New insights into Southern Caucasian glacial–interglacial climate conditions inferred from Quaternary gastropod fauna

In the present study, we performed gastropod analyses on loess–palaeosol sequences from northeast Armenia (Southern Caucasia) covering at least three glacial–interglacial cycles. The elaborated ecostratigraphy shows significant patterns of species composition related to the succession of pedocomplexes and loess, respectively. Pedocomplexes included species that can be associated with high-grass to forest-steppe biomes, indicating increased humidity for these sections compared to the loess layers. In contrast, loess layers that relate to glacial periods are associated with gastropod species of semidesert environments with shrub- and shortgrass-steppes, indicating semiarid to arid conditions. Furthermore, the loess deposits do not show any evidence for cold-adapted gastropod species. Therefore, we suggest that average July temperatures in the study area were above 10 °C, even during periods of loess deposition. Consequently, we propose that the limiting factor for tree growth during glacial periods was aridity, rather than temperature. In addition, we observe environmental differences between the various glacial times, with our results indicating a trend towards steadily increasing aridity in Southern Caucasia across the Middle to Late Pleistocene.     

Crystallization of melilite from CMAS-liquids and the formation of the melilite mantle of Type B1 CAIs: Experimental simulations

Type B CAIs are subdivided into B1s, with well-developed melilite mantles, and B2s, with randomly distributed melilite. Despite intensive study, the origin of the characteristic melilite mantle of the B1s remains unclear. Recently, we proposed that formation of the melilite mantle is caused by depletion of the droplet surface in volatile magnesium and silicon due to higher evaporation rates of volatile species compared to their slow diffusion rates in the melt, thus making possible crystallization of melilite at the edge of the CAI first, followed by its crystallization in the central parts at lower temperatures. Here, we present the results of an experimental study that aimed to reproduce the texture observed in natural Type B CAIs. First, we experimentally determined crystallization temperatures of melilite for three melt compositions, which, combined with literature data, allowed us to find a simple relationship between the melt composition, crystallization temperature, and composition of first crystallizing melilite. Second, we conducted a series of evaporation and cooling experiments exposing CAI-like melts to gas mixtures with different oxygen fugacities (f_O2). Cooling of the molten droplets in gases with logf_O2?IW-4 resulted in crystallization of randomly distributed melilite, while under more reducing conditions, melilite mantles have been formed. Chemical profiles through samples quenched right before melilite started to crystallize showed no chemical gradients in samples exposed to relatively oxidizing gases (logf_O2?IW-4), while the near-surface parts of the samples exposed to very reducing gases (logf_O2?IW-7) were depleted in volatile MgO and SiO₂, and enriched in refractory Al₂O₃. Using these experimental results and the fact that the evaporation rate of magnesium and silicon from CAI-like melts is proportional to , we estimate that Type B1 CAIs could be formed by evaporation of a partially molten precursor in a gas of solar composition with . Type B2 CAIs could form by slower evaporation of the same precursors in the same gas with .     

Talc-chlorite-amphibole felses of the KTB pilot hole,Oberpfalz, Bavaria: Protolith characteristics and phase relationships

Summary The metagabbro-amphibolite sequences in the KTB pilot hole contain intercalations of talc-chlorite-amphibole felses (or hosbachites), which show transitional contacts to the adjacent metagabbros. The hosbachites are characterized by relics of a primary igneous texture and still contain igneous minerals like clinopyroxene, biotite and pseudomorphs after olivine, while brown Ca-amphibole was presumably formed in a late-magmatic stage. The geological, textural, mineralogical and geochemical evidence indicates that the hösbachites were derived from ultramafic cumulates, differentiated from a basaltic magma, either in the inner parts of dolerite sills or in small gabbro intrusions. A pervasive metamorphic overprint under medium-pressure, amphibolite-facies conditions which was accompanied by penetrative deformation led to assemblages with green Ca-amphibole ± anthophyllite ± cummingtonite ± tremolite/actinolite + clinochlore + talc + olivine + ilmenite ± Cr-bearing spinel + sulfides. Phase relationships are consistent with a prograde P-T path leading to the formation of anthophyllite from olivine + talc at peak metamorphic temperatures of 640–700°C, at assumed pressures of 8-10 kbar, similar to those derived from mineral assemblages in the adjacent metabasites and metasediments. High-pressure relics locally present in coronitic metagabbros and retrograded eclogites of the KTB pilot hole were not recognized in the hosbachites. A retrograde overprint under greenschist-facies conditions led to the total replacement of igneous or metamorphic olivine by aggregates of antigorite + magnetite, chloritization of biotite and the formation of late tremolite/ actinolite.

