Undersaturated Conditions Research Articles

Carbon–climate feedbacks accelerate ocean acidification

Abstract. Carbon–climate feedbacks have the potential to significantly impact the future climate by altering atmospheric CO2 concentrations (Zaehle et al., 2010). By modifying the future atmospheric CO2 concentrations, the carbon–climate feedbacks will also influence the future ocean acidification trajectory. Here, we use the CO2 emissions scenarios from four representative concentration pathways (RCPs) with an Earth system model to project the future trajectories of ocean acidification with the inclusion of carbon–climate feedbacks. We show that simulated carbon–climate feedbacks can significantly impact the onset of undersaturated aragonite conditions in the Southern and Arctic oceans, the suitable habitat for tropical coral and the deepwater saturation states. Under the high-emissions scenarios (RCP8.5 and RCP6), the carbon–climate feedbacks advance the onset of surface water under saturation and the decline in suitable coral reef habitat by a decade or more. The impacts of the carbon–climate feedbacks are most significant for the medium- (RCP4.5) and low-emissions (RCP2.6) scenarios. For the RCP4.5 scenario, by 2100 the carbon–climate feedbacks nearly double the area of surface water undersaturated with respect to aragonite and reduce by 50 % the surface water suitable for coral reefs. For the RCP2.6 scenario, by 2100 the carbon–climate feedbacks reduce the area suitable for coral reefs by 40 % and increase the area of undersaturated surface water by 20 %. The sensitivity of ocean acidification to the carbon–climate feedbacks in the low to medium emission scenarios is important because recent CO2 emission reduction commitments are trying to transition emissions to such a scenario. Our study highlights the need to better characterise the carbon–climate feedbacks and ensure we do not underestimate the projected ocean acidification.

Polyphosphate-Accumulating Bacteria: Potential Contributors to Mineral Dissolution in the Oral Cavity.

ABSTRACTBacteria that accumulate polyphosphates have previously been shown to dynamically influence the solubility of phosphatic minerals in marine settings and wastewater. Here, we show that dental plaque, saliva, and carious lesions all contain abundant polyphosphate-accumulating bacteria. Saturation state modeling results, informed by phosphate uptake experiments using the model organism Lactobacillus rhamnosus, which is known to inhabit advanced carious lesions, suggest that polyphosphate accumulation can lead to undersaturated conditions with respect to hydroxyapatite under some oral cavity conditions. The cell densities of polyphosphate-accumulating bacteria we observed in some regions of oral biofilms are comparable to those that produce undersaturated conditions (i.e., those that thermodynamically favor mineral dissolution) in our phosphate uptake experiments with L. rhamnosus. These results suggest that the localized generation of undersaturated conditions by polyphosphate-accumulating bacteria constitutes a new potential mechanism of tooth dissolution that may augment the effects of metabolic acid production.IMPORTANCE Dental caries is a serious public health issue that can have negative impacts on overall quality of life and oral health. The role of oral bacteria in the dissolution of dental enamel and dentin that can result in carious lesions has long been solely ascribed to metabolic acid production. Here, we show that certain oral bacteria may act as a dynamic shunt for phosphate in dental biofilms via the accumulation of a polymer known as polyphosphate—potentially mediating phosphate-dependent conditions such as caries (dental decay).

Contrasting Impact of Future CO2 Emission Scenarios on the Extent of CaCO3 Mineral Undersaturation in the Humboldt Current System

