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USGS2024

Accelerating elevation gain indicates land loss associated with erosion in Mississippi River Deltaic Plain tidal wetlands

In recent years, the Mississippi River Deltaic Plain (MRDP) has experienced the highest rates of wetland loss in the USA. Although the process of vertical drowning has been heavily studied in coastal wetlands, less is known about the relationship between elevation change and land loss in wetlands that are experiencing lateral erosion and the contribution of erosion to land loss in the MRDP. We quantified relationships of elevation change and land change in ten submerging tidal wetlands and found that, despite significant land loss, elevation trajectories in seven of the land loss study sites were positive. Furthermore, we observed an acceleration in elevation gain preceding the conversion from vegetated marsh to open water. To identify regional contributions of lateral erosion to land loss, we quantified the relationship of elevation change and land change in 159 tidal marsh sites in the MRDP. Approximately half the sites were persistently losing land, and 82% of these sites were vulnerable to erosion, identifying erosion as a dominant mechanism of coastal wetland loss in this region. Notably, the sites that were vulnerable to erosion were experiencing land loss while also gaining elevation, and sites with the highest land loss exhibited accelerating elevation gain. Together, these data illustrate that (1) erosion is a dominant mechanism of wetland loss in the MRDP, (2) accelerated elevation gain is an indicator of erosion, and (3) consideration of elevation change trajectories within the context of land change is critical for providing accurate coastal wetland vulnerability assessments.

Louisiana↗

USGS2013

Factors affecting the movement and persistence of nitrate and pesticides in the surficial and upper Floridan aquifers in two agricultural areas in the southeastern United States

Differences in the degree of confinement, redox conditions, and dissolved organic carbon (DOC) are the main factors that control the persistence of nitrate and pesticides in the Upper Floridan aquifer (UFA) and overlying surficial aquifer beneath two agricultural areas in the southeastern US. Groundwater samples were collected multiple times from 66 wells during 1993–2007 in a study area in southwestern Georgia (ACFB) and from 48 wells in 1997–98 and 2007–08 in a study area in South Carolina (SANT) as part of the US Geological Survey National Water-Quality Assessment Program. In the ACFB study area, where karst features are prevalent, elevated nitrate-N concentrations in the oxic unconfined UFA (median 2.5 mg/L) were significantly (p = 0.03) higher than those in the overlying oxic surficial aquifer (median 1.5 mg/L). Concentrations of atrazine and deethylatrazine (DEA; the most frequently detected pesticide and degradate) were higher in more recent groundwater samples from the ACFB study area than in samples collected prior to 2000. Conversely, in the SANT study area, nitrate-N concentrations in the UFA were mostly <0.06 mg/L, resulting from anoxic conditions and elevated DOC concentrations that favored denitrification. Although most parts of the partially confined UFA in the SANT study area were anoxic or had mixed redox conditions, water from 28 % of the sampled wells was oxic and had low DOC concentrations. Based on the groundwater age information, nitrate concentrations reflect historic fertilizer N usage in both the study areas, but with a lag time of about 15–20 years. Simulated responses to future management scenarios of fertilizer N inputs indicated that elevated nitrate-N concentrations would likely persist in oxic parts of the surficial aquifer and UFA for decades even with substantial decreases in fertilizer N inputs over the next 40 years.

Florida;Georgia;South Carolina↗

USGS2021

Efficient genotyping with backwards compatibility: Converting a legacy microsatellite panel for muskellunge (Esox masquinongy) to genotyping-by-sequencing chemistry

Microsatellites have been a staple of population genetics research for over three decades, and many large datasets have been generated with these markers. Microsatellites have been used, for example, to conduct genetic monitoring and construct large multigeneration pedigrees as well as genotype thousands of individuals from a given species to create high-resolution baselines of spatial genetic structure. However, the capillary electrophoresis (CE) approach used to genotype microsatellites is inefficient compared to newer genotyping-by-sequencing (GBS) approaches, and researchers have begun transitioning away from CE. Backward compatibility between GBS and CE would facilitate a seamless transition to a more efficient chemistry, while ensuring that research based on CE panels could continue. Here, we explore the feasibility of converting a legacy panel of 15 microsatellites developed for muskellunge ( Esox masquinongy ) from CE to GBS chemistry. Muskellunge are an important sportfish in the Great Lakes region, and the existing microsatellite panel has been used to genotype thousands of samples to develop a region-wide baseline of genetic structure. We successfully converted all 15 microsatellites to GBS chemistry. GBS produced high genotyping rates (98%) and had high concordance with CE microsatellite genotypes (99%). Conversion to GBS required redesign of some primers and pairs to shorten amplicon length and adjust melting temperatures, optimization of primer concentrations, and comparisons with CE genotypes to optimize GBS genotyping parameters; however, none of these steps were especially onerous. Our results demonstrate that it is highly feasible to convert legacy CE panels to GBS, ensuring seamless continuation of important, often long-term research.

