Numerical simulation of deep convective cloud seeding using liquid carbon dioxide

In this study, a one-dimensional transient cumulonimbus cloud is modeled to be seeded by liquid CO₂. The model includes microphysical and dynamical processes associated with glaciogenic seeding by homogenous ice nucleation and two thermal terms associated with seeding by ?90 ºC liquid CO₂. For this model, the study concentrates on five types of hydrometeors, namely, cloud droplet, cloud ice, snow, hail/graupel, and rain. Point and horizontal seeding methods are implemented to observe their implications for rainfall enhancement, amount of hail/graupel production, vertical cloud extension, and radar’s reflectivity. In addition, the seeding temperature effects on the rainfall and microphysical processes are investigated. The results of the study show that, the rainfall enhancement and rainfall intensity in the point seeding case are more than those in the horizontal seeding. Moreover, the study reveals that, there is a vertical cloud extension enhancement of 0.5 km for clouds with top height of 10.5 km. The most important sources of the rain water production are found to be the accretion of cloud water by rain (P _RACW) and by snow (P _SACW), and for the graupel production is dry growth of the graupel (P _GDRY). The results of this study are confirmed by the results of other investigators and are found to be comparable with the recorded data at rain gauge stations.     

Cave air and hydrological controls on prior calcite precipitation and stalagmite growth rates: Implications for palaeoclimate reconstructions using speleothems

Hourly resolved cave air P_CO2 and cave drip water hydrochemical data illustrate that calcite deposition on stalagmites can be modulated by prior calcite precipitation (PCP) on extremely short timescales. A very clear second-order covariation between cave air P_CO2 and drip water Ca²⁺ concentrations during the winter months demonstrates the effects of degassing-induced PCP on drip water chemistry. Estimating the strength of the cave air P_CO2 control on PCP is possible because the PCP signal is so clear; at our drip site a one ppm shift in Ca²⁺ concentrations requires a P_CO2 shift of between 333 and 667 ppm. This value will undoubtedly vary from site to site, depending on drip water flow rate, residence time, drip water-cave air P_CO2 differential, and availability of low P_CO2 void spaces in the vadose zone above the cave. High-resolution cave environmental measurements were used to model calcite deposition on one stalagmite in Crag Cave, SW Ireland, and modelled growth over the study period (222 μm over 171 days) is extremely similar to the amount of actual calcite growth (240 μm) over the same time interval, strongly suggesting that equations used to estimate stalagmite growth rates are valid. Although cave air P_CO2 appears to control drip water hydrochemistry in the winter, drip water dilution caused by rain events may have played a larger role during the summer, as evidenced by a series of sudden drops in Ca²⁺ concentrations (dilution) followed by much more gradual increases in drip water Ca²⁺ concentrations (slow addition of diffuse water). This research demonstrates that PCP on stalactites, cave ceilings, and void spaces within the karst above the cave partially controls drip water chemistry, and that thorough characterisation of this process at individual caves is necessary to most accurately interpret climate records from those sites.     

Sr,Cd, Mn and Co distribution coefficients in calcite as a function of calcite precipitation rate

Distribution coefficients, as a function of precipitation rate, were determined for the metals Sr²⁺, Co²⁺, Mn²⁺ and Cd²⁺in calcite. A pH-stat was used to maintain a constant degree of-saturation, and hence precipitation rate, during each coprecipitation run. The precipitation rate was proportional to the degree of supersaturation and the mass of seed crystal introduced. Distribution coefficients (λ) as a function of rate were determined using radioactive isotopes for solutions with saturations $\text{Ω = 1}$ to $\text{Ω = 5.5}$. Strontium distribution coefficients increased with increasing precipitation rate, while Co, Mn and Cd distribution coefficients decreased with increasing precipitation rate. The following rate expressions (at 25°C) were derived: logλ_Sr = 0.249 log R ?1.57logλ_Mn = ?0.266 log R + 1.35logλ_Co = ?0.173 log R + 0.68logλ_Cd = ?0.194 log R + 1.46 where R is the observed precipitation rate in nmoles CaCO₃ per mg seed crystal per min.In separate experiments the uptake of radioactive isotopes was monitored during the recrystallization of calcite seed crystals. Rates of recrystallization were from 100 to 10, 000 times slower than the pH-stat experiments, but yielded distribution coefficients consistent with the above rate expressions.Using gross estimates of biogenic crystal growth rates, aragonite to calcite transformation rates, and the above Sr rate expression, biogenic calcite and diagenetic calcite Sr contents are estimated. These experiments indicate that in addition to solution composition, precipitation rate is a significant factor influencing the trace metal content of naturally occurring calcite.     

