An anchored astronomical time-scale for the Turonian reference sections in the Umbria-Marche Basin, Italy

In the Umbria-Marche basin, the aftermath of Ocean Anoxic Event 2 (OAE2, Bonarelli Level) is represented
by the Turonian part of the Scaglia Rossa Formation. The Scaglia Rossa pelagic limestones were studied in
the classic Contessa and Bottaccione sections near Gubbio, in the Umbria-Marche region of the northeastern
Apennines of Italy. Oscillations between radiolarian cherts interbedded with foram-coccolith pelagic limestones
are interpreted to follow the rhythm of precession and show hierarchical bundles, which are suggestive of
eccentricity-related grouping. Eccentricity-bundles are correlated amongst the two studied sections. Moreover, the
magnetic susceptibility signal of the Bottaccione section and the 18O and 13C record of both sections clearly
demonstrate the imprint of precession and eccentricity. Eccentricity minima are associated with relatively warm
periods (18O minima), characterized by an increased magnetic susceptibility signal and radiolarian blooms,
which are expressed by frequent chert beds. Radiolarian blooms seem to hamper primary productivity, given that
they correlate with 13C minima.
The delineated astronomical cycles constitute an eccentricity-based cyclostratigraphy for the Turonian part of the
Scaglia Rossa. Moreover, the constructed cyclostratigraphy is anchored to numerical time by calibration with
the astronomical solution La2010 (Laskar et al., 2011) and with recent radioisotopic ages from the Cenomanian-
Turonian boundary interval near the GSSP in Colorado, USA (Meyers et al., 2012). The numerical age (93.9
0.15 Ma; Meyers et al., 2012) of the Cenomanian-Turonian boundary (0.75 m above the top of the Bonarelli Level
in the Contessa section; Tsikos et al., 2004; Kennedy et al., 2005) is used as the pinpoint to which our astronomical
time-scale is anchored. Using the anchored astronomical time-scale for the Turonian of the Umbria-Marche
basin, the top of the Bonarelli Level is placed at 93.97 0.25 Ma, and the boundary between the Whiteinella
archaeocretacea and Helvetoglobotruncana helvetica planktonic foraminiferal zones is put at 93.48 0.25 Ma.
These ages are consistent with the numerical ages obtained from radioisotopic dating of the near-GSSP USGS #1
Portland core (Meyers et al., 2012).
High-resolution XRF geochemical analysis through the 82 cm thick Bonarelli Level in the Bottaccione section at
Gubbio, reveals four strong 21 cm thick cycles. Spectral analyses on the SiO2 and Al2O3 concentration and in
the Si/Al ratio suggest an eccentricity and precession signature if one assumes an average sedimentation rate of
2.0 m/Myr during the OAE2 in this pelagic basin. These results indicate a duration of 410 kyr for the Bonarelli
Level and place the bottom of this anoxic interval at 94.38 0.25 Ma. In the near future, the latter marker-bed
could be used to connect the astronomical time-scale presented in this abstract to the Cenomanian astronomical
time-scale based on the nearby Furlo section (Batenburg et al., this session), in order to obtain a considerably
long (> 5 Myr) astronomically calibrated time-scale across a an intriguing time interval in Earth history, with
unprecedented precision and accuracy.
References
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