Torsten Vennemann

Climatic and biotic upheavals following the end-Permian mass extinction

The recovery from the end-Permian mass extinction was slow and prolonged. A temperature reconstruction shows that further biotic crises during the recovery were associated with extreme warmth. Recovery from the end-Permian mass extinction is frequently described as delayed1,2,3, with complex ecological communities typically not found in the fossil record until the Middle Triassic epoch. However, the taxonomic diversity of a number of marine groups, ranging from ammonoids to benthic foraminifera, peaked rapidly in the Early Triassic4,5,6,7,8,9,10. These variations in biodiversity occur amidst pronounced excursions in the carbon isotope record, which are compatible with episodes of massive CO2 outgassing from the Siberian Large Igneous Province4,11,12,13. Here we present a high-resolution Early Triassic temperature record based on the oxygen isotope composition of pristine apatite from fossil conodonts. Our reconstruction shows that the beginning of the Smithian substage of the Early Triassic was marked by a cooler climate, followed by an interval of warmth lasting until the Spathian substage boundary. Cooler conditions resumed in the Spathian. We find the greatest increases in taxonomic diversity during the cooler phases of the early Smithian and early Spathian. In contrast, a period of extreme warmth in the middle and late Smithian was associated with floral ecological change and high faunal taxonomic turnover in the ocean. We suggest that climate upheaval and carbon-cycle perturbations due to volcanic outgassing were important drivers of Early Triassic biotic recovery.

Reconstructing paleoelevation in eroded orogens

Hydrogen isotope and 40Ar/39Ar geochronological data are presented from muscovite within a crustal-scale extensional detachment of the Shuswap Metamorphic Complex, North American Cordillera. The hydrogen isotope compositions (δDms) of precisely dated muscovite attain values as low as −156‰ in the detachment mylonite, whereas footwall quartzite has a δDms value of −81‰. The very low δDms values in the detachment are best explained by infiltration of meteoric water, with maximum δD values of −135‰ ± 3‰, during extensional unroofing of the orogen at 49.0–47.9 Ma. On the basis of the empirically determined relationship between elevation and isotopic composition of precipitation, the reconstructed early Eocene paleoelevations of the orogen are 4060 ± 250 m to 4320 ± 250 m, at least 1000 m higher than the highest present-day peaks. We propose that the isotopic composition of surface-derived waters in extensional detachments represents a newly recognized method to estimate maximum paleoelevations attained immediately preceding extensional orogenic collapse.

Solubility of water in magmas to 2 kbar

High water concentrations (4–6 wt%) are found in the quenched glass inclusions of phenocrysts of explosive silicic and intermediate eruptions. Accordingly, we have measured the solubility of water in a wide range of natural liquids up to 2 kbar. With data from the literature, we have developed a simple, thermodynamically based empirical model for the solubility of water as a function of temperature (700–1200 °C), pressure (1–2000 bar), and composition. The model works well for both pure H 2 O and mixed fluid compositions, fits the data to ± 0.5 wt% (2σ), and may be used to successfully model the degassing and eruption behavior in most volcanic systems, as well as the magmatic exsolution processes that lead to ore deposition. Due to the nature of the regression, however, it should not be extrapolated to conditions beyond the range of the data.

Preservation of an extreme transient geotherm in the Raft River detachment shear zone

Extensional detachment systems separate hot footwalls from cool hanging walls, but the degree to which this thermal gradient is the product of ductile or brittle deformation or a preserved original transient geotherm is unclear. Oxygen isotope thermometry using recrystallized quartz-muscovite pairs indicates a smooth thermal gradient (140 °C/100 m) across the gently dipping, quartzite-dominated detachment zone that bounds the Raft River core complex in northwest Utah (United States). Hydrogen isotope values of muscovite (δD Ms ~ –100‰) and fl uid inclusions in quartz (δD Fluid ~ –85‰) indicate the presence of meteoric fl uids during detachment dynamics. Recrystallized grain-shape fabrics and quartz c-axis fabric patterns reveal a large component of coaxial strain (pure shear), consistent with thinning of the detachment section. Therefore, the high thermal gradient preserved in the Raft River detachment refl ects the transient geotherm that developed owing to shearing, thinning, and the potentially prominent role of convective fl ow of surface fl uids.

