ALBERT

Description: An integrated magneto-biostratigraphic framework is presented for Middle Miocene sediments of DSDP Site 372 located in the Western Mediterranean. Detailed biostratigraphic analysis shows a nearly complete sequence of early Middle Miocene calcareous plankton bioevents in the Mediterranean, including the LCO (Last Common Occurrence) of the nannofossil Sphenolithus heteromorphus which has been astronomically dated in the Ras il Pellegrin (RIP) section on Malta Island [Abels, H.A., Hilgen, F.J., Krijgsman, W., Kruk, R.W., Raffi, I., Turco, E., Zachariasse, W.J., 2005. Long-period orbital control on middle Miocene global cooling: integrated stratigraphy and astronomical tuning of the Blue Clay Formation on Malta, Paleoceanography, 20, PA4012. doi:10.1029/2004PA001129. 11 pp]. Thermal demagnetization of discrete samples revealed a characteristic low-temperature component with dual polarities despite a weak paleomagnetic signal. The resultant magnetostratigraphic record, combined with the calcareous plankton biostratigraphy, is straightforwardly correlated to the geomagnetic polarity time scale (CK95) of Cande and Kent [Cande, S.C., Kent, D.V., 1995. Revised calibration of the Geomagnetic Polarity Time Scale for the Late Cretaceous and Cenozoic, J. Geophys. Res., 100, 6093-6095] and the Astronomical Tuned Neogene Time Scale (ATNTS04) of Lourens et al. [Lourens, L.J., Hilgen, F.J., Laskar, J., Shackleton, N.J., Wilson, D., 2004. The Neogene Period. In: Gradstein F.M., Ogg J.G., Smith A.G. (Eds.), A Geologic Time Scale, Cambridge Univ. Press, pp. 409-440]. The subchrons recorded in Site 372 succession range from C5Br up to C5AAr. To confirm the magnetostratigraphic calibration, 40Ar/39Ar dating was performed on feldspar of two volcanic ash layers. The radio-isotopic dating indicates a younger age for these two ash layers compared to the magnetostratigraphic calibrated ages according to the CK95 and ATNTS04 age models. However, if the astronomically calibrated age of 28.21 +/- 0.04 Ma is used for the Fish Canyon standard (FCs), the age for the older ash layer exactly matches its ATNTS04 age. Ages for bioevents were calculated assuming constant sedimentation rates between magnetostratigraphic age-tie points. The S. heteromorphus LCO has an age of 13.54 Ma and 13.63 Ma according to CK95 and ATNTS04, respectively, which is consistent with the astronomical tuned age of 13.65 Ma determined at RIP section [Abels, H.A., Hilgen, F.J., Krijgsman, W., Kruk, R.W., Raffi, I., Turco, E., Zachariasse, W.J., 2005. Long-period orbital control on middle Miocene global cooling: integrated stratigraphy and astronomical tuning of the Blue Clay Formation on Malta, Paleoceanography, 20, PA4012. doi:10.1029/2004PA001129. 11 pp]. We therefore conclude that the magnetostratigraphic calibration of DSDP Site 372 is correct and that, for the time being, this site can be considered as a reference section for the early Middle Miocene of the Mediterranean region.

Description: The astronomical-tuned time scale is rapidly extended into the Paleogene but, due to the existence of an Eocene gap, different tuning options had to be presented for the Paleocene. These options differ both in number and tuning of ~405-kyr eccentricity related cycles and are only partially consistent with recalculated 40Ar/39Ar constraints for the Cretaceous/Paleogene (K/Pg) and Paleocene/Eocene (P/E) boundaries. In this paper, we evaluate the cyclostratigraphic interpretation of records from ODP Leg 198 and 208 sites, and the Zumaia section to solve the problem of the different tuning options. We found that the interval between the K/Pg boundary and the early Late Paleocene biotic event (ELPE) comprises 17 instead of 16 * ~405-kyr eccentricity related cycles as previously proposed, while the entire Paleocene contains 25 * ~405-kyr cycles. Starting from 40Ar/39Ar age constraints for the K/Pg boundary, a new tuning to 405-kyr eccentricity is presented for the Paleocene and earliest Eocene, which results in ages of ~66.0 and ~ 56.0 Ma for the K/Pg and P/E boundaries, respectively. This tuning introduces considerable differences in age for a number of nannofossil events at ODP Sites 1209 and 1262 in the interval between 61 and 63 Ma, but eliminates large and abrupt changes in the seafloor spreading rate. The tuning seems further consistent with recalculated 40Ar/39Ar ages for ash layer -17 of early Eocene age. However, despite this apparent consistency with existing radio-isotopic constraints, an alternative 405-kyr younger or, less likely, older tuning cannot be excluded at this stage.

Description: The 40Ar/39Ar dating method is among the most versatile of geochronometers, having the potential to date a broad variety of K-bearing materials spanning from the time of Earth’s formation into the historical realm. Measurements using modern noble-gas mass spectrometers are now producing 40Ar/39Ar dates with analytical uncertainties of ∼0.1%, thereby providing precise time constraints for a wide range of geologic and extraterrestrial processes. Analyses of increasingly smaller subsamples have revealed age dispersion in many materials, including some minerals used as neutron fluence monitors. Accordingly, interpretive strategies are evolving to address observed dispersion in dates from a single sample. Moreover, inferring a geologically meaningful “age” from a measured “date” or set of dates is dependent on the geological problem being addressed and the salient assumptions associated with each set of data. We highlight requirements for collateral information that will better constrain the interpretation of 40Ar/39Ar data sets, including those associated with single-crystal fusion analyses, incremental heating experiments, and in situ analyses of microsampled domains. To ensure the utility and viability of published results, we emphasize previous recommendations for reporting 40Ar/39Ar data and the related essential metadata, with the amendment that data conform to evolving standards of being findable, accessible, interoperable, and reusable (FAIR) by both humans and computers. Our examples provide guidance for the presentation and interpretation of 40Ar/39Ar dates to maximize their interdisciplinary usage, reproducibility, and longevity.