AbstractThe eastern boundary upwelling systems are among those regions that are most vulnerable to an ocean acidification‐induced transition toward undersaturated conditions with respect to mineral CaCO3, but no assessment exists yet for the Humboldt Current System. Here we use a high‐resolution (∼7.5 km) regional ocean model to investigate past and future changes in ocean pH and CaCO3 saturation state in this system. We find that within the next few decades, the nearshore waters off Peru are projected to become corrosive year round with regard to aragonite, the more soluble form of CaCO3. The volume of aragonite undersaturated water off Peru will continue to increase in the future irrespective of the amount of CO2 emitted to the atmosphere. In contrast, the development of the saturation state with regard to calcite, a less soluble form of carbonate, depends strongly on the scenario followed. By 2050, calcite undersaturation appears in the nearshore waters off Peru occasionally, but by 2090 in a high‐emission scenario (RCP8.5), ∼60% of the water in the euphotic zone will become permanently calcite undersaturated. Most of this calcite undersaturation off Peru can likely be avoided if a low emission scenario (RCP2.6) consistent with the Paris Agreement is followed. The progression of ocean acidification off Chile follows a similar pattern, except that the saturation states are overall higher. But also here, calcite undersaturated waters will become common in the subsurface waters under the RCP8.5 scenario by the end of this century, while this can be avoided under the RCP2.6 scenario.

Past and future evolution of the marine carbonate system in a coastal zone of the Northern Antarctic Peninsula

It is arduous to gather a good spatial and temporal dataset of marine carbonate properties, especially in the Southern Ocean. In this study, we have reconstructed the carbonate system in the Gerlache Strait, a coastal zone of the Northern Antarctic Peninsula. We also analyzed the impact of ocean acidification by calculating the tipping points of the calcium carbonate saturation states and pH (i.e., when saturation state and pH goes below one and 7, respectively). Hydrographic and carbonate data from three distinct data sets (GOAL – 2013–2016, FRUELA – 1996, and World Ocean Database – 1965–2004) have been joined and used to reconstruct the carbonate system from the past 50 years. Temporal annual mean trends were determined depending on the water column depth-layer. The northern Gerlache Strait showed a significant increasing trend of total inorganic carbon concentrations (1.0024 ± 0.34 µmol kg−1) and related pH decreasing trend (−0.0026 ± 0.0009 sws) in the surface mixed layer (> 60 m). The properties variability is relatively different (magnitudes and signs) between the northern and southern sectors of the Gerlache Strait, which indicate that adjacent regions to the Gerlache Strait to the southwest and north, respectively, may major influence the regional carbonate dynamics. Results also show that episodic under-saturation conditions, in relation to aragonite within the surface mixed layer, may already occur, especially in regions close to large glaciers.

Crystallization-Induced Fouling during Boiling: Formation Mechanisms to Mitigation Approaches.

Boiling is significantly altered by the presence of dissolved salts. In particular, salts whose solubility decreases with temperature have the tendency to crystallize and adhere to the heat transfer surface and adversely affect the thermal performance. Scaling due to the precipitation of such salts poses serious operational and safety challenges in several practical applications, including heat exchangers, pipelines, and desalination. Here, we study the effect of dissolved salts on the dynamics of pool boiling and its impact on the heat transfer coefficient and critical heat flux (CHF). We find that even undersaturated conditions can lead to crystallization and scale buildup on the boiling surface and dramatically lower heat transfer performance. For example, the CHF for a salt solution that is 75% of the saturation concentration is found to be at least 2 times lower than that for deionized water. Using simultaneous high-speed optical and infrared imaging, we determine the interdependence between crystallization-induced scale formation and bubble evolution dynamics, including bubble nucleation, growth, and departure. We find that salt crystallizes in a "coffee-ring" pattern due to evaporation at the contact line of the bubble. On the basis of the role of the microlayer and triple contact line on scale formation, we propose manipulating surface wettability as a means to avoid scale formation and the associated decrease in the heat transfer coefficient. Surfaces with hybrid wettability are demonstrated as a means to mitigate the reduction in the heat transfer coefficient and CHF in the presence of dissolved salts.