The solubility of rhodochrosite (MnCO3) and siderite (FeCO3) in anaerobic aquatic environments

Natural groundwaters are often reported to be highly supersaturated with the carbonate minerals siderite (FeCO₃) and rhodochrosite (MnCO₃). The kinetics of precipitation and dissolution were determined in the light of new determinations of the solubility products of siderite and rhodochrosite. Laboratory experiments showed that the precipitation kinetics of siderite and rhodochrosite were much slower than that of calcite, and also much slower than their dissolution kinetics. Experiments with supersaturated solutions failed to reach steady state within 474 days in the case of siderite, whereas steady state for rhodochrosite was reached after 140 days. Suspensions of siderite and rhodochrosite crystals reached steady state after 10 and 80 days, respectively. The solubility product of siderite (−log K_S0(FeCO₃)) was 11.03 ± 0.10 for dried crystals and 10.43 ± 0.15 for wet crystals. For rhodochrosite the solubility product (−log K_S0(MnCO₃)) was 11.39 ± 0.14 for dried crystals and 12.51 ± 0.07 for wet crystals. The solubility product determined from supersaturated solutions was −log K_S0(MnCO₃)=11.65 ± 0.14. The observed slow precipitation kinetics of siderite and rhodochrosite might explain the apparent supersaturation that is often reported for anaerobic aquatic environments.     

Influence of microbial photosynthesis on tufa stromatolite formation and ambient water chemistry, SW Japan

Photosynthetic influences on tufa stromatolite formation and ambient water chemistry were investigated at two well-studied streams depositing tufa in Southwestern Japan (Shirokawa and Shimokuraida). The tufa stromatolites in both streams are composed of fine-grained calcite crystals showing annual lamination, and colonized by a number of filamentous cyanobacteria as well as non-phototrophic bacteria. Microelectrode measurements of pH, O₂, and Ca²⁺ near the stromatolite surface (the diffusive boundary layer; DBL) revealed that the investigated tufa stromatolites are formed by photosynthesis-induced CaCO₃ precipitation (PCP): cyanobacterial photosynthesis induces calcite precipitation under light conditions, while respiration of cyanobacteria and non-phototrophic bacteria inhibits precipitation in the dark. The bulk water chemistry at the lower sites of the investigated streams showed the daytime decreases of Ca²⁺ concentration and alkalinity that was expected for significant influence of PCP, while the other expected change, increased pH, was not observed. In order to examine this discrepancy, a novel approach using semi-in situ microelectrode measurements was applied to perform precise quantitative calculations. The calculation results demonstrated that the observed Ca²⁺ concentration and alkalinity decreases were caused by PCP, and that the concomitant pH increase was expected to be under the detection level of a conventional pH meter. Although the amount of PCP is supposed to be significantly affected by light intensity, observations in Shimokuraida revealed that the amount of PCP on cloudy day nonetheless accounted for about 80% of that on sunny day. Despite the significant role of PCP for tufa stromatolite formation, PCP accounted for only about 10% of the precipitated calcite in the investigated streams, which indicates that tufa stromatolites, the characteristic deposits in the streams, are responsible for only a small portion of calcite precipitation, and the rest is considered to precipitate inorganically at biofilm-free substrates.     

