Pb (II) Sorption with Calcite: A Radiotracer Study

A ²¹⁰Pb radiotracer was used to monitor Pb solid-aqueousphase partitioning in sorption experiments at ambient temperature, pH = 8.2, and atmospheric PCO₂ in 0.15 M NaNO₃ solutions. A 24 h isotherm is linear up to Pb concentrations of 4 × 10^-6 M, above which an increase in slope suggests precipitation. The effect of Pb concentration, calcite loading, and ionic strength on Pb sorption with time was monitored. Sorption kinetics are rapid, followed by a slower sorption step.At a constant calcite loading500 mg L^-1,fractional sorptiondecreases with increasing initialPb concentration. The reverse isobserved for surface coverage, with0.6, 5.6 and 40.2% of availableCa²⁺sites occupied at10^-8,10^-7 and10^-6 MPb after 96 h. At a constant Pb concentration of10^-6 M,fractional sorption increases with increasing particleloading, however surface coverage decrease with72.5 and 22.1%Ca²⁺sites occupied at 100 and200 mg L^-1calcite after 96 h.The adsorption coefficient (K_d) is approximately 10³,increases with initial Pb concentration, but remains unaffected by variable calcite loading. Absence of an ionic strength effect on Pb sorption is interpreted as the dominance of inner-sphere complexation. For desorption experiments conducted over a range of initial sorption times, an average desorption index > 0.8 but < 1 indicates that sorption is largely reversible, but is accompanied by slight incorporation. Solid-solution formation increases with time, as observed by slower initial desorption rates for samples with longer sorption times. These findings indicate that Pb may be effectively sequestered by calcite; however re-release via desorption is likely as Pb does not become significantly incorporated into the mineral structure.     

Using performance analysis to identify candidate operations for GIS software parallelization

This paper examines the potential impact of parallel computing on the computational performance of GIS. Performance analyses on existing GIS operations, vector topology creation and interpolation, in commercial GIS products in a serial environment are used to work out the critical computational factors that determine elapsed time performance. Analyses reveal that the extraction of all data necessary to perform a GIS operation at a particular geographical locality dominates the I/O and CPU resource usage. For GIS to benefit from parallel computing, the performance of this task must be addressed.     

THE POTENTIAL FOR INCREASING CARBON STORAGE IN UNITED STATES UNRESERVED TIMBERLANDS BY REDUCING FOREST FIRE FREQUENCY: AN ECONOMIC AND ECOLOGICAL ANALYSIS

In this paper, we link the impact of fire damage on forests with an economic model of the United States forest sector in order to determine if a reduction in forest fire frequency will increase the projected storage of carbon in United States forests. We first develop a model of forest mortality following fire. We then link this model to an inventory model of United States forests, which allows us to determine how changes in the frequency of fires will impact forest inventories. Changes in inventory levels can be used to project both the amount of carbon stored and an economic response. If fire frequency is reduced from its current level, measured as the average from 1984 to 1990, we find that carbon storage can be increased in unreserved U.S. timberlands over the period 1990 to 2040.     

Selecting conservation reserves using species-covering models: Adapting the ARC/INFO GIS

Conflicts between human development of the landscape and conservation of biodiversiry will continue to grow. Given this reality, there have been a number of attempts to model the optimal selection of conservation reserve sites such that maximum biodiversity protection can be attained within a limited budget for land acquisition. Here we adapt the Location-Allocation module of ARC/INFO to solve the problem of representing, or covering, as many species as possible in a fixed number of selected reserve sites. Resident ARC/INFO solution routines are applied to an innovative 'logical' network that converts the problem of optimal reserve selection into a problem of optimal facility placement, which the Location-Allocation module can recognize and solve. Use of this unique logical network structure as input to ARC/INFO's internal solvers makes possible, compared to previous methods, a much tighter integration of spatial optimization tools with mapping and database tools, all of which are internal to the GIS and accessed via a menu-driven interface. The main advantage is that users of public domain data (such as the US Gap Analysis data) can conduct their own explorations of possible reserve systems without having to acquire and master optimization packages and reformat model output data for GIS display and post-analysis of solutions. Our sample application uses species data from south-western California. We also present a second major form of species-covering model grounded in the same logical network. This enhanced modcl accommodates weighting of species by their conservation importance, thus allowing reserve systems to be designed around the protection of the most threatened or vulnerable biota.     

