Volcanic ash layers in the Upper Cretaceous of the Central Apennines and a numerical age for the early Campanian

At Montagna della Maiella and at Gola del Furlo (central Apennines) two discrete layers of bentonic clay are intercalated within the pelagic (Furlo) and turbiditic/pelagic limestones (Maiella) of the Upper Cretaceous basinal succession of the Umbrian basin (Scaglia facies). The bentonite layers are dated by planktonic foraminifera to the Globotruncanita elevata zone, early Campanian, and by calcareous nannofossils to the Aspidolithus parcus zone (CC 18); they fall into the reversed interval of chron 33. Detailed correlation shows the layers to be of exactly the same age. The upper layer is dated by U/Pb on magmatic zircons to 81.67±0.21 Ma, an age compatible with the Cretaceous time-scale of Obradovich. The mineralogy of the bentonitic clays is almost pure montmorillonite and contrasts sharply with the clay mineral assemblage of the enclosing pelagic and turbiditic limestones, which is dominated by soil-derived smectite and illite in different proportions. The bentonite seams are interpreted as the submarine alteration products of wind-borne volcanic ashes. They can be followed with only minor changes in thickness over 200 km and must be derived from distant volcanic sources and related to extreme volcanic events. A possible source area is present in the Dinarides where Upper Cretaceous subduction-related magmatic rocks are widespread.     

Petrochemical evidence on the probable origin of ferriferous metasediments in western Bushmanland

The mineralized Proterozoic metasediments of Bushmanland are characterized by the presence of ferriferous rocks. This includes banded and unbanded iron formations and various types of gossans. These units are not laterally extensive and occur in different stratigraphic levels. The prevalent minerals in the ferriferous rocks are hematite, magnetite, quartz, garnet, muscovite, biotite and sillimanite, but less common occurrences of graphite, alunite, plumbojarosite, gahnite and dufrenite have been noted. The chemical variation (wt%) is extensive: total Fe₂O₃ (1.3–93.5), SiO₂ (4–93), Al₂O₃ (0.2–14.0), CaO (0.02–20.7), MnO (0.0–14.3), MgO (0.0–5.7), TiO₂ (0.0–4.4), Na₂O (0.0–2.0), K₂O (0.0–1.5) and P₂O₅ (0.1–7.0). The preliminary nature of the data set precludes, however, firm conclusions regarding stratigraphic control of the chemical composition. The trace-element contents (ppm) extend over several orders of magnitude: Zn (0–7,000), Ba (0–5,200), Cu (0–1,400), Pb (0–1,070) and Ni (6–540). Collectively, the data indicate that most of the ferriferous rocks represent highly metamorphosed sediments.     

The amplitude of turbulent shear flow

A new formulation of the problem of the statistical stability of fully turbulent shear flow is proposed, in which one seeks mean fields that bound the observed flow from the stable side. In the spirit of maximum transport theory, this formulation admits a larger set of “flows” than are dynamically possible. A sequence of constraints derived from the equations of motion can narrow this set, permitting at each step the determination of a “most stable” field free of any empirical elements. Turbulent channel flow is proposed as the first application and test of this quantitative theory. Past deductive theories for this flow, from “mean field” to “transport upper bounds,” are assessed. It is shown why these theories do not retain the significant destabilizing mechanisms of the actual flow. The implications for turbulent flow of recent work on the nonlinear and three-dimensional instability of laminar shearing flow are described. In first exploration of the “decoupled mean” stability theory proposed here, approximate analytical and numerical stability methods are used to find an amplitude and structure for the averaged flow propoerties. The quantitative results differ by considerably less than two from the observed values, providing an incentive for a more complete numerical study and for further constraints on the admitted class of flows. In the language now current for nonlinear stability theory, evidence is advanced here that anN-dimensional central manifold is adjacent to the realized turbulent flow, whereN has the largest possible value compatible with the dynamical relations.     

Dynamical seasonal prediction of Southern African summer precipitation

Prediction skill for southern African (16°–33°E, 22°–35°S) summer precipitation in the Scale Interaction Experiment-Frontier coupled model is assessed for the period of 1982–2008. Using three different observation datasets, deterministic forecasts are evaluated by anomaly correlation coefficients, whereas scores of relative operating characteristic and relative operating level are used to evaluate probabilistic forecasts. We have found that these scores for December–February precipitation forecasts initialized on October 1st are significant at 95 % confidence level. On a local scale, the level of prediction skill in the northwestern and central parts of southern Africa is higher than that in northeastern South Africa. El Niño/Southern Oscillation (ENSO) provides the major source of predictability, but the relationship with ENSO is too strong in the model. The Benguela Niño, the basin mode in the tropical Indian Ocean, the subtropical dipole modes in the South Atlantic and the southern Indian Oceans and ENSO Modoki may provide additional sources of predictability. Within the wet season from October to the following April, the precipitation anomalies in December-February are the most predictable. This study presents promising results for seasonal prediction of precipitation anomaly in the extratropics, where seasonal prediction has been considered a difficult task.     

