An experimental study of the Fe oxidation states in garnet and clinopyroxene as a function of temperature in the system CaO–FeO–Fe2O3–MgO–Al2O3–SiO2: implications for garnet–clinopyroxene geothermometry

Samples with eclogitic composition in the system CaO–FeO–Fe₂O₃–MgO–Al₂O₃–SiO₂ were produced from various kinds of starting materials held in graphite-lined Pt capsules at a pressure of 2.5–3.0 GPa and temperatures of 800–1,300 °C using a piston-cylinder or Belt apparatus. Garnets and clinopyroxenes were characterized by analytical transmission electron microscopy and electron probe micro-analysis (EPMA). Fe³⁺/ΣFe ratios determined by electron energy-loss spectroscopy (EELS) decrease in clinopyroxene from 22.2 ± 3.4 % at 800 °C to 13.3 ± 5.4 % at 1,300 °C, while in garnet, they vary between 10.8 ± 1.5 and 15.4 ± 4.7 %, respectively. Temperature estimates according to Krogh (Contrib Mineral Petrol 99:44–48, 1988) reproduce the experimental temperature to ±60 °C without systematic deviations if total iron is used in the calculation. If only the Fe²⁺ content is used, which was obtained by combining EPMA and EELS results, the experimental temperature is underestimated by 33 °C on average at 800–1,200 °C and overestimated by 77 °C on average at 1,300 °C. These systematic deviations can be explained by the temperature-dependent ratio of Fe²⁺/ΣFe in garnet divided by that in clinopyroxene. Since the difference between the calculated and experimental temperature is relatively small, a Fe²⁺-based recalibration of the thermometer appears not to be necessary for the investigated system in the range of pressure, temperature and composition covered by the experiments of this study.     

Molecular isotopic evidence of environmental and ecological changes across the Cenomanian–Turonian boundary in the Levant Platform of central Jordan

We evaluated the structure of planktonic communities and paleoenvironmental conditions throughout the Cenomanian–Turonian Oceanic Anoxic Event (OAE2) by studying bulk geochemical properties and the molecular isotopic composition of source-specific hydrocarbons from organic-rich sediments deposited in an intrashelf basin at the Levant Platform, central Jordan. High concentrations of desmethyl and 4-methylsteranes as well as dinosteranes indicate that marine algae including dinoflagellates were the main primary producing organisms. The presence of 2-methylhopanes and ¹³C-enriched hopanes, in addition to isotopically enriched aryl isoprenoids, evidenced the contribution of cyanobacteria and green sulfur bacteria, respectively. Additionally, variable but fairly low δ¹⁵N values during OAE2 suggest the occurrence of diazotrophy as a likely important process fueling primary production during OAE2 in this stratified/anoxic continental platform. Variations in the relative contribution of biomarkers revealed changes in planktonic communities associated with sea level change and water column stratification. OAE2 was characterized by strong stratification, anoxic bottom waters and a deep chemocline, as evidenced by high gammacerane and homohopane indices and the absence of photic zone euxinia (PZE) markers, respectively. However, the presence of isorenieratane and its derivatives in post-OAE black shales points to a shoaling of the chemocline and to PZE. This interval was also characterized by an exceptionally high abundance of chlorophyll-derived pristane and phytane (up to 2 mg g^?1 TOC), likely as a result of highly enhanced primary production and organic matter preservation. Remarkably, this high productivity event co-occurs with an exceptionally high abundance of calcispheres reported elsewhere to be part of a global bio-event.     

Geochemical signatures of Variscan eclogites from the Saxonian Erzgebirge, central Europe

From the abundant metre to km-sized eclogite bodies in the Variscan crystalline complex of the Saxonian Erzgebirge we have investigated 19 samples from the ultrahigh pressure area at the Saidenbach reservoir. Twenty-two samples were from the south-western Erzgebirge, and from occurrences located only some km away from the reservoir. These samples were analysed for major and trace elements using X-ray fluorescence (XRF) spectrometry and inductively coupled plasma mass spectrometry (ICP-MS).The non-Saidenbach eclogites (SiO₂=49–53 wt%) can be derived from N-mid-ocean ridge basalts (MORBs) partially transitional to P-MORBs (e.g., (Nb)_N: 3–36; (Sr)_N: 4–17; (La/Sm)_N<1.5 (in most instances <0.7) and (Sm/Yb)_N around 1.2). Eclogites from the Saidenbach reservoir (SiO₂=49–61 wt%) are characterised by (Nb)_N: 20–170; (Sr)_N: 9–43; (La/Sm)_N: 1.2–3.0; (Sm/Yb)_N: 1.4–8.8, and a clear negative Eu anomaly for the Si-rich samples, thus, being significantly different from the other investigated eclogites. These signatures point to protoliths related to within plate igneous rocks. However, we also discuss the possibilities of (1) protoliths related to a magmatic arc along an active continental margin and (2) the formation by melting of crustal material in the deep mantle and final crystallisation in the lowermost continental crust similar to the adjacent diamondiferous quartzofeldspathic rocks.Due to the specific geochemical signatures of eclogites in the Saidenbach area including other facts, this ultrahigh pressure region is believed to represent a section of lowermost crust not outcropping in other portions of the Saxonian Erzgebirge.     