The interaction of magnesium with calcite during crystal growth at 25–90°C and one atmosphere

Calcite crystals were grown in a closed system by recrystallization of synthetic and natural aragonite crystals, in the presence of various CaCl2-MgCl2 solutions with and without NaCl.The distribution of Mg2+ between calcite and solution at the entire temperature range is heterogeneous, closely following the Doerner-Hoskins (Doerner and Hoskins, 1925) distribution law. λMg2+C is strongly dependent on temperature, being: 0·0573 ± 0·0017 at 25°C, 0·0681 ± 0·0019 at 35°C, 0·0778 ± 0.0022 at 50°C, 0·0973 ± 0·0021 at 70°C, and 0·1163 ±0 ·0034 at 90°C. λMg2+C is independent of the absolute concentration of Ca2+ in solution as well as of the presence of NaCl.Relatively high λMg2+C values are obtained during the initial reaction stages when too-highly reactive synthetic aragonites are recrystallized. SEM micrographs show that calcite crystals grown from such aragonites are imperfect and that their earlier formed Mg-rich cores redissolve later, resulting in apparently inconsistent λMg2+C values.Calculations applying the new λMg2+C value for 25°C and the solubility data for magnesian calcites (Chaveet al., 1962) demonstrate that although no calcite should be expected to precipate directly from open sea water, its direct precipitation (or recrystallization from aragonite) is possible in closed diagenetic systems which still contain marine solutions, provided a temporary increase in the dissolved calcium concentration takes place.The λMg2+C values obtained allow for a new insight into processes of calcite cementation of reefs and a variety of other carbonate sediments, and for a more precise definition of dedolomitization chemistry.     

Effects of freshwater Synechococcus sp. cyanobacteria pH buffering on CaCO3 precipitation: Implications for CO2 sequestration

In the present study, a mixed-flow steady-state bio-reactor was designed to biomineralize CO₂ as a consequence of photosynthesis from active Synechococcus sp. Dissolved CO₂, generated by constant air bubbling of inorganic and cyanobacteria stock solutions, was the only source of inorganic carbon. The release of hydroxide ion by cyanobacteria from photosynthesis maintained highly alkaline pH conditions. In the presence of Ca²⁺ and carbonate species, this led to calcite supersaturation under steady state conditions. Ca²⁺ remained constant throughout the experiments showing the presence of steady state conditions. Similarly, the Synechococcus sp. biomass concentration remained stable within uncertainty. A gradual pH decrease was observed for the highest Ca²⁺ condition coinciding with the formation of CaCO₃. The high degree of supersaturation, under steady-state conditions, contributed to the stabilization of calcite and maintained a constant driving force for the mineral nucleation and growth. For the highest Ca²⁺ condition a fast crystal growth rate was consistent with rapid calcite precipitation as suggested further by affinity calculations. Although saturation state based kinetic precipitation models cannot accurately reflect the controls on crystal growth kinetics or reliably predict growth mechanisms, the relatively reaction orders obtained from modeling of calcite precipitation rates as function of decreasing carbonate concentration suggest that the precipitation occurred via surface-controlled rate determining reactions. These high reaction orders support in addition the hypothesis that crystal growth proceeded through complex surface controlled mechanisms. In conclusion, the steady state supersaturated conditions generated by a constant cyanobacteria biomass and metabolic activity strongly suggest that these microorganisms could be used for the development of efficient CO₂ sequestration methods in a controlled large-scale environment.     

The transfer of oxygen isotopic signals from precipitation to drip water and modern calcite on the seasonal time scale in Yongxing Cave,central China