Conservation Genetics Resources↗

USGS2012

Incorporating incorporating economic models into seasonal pool conservation planning

Massachusetts, New Jersey, Connecticut, and Maine have adopted regulatory zones around seasonal (vernal) pools to conserve terrestrial habitat for pool-breeding amphibians. Most amphibians require access to distinct seasonal habitats in both terrestrial and aquatic ecosystems because of their complex life histories. These habitat requirements make them particularly vulnerable to land uses that destroy habitat or limit connectivity (or permeability) among habitats. Regulatory efforts focusing on breeding pools without consideration of terrestrial habitat needs will not ensure the persistence of pool-breeding amphibians. We used GIS to combine a discrete-choice, parcel-scale economic model of land conversion with a landscape permeability model based on known habitat requirements of wood frogs ( Lithobates sylvaticus ) in Maine (USA) to examine permeability among habitat elements for alternative future scenarios. The economic model predicts future landscapes under different subdivision open space and vernal pool regulatory requirements. Our model showed that even “no build” permit zones extending 76 m (250 ft) outward from the pool edge were insufficient to assure permeability among required habitat elements. Furthermore, effectiveness of permit zones may be inconsistent due to interactions with other growth management policies, highlighting the need for local and state planning for the long-term persistence of pool-breeding amphibians in developing landscapes.

Maine↗

USGS2012

Local and landscape associations between wintering dabbling ducks and wetland complexes in Mississippi

Landscape features influence distribution of waterbirds throughout their annual cycle. A conceptual model, the wetland habitat complex, may be useful in conservation of wetland habitats for dabbling ducks (Anatini). The foundation of this conceptual model is that ducks seek complexes of wetlands containing diverse resources to meet dynamic physiological needs. We included flooded croplands, wetlands and ponds, public-land waterfowl sanctuary, and diversity of habitats as key components of wetland habitat complexes and compared their relative influence at two spatial scales (i.e., local, 0.25-km radius; landscape, 4-km) on dabbling ducks wintering in western Mississippi, USA during winters 2002–2004. Distribution of mallard ( Anas platyrhynchos ) groups was positively associated with flooded cropland at local and landscape scales. Models representing flooded croplands at the landscape scale best explained occurrence of other dabbling ducks. Habitat complexity measured at both scales best explained group size of other dabbling ducks. Flooded croplands likely provided food that had decreased in availability due to conversion of wetlands to agriculture. Wetland complexes at landscape scales were more attractive to wintering ducks than single or structurally simple wetlands. Conservation of wetland complexes at large spatial scales (≥5,000 ha) on public and private lands will require coordination among multiple stakeholders.

Mississippi↗

USGS2016

Prerequisites for understanding climate-change impacts on northern prairie wetlands