Evaporation of Water from Gas Diffusion Layers

Evaporation of water from gas diffusion layers (GDL) is an important process for water management in polymer electrolyte fuel cells. At temperatures around 80 °C and locally under-saturated conditions, evaporation can have a significant impact on the GDL saturation. However, little is known about the evaporation rates of water in gas diffusion layers. First ex-situ results at room temperature and for low saturations have been reported in [1]. But for a better understanding of the evaporation rates under real operating temperatures, generic data is required at different temperatures and saturation levels. Using a special setup, which is applicable also for X-ray imaging of water saturation, water surface and GDL structure, evaporation rates are determined for well-controlled gas speeds (over the surface of the GDL),, temperatures of up to 80 °C and relevant liquid saturations. Figure 1 shows data (including reproducibility) for Toray TGPH 060 material as function of temperature up to 80 °C, at a capillary pressure of 10 mbar and gas velocity of 6 m/s. [1] I. V. Zenyuk, A. Lamibrac, J. Eller, D. Y. Parkinson, F. Marone, F. N. Büchi, and A. Z. Weber, J. Phys. Chem. C, 2016, 120 (50), pp 28701–28711 Figure 1: Evaporation rates from Toray TGPH 060 gas diffusion layer material a) as function of temperature at a gas velocity of 6.0 m/s; b) as function of gas velocity on the GDL surface at 60 °C; both for a dry gas stream and capillary pressure of 10 mbar; data shown based on preliminary set-up. Figure 1

Catalysis and chemical mechanisms of calcite dissolution in seawater

Near-equilibrium calcite dissolution in seawater contributes significantly to the regulation of atmospheric [Formula: see text] on 1,000-y timescales. Despite many studies on far-from-equilibrium dissolution, little is known about the detailed mechanisms responsible for calcite dissolution in seawater. In this paper, we dissolve 13C-labeled calcites in natural seawater. We show that the time-evolving enrichment of [Formula: see text] in solution is a direct measure of both dissolution and precipitation reactions across a large range of saturation states. Secondary Ion Mass Spectrometer profiles into the 13C-labeled solids confirm the presence of precipitated material even in undersaturated conditions. The close balance of precipitation and dissolution near equilibrium can alter the chemical composition of calcite deeper than one monolayer into the crystal. This balance of dissolution-precipitation shifts significantly toward a dissolution-dominated mechanism below about [Formula: see text] Finally, we show that the enzyme carbonic anhydrase (CA) increases the dissolution rate across all saturation states, and the effect is most pronounced close to equilibrium. This finding suggests that the rate of hydration of [Formula: see text] is a rate-limiting step for calcite dissolution in seawater. We then interpret our dissolution data in a framework that incorporates both solution chemistry and geometric constraints on the calcite solid. Near equilibrium, this framework demonstrates a lowered free energy barrier at the solid-solution interface in the presence of CA. This framework also indicates a significant change in dissolution mechanism at [Formula: see text], which we interpret as the onset of homogeneous etch pit nucleation.

Molecular and geochemical perspectives on the influence of CO2 on calcification in coral cell cultures

AbstractUnderstanding the cellular and molecular responses of stony corals to ocean acidification is key to predicting their ability to calcify under projected high CO2 conditions. Of specific interest are the links between biomineralization proteins and the precipitation of new calcium carbonate (CaCO3), which potentially can provide a better understanding of the biomineralization process. We have assessed the effects of increased CO2 on the calcification process in cell cultures of the stony coral, Stylophora pistillata, reared in nutrient‐enriched artificial seawater at four pCO2 levels and two glucose concentrations. Dispersed S. pistillata cells grown at low (400 ppmV) and moderate (700 ppmV) pCO2 re‐aggregate into proto‐polyps and precipitate CaCO3. When grown at pCO2 levels of 1000 ppmV and 2000 ppmV, the cells up‐regulate genes for two highly acidic proteins as well as a carbonic anhydrase, but down‐regulate long term cadherin protein production and minimize proto‐polyp formation, and exhibit a significant decrease in measurable CaCO3 precipitation. However, cell cultures precipitate CaCO3 in all treatments, even at slightly undersaturated conditions (Ωaragonite < 0.95). Glucose addition does not influence either biomineralization gene expression or calcification rate. Measured δ11B of the mineral phase, as a proxy of the pH at the calcifying sites, is out of equilibrium with the ambient seawater medium surrounding the cells and proto‐polyps, suggesting pH is elevated in the micro‐environment of the precipitating mineral. Our results suggest that coral cells possess molecular mechanisms to help compensate for the effects of ocean acidification within the bounds projected in the coming century.