Scaling impact melting and crater dimensions: Implications for the lunar cratering record

Abstract— The dimensions of large craters formed by impact are controlled to a large extent by gravity, whereas the volume of impact melt created during the same event is essentially independent of gravity. This “differential scaling” fosters size-dependent changes in the dynamics of impact-crater and basin formation as well as in the final morphologies of the resulting structures. A variety of such effects can be observed in the lunar cratering record, and some predictions can be made on the basis of calculations of impact melting and crater dimensions. Among them are the following: (1) as event magnitude increases, the volume of melt created relative to that of the crater will grow, and more will be retained inside the rim of the crater or basin. (2) The depth of melting will exceed the depth of excavation at diameters that essentially coincide with both the inflection in the depth-diameter trend and the simple-to-complex transition. (3) The volume of melt will exceed that of the transient cavity at a cavity diameter on the order of the diameter of the Moon; this would arguably correspond to a Moon-melting event. (4) Small lunar craters only rarely display exterior flows of impact melt because the relatively small volumes of melt created can become choked with clasts, increasing the melt's viscosity and chilling it rapidly. Larger craters and basins should suffer little from such a process. (5) Deep melting near the projectile's axis of penetration during larger events will yield a progression in central-structure morphology; with growing event magnitude, this sequence should range from single peaks through multiple peaks to peak rings. (6) The minimum depth of origin of central-peak material should coincide with the maximum depth of melting; the main central peak in a crater the size of Tycho should have had a preimpact depth of close to 15 km.     

A Strontium Isotopic Investigation of the Bjerkreim--Sokndal Layered Instrusion, Southwest Norway

The 230 km² Proterozoic Bjerkreim—Sokndal layered instrusionhas a monzonoritic bulk composition and comprises a 6 km thick,broadly leuconoritic Layered Series (LS) overlain by unlayeredmangerite and quartz mangerite. In the Bjerkreim lobe the LScomprises six megacyclic units (MCU 0, IA, IB, II, 11 and IV)in a syncline. This lobe is surrounded by migmatitic gneisses;the roof to the instrusion is missing. The mg-number opx, An% and whole-rock initial ⁸⁷Sr/⁸⁶Sr isotope ratios (Sr₀) displaybroadly parallel trends through MCUs II and III, and into MCUIV, with decreasing mg-number (75–58) and An % (50–40)accompanying increasing Sr₀ (0.7050–0.7085). This correlationbreaks down in the upper part of MCU IV and mangerite; the quartzmangerite has Sr₀ values of 0.7085. Abrupt reversals occur acrossthe MCU boundaries. The LS crystallized on the floor of a periodically replenishedmagma chamber that was continually assimilating country rockgneisses. Strong compositional zoning of the magma developedas a result of repeated replenishments with relatively dense,primitive ferrobasaltic magma (Sr₀ = 0.7049) along the floorand the development of extensive buoyant roof melts. Assimilationtook place on a massive scale (up to 50%) in the upper partsof the chamber. KEY WORDS: assimilation; fractional crystallization; hybrid magma; layered intrusion; Sr-isolopes ^*Corresponding author.     

Archean High-Mg Granodiorite: A Derivative of Light Rare Earth Element-enriched Monzodiorite of Mantle Origin