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Zusammenfassung Talk- Chlorit-Amphibol-Felse der KTB-Vorbohrung, Oberpfalz: Eduktcharakteristik und Phasenbeziehungen Die Metagabbro-Amphibolit-Folge in der KTB-Vorbohrung enthält Einschaltungen von Talk-Chlorit-Amphibol-Felsen (Hosbachite), die graduelle Übergänge zu den benachbarten Metagabbros aufweisen. Die Hosbachite sind durch Relikte von primären magmatischen Gefügen gekennzeichnet and führen noch magmatische Mineralrelikte wie Klinopyroxen, Biotit and Pseudomorphosen nach Olivin, während brauner CaAmphibol wahrscheinlich spdtmagmatisch gebildet wurde. Verbandsverhältnisse, Reliktgefüge und Reliktminerale sowie Haupt- und Spurenelement-Geochemie sprechen dafür, daß die Hosbachite auf ultramafische Kumulate zurückgehen, die aus einem basischen Magma differenziert wurden, and zwar entweder im Innern von doleritischen Lagergängen oder in kleinen Gabbro-Intrusionen.Eine durchgreifende metamorphe Überprägung unter Bedingungen der Mitteldruck Amphibolitfazies, die von einer penetrativen Deformation begleitet war, fuhrte zu Mineralparagenesen mit grünern Ca-Amphibol ± Anthophyllit ± Cummingtonit + Tremolit/Aktinolith + Klinochlor + Talk + Olivin + Ilmenit + Cr-haltigem Spinell + Sulfiden.Die Phasenbeziehungen weisen darauf hin, daß sich im Zuge eines prograden P-TPfades Anthophyllit aus der Paragenese Olivin + Talk bildete. Als P-T-Bedingungen beim Hbhepunkt der Metamorphose können Temperaturen von 640–700°C in einem angenommenen Druckbereich von 8-10 kbar abgeschätzt wurden, ähnlich wie sie auch aus den Mineralparagenesen in den angrenzenden Metabasiten and Metasedimenten der KTB-Vorbohrung ableitbar sind. Hochdruckrelikte, die gelegentlich in koronitischen Metagabbros and retrograd überprägten Eklogiten der KTB-Vorbohrung auftreten, wurden in den Hösbachiten nicht gefunden. Eine retrograde Überprägung unter grünschieferfaziellen Bedingungen führte zu einer vollständigen Verdrängung von magmatischem und metamorphem Olivin durch Aggregate von Antigorit + Magnetit, zur Chloritisierung von Biotit und zur Bildung einer späten Generation von Tremolit/ Aktinolith.

     

Proterozoic deformation of the East Saharan Craton in Southeast Libya, South Egypt and North Sudan

The basement areas in Southeast Libya, South Egypt and North Sudan, west of the Nile, between Gebel Uweinat and the Bayuda Desert, are part of an approximately 1000-km-wide, complexly folded, polymetamorphic zone with a regional N-NNE-NE-ENE trend of foliation and fold axis. Since this belt extends southwestward into the area of Zalingei in the southern Darfur block (West Sudan), it is named the Northern Zalingei fold zone. Sr and Nd isotopic studies suggest that this zone is older than Pan-African and further indicate that, apart from Archean rocks in the Gebel Uweinat area, this belt is of Early-Middle Proterozoic age. An Early-Middle Proterozoic three-stage deformational and anatectic event established the present-day fold and fault geometry in the western parts of this zone in the Gebel Uweinat—Gebel Kamil area. The Pan-African tectono-thermal episode was most effective in the eastern part of the belt, near the boundary with the Nubian Shield volcano-sedimentary-ophiolite-granitoid assemblages. It caused migmatization, granite emplacement, mylonitization and large-scale wrench faulting which was related to Late Proterozoic accretionary and collisional events of the Arabian-Nubian Shield with the margin of the East Saharan Craton.     

