An inter-comparison of regional climate models for Europe: model performance in present-day climate

The analysis of possible regional climate changes over Europe as simulated by 10 regional climate models within the context of PRUDENCE requires a careful investigation of possible systematic biases in the models. The purpose of this paper is to identify how the main model systematic biases vary across the different models. Two fundamental aspects of model validation are addressed here: the ability to simulate (1) the long-term (30 or 40 years) mean climate and (2) the inter-annual variability. The analysis concentrates on near-surface air temperature and precipitation over land and focuses mainly on winter and summer. In general, there is a warm bias with respect to the CRU data set in these extreme seasons and a tendency to cold biases in the transition seasons. In winter the typical spread (standard deviation) between the models is 1 K. During summer there is generally a better agreement between observed and simulated values of inter-annual variability although there is a relatively clear signal that the modeled temperature variability is larger than suggested by observations, while precipitation variability is closer to observations. The areas with warm (cold) bias in winter generally exhibit wet (dry) biases, whereas the relationship is the reverse during summer (though much less clear, coupling warm (cold) biases with dry (wet) ones). When comparing the RCMs with their driving GCM, they generally reproduce the large-scale circulation of the GCM though in some cases there are substantial differences between regional biases in surface temperature and precipitation.     

Hypogene Zn carbonate ores in the Angouran deposit,NW Iran

The world-class Angouran nonsulfide Zn–Pb deposit is one of the major Zn producers in Iran, with resources estimated at about 18 Mt at 28% Zn, mainly in the form of the Zn carbonate smithsonite. This study aims to characterize these carbonate ores by means of their mineralogy and geochemistry, which has also been extended to the host rocks of mineralization and other local carbonate rock types, including the prominent travertines in the Angouran district, as well as to the local spring waters. Petrographical, chemical, and stable isotope (O, H, C, Sr) data indicate that the genesis of the Zn carbonate ores at Angouran is fairly distinct from that of other “classical” nonsulfide Zn deposits that formed entirely by supergene processes. Mineralization occurred during two successive stages, with the zinc being derived from a preexisting sulfide ore body. A first, main stage of Zn carbonates (stage I carbonate ore) is associated with both preexisting and subordinate newly formed sulfides, whereas a second stage is characterized by supergene carbonates (Zn and minor Pb) coexisting with oxides and hydroxides (stage II carbonate ore). The coprecipitation of smithsonite with galena, pyrite and arsenopyrite, as well as the absence of Fe- and Mn-oxides/hydroxides and of any discernible oxidation or dissolution of the sphalerite-rich primary sulfide ore, shows that the fluids responsible for the main, stage I carbonate ores were relatively reduced and close to neutral to slightly basic pH with high fCO₂. Smithsonite δ¹⁸O_VSMOW values from stage I carbonate ore range from 18.3 to 23.6‰, while those of stage II carbonate ore show a much smaller range between 24.3 and 24.9‰. The δ¹³C values are fairly constant in smithsonite of stage I carbonate ore (3.2–6.0‰) but show a considerable spread towards lower δ¹³C_VPDB values (4.6 to −11.2‰) in stage II carbonate ore. This suggests a hypogene formation of stage I carbonate ore at Angouran from low-temperature hydrothermal fluids, probably mobilized during the waning stages of Tertiary–Quaternary volcanic activity in an environment characterized by abundant travertine systems throughout the whole region. Conversely, stage II carbonate ore is unambiguously related to supergene weathering, as evidenced by the absence of sulfides, the presence of Fe-Mn-oxides and arsenates, and by high δ¹⁸O values found in smithsonite II. The variable, but still relatively heavy carbon isotope values of supergene smithsonite II, suggests only a very minor contribution by organic soil carbon, as is generally the case in supergene nonsulfide deposits.     

Concordant ages for the giant Kipushi base metal deposit (DR Congo) from direct Rb–Sr and Re–Os dating of sulfides

We report concordant ages of 451.1 ± 6.0 and 450.5 ± 3.4 Ma from direct Rb–Sr and Re–Os isochron dating, respectively, of ore-stage Zn–Cu–Ge sulfides, including sphalerite for the giant carbonate-hosted Kipushi base metal (+Ge) deposit in the Neoproterozoic Lufilian Arc, DR Congo. This is the first example of a world-class sulfide deposit being directly dated by two independent isotopic methods. The 451 Ma age for Kipushi suggests that the ore-forming solutions did not evolve from metamorphogenic fluids mobilized syntectonically during the Pan-African-Lufilian orogeny but rather were generated in a Late Ordovician postorogenic, extensional setting. The homogeneous Pb isotopic composition of the sulfides indicates that both Cu–Ge- and Zn-rich orebodies of the Kipushi deposit formed contemporaneously from the same fluid system. The sulfide Pb isotope signatures in combination with initial ⁸⁷Sr/⁸⁶Sr and ¹⁸⁷Os/¹⁸⁸Os ratios defined by the isochrons point to metal sources located in the (upper) crust. The concordant Re–Os and Rb–Sr ages obtained in this study provide independent proof of the geological significance of direct Rb–Sr dating of sphalerite.     

Investigations into a small fraction of volatile hydrocarbons: III. Two diatom cultures produce ectocarpene, a pheromone of brown algae

Ectocarpene was extracted from cultures of Skeletonema costatum and probably in traces from cultures of Lithodesmium undulatum shortly after a change in the light regime which is known to induce formation of gametes.     