Stable isotope data of precipitation (δ¹⁸O_p and deuterium excess), drip water (δ¹⁸O_d), and modern calcite precipitates (δ¹⁸O_c and δ¹³C_c) from Yongxing Cave, central China, are presented, with monthly sampling intervals from June 2013 to September 2016. Moderate correlations between the monthly variation of δ¹⁸O_p values (from ??11.5 to ??0.7‰) and precipitation amount (r = ??0.59, n?=?34, p?<?0.01) and deuterium excess (r?=?0.39, n?=?31, p?<?0.01) imply a combined effect of changes in precipitation amount and atmospheric circulation. At five drip sites, the δ¹⁸O_d values have a much smaller variability (from ??9.1 to ??7.5‰), without seasonal signals, probably a consequence of the mixing in the karst reservoir with a deep aquifer. The mean δ¹⁸O_d value (??8.4‰) for all drip waters is significantly more negative than the mean δ¹⁸O_p value (??6.9‰) weighted by precipitation amount, but close to the wet season (May to September) mean value (??8.3‰), suggesting that a threshold of precipitation amount must be exceeded to provide recharge. Calculation based on the equilibrium fractionation factor indicates that the δ¹⁸O_c values are not in isotopic equilibrium with their corresponding drip waters, with a range of disequilibrium effects from 0.4 to 1.4‰. The δ¹⁸O_c and δ¹³C_c values generally increase progressively away from the locus of precipitation on glass plates. The disequilibrium effects in the cave are likely caused by progressive calcite precipitation and CO₂ degassing related to a high gradient of CO₂ concentration between drip waters and cave air. Our study provides an important reference to interpret δ¹⁸O_c records from the monsoon region of China.     

Effects of seed material and solution composition on calcite precipitation

We studied the effects of seed material and solution composition on calcite crystal precipitation using a pH-stat system. The seed materials investigated included quartz, dolomite, two calcites with different particle size and specific surface area, and two dried precipitates from precipitative softening water treatment plants. Our results indicated that, of the seed materials examined, only calcite had the ability to initiate calcite precipitation in a solution with a degree of supersaturation of 5.3 over a period of two hours, and that the precipitation rate was proportional to the available surface area of the seed. For different solution compositions with the same degree of supersaturation, the calcite precipitation rate increased with increasing carbonate/calcium ratio, which contradicts the generally accepted empirical rate expression that the degree of supersaturation is the sole factor controlling precipitation kinetics. By applying a surface complexation model, the surface concentrations of two species, >CO₃⁻ and >CaCO₃⁻, appear to be responsible for catalyzing calcite precipitation.     

Precipitation of amorphous CaCO3 (aragonite-like) by cyanobacteria: A STXM study of the influence of EPS on the nucleation process

An amorphous or nanocrystalline calcium carbonate (ACC) phase with aragonite-like short-range order was found to be a transient precursor phase of calcite precipitation mediated by cyanobacteria of the strain Synechococcus leopoliensis PCC 7942. Using scanning transmission X-ray microscopy (STXM), different Ca-species such as calcite, aragonite-like CaCO₃, and Ca adsorbed on extracellular polymers were discriminated and mapped, together with various organic compounds, at the 30 nm-scale. The nucleation of the amorphous aragonite-like CaCO₃ was found to take place within the tightly bound extracellular polymeric substances (EPS) produced by the cyanobacteria very close to the cell wall. The aragonite-like CaCO₃ is a type of ACC since it did not show either X-ray or electron diffraction peaks. The amount of aragonite-like CaCO₃ precipitated in the EPS was dependent on the nutrient supply during bacterial growth. Higher nutrient concentrations (both N and P) during the cultivation of the cyanobacteria resulted in higher amounts of precipitation of the aragonite-like CaCO₃, whereas the amount of Ca²⁺ adsorbed per volume of EPS was almost independent of the nutrient level. After the onset of the precipitation of the thermodynamically stable calcite and loss of supersaturation the aragonite-like CaCO₃ dissolved whereas Ca²⁺ remained sorbed to the EPS albeit at lower concentrations. Based on these observations a model describing the temporal and spatial evolution of calcite nucleation on the surface of S. leopoliensis was developed. In another set of STXM experiments the amount of aragonite-like CaCO₃ precipitated on the cell surface was found to depend on the culture growth phase: cells in the exponential growth phase adsorbed large amounts of Ca within the EPS and mediated nucleation of ACC, while cells at the stationary/death phase neither adsorbed large amounts of Ca²⁺ nor mediated the formation of aragonite-like CaCO₃. It is suggested that precipitation of an X-ray amorphous CaCO₃ layer by cyanobacteria could serve as a protection mechanism against uncontrolled precipitation of a thermodynamically stable phase calcite on their surface.     