Thermodynamic model of Ni(II) solubility, hydrolysis and complex formation with ISA

AbstractThe solubility of β–Ni(OH)2(cr) was investigated atT=(22±2)°C in the absence and presence of α-isosaccharinic acid (ISA), the main degradation product of cellulose under alkaline pH conditions. Batch solubility experiments were performed from undersaturation conditions under inert gas (Ar) atmosphere. Solubility experiments in the absence of ISA were conducted in 0.5 and 3.0 M NaCl–NaOH solutions at 7.5≤ pHm≤13 (with pHm=–log10[H+]). XRD analyses of selected solid phases collected after completing the solubility experiments (≈300 days) confirmed that β–Ni(OH)2(cr) remains as solid phase controlling the solubility of Ni(II) in all investigated conditions. Based on the slope analysis (log10[Ni] vs. pHm) of the solubility data and solid phase characterization, the equilibrium reactions β–Ni(OH)2(cr)+2 H+⇔Ni2++2 H2O(l) and β–Ni(OH)2(cr)⇔Ni(OH)2(aq) were identified as controlling the solubility of Ni(II) within the investigated pHmregion. The conditional equilibrium constants determined from the solubility experiments at different ionic strengths were evaluated with the specific ion interaction theory (SIT). In contrast to the current thermodynamic selection in the NEA–TDB, solubility data collected in the present work does not support the formation of the anionic hydrolysis species Ni(OH)3−up to pHm≤13.0. Solubility experiments in the presence of ISA were conducted in 0.5 M NaCl–NaOH–NaISA solutions with 0.01 M≤[NaISA] ≤0.2 M and 9≤ pHm≤13. XRD analyses confirmed that β–Ni(OH)2(cr) is also the solid phase controlling the solubility of Ni(II) in the presence of ISA. Solubility data of all investigated systems can be properly explained with chemical and thermodynamic models including the formation of the complexes NiOHISA(aq), Ni(OH)2ISA−and Ni(OH)3ISA2−. The reported data confirm the low solubility (<10−7M) of Ni(II) in hyperalkaline pH conditions representative of cementitious environments (10≤ pH ≤13), which increases to up to 10−5M in the presence of 0.2 M NaISA. These results significantly improve source term estimations for Ni(II) in environments relevant for the disposal of low and intermediate level radioactive waste (L/ILW). The chemical and thermodynamic models derived in this work can be implemented in geochemical models/calculations, and provide further confidence in the safety analysis of repositories for the disposal of L/ILW.

Gypsum and hydrohalite dynamics in sea ice brines

Mineral authigenesis from their dissolved sea salt matrix is an emergent feature of sea ice brines, fuelled by dramatic equilibrium solubility changes in the large sub-zero temperature range of this cryospheric system on the surface of high latitude oceans. The multi-electrolyte composition of seawater results in the potential for several minerals to precipitate in sea ice, each affecting the in-situ geochemical properties of the sea ice brine system, the habitat of sympagic biota. The solubility of two of these minerals, gypsum (CaSO4·2H2O) and hydrohalite (NaCl·2H2O), was investigated in high ionic strength multi-electrolyte solutions at below-zero temperatures to examine their dissolution–precipitation dynamics in the sea ice brine system. The gypsum dynamics in sea ice were found to be highly dependent on the solubilities of mirabilite and hydrohalite between 0.2 and -25.0°C. The hydrohalite solubility between -14.3 and -25.0°C exhibits a sharp change between undersaturated and supersaturated conditions, and, thus, distinct temperature fields of precipitation and dissolution in sea ice, with saturation occurring at -22.9°C. The sharp changes in hydrohalite solubility at temperatures ⩽−22.9°C result from the formation of an ice–hydrohalite aggregate, which alters the structural properties of brine inclusions in cold sea ice. Favourable conditions for gypsum precipitation in sea ice were determined to occur in the region of hydrohalite precipitation below -22.9°C and in conditions of metastable mirabilite supersaturation above -22.9°C (investigated at -7.1 and -8.2°C here) but gypsum is unlikely to persist once mirabilite forms at these warmer (>−22.9°C) temperatures. The dynamics of hydrohalite in sea ice brines based on its experimental solubility were consistent with that derived from thermodynamic modelling (FREZCHEM code) but the gypsum dynamics derived from the code were inconsistent with that indicated by its experimental solubility in this system. Incorporation of hydrohalite solubility into a 1D thermodynamic model of the growth of first-year Arctic sea ice showed its precipitation to initiate once the incoming shortwave radiation dropped to 0Wm−2, and that it can reach concentrations of 9.9gkg−1 within the upper and coldest layers of the ice pack. This suggests a limited effect of hydrohalite on the albedo of sea ice. The insights provided by the solubility measurements into the behaviour of gypsum and hydrohalite in the ice–brine system cannot be gleaned from field investigations at present.

