The effects of the geothermal gradient on amino acid racemization

The “second region” of the kinetic curve for the racemization (epimerization) of isoleucine in foraminifera is defined from previously published data corrected for the thermal history experienced by the samples. These kinetic parameters are applied to racemization data from Deep Sea Drilling Project Sites 332A and 333 from the Deep Drill Valley, Mid-Atlantic Ridge, and Site 148 from the Aves Ridge, in order to determine whether there are any observable effects due to the geothermal gradient. The data for Site 148 clearly show an increase of temperature with depth. The data for the two sites of the Mid-Atlantic Ridge do not show this clear relationship. When bottom water temperature variations are taken into consideration, the effects of the geothermal gradient become apparent. Since the degree of racemization is dependent upon both age and temperature, a knowledge of the age of a sample places constraints upon its thermal history, and hence on the heat flow at the location since deposition of the sample. The crude heat flow models thus developed are compatible with present geologic and geophysical information. It appears probable that detailed heat flow models may be developed by improving analytical precision.     

Rare earth partitioning between immiscible carbonate and silicate liquids and CO2 vapor: Results and implications for the formation of light rare earth-enriched rocks

Melting relations at 5 and 20 kbar on the composition join sanidine-potassium carbonate are dominated by a two-liquid region that covers over 60% of the join at 1,300 ° C. At this temperature, the silicate melt contains approximately 19 wt% carbonate component at 5 kbar and 32 wt% carbonate component at 20 kbar. The conjugate carbonate melt contains less than 5 wt% silicate component, and it varies less as a function of temperature than does the silicate melt.Partition coefficients for Ce, Sm, and Tm between the immiscible carbonate and silicate melts at 1,200 ° and 1,300 ° C at 5 and 20 kbar are in favor of the carbonate melt by a factor of 2–3 for light REE and 5–8 for heavy REE. The effect of pressure on partitioning cannot be evaluated independently because of complementary changes in melt compositions.Minimum REE partition coefficients for CO₂ vapor/carbonate melt and CO₂ vapor/silicate melt can be calculated from the carbonate melt/silicate melt partition coefficients, the known proportions of melt, and maximum estimates of the proportion of CO₂ vapor. The vapor phase is enriched in light REE relative to both melts at 20 kbar and enriched in all REE, especially the light elements, at 5 kbar. The enrichment of REE in CO₂ vapor relative to both melts is 3–4 orders of magnitude in excess of that in water vapor (Mysen, 1979) at 5 kbar and is approximately the same as that in water vapor at 20 kbar.Mantle metasomatism by a CO₂-rich vapor enriched in light REE, occurring as a precursor to magma genesis, may explain the enhanced REE contents and light REE enrichment of carbonatites, alkali-rich silicate melts, and kimberlites. Light REE enrichment in fenites and the granular suite of nodules from kimberlites attests to the mobility of REE in CO₂-rich fluids under both mantle and crustal conditions.     

An experimental investigation of the partitioning of REE between garnet and liquid with reference to the role of defect equilibria

