Improved time control on Cretaceous coastal deposits: new results from Sr isotope measurements using laser ablation

Sr-isotope measurements of unaltered oyster shell calcite using laser-ablation multi-collector ICPMS techniques represent a powerful tool capable of establishing a chronostratigraphic framework for fossil near-shore deposits. This approach is tested on two shoal-water successions from the Portuguese Algarve and Lusitanian basins covering Late Barremian to Albian strata. ⁸⁷Sr/⁸⁶Sr values derived from low-Mg calcite of oyster shells fluctuate between 0.70717 ± 0.00001 and 0.70746 ± 0.00002; absolute values as well as stratigraphic trends match well with the global open-marine seawater signal. The new Sr-isotope results in combination with existing bio- and chemostratigraphic records allow for identification of an Early Aptian biocalcification crisis at both studied localities and show that this event has severely affected coastal environments along the evolving Atlantic. Siliciclastic intervals in the Algarve section are interpreted as near-shore equivalents of enhanced detrital shedding into shelf and deep-sea environments as a consequence of increased chemical weathering rates during humid climate episodes.     

The Kelvin–Helmholtz instability at Venus: What is the unstable boundary?

The Kelvin–Helmholtz instability gained scientific attention after observations at Venus by the spacecraft Pioneer Venus Orbiter gave rise to speculations that the instability contributes to the loss of planetary ions through the formation of plasma clouds. Since then, a handful of studies were devoted to the Kelvin–Helmholtz instability at the ionopause and its implications for Venus. The aim of this study is to investigate the stability of the two instability-relevant boundary layers around Venus: the induced magnetopause and the ionopause. We solve the 2D magnetohydrodynamic equations with the total variation diminishing Lax–Friedrichs algorithm and perform simulation runs with different initial conditions representing the situation at the boundary layers around Venus. Our results show that the Kelvin–Helmholtz instability does not seem to be able to reach its nonlinear vortex phase at the ionopause due to the very effective stabilizing effect of a large density jump across this boundary layer. This seems also to be true for the induced magnetopause for low solar activity. During high solar activity, however, there could occur conditions at the induced magnetopause which are in favour of the nonlinear evolution of the instability. For this situation, we estimated roughly a growth rate for planetary oxygen ions of about 7.6 × 10²⁵ s⁻¹, which should be regarded as an upper limit for loss due to the Kelvin–Helmholtz instability.     

Comment on “A Comparison of Modified Fuzzy Weights of Evidence,Fuzzy Weights of Evidence,and Logistic Regression for Mapping Mineral Prospectivity” by Daojun Zhang,Frits Agterberg,Qiuming Cheng,and Renguang Zuo Math Geosci DOI 10.1007/s11004-013-9496-8

Despite a missing definition of equivalence of mathematical models or methods by Zhang et al. (Math Geosci, 2013), an “equivalence” (Zhang et al., Math Geosci, 2013, p. 6,7,8,14) of modified weights-of-evidence (Agterberg, Nat Resour Res 20:95–101, 2011) and logistic regression does not generally exist. Its alleged proof is based on a previously conjectured linear relationship between weights of evidence and logistic regression parameters (Deng, Nat Resour Res 18:249–258, 2009), which does not generally exist either (Schaeben and van den Boogaart, Nat Resour Res 20:401–406, 2011). In fact, an extremely simple linear relationship exists only if the predictor variables are conditionally independent given the target variable, in which case the contrasts, i.e., the differences of the weights, are equal to the logistic regression parameters. Thus, weights-of-evidence is the special case of logistic regression if the predictor variables are binary and conditionally independent given the target variable.     

索伦霍芬石灰岩中的翼龙：新发现和改变采掘行为的影响

The first discoveries of pterosaur fossils were made at the end of the 18th century and during the first decades of the 19th century in Bavaria, southern Germany. They originate from limestone quarries in the vicinity of the village of Solnhofen and adjacent hamlets as well as from limestone pits close to the town of Eichst?tt.     

Orientation Distribution Within a Single Hematite Crystal

While crystallography conventionally presumes that a single crystal carries a unique crystallographic orientation, modern experimental techniques reveal that a single crystal may exhibit an orientation distribution. However, this distribution is largely concentrated; it is extremely concentrated when compared with orientation distributions of polycrystalline specimen. A case study of a deformation experiment with a single hematite crystal is presented, where the experimental deformation induced twining, which in turn changed a largely concentrated unimodal “parent” orientation distribution into a multimodal orientation distribution with a major mode resembling the parent mode and three minor modes corresponding to the progressive twining. The free and open source software MTEX for texture analysis was used to compute and visualize orientations density functions from both integral orientation measurements, i.e. neutron diffraction pole intensity data, and individual orientation measurements, i.e. electron back scatter diffraction data. Thus it is exemplified that MTEX is capable of analysing orientation data from largely concentrated orientation distributions.     

The late Miocene onset of high productivity in the Benguela Current upwelling system as part of a global pattern

We have examined the history of the elevated primary productivity associated with the Benguela Current upwelling system off southwest Africa using sediments from 7.5 to 4.8 Ma at Ocean Drilling Program Site 1085 in the middle Cape Basin. Sedimentation rates are low until 6.9 Ma. Low accumulation rates of benthic foraminifers and organic carbon indicate that biological productivity was also low. Paleoproductivity dramatically increased at 6.7–6.5 Ma and was highly variable until 4.8 Ma with productivity maxima during cooler periods. The presence of radiolarian opal only between 5.8 and 5.2 Ma suggests an interlude of silica-rich intermediate water in the Cape Basin. The onset of heightened productivity under the Benguela Current is mirrored by similar increases reported between 6.9 and 6.7 Ma in the tropical eastern Pacific, the western and northern Pacific, and the Indian Ocean. The similarity between the patterns at Site 1085 and in the Pacific and Indian Oceans suggests that the dramatic productivity increase off southwest Africa is part of a global response to paleoceanographic changes.     

The Tetrahedral 4‐Body Problem with Rotation

We consider four bodies in space with same masses forming two binaries, each one symmetric with respect to a fixed axis and moving under Newtonian gravitation in opposite directions about this axis. It is given a direct proof that all singularities of this model are due to collisions, and it is proved that the singularities due to simultaneous double collisions are regularizable. The set of equilibrium points on the total collision manifold is studied as well as the possible connections among them. We show that the set of initial conditions on a given energy surface going to quadruple collision is a union of twenty submanifolds: twelve of them have dimension 2 and the others have dimension 3. Similarly for ejection orbits from quadruple collision. This revised version was published online in July 2006 with corrections to the Cover Date.     

Relative Sea‐Level Changes During Roman Times in the Northwest Mediterranean: The 1st Century A.D. Fish Tank of Forum Julii,Fréjus,France

Fish tanks become fashionable throughout the Mediterranean area between the 1st century B.C. and the 1st century A.D. Because of this narrow chronological window, and their link to former sea level, they constitute precious archives to investigate relative sea level (RSL) since the Roman period, especially when combined with fossilized marine benthos found attached to the fish tank walls. Here, we present new results from an integrated analysis of a fish tank located in the Roman colony of Fréjus, Southeastern France. The well‐preserved biological remains on the fish tank wall allow us to estimate an RSL rise of 40 ± 10 cm at Fréjus since Roman times, consistent with a recently published range of −32 to −58 ± 5 cm for the Northwestern Mediterranean for the same time. By contrast, the findings contradict the ∼150 cm of RSL change since Roman times reported for the Northwestern Mediterranean by some authors. © 2013 Wiley Periodicals, Inc.