The Roaring River Complex, Superior Province, Canada, containsrocks varying from diorite and monzodiorite to granodioritewhich are characterized by high mg-numbers (0.43–0.62),high abundances of Cr (150 ppm), Sr (500–2000 ppm), Ba(1000–2500ppm), and P2O5 (0.5 wt.%), low Rb/Sr ratios (001–0.02),and steeply fractionated, subparallel REE patterns (Ce_n =65–170,Yb_n = 3–6) without Eu anomalies. The continuous compositionalvariation of the rock suite provides a basis for testing thevarious processes thought to have been important in the extractionof granodiorite magmas from the mantle during the Archean. Weconsider (1) the relative roles of partial melting, crystallizationfractionation, and other processes; (2) the role of garnet orother phases in controlling the steep REE patterns of the rocks;and (3) the chemical and isotopic composition of the sourceregion. The subparallel and decreasing REE patterns with increasingsilica, and the ten-fold variation and high abundances of Crand Ni within the diorite-granodiorite series are not consistentwith different extents of melting of basic crust. The scatterin bivariate plots for closely spaced samples does not supportsimple two-component mixing or liquid immiscibility. The compositionalvariation can be explained by crystallization differentiation(from 0 to 90%) of monzodioritic magma through separation ofdioritic cumulates containing clinopyroxene, hornblende, biotite,plagioclase, K-feldspar, and accessories. The compatibilityof the REEs resulted principally from crystallization of spheneand apatite. The parental monzodioritic magmas with their high mg-numbers,Ni, and Cr contents were derived from peridotitic source rocks(mg-numbers>0.80) with low Rb/Sr ratios (<0.02) and light-REEenrichment relative to chondrites. The differences in the REEpatterns of monzodiorite samples do not support, nor rule out,garnet in the residue for melting. If the monzodioritic meltswere derivatives of other melts, the parent melts would havebeen similar to high-Mg monzodiorites (‘sanukitoids’)recognized as components of other diorite-granodiorite bodiesin the region. An Rb-Sr whole-rock isochron (n = 25) yields a minimum crystallizationage of 2623 Ma (?19) with initial ⁸⁷Sr/⁸⁶Sr = 070134 (?000004;MSWD=l.8). Sm-Nd isotope data for six rocks yield _Nd (2623)=+0.8 ?0.3. The isotope data indicate a source region with long-termRb/Sr of 0.02, similar to depleted mantle, and light-REE depletionrelative to chondrites. The peridotite source to the diorite-granodioriteseries became light-REE enriched before melting through theaddition of a light-REE component of a fluid or melt. In generating Archean granodiorite with suitably high mg-numbers,and Ni, Cr, Sr, Ba, P2O5, and light-REE contents, these dataindicate: (1) the importance of crystallization differentiationof high-Mg monzodioritic parent magmas, (2) that the steep REEpatterns may be a characteristic of the source rocks, and (3)light-REE-enriched, peridotitic sources were melting and contributingsiliceous material directly to the Archean crust.     

Igneous Petrology of the Synorogenic Fongen-Hyllingen Layered Basic Complex, South-Central Scandinavian Caledonides

The 160 km² Caledonian Fongen-Hyllingen complex is an extremelydifferentiated, layered, basic intrusion, synorogenically emplacedat 5–6 kb in the allochthonous Trondheim nappe complex,situated in the Trondheim region of Norway. A zone of gabbroic rocks without rythmic layering usually occursalong the margin and a supposed feeder to at least part of thecomplex is preserved. A wide variety of magmatic sedimentarystructures are present in the c. 10,000 m thick sequence ofrhythmically layered rocks which vary from olivine-picotitecumulates at the base to quartz-bearing ferrosyenites at thetop. Mineral compositions, fractionation trends, and the compositionof feeder rocks suggest a tholeiitic parent. Mineral compositions cover extreme ranges. Olivine varies fromFo_86·2 to Fo_0·2 with a hiatus between about Fo₇₁and Fo₆₁. Plagioclase ranges from An_79·5 to An_1·5,albite coexisting with orthoclase microperthite in the finaldifferentiates. Cumulus Ca-poor pyroxene (Wo_2.4En_66.8Fs_30.8-Wo_2·0En_17·0Fs_81·0)first shows sporadic inversion from pigeonite at the Fe-richcomposition of Fs₆₇ and the final Ca-poor pyroxenes are replacedby magmatic grunerite which reaches an Mg: Fe ratio of 12:88.Ca-rich pyroxenes (Wo_44·7En_43·8Fs_11·5-Wo_47·0En₀Fs_53·0)are highly calcic and have a slight Ca-minimum in the earlystages, unrelated to the disappearance of Ca-poor pyroxene.Calcic amphibole, a constant intercumulus phase in most of thecomplex, becomes a cumulus phase in the later stages and variesfrom titanian-pargasite to ferro-edenite. Magnetite and ilmenitejoin the cumulate assemblage at Fo₅₅ and ilmenite persists intothe final quartz-bearing ferrosyenite where it shows replacementby sphene. Apatite, biotite, zircon, quartz, K-feldspar andallanite join the final extreme differentiates in the namedsequence. The fractionation trend is, in many respects, transitionalbetween those typical of the tholeiitic and calc-alkaline series,and is interpreted as reflecting crystallization under moderate,increasing P_H2O. Cryptic layering shows several reversals to higher temperatureassemblages with increasing stratigraphic height. Successivereversals are to irregular compositions and measured in termsof olivine composition, can be up to about 30 mole per centFo. The minimum stratigraphic thickness to include the entirefractionation range is reduced to about 2200 m after ‘removal’of the compositional overlaps due to the reversals. Thus roughlythree-quarters of the present cumulate stratigraphic sequencerepresents magma replenishment. A mechanism involving the mixingof fresh magma batches with the residual, differentiated magmafrom the previous influx, is envisaged. The periodic influxof fresh magma took place into a chamber which was probablyclosed to the exit of material.     

