Zinc stable isotopes in seafloor hydrothermal vent fluids and chimneys

Many of the heaviest and lightest natural zinc (Zn) isotope ratios have been discovered in hydrothermal ore deposits. However, the processes responsible for fractionating Zn isotopes in hydrothermal systems are poorly understood. In order to better assess the total range of Zn isotopes in hydrothermal systems and to understand the factors which are responsible for this isotopic fractionation, we have measured Zn isotopes in seafloor hydrothermal fluids from numerous vents at 9–10°N and 21°N on the East Pacific Rise (EPR), the TAG hydrothermal field on the Mid-Atlantic Ridge, and in the Guaymas Basin. Fluid δ⁶⁶Zn values measured at these sites range from + 0.00‰ to + 1.04‰. Of the many physical and chemical parameters examined, only temperature was found to correlate with fluid δ⁶⁶Zn values. Lower temperature fluids (< 250 °C) had both heavier and more variable δ⁶⁶Zn values compared to higher temperature fluids from the same hydrothermal fields. We suggest that subsurface cooling of hydrothermal fluids leads to precipitation of isotopically light sphalerite (Zn sulfide), and that this process is a primary cause of Zn isotope variation in hydrothermal fluids. Thermodynamic calculations carried out to determine saturation state of sphalerite in the vent fluids support this hypothesis with isotopically heaviest Zn found in fluids that were calculated to be saturated with respect to sphalerite. We have also measured Zn isotopes in chimney sulfides recovered from a high-temperature (383 °C) and a low-temperature (203 °C) vent at 9–10°N on the EPR and, in both cases, found that the δ⁶⁶Zn of chimney minerals was lighter or similar to the fluid δ⁶⁶Zn. The first measurements of Zn isotopes in hydrothermal fluids have revealed large variations in hydrothermal fluid δ⁶⁶Zn, and suggest that subsurface Zn sulfide precipitation is a primary factor in causing variations in fluid δ⁶⁶Zn. By understanding how chemical processes that occur beneath the seafloor affect hydrothermal fluid δ⁶⁶Zn, Zn isotopes may be used as a tracer for studying hydrothermal processes.     

Latest Pleistocene and Holocene dune construction at the north-eastern edge of the Rub Al Khali, United Arab Emirates

Optical dating of two dune profiles developed in linear dunes in Ras Al Khaimah, United Arab Emirates, has been used to establish the timing and rate of dune accretion. One section at Awafi was over 17 m high, while that at Idhn was over 40 m high. The Awafi dune appears to have accumulated very rapidly ≈10 000 years ago, with a vertical accumulation rate of about 3·3 m ka⁻¹. The Idhn dune appears to have accumulated over the past 1000 years, with 20 m of sediment accumulating in a time period of about 270 years. The Awafi dune may have accumulated in response to the transgression of the Persian Gulf by rising sea levels in late Pleistocene and Holocene times. The Idhn dune may have accumulated rapidly because of intensified human activity, a short-lived climatic event, or because it may periodically be reactivated after erosion by fluvial action at its base.     

Parameter tests within cosmological simulations of galaxy formation

Numerical simulations of galaxy formation require a number of parameters. Some of these are intrinsic to the numerical integration scheme (e.g., the time-step), while others describe the physical model (e.g., the gas metallicity). In this paper we present results of a systematic exploration of the effects of varying a subset of these parameters on simulations of galaxy formation. We use N -body and 'Smoothed Particle Hydrodynamics' techniques to follow the evolution of cold dark matter and gas in a small volume. We compare a fiducial model with 24 different simulations, in which one parameter at a time is varied, focusing on properties such as the relative fraction of hot and cold gas, and the abundance and masses of galaxies. We find that for reasonable choices of numerical values, many parameters have relatively little effect on the galaxies, with the notable exception of the parameters that control the resolution of the simulation and the efficiency with which gas cools.     

