly or definitely non-selective (Jablonski, 2005) and affecting different paleoenvironments. Therefore, the Bartonian-Priabonian transition, aside for the relatively low intensity, seems to fit the requirements to be ascribed to a (minor) mass extinction event. A good chance to check wether the extinction events were contemporary or not in the shallow and deep water environments came by the work to find a section suitable for establishing the GSSP for the base of the Priabonian. A good pelagic section close to the type area of the Priabonian in northern Italy was intensively studied in Alano di Piave (northern Italy; Agnini et al., 2011). Here, several extinction and appearances among planktonic foraminifera and calcareous nannoplankton were carefully recorded, and the extinction of the muricate large acarininids and Morozovelloides were proposed as one of the possible markers of the base of the Priabonian. In the Alano di Piave section some turbiditic levels bear larger foraminifera, giving the chance to check the correlation with the plankton events. Only two of these levels contain enough material to be ascribed to the Shallow Benthic Zone (SBZ) 17 (Serra-Kiel et al., 1998). This is currently considered as lower Bartonian, but it has to be underlined that both turbidites are well below (at least 25 m) any of the plankton events. Another section in Varignano (northern Italy), some 80 km west of Alano di Piave, bear several turbiditic layers with larger foraminifera (Papazzoni et al., 2014) and allowed to recognize that the large nummulites, such as N. ex gr. millecaput, are still present above the plankton events. Because these large nummulites are indicative of the SBZ 18, considered as upper Bartonian, we have a clear diachrony between the main extinction events in the pelagic realm and in the shallow-water facies. This is further confirmed by the study (in progress) on the Urtszador section in Armenia, where the larger nummulites got extinct much later than the muricate forams. Therefore, the multidisciplinary study of the Bartonian-Priabonian transition reveal that the crisis in shallow and deep water are diachronous. References Agnini, C., Fornaciari, E., Giusberti, L., Grandesso, P., Lanci, L., Luciani, V., Muttoni, G., Pälike, H., Rio, D., Spofforth, D.J.A., Stefani, C., 2011. Integrated bio-magnetostratigraphy of the Alano section (NE Italy): a proposal for defining the middle-late Eocene boundary. Geological Society of America Bulletin, 123: 841-872. Armstrong, H., Brasier, M., 2005. Microfossils (2nd Edition). Blackwell, Malden Oxford Carlton, 296 pp. Jablonski, D., 2005. Mass extinctions and macroevolution. Paleobiology, 31 (2): 192-210. Papazzoni, C.A., Moretti, A., Luciani, V., Fornaciari, E., Giusberti, L., 2014. Correlation between shallow benthic zones and calcareous plankton zones at the Bartonian-Priabonian transition: preliminary results from the Varignano section (Trento Province, northern Italy). In: Rocha, R., Pais, J., Kullberg, J.C., Finney, S. (Eds.), STRATI 2013 First International Congress on Stratigraphy - At the Cutting Edge of Stratigraphy, Springer, pp. 127-130. Serra-Kiel, J., Hottinger, L., Caus, E., Drobne, K., Ferràndez, C., Jauhri, A.K., Less, Gy., Pavlovec, R., Pignatti, J., Samsó, J.M., Schaub, H., Sirel, E., Strougo, A., Tambareau, Y., Tosquella, J., Zakrevskaya, E., 1998. Larger foraminiferal biostratigraphy of the Tethyan Paleocene and Eocene. Bulletin de la Société géologique de France, 169 (2): 281-299. Zachos, J.C., Pagani, M., Sloan, L., Thomas, E., Billups, K., 2001. Trends, rhythms, and aberrations in global climate 65 Ma to present. Science, 292: 686-693.