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Controls on the precipitation of barite

Controls on the precipitation of barite (BaSO 4)crystals in calcite travertine at Twitya Spring,a warm sulphur spring in Canada's Northwest Territories

Sandy M.Bonny ?,Brian Jones 1

Department of Earth &Atmospheric Sciences,University of Alberta,Edmonton,Alberta,Canada T6G 2E3

Received 1May 2007;received in revised form 18October 2007;accepted 28October 2007

Abstract

Twitya Spring discharges warm (24°C),anoxic,sulphide-,calcium-(65ppm)and barium-(≥0.78ppm)rich spring water to a steep flow path that is inhabited by streamer and mat-forming microbes (Thiothrix ,Beggiatoa ,Oscillatoria ,Spirulina ,diatoms,rod shaped bacteria).Oxidation and CO 2degassing drive precipitation of elemental sulphur,barite,opaline silica,and calcite.A mound of travertine at the base of the flow path,dominantly composed of bedded barium-enriched crystallographic and noncrystallographic dendritic calcite crystals and calcite cements,hosts three types of barite crystals:type 1(T1)intergrown tabular crystals that formed in solution,type 2(T2)tabular and rhombic crystals that nucleated on calcite,and type 3(T3)subhedral and anhedral microcrystals that nucleated on microbial cell surfaces and in microbial extracellular polymeric substances.The formation and distribution of T1,T2,and T3barite in the Twitya Spring flow path are controlled by physiochemical gradients,calcite precipitation rates,and adsorption of barium to microbial biomass,all of which vary seasonally and episodically at Twitya Spring.The complex physiochemical and biological controls on barite formation at Twitya Spring both suggest that the classification of biogenic or inorganic sedimentary barite on the basis of crystal size and morphology may be oversimplified.There is also the potential that primary and authigenic barite crystals hosted in carbonates may yield information about the microbial ecology and ambient physiochemistry of their depositional environments.

Keywords:barite;calcite;warm spring;travertine;biologically mediated precipitation

1.Introduction

Sedimentary barite has high utility for paleoenvironmental reconstruction because of its link with marine productivity (Bishop,1988;Paytan et al.,1993;Bains et al.,2000;Greinert et al.,2002;Lu et al.,2004),and its geochemical sensitivity to changes in redox chemistry and local and global perturbations to the sulphur cycle (Miller et al.,1977;Kastner,1999;Hanor,2000;Bottrell and Newton,2006).Barite hosted in carbonate sediments may be especially useful for palaeoenvironmental geochemistry,as carbonates are prone to recrystallization and chemical and isotopic readjustment during diagenesis (Martin

et al.,1995;Kastner,1999).In highly altered carbonates,diag-enetically refractory barite microcrystals may provide the only accurate access to palaeogeochemical signals (Martin et al.,1995;Kiipli et al.,2004;Newton et al.,2004).

In clastic marine sediments,barite crystals formed through different mechanisms (i.e.biogenic vs.diagenetic)commonly preserve distinct geochemical signals,and are distinguishable on the basis of crystal size and habit (Paytan et al.,1993).Barite precipitated among plankton in the upper marine water column,for example,is morphologically distinct from barite precipitated at the sediment –water interface,allowing separation of barite from sediment core samples for environmentally specific δ34S analyses (Paradis and Lavoie,1996;Bains et al.,2000;Paytan et al.,2002).Morphologically based separation methods have limited usefulness in the geologic record because environmen-tal influences on barite crystal size and habit are unconstrained in many depositional environments,such as dysoxic marine

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Sedimentary Geology 203(2008)36–

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Corresponding author.Fax:+17804928190.

E-mail addresses:sandy.bonny@ualberta.ca (S.M.Bonny),brian.jones@ualberta.ca (B.Jones).1

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bottom waters,and hydrothermal and methane seeps (Fu et al.,1994;Martin et al.,1995;McManus et al.,1998;Alfonso et al.,2005;Riedinger et al.,2006).The potential utility of barite microcrystals hosted in limestones and dolostones is further limited by a lack of data pertaining to the influence of co-deposited carbonate on barite precipitation and preservation (Aharon,2000;Alfonso et al.,2005).

Twitya Spring in Canada's Northwest Territories (Fig.1)provides a rare opportunity to examine the controls on the distribution and morphology of barite crystals hosted in calcite.Twitya Spring is a warm (24°C),sulphur-and bicarbonate-rich spring which precipitates laminated carbonate travertine that contains up to 5%barite by volume.During the course of this study,detailed petrographic analysis revealed that barite crystal morphpology is correlated to precipitation substrate at Twitya Spring;type 1(T1)detrital intergrown tabular crystals are

deposited from solution;type 2(T2)tabular and rhombohedral crystals are nucleated on calcite;and type 3(T3)subhedral and anhedral crystals are nucleated on microbial cell surfaces and in microbial extracellular polymeric substances (EPS).This paper focuses on the controls on the formation and distribution of the different types of barite found in the Twitya travertine and explores potential implications for the interpretation of barite microcrystals in the geologic record.2.Study site

The Twitya Spring aquifer emerges from one main spring vent and two smaller seeps through surficial deposits of fractu-red shale approximately ～15m above the level of the Twitya River in the Sahtu Region of Canada's Mackenzie Mountains (Fig.1A).It is underlain by limestone and dolostone of

the

Fig.1.(A)Location map for Twitya Spring.(B –F)Photographs of Twitya Spring showing (B)overview;(C)surface of travertine mound;(D)detail of microgours on surface of travertine mound;(E)side view of bedding in travertine mound;(F)Detail of calcite dendrites in bedded travertine from image E.

