Annual Report 2010 of the Bartonian/Priabonian Boundary Stratotype Working Group

The Italian scientific community was very active in 2010. Investigations on GSSPs of the Middle-Upper Eocene transition have been completed by a number of researchers from several Italian Universities (i.e. Padua, Ferrara, Florence, Urbino, Milan) and CNR Institutes as well as from some European and USA Universities and Institutions of the “ALANO NET”, coordinated by Rio (University of Padua). The multidisciplinary studies on the Alano di Piave section (Veneto region, NE Italy), the potential candidate for defining the GSSP of the Middle/Upper Eocene, equated to the base of the Priabonian Stage, have been presented in an article entitled “Integrated bio-magnetostratigraphy of the Alano section (NE Italy): a proposal for defining the Middle/Late Eocene boundary” (co-authors Agnini, Fornaciari, Giusberti, Grandesso, Rio and Stefani (Univ of Padua), Lanci (Univ. of Urbino), Luciani (Univ. of Ferrara), Muttoni (Univ. of Milan), Palike & Spofforth (Univ. of Southampton, UK). Submitted and revised in Fall 2009, the article was accepted early this year by the Geological Society of America Bulletin. However, its publication was delaied by the GSA Editor pending the acceptance of the paper by Fornaciari et al., dealing with a detailed nannofossil biostratigraphy and chronostratigraphy, on which the main article deeply relies on. Fornaciari et al.’ paper is now accepted, then the Agnini et al. paper is expected to be published hopefully before the end of 2010 (galley proofs are on the way). In addition, Spofforth and co-authors published in 2010 the detailed oxygen and carbon isotope curves for the entire Alano section. The Alano section consists of ca. 120-130 m of bathyal gray marls interrupted in the lower part by 8 m-thick package of laminated dark to black marlstones. Intercalated in the section there are prominent marker beds, six of which are crystal tuff layers, whereas the other two are bioclastic rudites, useful for regional correlation and for an easy recognition of the various intervals of the section. The section is easily accessible, crops out continuously, is unaffected by any structural deformation, is rich in calcareous plankton and contains an expanded record of the critical interval for defining the GSSP of the Priabonian. Integrated calcareous plankton quantitative biostratigraphy (nannofossils and foraminifera), and a detailed magnetostratigraphic analysis have been conducted in high resolution especially across the critical intervals for defining the Priabonian Stage. The high resolution and solid biomagnetostratigraphic framework established at Alano has been compared with the coeval data already available (Wade, 2004) or acquired specifically from the deep sea ODP Site 1052. It is shown that the extinction of large muricate planktonic foraminifera, a major step in the evolution of this group during the Cenozoic, occurred in mid Chron C17n.3n and is probably a synchronous event over wide areas. Concomitant with this event major changes have been observed in the calcareous nannofossil assemblages with six biohorizons occurring across the Middle to Late Eocene transition, i.e. the highest occurrences (HOs) of Sphenolithus obtusus and Chiasmolithus grandis, the beginning and end of the Acme of Cribrocentrum erbae, and the beginning and end of the Isthmolithus recurvus spike. Meanwhile, it is pointed out that the lowest occurrence (LO) of Chiasmolithus oamaruensis and the HO of C. grandis must be used with extreme caution for accurate correlations and the LO of I. recurvus is much older than in previously estimates. Proposed Priabonian GSSP at Alano and its correlation potencial: The major result of the multidisciplinary study is the proposal of designating the base of the Tiziano bed, a prominent crystal tuff layer in the Alano section, NE Italy, as the GSSP of the Priabonian Stage, the standard chronostratigraphic unit of the Upper Eocene. For approximating the base of the Priabonian in the marine stratigraphic records over large areas and depositional settings the following events can be considered, (1) the extinction of the large muricate planktonic foraminifera, (2) the beginning of the Acme of the distinctive Cribrocentrum erbae, (3) the base of Chron C17n that allows also correlation with the continental records. Finally, it is very possible that the first appearance of Nummulites fabianii would remain a useful criterion for recognising the Priabonian in shallow water marine settings, even though the correlation between the larger foraminifera biostratigraphy to the GPTS must be improved. Work on the Alano section will continue in an attempt to establish an orbital tuning of the proposed GSSP and isotopic dating of the Tiziano bed, both of which would allow a much better time framing of the base of the Priabonian and improve the time scale of the Late Eocene. The following papers were published or accepted:

Spofforth, D.J.A., C. Agnini, H. Pälike, D. Rio, E. Fornaciari, L. Giusberti, V. Luciani, L. Lanci, and G. Muttoni, (2010). Organic carbon burial following the middle Eocene climatic optimum in the central western Tethys. Paleoceanography, 25, PA3210, doi:10.1029/2009PA001738.

Agnini C., Fornaciari E., Giusberti L., Grandesso P., Lanci L., Luciani V., Muttoni G., Palike H., Rio D., Spofforth D.J.A., Stefani C., (accepted). Integrated bio-magnetostratigraphy of the Alano section (NE Italy): a proposal for defining the Middle-Late Eocene boundary. Geological Society of America Bulletin.

Fornaciari E., Agnini C., Catanzariti R., Rio D., Bolla E.M., Valvasoni E., (accepted). Mid-latitude calcareous nannofossil Biostratigraphy and Biochronology across the middle to late Eocene transition. Micropaleontolgy.

Report by Isabella Premoli Silva, Chairwoman