Magnesium isotope fractionation in silicate melts by chemical and thermal diffusion

Two types of laboratory experiments were used to quantify magnesium isotopic fractionations associated with chemical and thermal (Soret) diffusion in silicate liquids. Chemical diffusion couples juxtaposing a molten natural basalt (SUNY MORB) and a molten natural rhyolite (Lake County Obsidian) were run in a piston cylinder apparatus and used to determine the isotopic fractionation of magnesium as it diffused from molten basalt to molten rhyolite. The thermal diffusion experiments were also run in a piston cylinder apparatus but with a sample made entirely of molten SUNY MORB displaced from the hotspot of the assembly furnace so that the sample would have a temperature difference of about 100-200 °C from one end to the other. The chemical diffusion experiments showed fractionations of ²⁶Mg/²⁴Mg by as much as 7‰, which resulted in an estimate for the mass dependence of the self-diffusion coefficients of the magnesium isotopes corresponding to D_26Mg/D_24Mg=(24/26)^β with β = 0.05. The thermal diffusion experiments showed that a temperature difference of about 100 °C resulted in the MgO, CaO, and FeO components of the basalt becoming slightly enriched by about 1 wt% in the colder end while SiO₂ was enriched by several wt% in the hotter end. The temperature gradient also fractionated the magnesium isotopes. A temperature difference of about 150 °C produced an 8‰ enrichment of ²⁶Mg/²⁴Mg at the colder end relative to the hotter end. The magnesium isotopic fractionation as a function of temperature in molten basalt corresponds to 3.6 × 10⁻²‰/°C/amu.     

Elemental and isotopic fractionation of Type B CAI-like liquids by evaporation

Vacuum evaporation experiments with Type B CAI-like starting compositions were carried out at temperatures of 1600, 1700, 1800, and 1900 °C to determine the evaporation kinetics and evaporation coefficients of silicon and magnesium as a function of temperature as well as the kinetic isotope fractionation factor for magnesium. The vacuum evaporation kinetics of silicon and magnesium are well characterized by a relation of the form J = J_oe^−E/RT with J_o = 4.17 × 10⁷ mol cm⁻² s⁻¹, E = 576 ± 36 kJ mol⁻¹ for magnesium, J_o = 3.81 × 10⁶ mol cm⁻² s⁻¹, E = 551 ± 63 kJ mol⁻¹ for silicon. These rates only apply to evaporation into vacuum whereas the actual Type B CAIs were almost certainly surrounded by a finite pressure of a hydrogen-dominated gas. A more general formulation for the evaporation kinetics of silicon and magnesium from a Type B CAI-like liquid that applies equally to vacuum and conditions of finite hydrogen pressure involves combining our determinations of the evaporation coefficients for these elements as a function of temperature (γ = γ₀e^−E/RT with γ₀ = 25.3, E = 92 ± 37 kJ mol⁻¹ for γ_Si; γ₀ = 143, E = 121 ± 53 kJ mol⁻¹ for γ_Mg) with a thermodynamic model for the saturation vapor pressures of Mg and SiO over the condensed phase. High-precision determinations of the magnesium isotopic composition of the evaporation residues from samples of different size and different evaporation temperature were made using a multicollector inductively coupled plasma mass spectrometer. The kinetic isotopic fractionation factors derived from this data set show that there is a distinct temperature effect, such that the isotopic fractionation for a given amount of magnesium evaporated is smaller at lower temperature. We did not find any significant change in the isotope fractionation factor related to sample size, which we interpret to mean that recondensation and finite chemical diffusion in the melt did not affect the isotopic fractionations. Extrapolating the magnesium kinetic isotope fractionations factors from the temperature range of our experiments to temperatures corresponding to partially molten Type B CAI compositions (1250-1400 °C) results in a value of α_Mg ≈ 0.991, which is significantly different from the commonly used value of .     