The Prairie Pothole Region (PPR) contains ecosystems that are typified by an extensive matrix of grasslands and depressional wetlands, which provide numerous ecosystem services. Over the past 150 years the PPR has experienced numerous landscape modifications resulting in agricultural conversion of 75–99 % of native prairie uplands and drainage of 50–90 % of wetlands. There is concern over how and where conservation dollars should be spent within the PPR to protect and restore wetland basins to support waterbird populations that will be robust to a changing climate. However, while hydrological impacts of landscape modifications appear substantial, they are still poorly understood. Previous modeling efforts addressing impacts of climate change on PPR wetlands have yet to fully incorporate interacting or potentially overshadowing impacts of landscape modification. We outlined several information needs for building more informative models to predict climate change effects on PPR wetlands. We reviewed how landscape modification influences wetland hydrology and present a conceptual model to describe how modified wetlands might respond to climate variability. We note that current climate projections do not incorporate cyclical variability in climate between wet and dry periods even though such dynamics have shaped the hydrology and ecology of PPR wetlands. We conclude that there are at least three prerequisite steps to making meaningful predictions about effects of climate change on PPR wetlands. Those evident to us are: 1) an understanding of how physical and watershed characteristics of wetland basins of similar hydroperiods vary across temperature and moisture gradients; 2) a mechanistic understanding of how wetlands respond to climate across a gradient of anthropogenic modifications; and 3) improved climate projections for the PPR that can meaningfully represent potential changes in climate variability including intensity and duration of wet and dry periods. Once these issues are addressed, we contend that modeling efforts will better inform and quantify ecosystem services provided by wetlands to meet needs of waterbird conservation and broader societal interests such as flood control and water quality.

Wetlands↗

USGS2020

Modeling the supporting ecosystem services of depressional wetlands

We explored how a geographic information system modeling approach could be used to quantify supporting ecosystem services related to the type, abundance, and distribution of landscape components. Specifically, we use the Integrated Valuation of Ecosystem Services and Tradeoffs model to quantify habitats that support amphibians and birds, floral resources that support pollinators, native-plant communities that support regional biodiversity, and above- and below-ground carbon stores in the Des Moines Lobe ecoregion of the U.S. We quantified services under two scenarios, one that represented the 2012 Des Moines Lobe landscape, and one that simulated the conversion to crop production of wetlands and surrounding uplands conserved under the USDA Agricultural Conservation Easement Program (ACEP). While ACEP easements only covered 0.35% of the ecoregion, preserved wetlands and grasslands provided for 19,020 ha of amphibian habitat, 21,462 ha of grassland-bird habitat, 18,798 ha of high-quality native wetland plants, and 27,882 ha of floral resources for pollinators. Additionally, ACEP protected lands stored 257,722 tonnes of carbon that, if released, would result in costs in excess of 45-million USD. An integrated approach using results from a GIS-based model in combination with process-based model quantifications will facilitate more informed decisions related to ecosystem service tradeoffs.

Iowa, Minnesota↗

USGS2024

Bayesian approaches to proxy uncertainty quantification in paleoecology: A mathematical justification and practical integration

Paleoenvironmental data are essential for reconstructing environmental conditions in the distant past, and these reconstructions strongly depend on proxies and age–depth models. Proxies are indirect measurements that substitute for variables that cannot be directly measured, such as past precipitation. Conversely, an age–depth model is a tool that correlates the observed proxy with a specific moment in time. Bayesian age–depth modelling has proved to be a powerful method for estimating sediment ages and their associated uncertainties. However, there remains considerable potential for further integration into proxy analysis. In this paper, we explore a mathematical justification and a computational approach that integrates uncertainty at the age–depth level and propagates it to the proxy scale in the form of a posterior predictive distribution. This method mitigates potential biases and errors by removing the need to assign a single age to a given proxy measurement. It allows for quantifying the likelihood that proxy data values correspond to modelled ages, thus enabling the quantification of uncertainty in both the temporal and proxy value domains. The use of Bayesian statistics in proxy analysis represents a relatively recent advancement. We aim to mathematically justify incorporating the Markov chain Monte Carlo output from age–depth models into proxy analysis and to present a novel methodology for constructing environmental reconstructions using this approach.

Journal of Agricultural, Biological and Environmen↗

USGS2013

Genetic structuring of northern myotis (Myotis septentrionalis) at multiple spatial scales

Although groups of bats may be genetically distinguishable at large spatial scales, the effects of forest disturbances, particularly permanent land use conversions on fine-scale population structure and gene flow of summer aggregations of philopatric bat species are less clear. We genotyped and analyzed variation at 10 nuclear DNA microsatellite markers in 182 individuals of the forest-dwelling northern myotis ( Myotis septentrionalis ) at multiple spatial scales, from within first-order watersheds scaling up to larger regional areas in West Virginia and New York. Our results indicate that groups of northern myotis were genetically indistinguishable at any spatial scale we considered, and the collective population maintained high genetic diversity. It is likely that the ability to migrate, exploit small forest patches, and use networks of mating sites located throughout the Appalachian Mountains, Interior Highlands, and elsewhere in the hibernation range have allowed northern myotis to maintain high genetic diversity and gene flow regardless of forest disturbances at local and regional spatial scales. A consequence of maintaining high gene flow might be the potential to minimize genetic founder effects following population declines caused currently by the enzootic White-nose Syndrome.