Groundwater mixing in the discharge area of San Vittorino Plain (Central Italy): geochemical characterization and implication for drinking uses

Discharge areas of carbonate fractured and karstified aquifers are a sensitive system of great interest, where frequently groundwater resources are tapped for drinking water supply. In geological settings affected by recent and/or active tectonics, mixing between fresh water coming from recharge areas and groundwater from deeper circuits, influenced by raising fluids, influences hydrogeochemistry. Surveys on major ions, trace elements and stable isotopes have been performed in the San Vittorino Plain (Central Italy), where the major source of drinking water for Rome is located (Peschiera Springs, mean discharge 18 m3 s−1, half of them tapped). Results of 21 springs revealed different contribution from recharge areas and deep flow paths, by increasing salinity and ion content, with particular references to Ca2+, HCO3 − and SO4 2−. Three main groups, respectively, related to fresh waters from recharge areas, groundwater from deep contribution and a mixing group between them, have been identified. Water stable isotopes allow to identify the common origin from rainfall and a very steady contribution with seasons and year, due to the huge extent of recharge area (>1000 km2). Saturation Indexes gave insight on the contribution of deep fluids, mainly CO2 and H2S, which turned groundwater to undersaturated conditions, facilitating rock dissolution. By PHREEQC software, the mixing between two considered end-members has been simulated, evaluating about 25% of deep contribution in the basal springs of San Vittorino Plain. Chemistry of Peschiera spring reveals a very limited percentage of deep flow paths (10%), which can lead to slight hydrochemistry changes even in possible drought conditions, when discharge can decrease until 15 m3 s−1.

Lawsonite-bearing eclogite from a tectonic mélange in the Ligurian Alps: new constraints for the subduction plate-interface evolution

AbstractLawsonite eclogites are rare rocks and have been described from only a few localities in the world. Lawsonite-bearing assemblages are highly unstable and physico-chemical processes linked to exhumation may destroy them; only aggregates interpreted as pseudomorphs after lawsonite could be often recognized. In this paper, we present a detailed structural and petrological study of an area in the northwestern sector of the metaophiolitic high-pressure Voltri Massif (Ligurian Western Alps, Italy). The study area is characterized by a lawsonite-bearing eclogitic metagabbro associated with carbonated serpentinites and glaucophanic metasediments. The metagabbro body reached eclogitic metamorphic peak conditions atT= 465–477°C andP= 20.9–24.4 kbar, with H2O continuously supplied to the system. H2O under-saturated conditions, with the occurrence of both lawsonite and epidote, characterized the exhumation path. Both the low temperature recorded by the body and the occurrence of variously carbonated serpentinites led us to interpret this area as a portion of the top of the subducted slab, coupled with a ‘cool’ mantle wedge, where both aqueous fluids and carbonate-rich fluids were present. The occurrence of rocks belonging to different paleogeographic domains (e.g. continent versus ocean) and the multiple deformations recorded by the metagabbro suggest that this area was nearby the slab–mantle interface. This sector was thus affected by a shear regime that acted in a low-viscosity serpentinite channel, bringing these high-pressure rocks back to the surface.