Simulations of an isolated two-dimensional thunderstorm: Sensitivity to cloud droplet size and the presence of graupel

Cloud Resolving Models (CRMs) which are used increasingly to make operational forecasts, employ Bulk Microphysics Schemes (BMSs) to describe cloud microphysical processes. In this study two BMSs are employed in a new Nonhydrostatic σ-coordinate Model to perform two hour simulations of convection initiated by a warm bubble, using a horizontal grid resolution of 500 m. Different configurations of the two BMSs are applied, to test the effects of the presence of graupel with one scheme (2-configurations) and of changing the cloud droplet sizes in the second scheme (4-configurations), on the simulation of idealised thunderstorms. Maximum updrafts in all the simulations are similar over the first 40 minutes, but start to differ beyond this point. The first scheme simulates the development of a second convective cell that is triggered by the cold pool that develops from the outflow of the first storm. The cold pool is more intense in the simulation with graupel because of melting of graupel particles, which results in relatively large raindrops, decreases the temperature through latent heat absorption, causing stronger downdrafts, which all contribute to the formation of a more intense cold pool. The second scheme simulates the development of a second cell in two of its configurations, while two other configurations do not simulate the redevelopment. Two configurations that simulate the secondary redevelopment produce a slightly stronger cold pool just before redevelopment. Our results show that small differences in the microphysics formulations result in simulations of storm dynamics that diverge, possibly due nonlinearities in the model.     

Phosphate: a neglected argument in studies of ancient glass technology

The phosphate content of ancient alkali-glass has been tested as an indicator for biogenic ash, used in glass production. The living tissue of plants and vertebrates accumulates phosphate that remains in native ash as a main component (1 to over 10 wt%) forming together with biologically essential element oxides a complex chemical fingerprint that characterises any glass made with it. If, instead of native plant ash its alkali extract is used, soluble oxides are concentrated in the extract whilst insoluble oxides get depleted, leading to a different elemental fingerprint of the glass. Geogenic raw-materials for glass production like quartz-rich sand, evaporitic natron, or limestone are virtually phosphate free (below detection limit up to ≈0.15 wt%). Several collections of ancient glass stemming from different archaeological excavations, regions and times have been analysed by instrumental methods (WD-XFA, ED-XFA, thermal techniques), i.e. flat- and hollow K–Ca glass dated 1699/1714 N = 340, Na–Ca glass 13th/14th cy N = 94, Na–Ca glass 7th/9th cy N = 110, and alkali-Ca glass La Tène N = 395. Although the phosphate content of native alkali plant ash gets diluted with quartz sand by a factor of roughly two in glass production, it remains a main component of the product glass. Wood-ash glass contains 1–3 wt% phosphate, whilst glass manufactured from halophyte ash contains around 0.2–1 wt%. When processed wood-ash is used, the phosphate content in glass sinks below 1 wt%, the depletion factor amounting to roughly 8–15. Glass made from evaporitic soda displays phosphate contents below 0.2 wt% in the cases examined. The phosphate content of ancient alkali glass thus indicates whether virtually phosphate-free evaporitic soda has been used as a flux, or native ash of halophile plants. Ternary melting diagrams reveal at least three groups of alkali glass: Naturally coloured forest glass displays a broad distributional pattern with minimum melting temperatures ranging from 1200 to ≥1450 °C, colourless potash-extract glass from 900 to ≈1200 °C, and the typical, tight cluster of Roman sodium-calcium glass ranges from 800 to ≈900 °C. The wide-spread sodium-lime glass made with native halophytic ash, contains besides Na₂O as the chemical main component always some K₂O, and melting temperatures range from 800 to ≈1000 °C.     

The bi-decadal rainfall cycle,Southern Annular Mode and tropical cyclones over the Limpopo River Basin,southern Africa

The association between bi-decadal rainfall variability over southern Africa and the rainfall contributed by tropical cyclonic systems from the Southwest Indian Ocean (SWIO) provides a potential means towards understanding decadal-scale variability over parts of the region. A multi-decadal period is considered, focusing on the anomalous tropospheric patterns that induced a particularly wet 8-year long sub-period over the Limpopo River Basin. The wet sub-period was also characterized by a larger contribution to rainfall by tropical cyclones and depressions. The findings suggest that a broadening of the Hadley circulation underpinned by an anomalous anticyclonic pattern to the east of southern Africa altered tropospheric steering flow, relative vorticity and moisture contents spatially during the sub-period of 8 years. These circulation modulations induced enhanced potential for tropical systems from the SWIO to cause precipitation over the Limpopo River Basin. The same patterns are also conducive to increasing rainfall over the larger subcontinent, therefore explaining the positive association in the bi-decadal rainfall cycle and rainfall contributed by tropical cyclonic systems from the SWIO. An overview of regional circulation anomlies during alternating near-decadal wet and dry epochs is given. The regional circulation anomalies are also explained in hemispheric context, specifically in relation to the Southern Annular Mode, towards understanding variation over other parts of the Southern Hemisphere at this time scale.