Bottom irregularities in the North Sea

Summary A pressure transducer fixed to a sledge was towed over the bottom of the sea. Pressure variations corresponding to bottom displacement variations were recorded along different profiles in the area west of Sylt. From the time series obtained power spectra of bottom irregularities were computed as a function of wavenumberk or wavelength . Total rms bottom displacement in the wavelength band 50 to 500 m ranges from 3 cm to 36 cm depending on the location of the profile. Presumedly this is due to different types of sediment. Therefore the area cannot be considered as uniform or isotropic concerning the spatial and directional distribution of bottom irregularities. The approximation of spectra byE(k)k– yields -values between 0.4 and 2.9.

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Bodenunebenheiten in der Nordsee

Zusammenfassung Mit Hilfe einer Druckdose, die von einem Schiff aus über den Meeresboden gezogen wurde, konnten auf ausgewählten Profilen westlich von Sylt Bodenunebenheiten registriert werden. Aus den gewonnenen Zeitserien wurden SpektrenE(k) für die Wellenzahlenk bzw. für die Wellenlänge berechnet. Hieraus lassen sich die Amplituden der Bodenunebenheiten im Wellenlängenbereich von 20 bis 500 m bestimmen. Die effektive Bodenwelligkeit im Bereich von 50 bis 500 m liegt je nach Profil zwischen 3 cm und 36 cm. Die Ursache dafür ist möglicherweise in der unterschiedlichen Sedimentzusammensetzung zu suchen. Eine Isotropie des Bodenspektrums im Meßgebiet ist nicht vorhanden. Die Annäherung der Spektren durchE(k)k– ergibt -Werte zwischen 0,4 und 2,9.

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Irrégularités du fond en Mer du Nord

Résumé Un manomètre enregistreur fixé à un traîneau a été remorqué sur le fond de la mer. Les variations de pression correspondant aux variations du fond ont été enregistrées le long de différents profils dans la zone Ouest de Sylt. A partir de la série de mesures obtenues, on a calculé le spectre de puissance des irrégularités du fond en fonction du nombre d'ondek ou de la longueur d'onde . Globalement, les variations du fond dans la bande de longueurs d'onde de 50 à 500 m varient de 3 à 36 cm en moyenne quadratique selon la situation du profil. On présume que cela est dû aux différents types de sédiments. Aussi, la zone ne peut pas être considérée comme uniforme ou isotrope en ce qui concerne la distribution dans l'espace et en direction des irrégularités du fond. L'approximation de spectres parE(k)k– donne des valeurs de comprises entre 0,4 et 2,9.

     

Equatorial Atlantic variability and its relation to mean state biases in CMIP5

Coupled general circulation model (GCM) simulations participating in the Coupled Model Intercomparison Project Phase 5 (CMIP5) are analyzed with respect to their performance in the equatorial Atlantic. In terms of the mean state, 29 out of 33 models examined continue to suffer from serious biases including an annual mean zonal equatorial SST gradient whose sign is opposite to observations. Westerly surface wind biases in boreal spring play an important role in the reversed SST gradient by deepening the thermocline in the eastern equatorial Atlantic and thus reducing upwelling efficiency and SST cooling in the following months. Both magnitude and seasonal evolution of the biases are very similar to what was found previously for CMIP3 models, indicating that improvements have only been modest. The weaker than observed equatorial easterlies are also simulated by atmospheric GCMs forced with observed SST. They are related to both continental convection and the latitudinal position of the intertropical convergence zone (ITCZ). Particularly the latter has a strong influence on equatorial zonal winds in both the seasonal cycle and interannual variability. The dependence of equatorial easterlies on ITCZ latitude shows a marked asymmetry. From the equator to 15°N, the equatorial easterlies intensify approximately linearly with ITCZ latitude. When the ITCZ is south of the equator, on the other hand, the equatorial easterlies are uniformly weak. Despite serious mean state biases, several models are able to capture some aspects of the equatorial mode of interannual SST variability, including amplitude, pattern, phase locking to boreal summer, and duration of events. The latitudinal position of the boreal spring ITCZ, through its influence on equatorial surface winds, appears to play an important role in initiating warm events.     