Acta Theriologica↗

USGS1976

Dynamics of storage of organochlorine pollutants in herring gulls

Several organochlorine pollutants were studied over the period of one annual cycle in caged juvenile and wild-collected adult herring gulls (Lagus argentatus) from Lake Michigan. Fish, mostly alewives (Alosa pseudoharengus), comprised the major year-round food items in the wild; alewives were also fed to the caged juveniles. Fish residues averaged around 3 mg/kg of p,p'-DDE, 2 mg/kg p,p'DDT + p,p'-TDE, and 2 mg/kg apparent PCBs. Juvenile body-burdens of DDE and PCBs showed a continual buildup after fledging, then a temporary dynamic equilibrium, related only in part to annual lipid deposition. Maximum body-burdens were reached in both juveniles and adults when winter fat deposits were declining prior to the breeding season?followed by a return to dynamic equilibrium. Residues of DDT and TDE followed closely the annual pattern of lipid deposition in both juveniles and adults. Total body-burdens in both age classes were similar after the buildups to equilibrium in juveniles in their eighth month after fledging. Seasonal variations of residues of DDE and PCBs were characterised by two phases in adults and three in juveniles, which gradually assumed the adult cyclic pattern. The maximum body-burdens attained by caged juveniles fed a diet of Lake Michigan alewives were 290 mg/kg DDE, 19 mg/kg DDT + TDE, and 200 mg/kg apparent PCBs. Residues in wild adults at the same time were 300, 4, and 200 mg/kg of the same residues. Apparent PCBs and DDE were highly accumulative, although DDE levels resulted from dietary DDE, as well as conversion from DDT.

Environmental Pollution↗

USGS1985

Sulphur in char and char desulphurization by acid leaching and hydropyrolysis

Sulphur compounds volatilized during pyrolysis of acid-leached char were measured to determine characteristics of char desulphurization reactions. Pyrolysis of char in a hydrogen atmosphere (hydropyrolysis) produced a much higher concentration of thiophenic organics compared with that produced during pyrolysis in a nitrogen atmosphere. Hydrogen sulphide gas evolution, at progressively increasing pyrolysis temperature in a helium atmosphere, was measured on five char samples: untreated char, hydrochloric acid-leached char, and three model chars: a demineralized char and two demineralized chars incorporated with sulphur via reactions with elemental sulphur. Hydrogen sulphide gas evolution in untreated char and acid-leached char was found to peak in three temperature regions; the maxima are thought to relate to sulphur in different bonding environments. The amounts of hydrogen sulphide volatilized were much higher for acid-leached char than for untreated char. The gas evolved from each of the remaining three samples showed a single peak region corresponding closely to one of the three peak regions observed for the first two chars. The results of this study indicate that elemental sulphur was produced during hydrochloric acid leaching of the untreated char and suggested that the improved rate of desulphurization observed in the char that had been acid-leached before hydropyrolysis was due in part to the conversion of strongly bound mineral sulphur forms to more weakly bound sulphur forms that are predominantly elemental sulphur in character, and are more easily removed by hydrogen.

Fuel↗

USGS1996

Effect of surface area and chemisorbed oxygen on the SO2 adsorption capacity of activated char

The objective of this study was to determine whether activated char produced from Illinois coal could be used effectively to remove sulfur dioxide from coal combustion flue gas. Chars were prepared from a high-volatile Illinois bituminous coal under a wide range of pyrolysis and activation conditions. A novel char preparation technique was developed to prepare chars with SO2 adsorption capacities significantly greater than that of a commercial activated carbon. In general, there was no correlation between SO2 adsorption capacity and surface area. Temperature-programmed desorption (TPD) was used to determine the nature and extent of carbon-oxygen (C-O) complexes formed on the char surface. TPD data revealed that SO2 adsorption was inversely proportional to the amount of C-O complex. The formation of a stable C-O complex during char preparation may have served only to occupy carbon sites that were otherwise reactive towards SO2 adsorption. A fleeting C(O) complex formed during SO2 adsorption is postulated to be the reaction intermediate necessary for conversion of SO2 to H2SO4. Copyright ?? 1996 Elsevier Science Ltd.