Paleoclimatic implications of late quaternary lacustrine sediments in Western Nubia, Sudan

Owing to the hypercontinental location of Western Nubia, secular fluctuations of climate have been filtered and wet phases can be considered as representative of conditions throughout the southeastern Sahara. The study area is crossed by the 20-mm isohyet; between 9300 and about 4000 yr B.P., however, there were widespread lake and swamp environments with freshwater molluscs, ostracods, and diatoms, and a species-rich savanna mammal fauna. The center of the West Nubian Basin (approx. 18°N), an area of about 20,000 km², was occupied by a semiaquatic landscape which was situated at the same latitude as Paleolake Chad. From extensive lake carbonates up to about 4 m thick, a long-term rise of the groudwater table is inferred. Environments developed that now exist at about latitude 13°N. Radiocarbon dates from lake sediment sequences cluster between 30,000 and 21,000 yr B.P., indicating a Pleistocene wet phase. A gap in radiocarbon dates between 21,000 and 11,000 yr B.P. signals a phase of hyperaridity, similar to the present hyperarid phase, with eolian deflation and deposits of sand being the dominant forms of erosion and accumulation.     

Grußwort des Befehlshabers der Flotte

Ohne Zusammenfassung     

Growth of chloritoid and garnet along a nearly isothermal burial path to 70 km depth: an example from the Bughea Metamorphic Complex,Leaota Massif,South Carpathians

Mineralogy and Petrology - Chloritoid-bearing micaschist occurs in the matrix of a subduction mélange (Bughea Complex) of the Leaota Massif (South Carpathians) containing blocks of...     

The Tolerable Windows Approach: Theoretical and Methodological Foundations

The tolerable windows (TW) approach is presented as a novel scheme for integrated assessment of climate change. The TW approach is based on the specification of a set of guardrails for climate evolution which refer to various climate-related attributes. These constraints, which define what we call tolerable windows, can be purely systemic in nature – like critical thresholds for the North Atlantic Deep Water formation – or of a normative type – like minimum standards for per-capita food production worldwide. Starting from this catalogue of knock-out criteria and using appropriate modeling techniques, those policy strategies which are compatible with all the constraints specified are sought to be identified. In addition to the discussion of the basic elements and the general theory of the TW approach, a modeling exercise is carried out, based on simple models and assumptions adopted from the German Advisory Council on Global Change (WBGU). The analysis shows that if the global mean temperature is restricted to 2°C beyond the preindustrial level, the cumulative emissions of CO2 are asymptotically limited to about 1550 Gt C. Yet the temporal distribution of these emissions is also determined by the climate and socio-economic constraints: using, for example, a maximal tolerable rate of temperature change of 0.2°C/dec and a smoothly varying emissions profile, we obtain the maximal cumulative emissions, amounting to 370 Gt C in 2050 and 585 Gt C in 2100.     

Late Quaternary palæoecology and palæoclimates of the eastern Sahara

Latest field research and palæoenvironmental reconstructions have revealed that within less than 6000 years the eastern Sahara experienced a dramatic climatic change similar to that in the western Sahara, passing from hyperaridity to semi-aridity (dry savanna) to its present hyperarid state. Groundwater levels started to rise about 9300 years before present (¹⁴C years BP), leading to the formation of a mosaic of freshwater lakes and swamps. Within a few decades, the aquifers were loaded and the palæopiezometric surface was as much as 25 m higher than it is today. The uplands generated up to 800 km long fluvial systems, which put an end to the endorheic drainage of the region and functioned as migration paths for large savanna mammals. These wetter conditions persisted in Western Nubia during the Holocene until ca 5000 years BP The climatic deterioration began around 5700 years BP as shown by evaporitic sediments. Reversal events prior to aridification during the Late Holocene were not recorded systematically in the sediments of the eastern Sahara because of the stability of the ecosystems. Changes in land-surface conditions such as palæolakes, swamps and vegetation created water vapour sources that generated local rainfall and buffered short dry spells. Radiocarbon-dated charcoal indicates that Neolithic human occupation culminated during this Early Holocene wet phase and ended ca 2000 years after the fading of the wet phase at about 3000 years BP, when the shallow aquifers were exhausted.     

Ozeane: Ergebnisse der 7. Sitzung der Kommission für nachhaltige Entwicklung (CSD) der Vereinten Nationen

Ocean Dynamics - Die 7. Sitzung der Kommission für nachhaltige Entwicklung (Commission on Sustainable Development, CSD) der Vereinten Nationen fand vom 19. bis 30. April 1999 im...