S.M.Bonny,B.Jones /Sedimentary Geology 203(2008)36–53

middle Devonian Arnica Formation,and calcareous shale,lime-stone,and chert from the upper Ordovician to Silurian Road River Formation (Blusson,1971).Structural faults and folds allow deep penetration of ground water throughout the Mac-kenzie Mountains and there are numerous cold and geothermal mineral springs in the Sahtu Region (Blusson,1971;Cecile et al.,1984).Many springs have developed extensive deposits of porous,microcrystalline carbonate tufa,and the Flybye Springs ～35km to the north have produced a deposit of rare micro-crystalline barite tufa (Cecile et al.,1984;Bonny and Jones,2007a ).The Twitya Spring site is regionally unique in that it contains barite hosted in macrocrystalline calcite travertine.The Twitya travertine reaches maximum thickness in a mound,～10m long and 2m high,beneath the main spring vent (Fig.1B-D).The mound is composed of parallel,undulatory beds of ‘feather ’dendrite calcite crystals 1–5cm long.Individual beds contain numerous internal growth laminations (Fig.1E),and grey and green chromatic banding (Fig.1F).The bright white travertine,skirted by a green microbially colonised drainage stream,provides a striking contrast to surrounding gray and brown limestone and shale,and is considered a sacred heritage site by the Sahtu Dene people.3.Methods 3.1.Field work

The geometry and extent of the Twitya Spring travertine deposit were documented by ground survey and aerial photo-graphy.The pH,conductivity,and dissolved oxygen (D.O.)content of the Twitya Spring water were measured in August,

2005,using a portable Accumet AP62pH/mV meter and an Orion Model 1230D.O./pH/mV probe.Water samples were collected at the spring vent and 10m downstream through a 0.45μm pore size FisherBrand water-testing membrane and stored in sterile 250ml Nalgene containers.Ten microbial samples were collected from the spring flow path that en-compassed the textural and chromatic variability of the com-munities.One half of each microbial sample was stored in spring water and the other half was preserved in a 6:3:1solution of water,95%alcohol and formalin with 5ml of glycerol added per 100ml.Thirty samples of fresh precipitates and calcite travertine were collected at intervals of ～1m across the flow path (four from zone 1,six from zone 2,eight from zone 3,and twelve from zone 4;see Fig.2)https://www.wendangku.net/doc/164951266.html,boratory work

Spring water composition was determined by inductively coupled mass spectrometry (ICP-MS)at the University of Sas-katchewan,and SOLMINEQ88was used to evaluate the sa-turation indices (S.I.=log [ion activity product/solubility constant])of relevant mineral phases.The mineralogy of so-lid samples was determined by optical petrography and pow-der X-ray diffraction (XRD)on a Rigaku Geigerflex Cobalt Tube Power Diffractometer with a graphite monochromator and JCPDS online mineral identification database.

Whole and acid etched (10%HCl)travertine samples were fractured,mounted on steel stubs,sputter coated with gold,and analyzed on a JEOL 6301field emission scanning electron microscope (SEM).Both secondary electron and backscattered electron SEM images were collected at accelerating voltages

Fig.2.Schematic cross-section of the Twitya Spring flow path showing zones defined by spring water physiochemistry (boxed data),microbiology (right hand side),and mineral precipitates (left hand side).

38S.M.Bonny,B.Jones /Sedimentary Geology 203(2008)36–53

5and 20kV,respectively.Backscattered electron SEM images reflect variations in atomic weight through image brightness —barium (atomic mass 137.3)is very bright in contrast to increasingly dark strontium (87.6),calcium (40.1),sulphur (32.1),silicon (28.1),and organic matter (～12.0).Qualitative elemental analyses were conducted using an energy dispersive X-ray analyzer coupled to the SEM.

Cut,polished,and carbon-coated thin sections of travertine were prepared for quantitative elemental analysis (as wt.%)on a JEOL 8900R electron microprobe (EMP)at an accelerating voltage of 15kV with a 10nA beam current and 1μm beam diameter.Backscattered electron images generated on the EMP (at an accelerating voltage of 20kV with a 30nA beam current)have gray levels that reflect image-specific variations in average atomic weight —darker areas correspond to relatively lower average atomic weights,and bright areas to relatively higher average atomic weights.