Migration of sharks into freshwater systems during the Miocene and implications for Alpine paleoelevation

Trace-element and isotopic compositions of fossilized shark teeth sampled from Miocene marine sediments of the north Alpine Molasse Basin, the Vienna Basin, and the Pannonian Basin generally show evidence of formation in a marine environment under conditions geochemically equivalent to the open ocean. In contrast, two of eight shark teeth from the Swiss Upper Marine Molasse locality of La Moliere have extremely low ?18O values (10.3‰ and 11.3‰) and low 87Sr/86Sr ratios (0.707840 and 0.707812) compared to other teeth from this locality (21.1‰–22.4‰ and 0.708421–0.708630). The rare earth element (REE) abundances and patterns from La Moliere not only differ between dentine and enameloid of the same tooth, but also between different teeth, supporting variable conditions of diagenesis at this site. However, the REE patterns of enameloid from the “exotic” teeth analyzed for O and Sr isotopic compositions are similar to those of teeth that have O and Sr isotopic compositions typical of a marine setting at this site. Collectively, this suggests that the two “exotic” teeth were formed while the sharks frequented a freshwater environment with very low 18O-content and Sr isotopic composition controlled by Mesozoic calcareous rocks. This is consistent with a paleogeography of high-elevation (2300 m) Miocene Alps adjacent to a marginal sea.

Evidence of growth and sector zoning in hydrothermal quartz from Alpine veins

Several quartz crystals from three different Alpine vein localities and of known petrologic setting and evolution have been examined for possible elemental sector zoning in order to help to constrain the mechanisms of such trace element incorporation. Using different in situ techniques (EMPA, LA-ICPMS, SIMS, FTIR-spectroscopy), it was established that Al and Li concentrations can exceed several hundreds of ppma for distinct growth zones within crystals formed at temperatures of about 300 °C or less and that also display patterns of cyclic growth when examined with cathodoluminescence. In contrast, crystals formed at temperatures closer to 400 °C and without visible cyclic growth have low concentrations of Al and Li as well as other trace elements. Al and Li contents are correlated along profiles measured within the crystals and in general their proportion does not change along the profiles. No relationships were found between Al, Na, and K, and germanium has a qualitative relationship with Al. FTIR spectra also show OH − absorption bands within the quartz, with higher amplitudes in zones rich in Al and Li. Sector zoning is present. It is most pronounced between prismatic and rhombohedral faces of the same growth zone, but also between the rhombohedral faces of r and z , which contain different amounts of trace elements. The sector zoning is also expressed by changes in the Li/Al ratio, with higher ratios in z compared to r faces. It is concluded that the incorporation of trace elements into hydrothermal quartz from Alpine veins is influenced by growth mechanisms and surface-structures of the growing quartz crystals, the influence of which may change as a function of temperature, pH, as well as the chemical composition of the fluid.

Metastable prograde mineral reactions in contact aureoles

Extrapolation of reaction paths and rates of metamorphic mineral growth from experimental to natural systems is complicated by a number of factors. Many of these factors are difficult to evaluate for natural systems. A combination of textural modeling and stable isotope analysis allows for a distinction between several possible reaction paths for olivine growth in a siliceous dolomite contact aureole. It is suggested that olivine forms directly from dolomite and quartz. The formation of olivine from this metastable reaction implies metamorphic crystallization far from equilibrium. Stable and metastable reaction paths predict the textures observed (calcite haloes around bladed olivine crystals) well. It is possible to discriminate between individual reaction paths only on the basis of the oxygen isotope compositions of the minerals involved. Products were found to be in stable isotope equilibrium, but in disequilibrium with the reactants. Only the metastable overall reaction dolomite + quartz → olivine + calcite + CO 2 produces no dolomite by local reactions, and hence agrees with the oxygen isotope data. Thus, significant mineral growth occurred far from equilibrium with respect to the thermodynamically stable reactions of the system. This amazing finding implies that metamorphism of contact aureoles has to be reinterpreted in a more complex, dynamic fashion, involving metastable reactions and metastable equilibria as well. The spatial distribution of metamorphic mineral assemblages in a contact aureole cannot be interpreted as a proxy for the temporal evolution of a single rock specimen, because each rock undergoes a different reaction path, depending on temperature, heating rate, and fluid-infiltration rate.