Conditions for biological precipitation of iron by Gallionella ferruginea in a slightly polluted ground water

A sand filter has been built as a pilot plant with the purpose of biological precipitation of Fe from ground water polluted with mainly chlorinated aliphatics. The ground water is pumped directly from a well in a polluted ground water aquifer in Esbjerg, Denmark. The pollution includes trichlorethylene and tetrachlorethylene together with smaller amounts of pesticides. Furthermore the best conditions for Fe precipitating bacteria were not expected to be present because of a relatively high O₂ content, up to 6.7 mg/l, a low Fe content, 0.2 mg/l and a pH of ∼5 in the ground water. Added FeSO₄ increased the Fe content of the ground water to about 4 mg/l. These rather extreme conditions for precipitating Fe were observed over a period of 3 months. The goal of the research was to observe the mechanism of Fe precipitation in a sand filter in the above-mentioned conditions comparative to normal conditions for biotic as well as abiotic Fe mineralization in sand filters of fresh water treatment plants. The Fe precipitating bacterium Gallionella ferrugenia was found to dominate the biotic Fe oxidation/precipitation process despite the extreme conditions. A huge amount of exopolymer from Gallionella was present. The precipitated Fe oxide was determined to be ferrihydrate. The rate of the Fe oxidation/precipitation was found to be about 1000 times faster than formerly found for abiotic physico-chemical oxidation/precipitation processes. The hydrophobic pesticides and some of their degradation products were not adsorbed in the filter. An added hydrophilic pesticide was adsorbed up to 40%. Trichlorethylene was not adsorbed in the filter. The reason for the poor adsorption of the hydrophobic compounds and trichlorethylene is due to the pronounced hydrophilic property of the exopolymers of Gallionella and the precipitated ferrihydrite.     

Stable isotopic compositions of waters in the karst environments of China: Climatic implications

A total of 117 water samples, including cave water, ground water, spring water and river water, collected from the monsoonal area of China have been analyzed for their H- and O-isotope composition. Overall, a δ¹⁸O–δD correlation is observed of δD = −4.45 + 6.6δ¹⁸O (R² = 0.90) and a significant evaporation effect observed for the southern sites. Average δ¹⁸O and δD site values generally correspond to those of precipitation in nearby cities, with correlations of δD = 2.18 + 7.23δ¹⁸O (R² = 0.95) for the sample sites and δD = 11.05 + 7.95δ¹⁸O (R² = 0.95) for the cities. The effects of rainfall amount and temperature on precipitation δ¹⁸O were calculated using a simplified theoretical model derived from the Rayleigh distillation equation, which demonstrated that the sign of δ¹⁸O_pvs. T correlation is dependent on precipitation intensity. The mean δ¹⁸O value of cave waters exhibit decreasing trends with increasing latitude and reveal a spatial pattern of positive correlation with annual mean temperature and precipitation, mainly reflecting isotopic fractionations in the moisture source traveling from the ocean side to the inland continent. This spatial pattern implies that the δ¹⁸O values recorded in the proxy climate records derived from speleothems might be influenced by shifts in monsoon boundary during the past, especially between glacial and interglacial intervals.     

CO clumping in speleothems: Observations from natural caves and precipitation experiments