Microbe samples were examined by light microscopy and soft-bodied microbes were identified to genus level by morpholo-gical criteria (Table 1)following Rippka et al.(1979),Larkin and Strohl (1983),Schlegel and Bowien (1987),Wher and Sheath (2003)and Teske and Nelson (2005).Microbes viewed by SEM analyses were given ‘tentative ’taxonomic assignments based on their similarity to fresh and preserved microbial samples.No attempt was made to identify submicron-sized Actinomy-

cetes or unicellular microbes revealed by scanning electron microscopy.4.Results

4.1.Spring water physiochemistry

Twitya Spring emerges at ～24°C,indicating a minimum circulation depth of 600m for the aquifer (van Everdingen,1972).The spring water is rich in sodium,calcium,bicarbo-nate and sulphate,and anomalously rich in strontium,barium,and soluble silica (Table 2)relative to regional stream and ground waters (Cecile et al.,1984;Day et al.,2005).At the vent,the spring water contains b 0.61mg L ?1dissolved oxygen and geochemical modeling with SOLMINEQ88indicates that it is saturated (S.I.≥0)with respect to calcite (0.06)and barite (1.33),and undersaturated with respect to more soluble celes-tite (?1.75),gypsum (?1.57),witherite (?2.26),and strontia-nite (?1.16).Turbulence facilitates atmospheric oxidation in the spring flow path and dissolved oxygen increases to N 2.44mg L ?1in the stream at the base of the travertine mound (Fig.2).CO 2-degassing is also indicated by an increase in pH from 7.26at the spring vent to 8.55at the base of the spring flow path (cf.Chafetz et al.,1991).The dissolved oxygen and pH of the spring water are inversely related to its dissolved sulphide

Table 1

Morphological characteristics and environmental preferences of microbial genera from Twitya Springs Group /Genus Morphology

ISD ESD ST Habitat

Tuitye Field Appearance

Sulphur oxidising bacteria Thiothrix Colourless cylindrical filaments;2–3μm wide;septa

every 4–5μm;unsheathed;occur in rosettes Yes

Yes Flowing dysoxic

water White fringes;streamers,pervasive from the vent to the top boundary of the travertine mound

Beggiatoa

Colourless cylindrical filaments;5μm wide;septae every 3–5μm;unsheathed;occur singly;form hormogonia

Yes No

Yes Oxic/anoxic interface,

low turbulence

Basal layers of floating mats in

shallow ponds,long white streamers near spillover from ponds to travertine mound

Purple sulphur bacteria

Chromatium Motile;oval to bean shaped cells 1–3μm long;pink in light microscope

Yes

Yes Dysoxic to anoxic

photic zone Red non-cohesive films in ponded water behind travertine mound

Cyanobacteria

Oscillatoria Olive-green filaments;3μm wide;septa every 1μm (or less);unsheathed;non refractory internal granules;form hormogonia by necritic cell development;gliding movement;some have sulphur globules attached to outer surface

Yes

Yes Flowing or stagnant oxygenated to dysoxic water,wide chemical and thermal tolerance Abundant in orange –green mats and

films behind travertine mound Spirulina Green unsheathed filaments;1μm wide forming tight coils ~10μm wide

Cool to warm oxygenated freshwater

Bright green non-cohesive films in ponded water;scarce above travertine mound,abundant below

Diatoms

Siliceous frustules;pennate,most b 10μm long Naviculoid,Achnanthoid and Nitzschiod types

Yes Flowing or ponded

oxic to dysoxic water

Abundant in ponded water above and below the travertine mound

Rod Shaped (likely sulphur reducing)Bacteria

Straight or arcuate rods,5μm long and 0.5μm wide,some have sulphur globules attached to outer surfaces

No Yes?Yes Flowing and ponded

dysoxic water

Throughout the flow path,most

abundant amongst Thiothrix filaments in upper flow path

ISD =‘intracellular sulphur deposition ’;ESD =‘extracellular sulphur deposition ’;ST =‘sulphide tolerant ’(after Rippka et al.,1979;Reichenback,1981;Larkin and Strohl 1983;Schlegel and Bowien 1987;Pentecost,1995;Wher and Sheath 2003;Teske and Nelson 2005).

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and CO 2content and,together with hydrologic parameters,these physiochemical variables define four gradational zones in the Twitya flow path,each of which are characterized by distinct microbial assemblages and mineral precipitates (Fig.2).

4.2.Microbes

Thiothrix (Fig.3A)forms white microbial streamers on submerged substrates in zone 1.The streamers also contain rod-shaped bacteria (RSB),elemental sulphur,and EPS.The RSB are probably sulphate-reducing bacteria (Douglas and Douglas,2001),but this cannot be confirmed by morphological criteria alone.In zone 2,Thiothrix streamers and RSB are joined in dysoxic tributaries by Oscillatoria and diatoms,locally forming green/orange films ≤5mm thick (Fig.3B-D).In zone 3,spring water forms pools,≤5cm deep,which contain loose,floating green –white mats of Beggiatoa and Oscillatoria (Fig.3C,D),diatoms,and RSB.Localized pink patches in these mats also contain Chromatium (Fig.3B),and bright green patches contain Spirulina and clusters of spherical unicellular bacteria and elemental sulphur beads (Fig.3E,F).Desiccated mats are colonized by unidentified hyaline fungal hyphae.In August 2005,the surface of the travertine mound (zone 4)was locally colonized by orange/green mats dominated by Oscillatoria (Fig.1C),and pools and drainage streams beneath the travertine mound were bright green,due to dense colonisation by Spirulina