Deep pooling of low degree melts and volatile fluxes at the 85°E segment of the Gakkel Ridge: Evidence from olivine-hosted melt inclusions and glasses

We present new analyses of volatile, major, and trace elements for a suite of glasses and melt inclusions from the 85°E segment of the ultra-slow spreading Gakkel Ridge. Samples from this segment include limu o pele and glass shards, proposed to result from CO₂-driven explosive activity. The major element and volatile compositions of the melt inclusions are more variable and consistently more primitive than the glass data. CO₂ contents in the melt inclusions extend to higher values (167–1596 ppm) than in the co-existing glasses (187–227 ppm), indicating that the melt inclusions were trapped at greater depths. These melt inclusions record the highest CO₂ melt concentrations observed for a ridge environment. Based on a vapor saturation model, we estimate that the melt inclusions were trapped between seafloor depths (～ 4 km) and ～ 9 km below the seafloor. However, the glasses are all in equilibrium with their eruption depths, which is inconsistent with the rapid magma ascent rates expected for explosive activity. Melting conditions inferred from thermobarometry suggest relatively deep (25–40 km) and cold (1240°–1325 °C) melting conditions, consistent with a thermal structure calculated for the Gakkel Ridge. The water contents and trace element compositions of the melt inclusions and glasses are remarkably homogeneous; this is an unexpected result for ultra-slow spreading ridges, where magma mixing is generally thought to be less efficient based on the assumption that steady-state crustal magma chambers are absent in these environments. All melts can be described by a single liquid line of descent originating from a pooled melt composition that is consistent with the aggregate melt calculated from a geodynamic model for the Gakkel Ridge. These data suggest a model in which deep, low degree melts are efficiently pooled in the upper mantle (9–20 km depth), after which crystallization commences and continues during ascent and eruption. Based on our melting model and the assumption that CO₂ is perfectly incompatible, we show that the highest CO₂ concentrations of the melt inclusions (～ 1600 ppm) are consistent with the calculated CO₂ concentrations of primary undegassed melts. The highest measured CO₂/Nb ratio (443) of Gakkel Ridge melt inclusions predicts a mantle CO₂ content of 134 ppm and would result in a global ridge flux of 2.0 × 10¹² mol CO₂/yr.     

A toxicity identification evaluation of silty marine harbor sediments to characterize persistent and non-persistent constituents

Sediment toxicity in silty marine harbor sediments is frequently dominated by ammonia or sulfide, leaving the adverse effects of persistent toxic substances unnoticed. To investigate the latter, we subjected interstitial water from three contaminated silty sediments to toxicity identification evaluation (TIE) phase I manipulations and tested for toxicity with four bioassays: the amphipod Corophium volutator (survival as an endpoint), the sea urchin Psammechinus miliaris (fertilization, embryo development) and the bacterium Vibrio fischeri (bioluminescence inhibition).The graduated pH manipulations identified the prominent toxicity of ammonia in the amphipod and sea urchin embryo tests, and also sulfide toxicity in the bacterium test. In two of the three samples tested with the amphipods, sea urchin embryos and bacteria, a small but significant reduction in interstitial water toxicity was achieved by removing persistent compounds through C(18) solid phase extraction. EDTA chelation resulted in a slight detoxification of the interstitial water for the amphipods and sea urchin embryos, but this was not related to any measured trace metals. Despite the presence of toxic levels of ammonia and sulfide in the harbor sediments, we established the adverse biological effects of persistent constituents by means of the TIE manipulations and in vivo interstitial water bioassays.     

Tributyltin release from harbour sediments--modelling the influence of sedimentation, bio-irrigation and diffusion using data from Bremerhaven

This study describes the potential release of TBT from harbour sediments based on model calculations for different sediment management scenarios applying a numerical one-dimensional FD-model. A conceptual model was developed focussing on the following processes involved in the transport of TBT: sorption equilibrium, diffusion, irrigation and sedimentation. Assuming TBT-concentration of 292 microg/kg in the sediment, the diffusive release of TBT from sediments into the bottom water was calculated to 7.4 x 10(-7)-6.9 x 10(-6)mol m(-2) after 1 year and 7.4 x 10(-6)-1.6 x 10(-4)mol m(-2) after 100 years. In these scenarios, neither sedimentation nor capping were considered. Assuming a sedimentation at a rate of 1cm/a, the polluted sediments will be covered with TBT-free suspended matter. A diffusive release of TBT will be prevented if no bio-irrigation takes place. Capping with a layer of sand of 50 cm thickness will decrease the diffusive release. If a reactive capping could be applied, the release of TBT into the bottom water would be stopped almost completely. The study shows that our model calculations enables us to compare different scenarios in sediment management based on available data. Additionally, it can provide information for risk assessment in the absence of data.