Intra-Specific Variation Reveals Potential for Adaptation to Ocean Acidification in a Cold-Water Coral from the Gulf of Mexico

Ocean acidification, the decrease in seawater pH due to the absorption of atmospheric CO2, profoundly threatens the survival of a large number of marine species. Cold-water corals are considered to be among the most vulnerable organisms to ocean acidification because they are already exposed to relatively low pH and corresponding low calcium carbonate saturation states (Ω). Lophelia pertusa is a globally distributed cold-water scleractinian coral that provides critical three-dimensional habitat for many ecologically and economically significant species. In this study, four different genotypes of L. pertusa were exposed to three pH treatments (pH=7.60, 7.75, and 7.90) over a short (two-week) experimental period, and six genotypes were exposed to two pH treatments (pH=7.60, and 7.90) over a long (six-month) experimental period. Their physiological response was measured as net calcification rate and the activity of carbonic anhydrase, a key enzyme in the calcification pathway. In the short-term experiment, net calcification rates did not significantly change with pH, although they were highly variable in the low pH treatment, including some genotypes that maintained positive net calcification in undersaturated conditions. In the six-month experiment, average net calcification was significantly reduced at low pH, with corals exhibiting net dissolution of skeleton. However, one of the same genotypes that maintained positive net calcification (+0.04% day-1) under the low pH treatment in the short-term experiment also maintained positive net calcification longer than the other genotypes in the long-term experiment, although none of the corals maintained positive calcification for the entire 6 months. Average carbonic anhydrase activity was not affected by pH, although some genotypes exhibited small, insignificant, increases in activity after the sixth month. Our results suggest that while net calcification in L. pertusa is adversely affected by ocean acidification in the long term, it is possible that some genotypes may prove to be more resilient than others, particularly to short perturbations of the carbonate system. These results provide evidence that populations of L. pertusa in the Gulf of Mexico may contain the genetic variability necessary to support an adaptive response to future ocean acidification.

Silica nanoparticles as sources of silicic acid favoring wound healing in vitro

There is good evidence that certain silicon-containing materials promote would healing and their common feature is the delivery of orthosilicic acid (Si(OH)4) either directly or following metabolism. In this respect, amorphous silica nanoparticles (NP), which dissolve in aqueous environments releasing up to 2mM orthosilicic acid, may be appropriate ‘slow release’ vehicles for bioactive silicon. Here we studied the impact of silica NP suspensions (primary particles∼10nm) in undersaturated conditions (below 2mM Si) with differing degrees of surface charge and dissolution rate on human dermal fibroblasts (CCD-25SK cells) viability, proliferation and migration in a cellular wound model. Silica was shown to be non-toxic for all forms and concentrations tested and whilst the anticipated stimulatory effect of orthosilicic acid was observed, the silica NPs also stimulated fibroblast proliferation and migration. In particular, the amine-functionalized particles promoted wound closure more rapidly than soluble orthosilicic acid alone. We suggest that this effect is related to easy cellular internalization of these particles followed by their intracellular dissolution releasing silicic acid at a faster rate than its direct uptake from the medium. Our findings indicate that amorphous silica-based NPs may favour the delivery and release of bioactive silicic acid to cells, promoting wound healing.

Exposure to CO2 influences metabolism, calcification and gene expression of the thecosome pteropod Limacina retroversa.

Thecosomatous pteropods, a group of aragonite shell-bearing zooplankton, are becoming an important sentinel organism for understanding the influence of ocean acidification on pelagic organisms. These animals show vulnerability to changing carbonate chemistry conditions, are geographically widespread, and are both biogeochemically and trophically important. The objective of this study was to determine how increasing duration and severity of CO2 treatment influence the physiology of the thecosome Limacina retroversa, integrating both gene expression and organism-level (respiration and calcification) metrics. We exposed pteropods to over-saturated, near-saturated or under-saturated conditions and sampled individuals at 1, 3, 7, 14 and 21 days of exposure to test for the effect of duration. We found that calcification was affected by borderline and under-saturated conditions by week two, while respiration appeared to be more strongly influenced by an interaction between severity and duration of exposure, showing complex changes by one week of exposure. The organismal metrics were corroborated by specific gene expression responses, with increased expression of biomineralization-associated genes in the medium and high treatments throughout and complex changes in metabolic genes corresponding to both captivity and CO2 treatment. Genes associated with other physiological processes such as lipid metabolism, neural function and ion pumping had complex responses, influenced by both duration and severity. Beyond these responses, our findings detail the captivity effects for these pelagic organisms, providing information to contextualize the conclusions of previous studies, and emphasizing a need for better culturing protocols.