Fig.3.Light microscope photographs of microbes from Twitya Spring.(A)Thiothrix filaments joined in a rosette (arrows).(B)Oscillatoria (Osc -arrows),rod-shaped bacteria (RB-arrows)and Chromatium (Chr -arrows).(C)Diatoms and filaments of Oscillatoria and Beggiatoa (Begg -arrow).(D)Detail of Oscillatoria with external elemental sulphur (arrows)and Beggiatoa with internal elemental sulphur (arrows).(E)Elemental sulphur and Spirulina (arrows).(F)Detail of Spirulina (Spr -arrow)with Beggiatoa and a clumped elemental sulphur and unicellular bacteria (S/B-arrow).

Table 2

Chemistry of Twitya Spring water measured by ICP-MS Element Concentration (ppm)(meq/L)Na +104 4.52K +120.31Ca 2+65 3.24Mg 2+23 1.89Sr 2+ 1.380.03Ba 2+0.780.01Al 3+b 0.005–Fe 3+b 0.001–Cl ?46 1.3HCO 3

?356 5.83NO 3

2?0.130.002SO 42?140 2.91SiO 2

10.60.189

Total Alkalinity 292Analysis pH

7.74

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4.3.Mineral precipitates

The distribution,abundance,and habit of precipitates in the Twitya Spring flow path vary downstream with changing phy-siochemical conditions (Fig.2).

Elemental sulphur is found as single and chained rhombohe-dral crystals up to 10μm long,as spherical beads 0.5–10μm in diameter (Fig.4A),as crystalline sulphur rhombohedra that surround elemental sulphur beads (Fig.4B),and locally as chains of sulphur rhombohedra (Fig.4C).Elemental sulphur is present in decreasing abundance in zones 1through 4.In zones 1and 2it is found among Thiothrix streamers and forms a thin coating on rocks and branches.In zones 3and 4,it is deposited among microbial mats,and incorporated in calcite travertine.Calcite precipitates in zones 2and 3,forming laminated rinds on the downstream side of submerged branches and rocks.In zone 2,laminae b 1cm thick are composed of splaying trigonal calcite crystal bundles with shrub-like pseudo-dendritic habit (cf.Jones et al.,2000)(Fig.4D –F).The bundles are up to 20μm high,and are commonly interlaminated with thin layers of micrite and organic detritus (Fig.4E).In zone 3,laminae 0.5–2cm thick are composed of symmetric and subsymmetric crystallographic calcite dendrites (cf.Jones and Renaut,1995)with width to height ratios of 0.5–0.1,two to three orders of branching,and distinct chevron growth bands (Fig.5A –E).In the travertine mound (zone 4)convex and sub-vertical laminae,1–5cm thick,are composed of asymmetric,non-crystal-lographic dendrites (cf.Jones and Renaut,1995),with

isolated

Fig.4.Mineral precipitates.(A)Rhombohedra (r)and beads (b)of elemental sulphur.Rhombohedra surrounding beads are indicated by arrows.(B)Detail of rhombohedra surrounding bead.(C)Chains of elemental sulphur rhombohedra (arrows).(D)Cross-section of laminated calcite rind surrounding twig (arrows).(E)Thin section photograph of splaying calcite pseudo-dendrites from zone 2,arrows indicate growth bands demarked by detrital organics.(F)Splaying calcite pseudo-dendrites with trigonal cross-section.

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discontinuous laminae of crystallographic dendrites.The non-cry-stallographic dendrites have a macroscopic ‘feathery ’appearance (Fig.1F),exhibit up to four levels of branching (Fig.5F,G),and gain asymmetry by preferential growth of downstream branches.Calcite dendrites are backfilled by compositionally distinct secondary calcite cements (Fig.6).Cements and primary dendritic calcite boundaries are reflected by sharp grey-level shifts in backscattered EMP images,which are attributable to

variations

Fig.5.Dendritic calcite.(A)Cut hand sample showing calcite dendrites from zone 3with distinct growth laminations (arrows).(B)SEM image of etched travertine sample,arrows indicate individual dendrite crystals.(C)EMP image of crystallographic calcite dendrites from zone 3.(D)Crossed polar thin section photograph showing unit extinction of crystallographic dendrites.(E)Elongate crystallographic dendrite with ‘chevron ’growth bands (arrows).(F)Thin section photograph of non-crystallographic calcite dendrites from zone 4.(G)Crossed polar thin section photograph showing sweeping extinction of non-crystallographic dendrites.

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in the trace abundance of barium,strontium,and magnesium co-precipitated with the calcite (Fig.6).Primary dendrite calcite contains up to 0.58and 0.53wt.%Ba 2+and Sr 2+,respectively,whereas cements have maximum concentrations of 0.12and 0.04wt.%Ba 2+and Sr 2+.Mg 2+has a reversed distribution,with enrichment up to 1.26wt.%compared to 0.26wt.%in dendritic calcite.Traverses across compositional boundaries show co-

variation of Ba 2+and Sr 2+,which vary inversely with Mg 2+(Figs.6C and 7).Compositional variations at scales ≤5μm are not represented in Fig.7because they approach the combined precision limit of the microprobe beam spot size (1μm)and the automated stage positioner (+/?1.5μm).