The partitioning of samarium and thulium between garnets and melts in the systems Mg₃Al₂-Si₃O₁₂-H₂O and Ca₃Al₂Si₃O₁₂-H₂O has been studied as a function of REE concentration in the garnets at 30 kbar pressure. Synthesis experiments of variable time under constant P, T conditions indicate that garnet initially crystallizes rapidly to produce apparent values of D _Sm (D _Sm=concentration of Sm in garnet/concentration of Sm in liquid) which are too large in the case of pyrope and too small in the case of grossular. As the experiment proceeds, Sm diffuses out of or into the garnet and the equilibrium value of D _Sm is approached. Approximate values of diffusion coefficients for Sm in pyrope garnet obtained by this method are 6 × 10^–13 cm² s^–1 at 1,300 ° C and 2 × 10^–12 cm² s^–1 at 1,500 ° C, and for grossular, 8.3 × 10^–12 cm² s^–1 at 1,200 ° C and 4.6 × 10^–11 cm² s^–1 at 1,300 ° C. The equilibrium values of D _Sm have been reversed by experiments with Sm-free pyrope and Sm-bearing glass, and with Sm-bearing grossular and Sm-free glass.Between 12 ppm and 1,000 ppm Sm in pyrope at 1,300 ° C and between 80 ppm and >2 wt.% Tm in pyrope at 1,500 ° C, partition coefficients are constant and independent of REE concentration. Above 100 ppm of Sm in garnet at 1,500 ° C, partition coefficients are independent of Sm concentration. At lower concentrations, however, D _Sm is dependent upon the Sm content of the garnet. The two regions may be interpreted in terms of charge-balanced substitution of Sm₃Al₅O₁₂ in the garnet at high Sm concentrations and defect equilibria involving cation vacancies at low concentrations. At very low REE concentrations (< 1 ppm Tm in grossular at 1,300 ° C) D_REE garnet/liquid again becomes constant with an apparent Henry's Law value greater than that at high concentrations. This may be interpreted in terms of a large abundance of cation vacancies relative to the number of REE ions.The importance of defects in the low concentration region has been confirmed by adding other REE (at 80 ppm level) to the system Mg₃Al₂Si₃O₁₂-H₂O at low Sm concentrations. These change D _Sm in the defect region, demonstrating their role in the production of vacancies.Experiments on a natural pyropic garnet indicate that defect equilibria are of importance to REE partitioning within the concentration ranges found in nature.     

REE-bearing minerals in a Ti-rich vein from the Adamello contact aureole (Italy)

Zirconolite, aeschynite-(Ce), titanite and apatite have been found as minor or accessory minerals in a Ti-rich (TiO₂=2.1–4.5 wt.%) hydrothermal vein occurring in dolomite marbles at the contact with a tonalite intrusion of the Tertiary Adamello batholith (northern Italy). The vein consists of four distinct mineral zones, comprising from margin to center: (1) forsterite+calcite, (2) pargasite+calcite+titanite+sulfides, (3) phlogopite +calcite+titanite+sulfides, and (4) titanian clinohumite +spinel+calcite+sulfides. Zirconolite occurs in two vein zones only: in the phlogopite zone it is invariably anhedral, often corroded, and exhibits complex chemical zonation patterns. In the titanian clinohumite zone zirconolite is idiomorphic and characterized by a pronounced discontinous chemical zoning, but shows no evidence of corrosion. The considerable compositional variation observed for zirconolite (in wt.%: (REE₂O₃)=0.74–16.8, UO₂=0.59–24.0, ThO₂=0.67–17.1) is due to the zoning, and may be attributed to four major substitutions described by the exchange vectors:

Perception visual range and sky background

Abstract

The meteorological visual range V_m and the perception visual range V_p have been measured for a variety of horizon sky backgrounds and atmospheric extinction conditions. Comparison with the standard visibility V_n shows that in general V_p > V_m > V_n . Model calculations of V_p, V_m and V_n for different synthetic sky backgrounds and for an extinction coefficient function of the form σ(λ) = k λ^‐α in which a was varied from ‐1 to 4 show that the wavelength of maximum perception can vary between ～540 and ～630 nm. Under typically turbid or polluted atmospheric conditions and for a blue or white sky background the extinction coefficient should be measured at ‐570 nm if prevailing visibility is to be reliably estimated.     

Modeling biodiversity benefits and external costs from a keystone predator reintroduction policy

In this paper, an economic model was constructed to determine the optimal wolf population and distribution across the Northern Rocky Mountains. Both ecological and economic concepts were incorporated in an implicitly spatial social welfare maximization problem. This interdisciplinary model relies on multiple data sources, including current wolf population and distribution information, opportunity cost to local landowners, and contingent valuation studies to determine willingness-to-pay for wolves. Economic models tend to externalize ecological concerns and ecological models often omit the complex human dimensions of conservation policy. Accordingly, this model can serve as a guide for integrating best practices from both fields. The model presented here is sufficiently general to apply to wolves in other ecosystems and to other highly interacting species such as beavers and bison. The Northern Rocky Mountain wolf was used as an example of how this economic model works, but this model can be applied far more broadly.     