Fuel↗

USGS1956

Silica in hot-spring waters

The silica in hot-spring waters and in a few cold waters was studied by moans of the colorimetrie ammonium-molybdate method of analysis. Murata found in 1947 that only a part of the total silica in aged samples of high-silica waters was determinable by the colorimetric method. Weitz , franck And schuchard later showed that ammonium molybdate reacts readily with the monomeric form of silica (probably H 4 SiO 4 ) but very slowly with polymeric silica. If the colorimetric measurement is completed in two or three minutes, only the monomer is determined. Nearly all silica of hot springs is in the monomeric form. Solubility equilibrium exists between dissolved (monomeric) and amorphous silica. For the hot springs that were studied, the solubility is about 315 p.p.m. at 90°C and 110 p.p.m. at 25°C, which is very similar to Krauskopf's experimental data. Monomeric silica polymerizes so slowly to colloidal silica that many waters are supersaturated with respect to amorphous silica. The rate of polymerization is influenced by pH, temperature, degree of supersaturation, presence of previously formed colloidal and gelatinous silica and contact with opal and other substances. Supersaturated acid waters and alkaline waters with less than 100% supersaturation tend to remain supersaturated almost indefinitely, with little or no change. Precipitation of colloidal silica is favoured by high temperature and contact with opal. Many connate and other ground waters, including some thermal springs, are much below saturation with respect to amorphous silica, probably because low-solubility quartz and chalcedony have been precipitating. Quartz is favoured by relatively high temperature, slow rale of precipitation, and low degree of supersaturation, and is believed to form by deposition of monomeric molecules. Chalcedony is probably deposited when the degree of supersaturation is moderately high and the rate of deposition is relatively fast. The ranges of temperature over which quartz and chalcedony deposit no doubt overlap, but, if other factors are equal, quartz is favoured by high temperature. Opal is favoured by relatively low temperature and rapid rate of precipitation. Although opal has probably been deposited at temperatures as high as 140°C, it is unstable and is slowly converted to chalcedony or quartz. Water that is saturated with respect to opal is highly supersaturated with respect to quartz. Opal is probably formed from monomeric or more probably, the smaller polymeric molecules of silica, retaining some of their water content. Evidence is lacking for the direct conversion of gelatinous silica to opal. Some differences in solubility probably exist between amorphous opal and opal that shows X-ray patterns like that of cristobalite. The suggestion is made that clay minerals form by combination of monomeric silica and a comparable form of monomeric alumina, which must have very low solubility in waters within the pH range of 5 to 9. Because of the abundance and relatively high solubility of silica, the proposed reaction, dissolved alumina + dissolved silica ⇌ clay, is ordinarily displaced strongly to the right in hydrothermal alteration and in ordinary soil formation. With removal of free silica, aided by tropical rainfall and temperatures, the reaction may be displaced to the left by dissolution and removal of silica from the system. Alumina, because of its very low solubility, remains as bauxite.