Calcite dendrites are crystallographically continuous across many laminae in the Twitya travertine (Fig.8A)but are

locally

Fig.6.Backscattered EMP images showing variation in average atomic weight between primary calcite dendrites (P)and secondary calcite cements (S).(A)Crystallographic dendrite.(B)Non-crystallographic dendrite.(C)Detail of non-crystallographic dendrite with heavy (bright)primary branches (P)infilled by lighter (dark)secondary cements (S).Transect line X –Y corresponds to Fig.7.

S.M.Bonny,B.Jones /Sedimentary Geology 203(2008)36–53

interrupted by detrital laminae (Fig.8B),and irregular dissolu-tion bands (Fig.8C).Despite the presence of diatoms in mi-crobial samples,no preserved diatom frustules were found in the Twitya travertine,indicating that they undergo rapid dis-solution.Growth bands between and within calcite dendrite beds do,however,locally contain secondary calcite cement,micritic grains,detrital clay and organics,microbial cells and filaments,elemental sulphur (Fig.8D),opaline silica (Fig.8E),and barite (Fig.8A).

Barite crystals are distributed throughout the Twitya traver-tine,approaching up to 5%by volume along growth bands and in pockets of trapped microbial biomass.The Twitya barite is relatively pure (containing 0.9–7.6wt.%Ca 2+,0.2–3.9wt.%Sr 2+,and b 0.1wt.%Mg 2+)and can be divided into type groups T1,T2,and T3based on crystal habit and nucleation substrate.T1—Single or more commonly intergrown euhedral barite plates,5–10μm long and b 1μm wide,are found suspended in solution and trapped in microbial biomass and calcite rinds in zones 1and 2(Fig.9A,B).T1barite is present in zone 3calcite rinds,but a rare detrital component in the zone 4tra-vertine mound.

T2—Single and rarely intergrown tabular barite crystals,up to 50μm long and 10μm wide,and blocky barite rhombohedra,≤20μm wide,nucleate on calcite in zones 2,3,and 4(Fig.9C,D).T2barite crystals have sharp compositional boundaries with calcite substrates (Fig.9E)and rhombohedral T2crystals are commonly associated with etched elemental sulphur (Fig.9F).T2barite crystals are most abundant in the travertine mound,where they are localized between calcite dendrites and commonly nucleated on secondary calcite cements.

T3\Consistently b 5μm in their longest dimension,T3barite crystals are found exclusively in association with microbial biomass (Fig.10).They are generally anhedral on and within EPS (Fig.10D),but subhedral-tabular on microbial filaments with little associated EPS (Fig.10E).T3barite is

localized in microbial biomass found along laminar boundaries and in pocket porosity between calcite dendrites.The microbes that host T3barite are most commonly long,non-branching filaments with collapsed width of 2–3μm (most likely de-siccated filaments of Thiothrix and Oscillatoria ),as well as branching fungal hyphae.T3barite crystals nucleated on fila-ments and hyphae commonly grow around them (Fig.10F).5.Interpretation

Mineral precipitation in the Twitya Spring flow path takes place along dynamic physiochemical gradients established by equilibration of the spring water to atmospheric conditions,mineral precipitation,and the metabolism of resident microbes.Elemental sulphur beads are a by-product of sulphur redox metabolism for diverse microbial genera,including Thiothrix ,Beggiatoa ,and Chromatium at Twitya Spring (Fig.3).In contrast,crystalline elemental sulphur is usually produced by inorganic redox reactions and probably formed by spontaneous oxidation of dissolved sulphide and by recrystallization of metastable biogenic sulphur beads at Twitya Spring (cf.Steudel,1987;Douglas and Douglas,2001).

Pseudo-crystallographic,crystallographic,and non-crystal-lographic calcite dendrites are all known to form in carbonate spring flow paths by disequilibrium precipitation from strongly supersaturated solutions (Jones et al.,2000;Pentecost,2005).Their morphology may be influenced by the presence of mic-robial cells and/or biofilms (Chaftez and Guidry,1999),but dendritic calcite growth is thought to be initiated by specific physiochemical conditions,including rapid CO 2degassing (Jones and Renaut,1995;Chaftez and Guidry,1999),the presence of organic impurities (Pentecost,2005),an excess of bicarbonate to calcium (Jones et al.,2005),and the presence of divalent metal ions larger than calcium (Astilleros et al.,2000;Fernandez-Diaz et al.,2006).In the lower Twitya Spring flow path a Ca:CO3ratio of 0.18,relatively high concentrations of strontium and barium (Table 1),the presence of detrital and microbially derived organic compounds,and rapid,turbulence-incited CO2-degassing combine to promote precipitation of calcite dendrites.