Opportunistic Feeding Strategy for the Earliest Old World Hypsodont Equids: Evidence from Stable Isotope and Dental Wear Proxies

Background The equid Hippotherium primigenium, with moderately hypsodont cheek teeth, rapidly dispersed through Eurasia in the early late Miocene. This dispersal of hipparions into the Old World represents a major faunal event during the Neogene. The reasons for this fast dispersal of H. primigenium within Europe are still unclear. Based on its hypsodonty, a high specialization in grazing is assumed although the feeding ecology of the earliest European hipparionines within a pure C3 plant ecosystem remains to be investigated. Methodology/Principal Findings A multi-proxy approach, combining carbon and oxygen isotopes from enamel as well as dental meso- and microwear analyses of cheek teeth, was used to characterize the diet of the earliest European H. primigenium populations from four early Late Miocene localities in Germany (Eppelsheim, Höwenegg), Switzerland (Charmoille), and France (Soblay). Enamel δ13C values indicate a pure C3 plant diet with small (<1.4‰) seasonal variations for all four H. primigenium populations. Dental wear and carbon isotope compositions are compatible with dietary differences. Except for the Höwenegg hipparionines, dental microwear data indicate a browse-dominated diet. By contrast, the tooth mesowear patterns of all populations range from low to high abrasion suggesting a wide spectrum of food resources. Conclusions/Significance Combined dental wear and stable isotope analysis enables refined palaeodietary reconstructions in C3 ecosystems. Different H. primigenium populations in Europe had a large spectrum of feeding habits with a high browsing component. The combination of specialized phenotypes such as hypsodont cheek teeth with a wide spectrum of diet illustrates a new example of the Liem’s paradox. This dietary flexibility associated with the capability to exploit abrasive food such as grasses probably contributed to the rapid dispersal of hipparionines from North America into Eurasia and the fast replacement of the brachydont equid Anchitherium by the hypsodont H. primigenium in Europe.

Tinderet volcano, Kenya: an altered natrocarbonatite locality?

Abstract The Tinderet volcano (19.9 to 5.5 Ma), located within the Kavirondo rift in Kenya, contains blocks of carbonatite lavas with calcite, minor apatite, fluorite, spinel-group minerals, accessory perovskite and ‘plumbopyrochlore’; nyerereite is present as inclusions in the perovskite. At least four types of calcite are present in the carbonatite lavas; they differ in morphology, composition and origin. The dominant variety is secondary type-II calcite, which is enriched in sodium (up to 1.1 wt.% Na2O) and strontium (up to 1.3 wt.% SrO). The spinel-group minerals are manganese-bearing and include Mn-rich magnetite, magnesioferrite and jacobsite. Oxygen isotope data for bulk carbonatite samples (δ18O = +16.2‰ to +22.6‰ VSMOW) support a low crystallization temperature for the secondary calcite. Petrographic, mineralogical and isotopic data indicate that the Tinderet carbonatites are similar to natrocarbonatites from the Oldoinyo Lengai and Kerimasi volcanoes that have altered and recrystallized to form calcite carbonatites. These data support the hypothesis that some of the Tinderet carbonatites were originally alkali-rich rocks which contained primary nyerereite.

Oxygen isotope sector zoning in natural hydrothermal quartz

Abstract Oxygen isotope measurements using SIMS and laser-fluorination methods confirm the presence of concentric and sector zoning in low-temperature (200ºC to <400ºC) hydrothermal quartz from Alpine veins. While concentric zoning is most readily explained by changes in the chemical composition of the fluid or temperature of crystallization, the reasons for sector zoning are more difficult to explain. Relative enrichment in 18O for crystallographically different sectors of quartz corresponds to m >r >z. Sector zoning is, however, largely limited to the exterior zones of crystals and/or to crystals with large Al (>1000 ppm) and trace element contents, probably formed at temperatures <250ºC. Differences in δ18O between the prismatic (m) relative to the rhombohedral (r and z) growth sectors of up to 2‰ can be explained by a combination of a face-related crystallographic and/or a growth rate control. In contrast, isotopic sector zoning of up to about 1.5‰ amongst the different rhombohedral faces increases in parallel with the trace element content and is likely to represent disequilibrium growth. This is indicated by non-systematic, up to 2‰, differences within single growth zones and the irregular, larger or smaller, δ18O values (of several permil) of the exterior compared to the inner zones of the same crystals. Disequilibrium growth may be related to the large trace element content incorporated into the growing quartz at lower temperatures (<250ºC) and/or be related to fluid-vapour separation, allowing crystal growth from both a vapour as well as a liquid phase.