Magnesian calcite stabilities: A reevaluation

Equilibrium constants at stoichiometric saturation with respect to various magnesian calcite compositions were measured using free-drift dissolution rate data and inverse time plots to estimate equilibrium pH. The equilibrium constants determined for two ultrasonically cleaned and annealed biogenic magnesian calcites (12 and 18 mole % MgCO₃) in CaCl₂ + MgCl₂ media at two Mg:Ca molar ratios (1:5 and 5:1) are about three times smaller than those previously reported by Plummer and Mackenzie (1974). These equilibrium constants are not affected by changes in initial pH value, solid:solution ratio, or solution Mg:Ca molar ratio when the ion activity product is expressed in the fractional exponent form. Other models for expression of the equilibrium ion activity product fail to yield consistent values in solutions of different Mg:Ca molar ratios.Experiments performed using crushed samples not ultrasonically cleaned and annealed yield equilibrium constants which vary with solid:solution ratio. Those performed at high solid:solution ratios yield values which approach those previously reported. Submicron size particles and crystal strain induced by crushing the biogenic carbonates may cause more rapid dissolution rates and, hence, overestimation of the solubility of samples not prepared so as to minimize these effects. Thus, the large range in reported solubilities of magnesium calcites may be a result of differences in sample preparation procedure.The results of these measurements shift the thermodynamic equivalence point of aragonite and magnesian calcite from 7.5 mole % MgCO₃ up to 12 mole % MgCO₃ and prompt a reassessment of models for carbonate diagenetic reactions in natural environments.     

Distinctive microbial structures and the pre-phanerozoic fossil record

The live and sedimentary components of the flat laminated microbial mat at Laguna Figueroa have been studied since the late sixties. This paper reports the observation and isolation of a variety of micro-organisms, both prokaryotes and eukaryotes. The microbes were taken from the flat laminated mats submerged under at least one meter of water due to the spring rains of 1979 and 1980. Both in situ and enrichment culture observations were made using light and electron microscopic techniques. New strains of the following microbes are reported here: Bacillus megaterium, Bacillus licheniformis, Arthrobacter simplex and Paratetramitus jugosus. Several pseudomonads were isolated, some of which form distinctive subsurface colonial structures. Regular distinctive colony morphologies and desiccation resistant cysts were often observed, several types of which grow to characteristically large sizes and resemble objects found in the pre-Phanerozoic fossil record.Some colonies of manganese oxidizing bacilli and other bacteria are reminiscent of microfossils of the 2 Ga-old Gunflint Iron formation such as Metallogenium, Eosphaera tyleri, Eoastrion and Huronispora. Some manganese oxidizing bacteria form colonial structures that might be mistaken for individual organisms in the tens to hundreds of microns size range. The diversity of microbial structures, including those with preservation potential, must be kept in mind when interpreting the microfossil record.     

Strain and vorticity analysis using small-scale faults and associated drag folds

Small-scale faults with associated drag folds in brittle-ductile rocks can retain detailed information on the kinematics and amount of deformation the host rock experienced. Measured fault orientation (α), drag angle (β) and the ratio of the thickness of deflected layers at the fault (L) and further away (T) can be compared with α, β and L/T values that are calculated with a simple analytical model. Using graphs or a numerical best-fit routine, one can then determine the kinematic vorticity number and initial fault orientation that best fits the data. The proposed method was successfully tested on both analogue experiments and numerical simulations with BASIL. Using this method, a kinematic vorticity number of one (dextral simple shear) and a minimum finite strain of 2.5–3.8 was obtained for a population of antithetic faults with associated drag folds in a case study area at Mas Rabassers de Dalt on Cap de Creus in the Variscan of the easternmost Pyrenees, Spain.     

Migration‐as‐development repackaged? The globalizing imperative of the Singaporean state's diaspora strategies

Diaspora strategizing is becoming an important field of public policy in countries that seek to advance development through migration. Diaspora strategies present a way of complicating interpretations of development, as countries that represent different levels of development seek to mobilize diaspora networks nonetheless. While Singapore's diaspora strategies prioritize diaspora knowledge networks, it bears the stamp of the country's colonial history, postcolonial priorities and developmental state apparatus. We suggest that these factors result in a distinctive diaspora strategy characterized by a centralized and technocratic approach more similar to diaspora strategies found in emerging economies. Earlier research on diaspora strategies has been concerned with assessing successes or failures, but a more critical scholarship that questions the logics and outcomes of diaspora strategizing is emerging. We bring the Singaporean case into dialogue with four such critiques: first, the intellectual foundations of diaspora‐centred development; second, the hyperextension of state infrastructure for emigration and stakeholder alignment; third, the selective mobilization of the idea of diaspora; and fourth, the evidence base upon which diaspora‐centred development is predicated. Our goal is to further critical interrogation into the logics, efficacy and sustainability of diaspora strategizing. We situate our analyses in the international literature on diaspora knowledge mobilization.     