Resistance of Hong Kong waters to nutrient enrichment: assessment of the role of physical processes in reducing eutrophication

Hong Kong waters receive high nutrient loading from year-round sewage effluent and Pearl River discharge during the summer wet season. We assessed the role of physical processes in reducing eutrophication by calculating a eutrophication reduction index for four different hydrographical areas and four seasons. We used outdoor incubation experiments to assess the response of phytoplankton when physical (mixing and dilution) processes and mesozooplankton grazing were reduced. The primary regulator of phytoplankton growth in low nutrient eastern waters (reference site) shifted from nutrients in the wet season to increased vertical mixing in the dry season. In the highly flushed western waters and Victoria Harbour, the majority (>86?%) of the eutrophication impacts were reduced by strong hydrodynamic mixing (turbulence, vertical mixing, and flushing effects) all year. In southern waters, eutrophication effects were severe (chlorophyll a of up to ~73?μg?L^?1) and was regulated by the ambient phosphate (PO₄) concentration (~0.1?μM) during summer. In contrast, 62–96?% of the potential eutrophication impacts were reduced by physical processes during other seasons. Bioassays also revealed that the yield of chlorophyll from dissolved inorganic nitrogen (DIN) that was taken up by phytoplankton [1.1–3.3?g?Chl (mol?N)^?1] was not significantly different in both N- and P-limited cases. In contrast, the uptake ratios of DIN:PO₄ (26:1–105:1) and Chl:P ratios [42–150?g?Chl (mol?P)^?1] in the P-limited cases were significantly (p?<?0.05, t test) higher than the N-limited cases [~16DIN:1P and 22–48?g?Chl (mol?P)^?1]. The C:Chl ratios ranged from 32 to 87?g?g^?1. These potential ranges in ratios need to be considered in future nutrient models.     

Surface measurement system for the atmospheric electrical vertical conduction current density, with displacement current density correction

Global thunderstorm and shower cloud activity generate the global electric potential difference between the Earth's surface and the lower ionosphere. The finite conductivity of atmospheric air, which arises from cosmic ray and natural radioactive ionisation, permits a vertical conduction current density (1 pA m⁻²) between the lower ionosphere and the surface during fair-weather conditions; this current provides a physical link between the upper and lower atmospheres. A new instrument system is described to measure the conduction current density at the surface (the “air–Earth current”), which operates on a novel principle using two collecting electrodes of different geometry. Simultaneous measurements from two independent co-located systems using the geometrical principle show close agreement (correlation of 0.96 during 2.5 h of 5 min measurements). The sensor design described is durable and successful measurements in fair and disturbed weather have been obtained in air temperatures between −6 and 35 °C, relative humidity between 44% and 100%, fog, rain and snowfall. The uncertainty in conduction current density determinations is 0.20 pA m⁻².     

Short communication: Recognising the perils of landslide-generated tsunamis in the Asia–Pacific region

The objective of the paper was to characterise the temporal and spatial variability of winter warm spells in Central Europe in the years 1966/1967–2015/2016 and to determine the circulation conditions of their occurrence. The applied data were obtained from the Polish Institute of Meteorology and Water Management, Deutscher Wetterdienst and the National Centre for Environmental Prediction/National Centre for Atmospheric Research. A warm spell was defined as a sequence of at least three warm days, i.e. when the maximum air temperature is higher than the 95th percentile of the probability density function designated from observation. The research has proven that over the study period the air temperature increased in the winter season in Central Europe and this translated into an increase in the number of warm days. An average of 3–5 warm spells was recorded per 10 years. The most numerous warm spells occurred during three winter seasons, i.e. 1989/1990, 2006/2007 and 2015/2016. The occurrence of warm spells was related to positive anomalies of geopotential heights over the study area in the cross section of the entire troposphere. Maximum anomalies appeared at 250 hPa geopotential height, and they developed on average 9 days before the commencement of warm spells over the study area.