Geochimica et Cosmochimica Acta↗

USGS1962

Behaviour of calcium carbonate in sea water

Anomalies in the behaviour of calcium carbonate in natural solutions diminish when considered in context. Best values found by traditional oceanographie methods for the apparent solubility product constant K ' CaCO 3 in sea water at atmospheric pressure are consistent mineralogically—at 36 parts per thousand salinity and T-25°C, K ' aragonlte is estimated as 1.12 × 10 −6 and K ' calcite as 0.61 × 10 −6 . At 30°C the corresponding values are 0.98 × 10 −6 for aragonite and 0.53 × 10 −6 for calcite. Because the K ' computations do not compensate for ionic activity, however, they cannot give thermodynamically satisfactory results. It is of interest, therefore, that approximate methods and information now available permit the estimation from the same basic data of an activity product constant K CaCO 3 close to that found in solutions to which Debye-Hückel theory applies. Such methods indicate approximate K aragonite 7.8 × 10 −9 for surface sea water at 29°C; K calcite would be proportionately lower. Field data and experimental results indicate that the mineralogy of precipitated CaCO 3 depends primarily on degree of supersaturation, thus also on kinetic or biologic factors that facilitate or inhibit a high degree of supersaturation. The shallow, generally hypersaline bank waters west of Andros Island yield aragonitic sediments with O 18 O 16 "> O18O16 ratios that imply precipitation mainly during the warmer months, when the combination of a high rate of evaporation, increasing salinity (and ionic strength), maximal temperatures and photosynthetic removal of CO 2 result in high apparent supersaturation. The usual precipitate from solutions of low ionic strength is calcite, except where the aragonite level of supersaturation is reached as a result of diffusion phenomena ( e.g. dripstones), gradual and marked evaporation, or biologic intervention. Published data also suggest the possibility of distinct chemical milieus for crystallographic variations in skeletal calcium carbonate. It appears that in nature aragonite precipitates from solutions that are supersaturated with respect to both calcite and aragonite and calcite between saturation levels for the two species. Such a relation is consistent with Ostwald's rule of successive reactions. Aragonitc of marine origin persists in contact with supersaturated interstitial solutions at ordinary temperature and pressure. Conversion to calcite follows transfer to solutions undersaturated with respect to aragonite or upon exposure to the moist atmosphere.

Geochimica et Cosmochimica Acta↗

USGS1969

Geochemistry and origin of formation waters in the western Canada sedimentary basin-I. Stable isotopes of hydrogen and oxygen

Stable isotopes of hydrogen and oxygen, together with chemical analyses, were determined for 20 surface waters, 8 shallow potable formation waters, and 79 formation waters from oil fields and gas fields. The observed isotope ratios can be explained by mixing of surface water and diagenetically modified sea water, accompanied by a process which enriches the heavy oxygen isotope. Mass balances for deuterium and total dissolved solids in the western Canada sedimentary basin demonstrate that the present distribution of deuterium in formation waters of the basin can be derived through mixing of the diagenetically modified sea water with not more than 2.9 times as much fresh water at the same latitude, and that the movement of fresh water through the basin has redistributed the dissolved solids of the modified sea water into the observed salinity variations. Statistical analysis of the isotope data indicates that although exchange of deuterium between water and hydrogen sulphide takes place within the basin, the effect is minimized because of an insignificant mass of hydrogen sulphide compared to the mass of formation water. Conversely, exchange of oxygen isotopes between water and carbonate minerals causes a major oxygen-18 enrichment of formation waters, depending on the relative masses of water and carbonate. Qualitative evidence confirms the isotopic fractionation of deuterium on passage of water through micropores in shales.

Alberta↗

USGS1981

Fractionation of carbon and hydrogen isotopes by methane-oxidizing bacteria

Carbon isotopic analysis of methane has become a popular technique in the exploration for oil and gas because it can be used to differentiate between thermogenic and microbial gas and can sometimes be used for gas-source rock correlations. Methane-oxidizing bacteria, however, can significantly change the carbon isotopic composition of methane; the origin of gas that has been partially oxidized by these bacteria could therefore be misinterpreted. We cultured methane-oxidizing bacteria at two different temperatures and monitored the carbon and hydrogen isotopic compositions of the residual methane. The residual methane was enriched in both 13 C and D. For both isotopic species, the enrichment at equivalent levels of conversion was greater at 26°C than at 11.5°C. The change in δD relative to the change in δ 13 C was independent of temperature within the range studied. One culture exhibited a change in the fractionation pattern for carbon (but not for hydrogen) midway through the experiment, suggesting that bacterial oxidation of methane may occur via more than one pathway. The change in the δD value for the residual methane was from 8 to 14 times greater than the change in the δ 13 C value, indicating that combined carbon and hydrogen isotopic analysis may be an effective way of identifying methane which has been subjected to partial oxidation by bacteria.