What controls equatorial Atlantic winds in boreal spring?

The factors controlling equatorial Atlantic winds in boreal spring are examined using both observations and general circulation model (GCM) simulations from the coupled model intercomparison phase 5. The results show that the prevailing surface easterlies flow against the attendant pressure gradient and must therefore be maintained by other terms in the momentum budget. An important contribution comes from meridional advection of zonal momentum but the dominant contribution is the vertical transport of zonal momentum from the free troposphere to the surface. This implies that surface winds are strongly influenced by conditions in the free troposphere, chiefly pressure gradients and, to a lesser extent, meridional advection. Both factors are linked to the patterns of deep convection. Applying these findings to GCM errors indicates, that, consistent with the results of previous studies, the persistent westerly surface wind bias found in most GCMs is due mostly to precipitation errors, in particular excessive precipitation south of the equator over the ocean and deficient precipitation over equatorial South America. Free tropospheric influences also dominate the interannual variability of surface winds in boreal spring. GCM experiments with prescribed climatological sea-surface temperatures (SSTs) indicate that the free tropospheric influences are mostly associated with internal atmospheric variability. Since the surface wind anomalies in boreal spring are crucial to the development of warm SST events (Atlantic Niños), the results imply that interannual variability in the region may rely far less on coupled air–sea feedbacks than is the case in the tropical Pacific.     

Diagnosing southeast tropical Atlantic SST and ocean circulation biases in the CMIP5 ensemble

Warm sea-surface temperature (SST) biases in the southeastern tropical Atlantic (SETA), which is defined by a region from 5°E to the west coast of southern Africa and from 10°S to 30°S, are a common problem in many current and previous generation climate models. The Coupled Model Intercomparison Project Phase 5 (CMIP5) ensemble provides a useful framework to tackle the complex issues concerning causes of the SST bias. In this study, we tested a number of previously proposed mechanisms responsible for the SETA SST bias and found the following results. First, the multi-model ensemble mean shows a positive shortwave radiation bias of ~20 W m^?2, consistent with models’ deficiency in simulating low-level clouds. This shortwave radiation error, however, is overwhelmed by larger errors in the simulated surface turbulent heat and longwave radiation fluxes, resulting in excessive heat loss from the ocean. The result holds for atmosphere-only model simulations from the same multi-model ensemble, where the effect of SST biases on surface heat fluxes is removed, and is not sensitive to whether the analysis region is chosen to coincide with the maximum warm SST bias along the coast or with the main SETA stratocumulus deck away from the coast. This combined with the fact that there is no statistically significant relationship between simulated SST biases and surface heat flux biases among CMIP5 models suggests that the shortwave radiation bias caused by poorly simulated low-level clouds is not the leading cause of the warm SST bias. Second, the majority of CMIP5 models underestimate upwelling strength along the Benguela coast, which is linked to the unrealistically weak alongshore wind stress simulated by the models. However, a correlation analysis between the model simulated vertical velocities and SST biases does not reveal a statistically significant relationship between the two, suggesting that the deficient coastal upwelling in the models is not simply related to the warm SST bias via vertical heat advection. Third, SETA SST biases in CMIP5 models are correlated with surface and subsurface ocean temperature biases in the equatorial region, suggesting that the equatorial temperature bias remotely contributes to the SETA SST bias. Finally, we found that all CMIP5 models simulate a southward displaced Angola–Benguela front (ABF), which in many models is more than 10° south of its observed location. Furthermore, SETA SST biases are most significantly correlated with ABF latitude, which suggests that the inability of CMIP5 models to accurately simulate the ABF is a leading cause of the SETA SST bias. This is supported by simulations with the oceanic component of one of the CMIP5 models, which is forced with observationally derived surface fluxes. The results show that even with the observationally derived surface atmospheric forcing, the ocean model generates a significant warm SST bias near the ABF, underlining the important role of ocean dynamics in SETA SST bias problem. Further model simulations were conducted to address the impact of the SETA SST biases. The results indicate a significant remote influence of the SETA SST bias on global model simulations of tropical climate, underscoring the importance and urgency to reduce the SETA SST bias in global climate models.