The oxygen isotope composition of speleothems is an important proxy of continental paleoenvironments, because of its sensitivity to variations in cave temperature and drip water δ¹⁸O. Interpreting speleothem δ¹⁸O records in terms of absolute paleotemperatures and δ¹⁸O values of paleo-precipitation requires quantitative separation of the effects of these two parameters, and correcting for possible kinetic isotope fractionation associated with precipitation of calcite out of thermodynamic equilibrium. Carbonate clumped-isotope thermometry, based on measurements of Δ₄₇ (a geochemical variable reflecting the statistical overabundance of ¹³C¹⁸O bonds in CO₂ evolved from phosphoric acid digestion of carbonate minerals), potentially provides a method for absolute speleothem paleotemperature reconstructions independent of drip water composition. Application of this new technique to karst records is currently limited by the scarcity of published clumped-isotope studies of modern speleothems. The only modern stalagmite reported so far in the literature yielded a lower Δ₄₇ value than expected for equilibrium precipitation, possibly due to kinetic isotope fractionation.Here we report Δ₄₇ values measured in natural speleothems from various cave settings, in carbonate produced by cave precipitation experiments, and in synthetic stalagmite analogs precipitated in controlled laboratory conditions designed to mimic natural cave processes. All samples yield lower Δ₄₇ and heavier δ¹⁸O values than predicted by experimental calibrations of thermodynamic equilibrium in inorganic calcite. The amplitudes of these isotopic disequilibria vary between samples, but there is clear correlation between the amount of Δ₄₇ disequilibrium and that of δ¹⁸O. Even pool carbonates believed to offer excellent conditions for equilibrium precipitation of calcite display out-of-equilibrium δ¹⁸O and Δ₄₇ values, probably inherited from prior degassing within the cave system.In addition to these modern observations, clumped-isotope analyses of a flowstone from Villars cave (France) offer evidence that the amount of disequilibrium affecting Δ₄₇ in a single speleothem can experience large variations at time scales of 10 kyr. Application of clumped-isotope thermometry to speleothem records calls for an improved physical understanding of DIC fractionation processes in karst waters, and for the resolution of important issues regarding equilibrium calibration of Δ₄₇ in inorganic carbonates.     

Influences of air CO<Subscript>2</Subscript> on hydrochemistry of drip water and implications for paleoclimate study in a stream-developed cave,SW China

Cave air CO₂ is a vital part of the cave environment. Most studies about cave air CO₂ variations are performed in caves with no streams; there are few studies to date regarding the relationship of cave air CO₂ variations and drip water hydrochemistry in underground stream–developed caves. To study the relationship of underground stream, drip water, and cave air CO₂, monthly and daily monitoring of air CO₂ and of underground stream and drip water was performed in Xueyu Cave from 2012 to 2013. The results revealed that there was marked seasonal variation of air CO₂ and stream hydrochemistry in the cave. Daily variations of cave air CO₂, and of stream and drip water hydrochemistry, were notable during continuous monitoring. A dilution effect was observed by analyzing hydrochemical variations in underground stream and drip water after rainfall. High cave air CO₂ along with low pH and low δ¹³C_DIC in stream and drip water indicated that air CO₂ was one of the dominant factors controlling stream and drip water hydrochemistry on a daily scale. On a seasonal scale, stream flows may promote increased cave air CO₂ in summer; in turn, the higher cave air CO₂ could inhibit degassing of drip water and make calcite δ¹³C more negative. Variation of calcite δ¹³C (precipitated from drip water) was in reverse of monthly temperature, soil CO₂, and cave air CO₂. Therefore, calcite δ¹³C in Xueyu Cave could be used to determine monthly changes outside the cave. However, considering the different precipitation rate of sediment in different seasons, it was difficult to use stalagmites to reconstruct environmental change on a seasonal scale.     

Seasonal evolution of the isotopic composition of atmospheric water vapour above a tropical lake: Deuterium excess and implication for water recycling

We present an analysis of the rainfall-evaporation-atmospheric moisture cycle in a semi-arid tropical zone (southwestern Madagascar) to quantify the recycling and mixing processes that occur above an endorheic lake system (Lake Ihotry) during an annual cycle. The study combines an isotope mass balance with a detailed field investigation of the lake system and a previously established daily time-step lake water balance model. The mass balance and Craig-Gordon equations are used to calculate the isotopic composition of the evaporative flux from the lake surface (δ_E) and to derive a daily time series of the ambient atmospheric water vapour composition above the lake (δ_AL) during a 8-month dry season. Calculated δ_AL results from a mixing between regional moisture (δ_AR) and locally evaporated water (δ_E), the latter representing 50% of δ_AL at the end of the dry season. The contribution of recycled moisture to on-lake precipitation during the wet season is estimated to ?16%. We show that, as expected, the deuterium excess is high in recycled precipitation and low in evaporated precipitation, but also that the recycled moisture in an endorheic system may have a low deuterium excess resulting from the low deuterium excess in regional precipitation. In case of a long evaporative season, the atmospheric moisture is not in isotopic equilibrium with the annual composition of precipitation because of the contribution of the recycled vapour to the local atmospheric pool. Our approach demonstrates the importance of water recycling on the atmospheric moisture cycle and precipitation in a tropical semi-arid system, and can be applied to other natural systems, enlarging the potential range of investigation of the atmospheric vapour cycle and rainfall sources in tropical lands. It may also represent a valuable complement to direct water vapour sampling, in yielding the long-term evolution of the atmospheric vapour composition with spatially averaged values and smoothed temporal variations.     