Geochimica et Cosmochimica Acta↗

USGS1987

Thermodynamics of aragonite-strontianite solid solutions: Results from stoichiometric solubility at 25 and 76°C

Dissolution of synthetic strontianite-aragonite solid solutions was followed analytically to stoichiometric saturation using large solid to solution ratios in CO 2 -H 2 O solution at 25 and 76°C. The compositional dependence of the equilibrium constant was calculated from the composition of saturated (stoichiometric) solutions and used to calculate the activities and activity coefficients of CaCO 3 and SrCO 3 in the solid Ca (1− x ) Sr x CO 3 at 25 and 76°C. The results show that the solid-solution is not regular but unsymmetrical. The excess free energy of mixing is closely modeled for all compositions by the relation where A 0 is 8.49 ± 0.30 and 7.71 ± 0.20 KJ/mole and A 1 is −4.51 ± 0.20 and −3.36 ± 0.40 KJ/mole at 25 and 76°C, respectively. The equilibrium constant is denned as a function of the SrCO 3 mole fraction, x , by the relation where R is the gas constant, T is in Kelvins and K A and K S are the aragonite and strontianite equilibrium constants. The experimental results indicate the Henry's law coefficients of SrCO 3 in aragonites containing 0 to 6 mole percent SrCO 3 are approximately 91± 8 and 23 ± 1 at 25 and 76°C, respectively and for strontianites the Henry's law coefficients and applicable compositional ranges are approximately 7.3 ± 0.3 (0.84 ≤ x ≤ 1.00) and 3.3 ± 0.5 (0.50 ≤ x ≤ 1.00) at 25 and 76°C, respectively. Substitution of small amounts of Sr in aragonite and Ca in strontianite initially increases the stability of the solid. The most stable aragonites and strontianites contain 0.58 ± 0.03 and 12.5 ± 1.1 mole percent SrCO 3 and CaCO 3 at 25°C and 3.1 ± 0.3 and 17.2 ± 1.1 mole percent SrCO 3 and CaCO 3 at 76°C, respectively. The spinode occurs over the regions 0.065 ± 0.001 ≤ x ≤ 0.620 ± 0.014 at 25°C and 0.103 ± 0.007 ≤ x ≤ 0.585 ± 0.019 at 76°C where all compositions are unstable. A miscibility gap occurs over the compositional ranges 0.0058 ± 0.0003 ≤ x ≤ 0.875 ± 0.011 at 25°C and 0.031 ± 0.003 ≤ x ≤ 0.828 ± 0.011 at 76°C and is in reasonable agreement with reported compositions of natural aragonites and strontianites. Marine aragonites are neither at equilibrium nor stoichiometric saturation with surface seawater. The experimentally observed distribution coefficient of Sr in aragonite is 12 times larger than the calculated equilibrium value (0.095) at 25°C. Naturally occurring strontianites contain large amounts of calcium primarily because Ca/Sr ratios in natural waters are typically large. Neither equilibrium nor stoichiometric saturation is observed at 76°C during laboratory recrystallization of strontianite-aragonite solid solutions even after apparent 100 percent conversion to a narrow secondary composition and demonstration of a nearly constant composition system for periods of 300 hours.

Geochimica et Cosmochimica Acta↗

USGS1987

Synthesis and stability of hetaerolite, ZnMn2O4, at 25°C

A precipitate of nearly pure hetaerolite, ZnMn 2 O 4 , a spinel-structured analog of hausmannite, Mn 3 O 4 , was prepared by an irreversible wprecipitation of zinc with manganese at 25°C. The synthesis technique entailed constant slow addition of a dilute solution of Mn 2+ and Zn 2+ chlorides having a Mn/Zn ratio of 2:1 to a reaction vessel that initially contained distilled deionized water, maintained at a pH of 8.50 by addition of dilute NaOH by an automated pH stat, with continuous bubbling of CO 2 -free air. The solid was identified by means of X-ray diffraction and transmission electron microscopy and consisted of bipyramidal crystals generally less than 0.10 μm in diameter. Zn 2+ ions are able to substitute extensively for Mn 2+ ions that occupy tetrahedral sites in the hausmannite structure. Hetaerolite appears to be more stable than hausmannite with respect to spontaneous conversion to γMnOOH. The value of the standard free energy of formation of hetaerolite was estimated from the experimental data to be −289.4 ± 0.8 kcal per mole. Solids intermediate in composition between hetaerolite and hausmannite can be prepared by altering the Mn/Zn ratio in the feed solution.

Geochimica et Cosmochimica Acta↗