What is the Real ΔGr Function of Albite Dissolution?

One of the most significant environmental variables that affect albite dissolution rate is deviation from equilibrium (ΔGr). Most of the studies that describe the effect of ΔGr on albite dissolution rate were conducted under alkline pH (8.8-9.2), elvetad temperatures (80-300 °C) and for Amelia Courthouse (USA) albite. Comparison of the f(ΔGr) derived from those studies to the only f(ΔGr) from low temperature (25 °C), circum neutral pH and EVJE (Norway) albite leads to different ΔGr functions. Yet, except for the study of1, rate laws for silicate minerals dissolution, which are based on experiments conducted under ambient conditions, are lacking. Here we present new experimental results of single point batch experiments (SPBE) of albite dissolution in a isotopically spiked solution. The novel method that use Si isotopes2 enables detecting rates that otherwise can’t be detected under ambient conditions. Albite dissolution rates were determined under neutral-acidic pH, temperatures of 3.6, 25 and 50 °C, and a wide range of undersaturation conditions. The dependency of the dissolution rate on deviation from equilibrium (f(ΔGr)) was in agreement with the prediction of previous studies. However, the value of ΔGcrit was found to be significantly different, suggesting an effect of extrinsic properties of the environment.

Estimating the Impacts of Bus Stops and Transit Signal Priority on Intersection Operations: Queuing and Variational Theory Approach

Transit signal priority (TSP) can be used to improve bus operations at signalized intersections, often to the detriment of general car traffic. However, the impacts of TSP treatments applied to intersections with nearby bus stop locations are currently unknown. This paper quantifies changes in intersection capacity, car delay, and bus delay when priority is provided to buses that dwell at near- or farside bus stop locations through green extension or red truncation. Variational and kinematic wave theories are used to estimate car capacity and bus delay for oversaturated traffic conditions; queuing theory is used to estimate car and bus delays for undersaturated conditions. Numerical analyses are conducted to explore the impacts on various bus stop locations and bus dwell time durations. These results illustrate clear trade-offs between reduced bus delays and increased car delays or reduced intersection capacities that can be quantified with the proposed method. The results also reveal that the effects of TSP vary dramatically with bus dwell times for a given bus stop location. The proposed method and associated results can be used to implement TSP strategies to meet the specific needs of local agencies.

Iterative tuning strategy for setting phase splits with anticipation of traffic demand in urban traffic network

Facing large-scale urban traffic network, countless effort has been made toward intelligent and efficient urban traffic control system to better use existing traffic infrastructures. Recently, a novelty pre-timed traffic signal control strategy known as iterative tuning (IT) has been developed by exploiting repetitive characteristic of junction's vehicle throughput on working days, which is sufficiently efficient in under-saturated traffic conditions. This study further improves IT strategy in saturated traffic conditions with consideration of traffic demand including vehicle throughput and residual queued vehicles. Unlike conventional pre-timed strategies, preparation of signal schedules is not required in IT strategy and fine-tuning process executes iteratively and automatically. This study proposes a generalised traffic model to describe urban network dynamics and explicit split tuning algorithm. Without specific control trajectories, rigorous analysis generates sufficient condition for guaranteeing the convergence of IT strategy globally over repetitions. The robustness of IT controller to variations of traffic flow patterns and errors of initial conditions is also analysed in details. Commonwealth Avenue, an area with heavy traffic in Singapore, is demonstrated in simulations and simulation results indicate the effectiveness and robustness of IT strategy.

