The Unique Significance and Origin of the Cretaceous-Tertiary Boundary: Historical Context and Burdens of Proof
The abruptness and intensity of the Cretaceous-Tertiary boundary have been deemphasized by some authors over recent years, mainly by those skeptical of an impact origin for the boundary. However, it was recognized at the birth of stratigraphy as both abrupt and of major importance. It was used to define the change from the Mesozoic to the Cenozoic; the boundary has become continually more precisely defined and its global sequences more correlatable. It is now unique in being an event boundary marked by an iridium-bearing layer of global extent, rather than an arbitrary boundary in a sequence of little change. The Permian-Triassic boundary, in contrast, is arbitrary and the transition is not yet proven to be abrupt, the extinctions that define it perhaps having taken place in pulses over several millions of years. Some of those who have denied the importance (and in some cases even the existence) of an impact in the Cretaceous-Tertiary extinctions have placed burdens of proof on the impact hypothesis that they do not place on strictly terrestrial mechanisms. Terrestrial mechanisms have always been unsatisfactory (or at least unconvincing for global, massive, multienvironment faunal change) and are now even more so. Some authors have required of the impact hypothesis attributes that are not inherent in it, including particular patterns of extinction selectivity and timing.
Stable-Isotope and Trace Element Stratigraphy of the Jurassic/Cretaceous Boundary, Bosso River Gorge, Italy
Traditionally, the Jurassic/Cretaceous (J/K) boundary has not been well defined. Recently, a large impact structure in South Africa, the Morokweng impact structure, was found to have a radiometric age (145 Ma) indistinguishable from the stratigraphic age of the J/K boundary. Thus, it is conceivable that at the boundary, if indeed coeval with this impact structure, a record of the environmental changes associated with the impact event might exist. This could be manifested in the form of stratigraphic and geochemical features, such as sudden stable-isotope shifts, but also by the presence of impact markers, such as siderophile element anomalies. Prompted by the age similarity between Morokweng and the J/K boundary, we searched for possible impact signatures in a set of samples spanning about 40 m across the J/K boundary at the Bosso River Gorge in the Marche region of northern Italy. Various stratigraphic levels have been proposed for the boundary: by calcareous nannofossils (at 329 m), calpionellids (at 334.1 m), and magnetostratigraphy (at 318 m, in this stratigraphic interval). Carbon and oxygen stable-isotope analyses were performed on 110 carbonate samples, and trace element contents were determined in corresponding decarbonated samples. In the lower part of the sampled section, δ13C and δ18O values show only limited variations between 1.3‰ and 1.6‰ and a gradual shift from -1.2‰ to -2.0‰, respectively; at 333.5 m both values decrease significantly by 0.3‰ and 1.0‰, respectively. Most trace element abundances increase slightly across the lower third of the stratigraphic interval, onto which highly variable values are superimposed, probably as a result of variable amounts of a silicate or clay fraction. Very close to the boundary as defined by calcareous nannofossils (at 329 m), the abundances of Fe, Co, and Ni, and, to a lesser degree, Cr, show a marked enrichment compared to other samples. Another minor anomaly is present at 333.5 m for Ir and Cr. Thus, at best, the present data might be interpreted to hint at - but not to confirm - the presence of an impactoclastic layer at the Bosso River Gorge.
On the use of the Hardening Soil Small Strain model in geotechnical practice
This article highlights the importance of using advanced constitutive soil models in numerical modelling as daily geotechnical practice. In this context, the Hardening Soil (HS) model is presented as the most advanced soil model implemented in the ZSoil finite element code. The article recalls the basic features of the HS model and it explains their role in numerical simulations. The role of pre-failure non-linear behaviour of soils is also discussed. An application of the HS model is illustrated on typical geotechnical problems such as retaining wall excavation, tunnel excavation and undrained loading followed by consolidation.