Massive barite deposits and carbonate mineralization in the Derugin Basin, Sea of Okhotsk: precipitation processes at cold seep sites

An area of massive barite precipitations was studied at a tectonic horst in 1500 m water depth in the Derugin Basin, Sea of Okhotsk. Seafloor observations and dredge samples showed irregular, block- to column-shaped barite build-ups up to 10 m high which were scattered over the seafloor along an observation track 3.5 km long. High methane concentrations in the water column show that methane expulsion and probably carbonate precipitation is a recently active process. Small fields of chemoautotrophic clams (Calyptogena sp., Acharax sp.) at the seafloor provide additional evidence for active fluid venting. The white to yellow barites show a very porous and often layered internal fabric, and are typically covered by dark-brown Mn-rich sediment; electron microprobe spectroscopy measurements of barite sub-samples show a Ba substitution of up to 10.5 mol% of Sr. Rare idiomorphic pyrite crystals (∼1%) in the barite fabric imply the presence of H₂S. This was confirmed by clusters of living chemoautotrophic tube worms (1 mm in diameter) found in pores and channels within the barite. Microscopic examination showed that micritic aragonite and Mg-calcite aggregates or crusts are common authigenic precipitations within the barite fabric. Equivalent micritic carbonates and barite carbonate cemented worm tubes were recovered from sediment cores taken in the vicinity of the barite build-up area. Negative δ¹³C values of these carbonates (>−43.5‰ PDB) indicate methane as major carbon source; δ¹⁸O values between 4.04 and 5.88‰ PDB correspond to formation temperatures, which are certainly below 5°C. One core also contained shells of Calyptogena sp. at different core depths with ¹⁴C-ages ranging from 20?680 to >49?080 yr. Pore water analyses revealed that fluids also contain high amounts of Ba; they also show decreasing SO₄^2- concentrations and a parallel increase of H₂S with depth. Additionally, S and O isotope data of barite sulfate (δ³⁴S: 21.0-38.6‰ CDT; δ¹⁸O: 9.0-17.6‰ SMOW) strongly point to biological sulfate reduction processes. The isotope ranges of both S and O can be exclusively explained as the result of a mixture of residual sulfate after a biological sulfate reduction and isotopic fractionation with ‘normal’ seawater sulfate. While massive barite deposits are commonly assumed to be of hydrothermal origin, the assemblage of cheomautotrophic clams, methane-derived carbonates, and non-thermally equilibrated barite sulfate strongly implies that these barites have formed at ambient bottom water temperatures and form the features of a Giant Cold Seep setting that has been active for at least 49?000 yr.     

Subduction-related lithium metasomatism during exhumation of the Alpe Arami ultrahigh-pressure garnet peridotite (Central Alps, Switzerland)

The inter- and intragrain distribution of Li and Be in the subduction-related ultrahigh-pressure (UHP) garnet peridotite from Alpe Arami, Central Swiss Alps, was studied using secondary ion mass spectrometry. The data indicate substantial Li infiltration during exhumation of this ultramafic body. Orthopyroxene porphyroclasts and neoblasts are characterised by low Li contents (0.11-0.36 µg/g) typical of depleted peridotites, whereas Li zonation profiles across porphyroclasts of garnet and clinopyroxene document a metasomatic addition of Li. Small clinopyroxene grains in the matrix contain extremely high and variable abundances of Li (4-16 µg/g). In marked contrast to the behaviour of Li, the abundances of Be (77-134 ng/g) are similar in all textural types of clinopyroxene. Olivine porphyroclasts and neoblasts are characterised by somewhat elevated Li contents (0.95-1.79 µg/g), typical of fertile lherzolites. All textural types of clinopyroxene in the Alpe Arami peridotite are enriched in Li, providing evidence for infiltration of Li-rich and Be-poor aqueous solutions after the peak of UHP metamorphism. The lack of Li enrichment in orthopyroxene is consistent with orthopyroxene dissolution and formation of secondary olivine and clinopyroxene during metasomatism. Cr-diopside pyroxenite veins and boudins within the peridotite show low abundances of Li, with 0.7-2.5 µg/g in clinopyroxene and 1.1-1.5 µg/g in olivine. These pyroxenites likely represent precipitates from aqueous solutions which infiltrated the host peridotite after Li enrichment of the peridotite. A slab-derived nature of the metasomatic agent is suggested by the general lack of Ti enrichment in the Alpe Arami rocks.     

Partitioning and transport of metals across the O2H2S interface in a permanently anoxic basin: Framvaren Fjord,Norway

The geochemical processes operating on metals in anoxic marine waters influence metal mobility and mode of transport to the sediments in a manner different from that observed in oxic regimes. In order to better understand these processes, dissolved and particulate Mn, Fe, Co, Ni, Cu, Zn, and Cd concentrations were determined in the water column of a permanently anoxic basin, Framvaren Fjord, Norway. Class specific behavior determines the degree to which these metals are involved in the processes of redox cycling at the $\text{O}{}_2\text{H}{}_2\text{S}$ interface and metal sulfide precipitation in the sulfidic water. Metal sulfide precipitation influences the magnitude of metal enrichment in the sediments. The transition metals, Mn, Fe, and Co, show active involvement in redox cycling, characterized by dissolved maxima just below the $\text{O}{}_2\text{H}{}_2\text{S}$ interface. Nickel concentrations appear unaffected by processes influencing the profiles of the other metals. The metals, Cu, Zn, and Cd, display a dramatic solubility decrease across the interface, are not involved in redox cycling, and are enriched in the sediments relative to a lithogenic component by factors of 11, 105, and 420, respectively. Ion activity products of the metals and sulfide provide evidence that chemical equilibria with a pure metal sulfide solid phase is not the dominant process controlling dissolved metal concentrations in the sulfide containing waters.