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The oceans at the time of the Cenomanian–Turonian transition were abruptly perturbed by a period of bottom-water anoxia. This led to the brief but widespread deposition of black organic-rich shales, such as the Livello Bonarelli in the Umbria–Marche Basin (Italy). Despite intensive studies, the origin and exact timing of this event are still debated. In this study, we assess leading hypotheses about the inception of oceanic anoxia in the Late Cretaceous greenhouse world by providing a 6 Myr long astronomically tuned timescale across the Cenomanian–Turonian boundary. We procure insights into the relationship between orbital forcing and the Late Cretaceous carbon cycle by deciphering the imprint of astronomical cycles on lithologic, physical properties, and stable isotope records, obtained from the Bottaccione, Contessa and Furlo sections in the Umbria–Marche Basin. The deposition of black shales and cherts, as well as the onset of oceanic anoxia, is related to maxima in the 405 kyr cycle of eccentricity-modulated precession. Correlation to radioisotopic ages from the Western Interior (USA) provides unprecedented age control for the studied Italian successions. The most likely tuned age for the base of the Livello Bonarelli is 94.17 ± 0.15 Ma (tuning 1); however, a 405 kyr older age cannot be excluded (tuning 2) due to uncertainties in stratigraphic correlation, radioisotopic dating, and orbital configuration. Our cyclostratigraphic framework suggests that the exact timing of major carbon cycle perturbations during the Cretaceous may be linked to increased variability in seasonality (i.e. a 405 kyr eccentricity maximum) after the prolonged avoidance of seasonal extremes (i.e. a 2.4 Myr eccentricity minimum). Volcanism is probably the ultimate driver of oceanic anoxia, but orbital periodicities determine the exact timing of carbon cycle perturbations in the Late Cretaceous. This unites two leading hypotheses about the inception of oceanic anoxia in the Late Cretaceous greenhouse world.
The organic rich Livello Bonarelli formed as a result of oxygen deficiency and carbonate dissolution in the oceans during the Cenomanian/Turonian (C/T) transition. During this Ocean Anoxic Event 2 (OAE2), a combination of factors caused increased productivity, incomplete decomposition of organic matter and widespread deposition of black shales. Although these sediments are extensively studied, the exact extent, cause, timing and duration of oceanic anoxia are debated (Sinton and Duncan, 1997; Mitchell et al., 2008). Contrasting causal mechanisms have been suggested, including stratification of the water column (Lanci et al., 2010) versus intensification of the hydrological cycle driving a dynamic ocean circulation (Trabucho-Alexandre et al., 2010). Studies on trace-elemental and (radiogenic) isotope compositions of Cenomanian marine successions have suggested a volcanic origin of OAE2, by delivering nutrients to the semi-enclosed proto-North Atlantic (Zheng et al., 2013, and references therein; Du Vivier et al., 2014). Deciphering the importance of volcanic and oceanographic processes requires tight constraints on their relative timing. Regularly occurring black cherts and shales below the Livello Bonarelli demonstrate that oceanic conditions in the Umbria-Marche Basin were punctuated by episodes of regional anoxia from the mid-Cenomanian onwards. Their hierarchical stacking pattern suggests an orbital control on the deposition of organic rich horizons (Mitchell et al., 2008; Lanci et al., 2010). Stable carbon isotope data reveal that long-term 15 variations in eccentricity paced the carbon cycle (Sprovieri et al., 2013) and sea level changes (Voigt et al., 2006) of the Late Cretaceous. Here we investigate the role of orbital forcing on climate and the carbon cycle, and, specifically, on organic-rich sedimentation prior, during, and after OAE2.
We also explore the potential for establishing an anchored astrochronology for the C/T interval in Europe. Recent improvements in the astronomical solution (La2011; Laskar et al., 2011b) and in the intercalibration of radiometric and astronomical dating techniques (Kuiper et al., 2008; Renne et al., 2013) allow the extension of the astronomical time scale into the Cretaceous. The C/T boundary in the Western Interior (USA) has been dated at 93.90 ± 0.15 Ma by intercalibration of radio-isotopic and astrochronologic time scales (Meyers et al., 2012b). Also, reinterpretation of proxy records spanning the C/T interval seems to resolve discrepancies in reported durations of the OAE2 (Sageman et al., 2006; Meyers et al., 2012a). The well-documented Italian rhythmic successions, reference sections for climatic processes in the Tethyan realm, need to be tied in with the absolute time scale. Biostratigraphic correlation to radioisotopically-dated ash beds in the Western Interior is complicated by the provinciality of faunas and floras. However, δ13C stratigraphy provides a reliable correlation tool (Gale et al., 2005) and we present a new 40Ar/39Ar age for the Thatcher bentonite from the Western Interior occurring within the mid-Cenomanian δ13C event (MCE). This study integrates the well-developed cyclostratigraphy from the Umbria-Marche Basin with radioisotopic ages from the Western Interior and derives a numerical timescale for this critical interval in Earth’s history.