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This article reviews the most recent results concerning second harmonic generation (SHG) experiments of non-phase matchable and phase matchable powder samples at high pressures and explains the pressure dependence of the intensity of the SHG signal by correlating it to the ratio between the average coherence length and the average particle size. The examples discussed here include pressure-induced structural changes in quartz, ZnO, ice VII and KIO3. It is shown that the second harmonic generation technique is a unique tool for the detection of pressure-induced structural phase transitions. It is laboratory based and allows fast measurements. It is complementary to X-ray diffraction and provides additional information about the presence of an inversion center for unknown or controversially discussed structures at high pressure.
The forest, savanna, and grassland biomes, and the transitions between them, are expected to undergo major changes in the future due to global climate change. Dynamic global vegetation models (DGVMs) are very useful for understanding vegetation dynamics under the present climate, and for predicting its changes under future conditions. However, several DGVMs display high uncertainty in predicting vegetation in tropical areas. Here we perform a comparative analysis of three different DGVMs (JSBACH, LPJ-GUESS-SPITFIRE and aDGVM) with regard to their representation of the ecological mechanisms and feedbacks that determine the forest, savanna, and grassland biomes, in an attempt to bridge the knowledge gap between ecology and global modeling. The outcomes of the models, which include different mechanisms, are compared to observed tree cover along a mean annual precipitation gradient in Africa. By drawing on the large number of recent studies that have delivered new insights into the ecology of tropical ecosystems in general, and of savannas in particular, we identify two main mechanisms that need improved representation in the examined DGVMs. The first mechanism includes water limitation to tree growth, and tree–grass competition for water, which are key factors in determining savanna presence in arid and semi-arid areas. The second is a grass–fire feedback, which maintains both forest and savanna presence in mesic areas. Grasses constitute the majority of the fuel load, and at the same time benefit from the openness of the landscape after fires, since they recover faster than trees. Additionally, these two mechanisms are better represented when the models also include tree life stages (adults and seedlings), and distinguish between fire-prone and shade-tolerant forest trees, and fire-resistant and shade-intolerant savanna trees. Including these basic elements could improve the predictive ability of the DGVMs, not only under current climate conditions but also and especially under future scenarios.
The forest, savanna, and grassland biomes, and the transitions between them, are expected to undergo major changes in the future, due to global climate change. Dynamic Global Vegetation Models (DGVMs) are very useful to understand vegetation dynamics under present climate, and to predict its changes under future conditions. However, several DGVMs display high uncertainty in predicting vegetation in tropical areas. Here we perform a comparative analysis of three different DGVMs (JSBACH, LPJ-GUESS-SPITFIRE and aDGVM) with regard to their representation of the ecological mechanisms and feedbacks that determine the forest, savanna and grassland biomes, in an attempt to bridge the knowledge gap between ecology and global modelling. Model outcomes, obtained including different mechanisms, are compared to observed tree cover along a mean annual precipitation gradient in Africa. Through these comparisons, and by drawing on the large number of recent studies that have delivered new insights into the ecology of tropical ecosystems in general, and of savannas in particular, we identify two main mechanisms that need an improved representation in the DGVMs. The first mechanism includes water limitation to tree growth, and tree-grass competition for water, which are key factors in determining savanna presence in arid and semi-arid areas. The second is a grass-fire feedback, which maintains both forest and savanna occurrences in mesic areas. Grasses constitute the majority of the fuel load, and at the same time benefit from the openness of the landscape after fires, since they recover faster than trees. Additionally, these two mechanisms are better represented when the models also include tree life stages (adults and seedlings), and distinguish between fire-prone and shade-tolerant savanna trees, and fire-resistant and shade-intolerant forest trees. Including these basic elements could improve the predictive ability of the DGVMs, not only under current climate conditions but also and especially under future scenarios.