The Finding of Nondissolving Lysozyme Crystals and Its Significance for the Study of Hard-to-Crystallize Biological Macromolecules

Major discoveries in structural biology depend on obtaining well-diffracting macromolecular crystals. This necessity has motivated many fundamental studies on protein crystallization using lysozyme as a model system. In the present contribution, we report the unprecedented observation of lysozyme crystals that stop dissolving under undersaturated conditions imposed to sub-microliter crystallization drops at mild temperatures. Subsequent growth of the same crystals is apparently undisturbed after the drops are cooled below the saturation temperature. The succession of heating/cooling cycles only partially recovers crystal dissolution while crystal growth becomes gradually slower. Ultimately, increasing and decreasing the temperature between 10 and 37 °C has no visible effect on the size of the crystals. We ascribe this phenomenon to the partial denaturation of the soluble protein in the drop as evidenced by the decreasing glycoside hydrolase activity of lysozyme over the incubation time. The disturbances i...

Hydrothermal Transport of Ag, Au, Cu, Pb, Te, Zn, and Other Metals and Metalloids in New Zealand Geothermal Systems: Spatial Patterns, Fluid-Mineral Equilibria, and Implications for Epithermal Mineralization

In order to analyze the concentrations of Ag, As, Au, Cd, Cu, Hg, Mn, Mo, Ni, Pb, Se, Sb, Te, Tl, W, and Zn in deep hydrothermal chloride waters at 195° to 320°C, and in shallow boiled chloride waters at 160° to 230°C, we sampled wells drilled to 3-km depth in geothermal systems of the North Island, New Zealand. Six of the systems are located in a segment of volcanic arc in the central Taupo Volcanic Zone, and the seventh system is associated with an intraplate mafic (felsic) volcanic center. The concentrations of metals range widely from 0.1 to >1,000 μ g/kg with a large degree of intersystem variability. Some of the largest contrasts in Au, Ag, Pb, and Te concentrations are observed in the two nearest systems, Rotokawa and Wairakei, which are only 10 km apart. The correlations between metals are poor, except for Ag-Au-Pb-Te, and As-Sb. The correlations between metals, Cl, and H2S are also poor, with the exception of Rotokawa where the highest concentrations of Ag, Au, Cu, and Te correlate with the highest concentration of aqueous H2S. Speciation calculations indicate that the dominant aqueous species of Ag, Au, Cu, Pb, and Zn involve HS− complexes. The calculations also show that the states of saturation range from undersaturated conditions for acanthite, arsenopyrite, and gold to oversaturated conditions for chalcopyrite, sphalerite, and tellurides. Notably, the Au- and Ag-transporting capacities of the deep chloride waters are much larger than the measured aqueous concentrations. These results suggest that fluid-mineral equilibria and the concentrations of ligands exert weak influence on metal concentrations at the temperatures and depths of sampling. The complex trends in hydrothermal metal concentrations strongly suggest that the deep-seated sources of metals, comprising magmatic intrusions, deep country rock, and their related fluids, limit the hydrothermal supplies of metals. Between the geothermal systems, hydrothermal fluxes of Ag (6–8,000 kg/y), Au (0.9–66 kg/y), Cu (30–23,500 kg/y), and Te (2–10,400 kg/y) are variable. The highest concentrations and fluxes of Ag, Au, and Te in Rotokawa and Mokai are attributed to direct fluid inputs from intrusions of andesitic and basaltic magmas, respectively. Compared to their deep counterparts, boiled chloride waters are strongly depleted in Ag, Au, Cd, Cu, Pb, and Te, because these metals deposit in sharp response to gas loss and cooling in the well. By comparison, the As, Mn, Mo, Ni, Sb, Tl, and Zn concentrations are measurably less depleted in boiled waters, making them available to form metal anomalies at shallow depths and in the peripheral parts of the epithermal environment. Periods of strong metal flux through the geothermal system combined with deep boiling favor epithermal ore formation and the development of large precious metal deposits. However, even moderate metal fluxes can produce Au and Ag mineralization as long as the duration of focused fluid flow and boiling can be sustained. There is no evidence that the compositions of deep magmatic intrusions, mafic or intermediate, limit the ore-forming potential of a geothermal system.