Assessing the biogenicity or abiogenicity of specific mor-phological features in dendritic calcite at Twitya Spring is beyond the scope of this paper.The downstream progression of dendrite crystals at Twitya Spring does,however,parallel Jones and Renaut's (1995)correlation of dendrite type to disequili-brium level:pseudo-dendrites precipitate at relatively lower disequilibrium than crystallographic dendrites,whereas non-crystallographic dendrites precipitate at higher levels of dise-quilibrium.The dominance of non-crystallographic dendrites in zone 4thus indicates that highest levels of disequilibrium,i.e.rates of CO 2degassing (Jones et al.,2005),are achieved over the surface of the travertine mound.Elevation in pH by CO 2de-gassing in zone 4would also promote precipitation of silica (Jones et al.,2005)generated by dissolution of diatom frustules upstream.

In contrast to calcite and silica,the Twitya Spring water is supersaturated with barite in zone 1,where T1barite

represents

Fig.7.Variation in average weight %magnesium,barium and strontium along transect X –Y (Fig.6).Vertical lines correspond to data points collected from primary dendritic calcite (P)and secondary infilling calcite cement (S).

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the first precipitation product of dissolved barium with atmo-spherically oxidized sulphate.Barite crystal habit varies as a function of the degree of oxygenation,ambient chemistry and the viscosity of the precipitating solutions (Sasaki and Minato 1982;Radanovic-Guzvica,1999;Su et al.,2002;Torres et al.,2003)and T1barite's intergrown euhedral plates are typical of barite microcrystals precipitated at interfaces between oxidized and reduced solutions (Stark et al.,2004;Wagner et al.,2005

Fig.8.Dendrite growth bands.(A)Backscattered SEM image of etched cacite dendrites,arrow indicate growth bands,*indicates area rich in barite crystals (bright speckles).(B)Fine scale laminations composed of clay and organic detritus (arrows).(C)Irregular dissolution boundaries marked by organic detritus (arrows).(D)Detrital elemental sulphur (arrows),and microbial filaments from a dendrite growth band.(E)Beads of opaline silica (arrows)nucleated on EPS along a dendrite growth band.

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The fact that T1barite is increasingly scarce beyond zone 2indicates that spontaneous nucleation of barite in solution is inhibited,probably by the combined effects of decreasing am-bient barium concentration (due to upstream precipitation as T1barite and co-precipitation in calcite),CO 2-degassing (cf.Lindgren,1933;Hamm and Merritt,1944;Bolze et al.,1974),adsorption of barium to microbial biomass (Tazaki and Watanabe,2004),and the presence of barite nucleation inhibiting humic acids derived from detrital organic matter (Smith et al.,2004

Fig.9.SEM images of travertine-hosted barite.(A)T1barite crystals trapped in EPS.(B)T1barite crystals (b)deposited on calcite (c).(C)T2barite crystal (b)nucleated on calcite cement (c).(D)T2barite crystals (b)surrounded in an etched travertine sample,arrows indicate irregular faces on T2crystals where they abutted calcite (c)that has been dissolved.(E)EMP image of T2barite showing sharp compositional boundary between barite (b)and calcite (c).(F)Rhombohedral T2barite (b)nucleated on calcite (c)and associated with oxidizing elemental sulphur (s).

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The distribution and abundance of T2(calcite-nucleated)and T3(microbially associated)barite in the Twitya travertine depends on competition for dissolved barium between calcite dendrite lattice sites,calcite cement lattice sites,and negatively charged sites hosted by microbial cells and EPS.Barium ad-sorption is maximized in oxygenated water,with pH N 7,where free,high charge density Ba 2+ions are more abundant in so-lution than weakly adsorptive BaOH +and BaHS +

complexes

Fig.10.SEM images of T3barite.(A)Backscattered SEM image of calcite dendrites with pockets and drapes or barium-enriched biomass (arrows).(B)Detail SEM image of barium-enriched drape from image A.(C)Backscattered image of the same drape,showing barite mineralisation along microbial filaments and scattered through EPS.(D)Detail backscatter image of anhedral barite (arrows)nucleated on microbial filaments surrounded by EPS (eps).(E)Backscattered image of subhedral rhombohedra of barite on fungal hyphae (arrows).(F)Detail of subhedral barite surrounding an EPS-free fungal hypha.

S.M.Bonny,B.Jones /Sedimentary Geology 203(2008)36–53

(Langmuir,1997).Increasing pH and dissolved oxygen in zones 3and4,thus,promote adsorption of barium to calcite and microbial surfaces.Sequestration of cations to sorptive sites can either promote or inhibit mineral precipitation,depending on ambient chemistry and reactant concentrations(Arp et al., 1999).Predictive models exist that can be used to assess the relative sorptive capacities of microbial surfaces vs.mineral surfaces and the efficacy of each to host new precipitate phases (e.g.Fein et al.,2001).A complication to modeling the fate of barium at Twitya Spring,however,is that many of the microbes identified in the flow path are known barium bioaccmulators, including Thiothrix,Beggiatoa,and Chromatium(Bonny and Jones,2007a);Oscillatoria(Younger,1986;Douglas and Douglas,2001;Bonny and Jones,2007a);Spirulina(Younger, 1986),and fungi(Arguete et al.,1998).Bioaccumulation can have a strong and measurable impact on mineral saturation gradients(Ganeshram et al.,2003;Bonny and Jones,2007b) and thus precludes application of predictive adsorptive models at Twitya Spring.Based on data gathered in this study,barium remaining in solution beyond zone2can be(1)incorporated into dendritic calcite and calcite cements;(2)precipitated as T2 barite nucleated on calcite;and(3)precipitated as T3barite in association with microbial biomass.