Temporal variations of reference evapotranspiration and controlling factors: Implications for climatic drought in karst areas

Variations in reference evapotranspiration (ET₀) and drought characteristics play a key role in the effect of climate change on water cycle and associated ecohydrological patterns. The accurate estimation of ET₀ is still a challenge due to the lack of meteorological data and the heterogeneity of hydrological system. Although there is an increasing trend in extreme drought events with global climate change, the relationship between ET₀ and aridity index in karst areas has been poorly studied. In this study, we used the Penman–Monteith method based on a long time series of meteorological data from 1951 to 2015 to calculate ET₀ in a typical karst area, Guilin, Southwest China. The temporal variations in climate variables, ET₀ and aridity index (AI) were analyzed with the Mann–Kendall trend test and linear regression to determine the climatic characteristics, associated controlling factors of ET₀ variations, and further to estimate the relationship between ET₀ and AI. We found that the mean, maximum and minimum temperatures had increased significantly during the 65-year study period, while sunshine duration, wind speed and relative humidity exhibited significant decreasing trends. The annual ET₀ showed a significant decreasing trend at the rate of ?8.02 mm/10a. However, significant increase in air temperature should have contributed to the enhancement of ET₀, indicating an “evaporation paradox”. In comparison, AI showed a slightly declining trend of ?0.0005/a during 1951–2015. The change in sunshine duration was the major factor causing the decrease in ET₀, followed by wind speed. AI had a higher correlation with precipitation amount, indicating that the variations of AI was more dependent on precipitation, but not substantially dependent on the ET₀. Although AI was not directly related to ET₀, ET₀ had a major contribution to seasonal AI changes. The seasonal variations of ET₀ played a critical role in dryness/wetness changes to regulate water and energy supply, which can lead to seasonal droughts or water shortages in karst areas. Overall, these findings provide an important reference for the management of agricultural production and water resources, and have an important implication for drought in karst regions of China.     

Expériences sur la précipitation directe de l'apatite dans l'eau de mer: Implication dans la genèse des phosphorites

The direct precipitation of apatite in seawater is inhibited by Mg ions. The action of various factors on this precipitation is studied: addition of Ca, addition of F, reduction of pH and simultaneous effect of reduction of pH and addition either of F or of Ca. It is established that the reduction of the pH of seawater to 7 allows the precipitation of the apatite with a Ca/Mg ratio in the water of 1.2. The favourable environment for the settlement of natural apatite must then be either neutral or acid. This result is corroborated by the re-study of Gulbrandsen's equation. Such an acid environment with a high phosphorus content may be the result of oxidation of large quantities of organic matter due to previous intense biological activity. The acidity is probably brought about by bubbling of CO₂.     

Comparison of two potential strategies of planktonic foraminifera for house building: Mg or H removal?

Marine organisms must possess strategies enabling them to initiate calcite precipitation despite the unfavorable conditions for inorganic precipitation in surface seawater. These strategies are poorly understood. Here we compare two potential strategies of marine calcifyers to manipulate seawater chemistry in order to initiate calcite precipitation: Removal of Mg²⁺ and H⁺ ions from seawater solutions. An experimental setup was used to monitor the onset of inorganic precipitation on seed crystals as a function of the Mg²⁺ concentration and pH in artificial seawater. We focused on precipitation rates typical for biogenic calcification in planktonic foraminifera (∼10⁻³ mol m⁻² h⁻¹) and time scales typical for the initiation of calcification in these organisms (minutes to hours). We find that the carbonate ion concentration has to increase by a factor of ∼13 when [Mg²⁺] increases from 0 to 53 mmol kg⁻¹ in order to maintain a typical biogenic precipitation rate. Model calculations for the energy requirement for various scenarios of Mg²⁺ and H⁺ removal including Ca²⁺ exchange and CO₂ diffusion are presented. We conclude that the more cost-effective strategy to initiate calcite precipitation in foraminifera is H⁺ removal, rather than Mg²⁺ removal.     