This research was conducted in the Rwenzori Region of the Western Branch, East African Rift System (EARS). The EARS is a tectonic structure extending over a length of more than 3000 km from the Afar Triple Junction, in Ethiopia, to Lake Malawi in the south. The Western Rift System is a roughly NE to ENE trending sector of the EARS, which runs along the western boundary of Uganda and the neighboring Democratic Republic of Congo (D.R.C). It stretches 2100 km from Nimule, NW on Uganda-Sudan border, extending to Lake Malawi in the SE of Africa. The unusual uplift of the Rwenzori Mountains within an extensional regime and the mechanisms associated with the high frequency of seismic activity in the region was hardly understood and therefore, had remained a subject of contention that needed to be critically addressed in detail. To my knowledge, this was probably the first study to be performed and documented in great depth within the domains of seismic noise variation, seismic anisotropy and b value analyses beneath the Rwenzori Region. After about six years of operation (2006-2012), the seismology group of the RIFTLINK Research Project (www.riftlink.org) acquired a vast amount of high-quality, digital data that were collected using a seismic network of well calibrated seismic equipment. The project was divided into two phases. Phase I, that operated between February 2006 - September 2007, consisted of thirty-two temporary seismic stations, which were selectively spread out in the Rwenzori Region on the Ugandan side, to detect and record extremely weak as well as strong naturally occurring earthquakes. The seismic equipment used included EDL and REFTEK digitizers, which were coupled with Güralp and MARK sensors respectively (REFTEKS: only short-period MARK sensors, EDLs: short-period MARK plus few broadband Güralp Sensors). Exactly 22375 earthquakes were recorded. The data were processed using the SEISAN software package. About 14413 earthquakes were carefully localized using the velocity model of Bram (1975) that implements a Vp=Vs ratio fixed at 1.74. Phase II, that extended between 2009-2012 consisted of thirty-two seismic stations, which were spread out around the Rwenzori Mountains, both on the Ugandan side and the neighboring D.R.C. Only Taurus digitizers that were coupled with Trillium sensors were used in the D.R.C. On the Ugandan side however, both EDL and Taurus digitizers, which were coupled with Trillium and Güralp sensors were used. ...
During the Late Cretaceous and early Cenozoic the Earth experienced prolonged climatic cooling most likely caused by decreasing volcanic activity and atmospheric CO2 levels. However, the causes and mechanisms of subsequent major global warming culminating in the late Paleocene to Eocene greenhouse climate remain enigmatic. We present deep and intermediate water Nd-isotope records from the North and South Atlantic to decipher the control of the opening Atlantic Ocean on ocean circulation and its linkages to the evolution of global climate. The marked convergence of Nd-isotope signatures 59 million years ago indicates a major intensification of deep-water exchange between the North and South Atlantic, which coincided with the turning point of deep-water temperatures towards early Paleogene warming. We propose that this intensification of Atlantic overturning circulation in concert with increased atmospheric CO2 from continental rifting marked a climatic tipping point contributing to a more efficient distribution of heat over the planet.
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.
Buchbesprechungen
(1996)
Besprochen werden die beiden folgenden Werke:
(1) Handbuch der Bodenkunde - Grundwerk. Von H. P. Blume , P. Felix-Henningsen, W.R. Fischer, H.-G. Frede, R. Horn u. K. Stahr.
(2) Thienemann, Johannes: Rossitten - drei Jahrzehnte auf der Kurischen Nehrung. Reprint der Ausgabe Melsungen, Neumann-Neudamm von 1930 (3.Aufl.).
Clastic deposits related to alluvial and estuarine environments sedimented during the early Liassic in the Veillon area (south of Talmont-Saint-Hilaire, Vendée, France). A reptile fauna including various taxa, just known by innumerable footprints, lived in that environment, in rather hot and dry climatic conditions.
Tetra-auricupride, ideally AuCu, represents the only species showing the coexistence of Au with an elevated level of Pt, as in the case of a detrital grain studied structurally for the first time, from an ophiolite-associated placer at Bolshoy Khailyk, western Sayans, Russia. We infer that tetra-auricupride can incorporate as much as ~30 mol. % of a “PtCu” component, apparently without significant modification of the unit cell. The unit-cell parameters of platiniferous tetra-auricupride are: a 2.790(1) Å, c 3.641(4) Å, with c/a = 1.305, which are close to those reported for ordered AuCu(I) in the system Au–Cu, and close also to the cell parameters of tetraferroplatinum (PtFe), which both appear to crystallize in the same space group, P4/mmm. These intermetallic compounds and natural alloys are thus isostructural. The closeness of their structures presumably allows Pt to replace Au atoms so readily. The high extent of Cu + Au enrichment is considered to be a reflection of geochemical evolution and buildup in levels of the incompatible Cu and Au with subordinate Pt in a remaining volume of melt at low levels of fO2 and fS2 in the system.