Incorporation of cations with larger ionic radius than calcium (1.00?)into calcite is positively correlated to crystal growth rate(Tesoiero and Pankow,1996).This may explain why ra-pidly precipitated dendritic calcite from Twitya Spring is barium (1.47?)enriched relative to secondary calcite cements(Figs.7 and8).Incorporation of barium into calcite dendrites may limit precipitation of barite,whereas slower growing calcite cements would selectively exclude barium(Tesoiero and Pankow,1996), leaving it available for barite precipitation.

T2barite's rhombohedral and tabular crystals are typical of slow precipitation via surface reaction growth mechanisms (Shikazono,1994;Greinert et al.,2002).Tabular barite forms are also favoured by the presence of Ca2+and Mg2+,con-centrations of which are linearly correlated to increased ex-tension of barite's b axis at the expense of the a and c axes (Hennessy and Graham,2002).The Twitya Spring water is rich in calcium and magnesium relative to barium(Ca:Mg: Ba=83:29:1),and thus favours growth of tabular barite crystals. The association of etched elemental sulphur with rhombohedral T2barite microcrystals(Fig.9E,F)indicates that it formed as a secondary precipitate.Elemental sulphur is unstable in well-oxygenated solutions(Pentecost,2005),and will spontaneously oxidize to sulphate upon desiccation(Brigmon et al.,1994). Rhombohedral T2barite may,thus,form at the expense of detrital elemental sulphur in parts of the Twitya travertine mound that become aerially exposed,or when the spring flow path is flushed by oxygenated meteoric waters.

During summer,microbes flourish in pools of Twitya Spring water,form mats on hydrated portions of the travertine mound, and inhabit hydrated pore space in the upper layers of the travertine—clearly,there are abundant substrates for T3barite precipitation.At pH7–8,microbial cells and EPS both host deprotenated carboxyl and phosphoryl groups capable of ad-sorbing divalent metal cations(Phoenix et al.,2002;Yee et al.,2003).At Twitya Springs,microbial surfaces undoubtedly ad-sorb divalent cations other than Ba2+(e.g.Ca2+,Mg2+,Sr2+—Table2),but the solubility of barite at ambient temperature (logK BaSO4=9.97)is so low that barite nuclei form more readily than other,more soluble sulphate and carbonate salts (log K CaSO4=4.85,CaCO3=8.47,SrSO4=6.47,SrSO4=9.25, BaCO3=8.59).T3barite at Twitya Spring probably forms by reaction of barium adsorbed to(and/or released from)microbial cells and EPS with sulphate derived by oxidation of sulphide in pore/mat waters,organic sulphur compounds,and biogenic elemental sulphur(cf.Stoner et al.,1994;Glamoclija et al., 2004;Sanchez-Moral et al.,2004).T3barite crystals nucleated on EPS-free microbes are subhedral,whereas barite crystals formed on and in microbial EPS are anhedral.This suggests that EPS may prevent development of crystalline barite habits by retarding diffusion of ions to growing crystal faces(cf. Buczynski and Chafetz,1991;Arp et al.,1999).Organic acids and phosphates present in degrading biomass also inhibit barite crystal growth(Smith et al.,2004)and may be partly responsible for the anhedral and subhedral T3barite forms.

T3barite is localized at laminar boundaries in the travertine, suggesting that microbial colonization of the travertine mound takes place when calcite dendrites are not growing.This is somewhat intuitive because dendrites grow very quickly and might outpace the rate at which microbial mats could form over them,and because,as dendritic crystals grow only at nucleation sites on branch tips,the presence of microbial mats may impede their dendritic crystal growth(Jones and Renaut,1995).Twitya Spring is located just south of the Arctic Circle and experien-ces wide seasonal variation in temperature and incident light, which re almost certainly translated into variations in micro-bial growth rate.In summertime,microbes are abundant in the Twitya flow path(Fig.1C).In fall,dropping temperatures and decreased daylight hours likely reduce microbial growth (cf.Suganuma,1928;Jones et al.,2005).At northern latitudes, lowest spring water discharge rates and calcite precipitation rates are manifest during fall–winter conditions(van Ever-dingen,1972;Kano et al.,2003).In spring,the introduction of snowmelt to aquifer conduits can produce temporary increases in spring water volume that produce increased turbulence in the spring flow path(van Everdingen,1972).Increased turbulence favours both rapid CO2degassing and spring water oxidation, which,at Twitya Spring,would promote precipitation of ba-rium-enriched dendritic calcite in zone4(cf.Jones et al.,2005) and T1barite crystals in zones1and2.