Nepheloid and suspended particulate matter in south-eastern Lake Michigan*

A persistent benthic nepheloid layer with high total suspended matter (TSM) and high total particulate surface area was observed in south-eastern Lake Michigan. The layer thickens from a few metres near the shelf-slope boundary to greater than 10 m at the base of the slope. When compared to the hypolimnion, TSM increases by a factor of 2-20 at 1 m above the bottom, the greatest increase detected at the slope-basin boundary. Sediment trap profiles within the nepheloid layer show that the particulate flux increases exponentially from about 10 m above the bottom to 1 m above the bottom, suggesting that a large fraction of the collected material came from resuspension. A nepheloid layer is created during the formation of the thermal bar and maintained during the stratified period, apparently through the action of weak but persistent currents. This layer is supplemented by lakeward transport of fine particles resuspended near the shelf-slope boundary due to impingement of the thermocline on the bottom, or during higher energy events.     

Crystallization and Layering of the Skaergaard Intrusion

Solidification of large slowly cooled intrusions is a complexprocess entailing progressive changes of rheological propertiesas the crystallizing magma passes through successive stagesbetween a viscous Newtonian fluid and a brittle solid rock.Studies of this transition in the Skaergaard intrusion indicatethat most crystallization took place in an advancing front ofsolidification against the floor, walls, and roof where crystalsnucleated and grew in a static boundary layer, much in the mannerproposed by Jackson in 1961. The non-Newtonian properties ofthe crystallizing magma account for the fact that plagioclase,which was lighter than the liquid, is a major component of rockson the floor, while mafic minerals that were heavier than theliquid accumulated under the roof. Crystals that nucleated andgrew in these zones were trapped by an increasingly rigid zonethat advanced more rapidly than the crystals sank or floated.If any crystals escaped entrapment, they were those of the largestsize and density contrast. The rates of accumulation in different parts of the intrusionwere not governed by rates of gravitational accumulation somuch as by the nature of convection and heat transfer. Cumulatetextures, preferred orientations of crystals, and layering,all of which have been taken as evidence of sedimentation, canbe explained in terms of in situ crystallization. Layering cannothave been caused by density currents sweeping across the floor;it is well developed on the walls and under the roof, lacksthe size and density grading and mineralogical compositionsthat would be expected, and shows no evidence of having beenaffected by obstructions in the paths of the currents. We propose an alternative origin of layering that is based onprocesses governed by the relative rates of chemical and thermaldiffusion during cooling. Intermittent layering resulted fromgravitational stratification of the liquid, and cyclic layeringwas produced by an oscillatory process of nucleation and crystalgrowth. The effects of differentiation during in situ crystallizationare strongly dependent on relative rates of diffusion of individualcomponents, and some of the compositional variations in differentparts of the intrusion can be explained in terms of these differences.