Analysis of stability parameters in relation to precipitation associated with pre-monsoon thunderstorms over Kolkata,India

The upper air RS/RW (Radio Sonde/Radio Wind) observations at Kolkata (22.65N, 88.45E) during pre-monsoon season March–May, 2005–2012 is used to compute some important dynamic/thermodynamic parameters and are analysed in relation to the precipitation associated with the thunderstorms over Kolkata, India. For this purpose, the pre-monsoon thunderstorms are classified as light precipitation (LP), moderate precipitation (MP) and heavy precipitation (HP) thunderstorms based on the magnitude of associated precipitation. Richardson number in non-uniformly saturated (R _i *) and saturated atmosphere (R _i ); vertical shear of horizontal wind in 0–3, 0–6 and 3–7 km atmospheric layers; energy-helicity index (EHI) and vorticity generation parameter (VGP) are considered for the analysis. The instability measured in terms of Richardson number in non-uniformly saturated atmosphere ( \(R_{i}^{\mathrm {\ast } })\) well indicate the occurrence of thunderstorms about 2 hours in advance. Moderate vertical wind shear in lower troposphere (0–3 km) and weak shear in middle troposphere (3–7 km) leads to heavy precipitation thunderstorms. The wind shear in 3–7 km atmospheric layers, EHI and VGP are good predictors of precipitation associated with thunderstorm. Lower tropospheric wind shear and Richardson number is a poor discriminator of the three classified thunderstorms.     

Evolution of the isotopic composition of carbon and oxygen in a calcite precipitating H2O-CO2-CaCO3 solution and the related isotopic composition of calcite in stalagmites

The isotopic composition of carbon and oxygen in a calcite precipitating CO₂-H₂O-CaCO₃ solution is preserved in the calcite precipitated. For the interpretation of isotopic proxies from stalagmites knowledge of the evolution of δ¹³C and δ¹⁸O in the solution during precipitation is required. A system of differential equations is presented from which this evolution can be derived. Both, irreversible loss of carbon and oxygen from the solution with precipitation time τ and exchange of oxygen in the carbonates with the oxygen in the water with exchange time T are considered. For carbon, where no exchange is active, a modified equation of Rayleigh-distillation is found, which takes into account that precipitation stops at c_eq, the saturation concentration of DIC with respect to calcite, and that c_eq as well as the precipitation time τ is slightly different for the heavy and the light isotope. This, however, requires introducing a new parameter γ = (A_eq/B_eq)/(A₀/B₀), which has to be determined experimentally. (A_eq/B_eq) is the isotopic ratio for the heavy (A) and the light isotope (B) at both chemical and isotopic equilibrium and (A₀/B₀) is the initial isotopic ratio of the solution. In the case of oxygen, where exchange is present, the isotopic shifts are reduced with increasing values of the precipitation time τ. For τ ? T the solution stays in isotopic equilibrium with the oxygen in the water during the entire time in which precipitation is active. The isotopic ratios in a calcite precipitating solution R(t)/R₀ = (1 + δ(t)/1000) for carbon are plotted versus those of oxygen. R₀ is the isotopic ratio at time t = 0, when precipitation starts and δ(t) the isotopic shift in the solution after time t. These show positive correlations for the first 50% of calcite, which can precipitate. Their slopes increase with increasing values of τ and they closely resemble Hendy-tests performed along growth layers of stalagmites. Our results show that stalagmites, which grow by high supply of water with drip times less than 50 s, exhibit positive correlations between δ¹³C and δ¹⁸O along a growth layer. But in spite of this the isotopic composition of oxygen in the solution at the apex is in isotopic equilibrium with the oxygen in the water, and therefore also that of calcite deposited at the apex.