Dedicated to the father of modern chronostratigraphy

The journal Lethaia has published in July, 2017, a thematic issue entitled “The contribution of fossils to chronostratigraphy, 150 years after Albert Oppel” and dedicated to the importance of fossils for dating and correlating of sedimentary rocks. In this issue, as explained in the forewords by Marco Balini, Annalisa Ferretti, Stan Finney and Simonetta Monechi, Oppel’s significant contribution to modern chronostratigraphy was analysed. The Phanerozoic through all its major fossil groups is then explored in a series of paper, to verify if fossils are still able to make a significant contribution to chronostratigraphy.

Albert Oppel (1831–65), a specialist on Jurassic ammonoids widely aknowledged as the father of modern bio- and chronostratigraphy.

The extraordinary merit of Oppel’s work has been the demonstration that fossils can be used to sub-divide sedimentary sequences into zones, which in turn might be organized in higher chronostratigraphical units. The zone for Oppel is characterized by the distinctive fossil content, and his view strongly influenced the development of the standard chronostratigraphical scale for about one century, until the introduction, in the 1950s, of the log-based range chart as the common practice to study the fossil record of sedimentary successions. This approach forced the stratigraphers to shift the focus from the fossil content of the zones to their boundaries, in turn allowing for the introduction of new kind of zones with precisely defined boundaries based on bioevents and to the decline of the Oppel Zone. This turning point in the history of chronostratigraphy was fuelled by the International Commission on Stratigraphy programme of definition of the units of the International Chronostratigraphic Chart based on the boundary stratotype and point (GSSP) concept, which started in 1973.

Of particular interest for the Subcommission on Paleogene Stratigraphy is the contribution by Claudia Agnini, Simonetta Monechi and Isabella Raffi on the historical background of calcareous nannofossil biostratigraphy and its application in Cenozoic chronostratigraphy.

Calcareous nannofossil evolutionary rates and diatom diversity plotted against selected physical environmental parameters. A, species-richness, speciation rate and extinction rate of calcareous nannofossils through the last 220 Myr (Bown et al. 2004); B, aragonite and Calcite seas and major sediment producers (Stanley & Hardie 1998); C, diatom diversity (Spencer-Cervato 1999); D, atmospheric CO2 (Royer et al. 2004; Pagani et al. 2005); E, temperature versus today value (Zachos et al. 2001; Royer et al. 2004). — Calcareous nannofossil evolutionary rates and diatom diversity plotted against selected physical environmental parameters. A, species-richness, speciation rate and extinction rate of calcareous nannofossils through the last 220 Myr; B, aragonite and Calcite seas and major sediment producers; C, diatom diversity; D, atmospheric CO2; E, temperature versus today value.

Starting from the 1950s, changes in calcareous nannofossil assemblages have been used to date rocks and sediments and a fundamental step was achieved two decades later with the publication of the first comprehensive biostratigraphical schemes. Standardized quantitative counting methods, unambiguous taxonomy as well as highly resolved data sets provide high-quality biostratigraphical datums which, in turn, result in the precise positioning of calcareous nannofossil biohorizons and in the construction of reliable biostratigraphical frameworks. Recently published Cenozoic biozonations are then used as a framework to present an overview of the calcareous nannofossil biohorizons which are used in chronostratigraphy to denote Cenozoic Global Standard Stratotype-section and Point.

Neogene calcareous nannofossil biozonation (modified from Backman et al. 2012): CP (Okada & Bukry 1980), NP (Martini 1971), CN (Backman et al. 2012). The Geomagnetic Polarity Time Scale (GPTS) is from Lourens et al. (2004). Grey box and dashed lines show the uncertainty in defining biozone boundaries. Biochronology is after Backman et al. (2012). On the right, images of CN stage index-species are taken from literature. * = Base; + = Top; x = crossover; ° = acme end. — Neogene calcareous nannofossil biozonation: CP, NP, CN, plotted against the Geomagnetic Polarity Time Scale (GPTS). Grey box and dashed lines show the uncertainty in defining biozone boundaries. On the right, images of CN stage index-species are taken from literature. * = Base; + = Top; x = crossover; ° = acme end. See Agnini et al., 2017, for full references.

References:

Balini, M., Ferretti, A., Finney, S., Monechi, S. (2017). The contribution of fossils to chronostratigraphy, 150 years after Albert Oppel. Lethaia 50, 323–335, doi: 10.1111/let.12224.

Agnini, C., Monechi, S., Raffi, I. (2017). Calcareous nannofossil biostratigraphy: historical background and application in Cenozoic chronostratigraphy. Lethaia 50, 447-463, doi: 10.1111/let.12218.