Improving geospatial query performance of an interoperable geographic situation‐awareness system for disaster response

Disaster response operations require fast and coordinated actions based on real‐time disaster situation information. Although crowdsourced geospatial data applications have been demonstrated to be valuable tools for gathering real‐time disaster situation information, they only provide limited utility for disaster response coordination because of the lack of semantic compatibility and interoperability. To help overcome the semantic incompatibility and heterogeneity problems, we use Geospatial Semantic Web (GSW) technologies. We then combine GSW technologies with Web Feature Service requests to access multiple servers. However, a GSW‐based geographic information system often has poor performance due to the complex geometric computations required. The objective of this research is to explore how to use optimization techniques to improve the performance of an interoperable geographic situation‐awareness system (IGSAS) based on GSW technologies for disaster response. We conducted experiments to evaluate various client‐side optimization techniques for improving the performance of an IGSAS prototype for flood disaster response in New Haven, Connecticut. Our experimental results show that the developed prototype can greatly reduce the runtime costs of geospatial semantic queries through on‐the‐fly spatial indexing, tile‐based rendering, efficient algorithms for spatial join, and caching, especially for those spatial‐join geospatial queries that involve a large number of spatial features and heavy geometric computation.     

Geoarchaeologic analysis of South Platte River terraces: Kersey,Colorado

Soil-sediment records and radiometric dating allow the development of environmental histories of three South Platte River alluvial terraces in the vicinity of Kersey, Colorado. These advocate a correlation with Holocene glacial records for the Colorado Front Range (Benedict, 1981, 1985). The archaeological potential of the Kersey fill, the Kuner strath, and the Hardin fill depends upon their age and sediment context. The oldest and most extensive terrace is the Kersey fill. The position of cultural components on the Kersey terrace implies an association of older Paleoindian sites (11,500–10,000 B.P.) with channel banks and bars on the terrace, younger Paleoindian sites (<10,000 B.P.) with terrace margins near the river, and Archaic and younger sites with eolian deposits on the terrace. An association of Clovis components with both Kersey alluvium and adjacent eolian dune fields indicates that eolian deposition began prior to 11,000 B.P. and that sediment availability influenced early Holocene eolian deposition. Examination of 150 cores and 75 backhoe test units along an 8-km study corridor demonstrates that Paleoindian sites are not as abundant on the Kersey terrace as previous researchers have proposed. Although the incision of the Kuner strath began earlier than 9600 B.P., we propose that its greatest potential is to yield cultural components that postdate ca. 7250 B.P. In turn, the Hardin fill may yield cultural components dating to the Kuner abandonment (ca. 6380 B.P.). However, Hardin sediment and soil records recommend that this fill terrace's highest potential is to yield in situ cultural components dating from ca. 1900 to 120 B.P. © 1994 John Wiley & Sons, Inc.     

On the Variability of the Fluxes of Momentum and Sensible Heat

Direct measurements of the air–sea turbulent fluxes of momentum and heat, along with surface currents, waves and supporting meteorological variables, were acquired during a recent field campaign. Surface currents, measured from a very high frequency radar, were found to steer the stress away from the mean wind direction. Although this effect has been reported in a recent scatterometer study, this is the first time it has been observed in an in situ study with co-located flux, wind and surface current measurements. Data collected during a week of stationary conditions are used to investigate and quantify the sampling variability of the air–sea fluxes of momentum and sensible heat.     

Effect of orthophosphate on the dissolution kinetics of biogenic magnesian calcites

The dissolution behavior of two biogenic Mg-calcites, the echinoid, Tripneustes (12 mol% MgCO₃), and the red alga, Neogoniolithon (18 mol% MgCO₃ plus brucite), was studied using free-drift methods in distilled water and phosphate-spiked solutions at 25°C and P_CO2 = 1 atm. Small concentrations of phosphate strongly inhibit dissolution rates. Inhibition increases with increased phosphate levels and with approach toward saturation. Near saturation, dissolution rates are reduced by 10³ by the presence of 0.6 μmol adsorbed-P/m². The magnitude of phosphate inhibition is similar to that observed for low-Mg calcite, and like calcite, the mechanism of inhibition is probably by adsorption of P at surface kink sites. Phosphate appears to inhibit removal of Mg and Ca equally during Mg-calcite dissolution. Rates of brucite dissolution are unaffected by phosphate.Mg-calcites containing >8.5 mol% MgCO₃ should be thermodynamically unstable relative to aragonite in most natural waters. Recent work, however, shows that in contrast to its effect on the behavior of Mg-calcites. phosphate does not inhibit aragonite dissolution. The presence of phosphate might thus reverse the relative stability of these two minerals during diagenesis of shallow marine carbonate sediments.