It is tempting to interpret T2and T3barite as fall and summer precipitates,respectively—T2precipitating in fall when both dendrite growth and microbial growth wane,and T3precipitating in the summer when reduced dendrite growth allows microbes to colonize the travertine mound.Growth laminations within and between calcite dendrite beds indicate,however,that the Twitya flow path is also subject to episodic,subseasonal physiochemical variation.Detrital clay bands,for example,may represent rainfall or snowmelt events during which detrital sediment washed onto the surface of the travertine mound(cf.Kano et al.,2004). Flushing of the travertine by meteoric water may also be responsible for dissolution growth bands(cf.Pentecost,2005),

48S.M.Bonny,B.Jones/Sedimentary Geology203(2008)36–53

and bands rich in organic carbon may represent brief interludes

where microbial mat growth outpaced calcite dendrite growth

during spring/summer transitional periods.Animal trampling,

frost wedging,and dissolution of calcite by humic acids are other

stresses that affect and complicate interpretation of seasonal

precipitation patterns at Twitya Spring.

6.Discussion

Barite's precipitation behaviour is commonly assumed to

be controlled by inorganic physiochemical variables(Hanor,

2000);indeed,its solubility is so low that thermodynamic mo-

dels predict barite supersaturation when even trace quantities of

barium and sulphate come into contact(SOLMINEQ88).In

natural solutions,however,barite precipitation is commonly

inhibited by the presence of other divalent metal ions,dissolved

sulphide,carbonate,phosphate,and/or organic acids(Lindgren,

1933;Hamm and Merritt,1944;Davis and Collins,1971;Bolze

et al.,1974;Smith et al.,2004).Oxidation of barium-bearing

solutions is also commonly paired to other physiochemical

changes,such as cooling,degassing,and changes in pressure or

hydrodynamics,which may induce precipitation of other mine-

ral phases(Greinert et al.,2002;Canet et al.,2005).

Significant amounts of barium can be accommodated in

carbonate,phosphate,and hydroxide minerals by substitution

(Younger,1986;Tesoiero and Pankow,1996;Kastner,1999).In

barium-limited solutions,therefore,barite precipitation may

require not only the presence of sulphate,but also a means to

concentrate barium to a level at which it can form as a separate

mineral phase(Younger,1986;Bonny and Jones,2007b).At

Twitya Spring,T1barite precipitates in solution,but adsorption

of barium to calcite surfaces catalyzes precipitation of T2barite,

and T3barite forms only in association with microbes that are

capable of adsorbing and bioaccumulating barium.

Microbially mediated barite precipitation has only recently

come under investigation(Gonzalez-Munoz et al.,2003;

Glamoclija et al.,2004;Sanchez-Moral et al.,2004;Senko

et al.,2004;Bonny and Jones,2007b),but the presence of ‘thermodynamically unstable’barite in diverse natural environ-ments indicates that it may be more important than thermo-

dynamic models predict(Riedinger et al.,2006).Microbes do not

catalyze‘impossible’precipitation,but establish microgradients

around their cells,through surface adsorption reactions,

bioaccumulation,and/or metabolic processes,that support ther-

modynamically favourable microenvironments for mineral

precipitation(Aharon,2000).Barite is commonly co-deposited

with microbially mediated carbonate minerals in marine and

lacustrine sediments and at microbially colonized methane

seeps,hydrothermal vents,and subaerial springs(Tazaki et al.,

1997;Arenas et al.,2000;Burhan et al.,2000;Torres et al.,

2003;Canet et al.,2005;Schwartz,2005)but biological

influences on barite crystal habit and distribution have received

little attention.This is due partly to the traditional assumption

that barite precipitation is not microbially mediated,and partly to

the logistic difficulties involved in studying diagenetic,methane

seep,and hydrothermal vent minerals in situ(Kastner,1999;

Aharon,2000).In contrast,subaerial springs are more accessible venues for the examination of barite precipitation and preserva-tion in carbonate.

Euhdral barite crystals similar to T1and T2barite from Twitya Spring are reported as accessory minerals in many continental and shallow marine spring deposits.They are ge-nerally found as tabular or intergrown platy barite microcrys-tals nucleated on or trapped in volumetrically dominant calcite, and/or silica(Borneuf,1983;Younger,1986;Arenas et al.,2000; Canet et al.,2005;Jones and Renaut,2006).Microbially mediated barites have also been reported from at least two other continental carbonate-precipitating springs,Stinking Springs in Utah and Doughty Spring in Colorado.At Stinking Springs, barite precipitation among volumetrically dominant calcite is mediated by bioaccumulation of barium in diatoms(Bonny and Jones,2007b).At Doughty Spring,barium is co-precipitated in calcite,locally forming a mixed phase barytocalcite((Ba,Ca) CO3),but forms a pure barium sulphate phase around barium-enriched cyanobacterial cells(Younger,1986).As at Twitya Spring,microbially mediated barites at Stinking Springs and Doughty Springs are preserved in carbonates together with morphologically distinct inorganic barium phases(Cadigan and Felmlee,1977;Younger,1986;Bonny and Jones,2007b).

Although conditions at subaerial springs are quite different from the marine and lacustrine environments where most sedi-mentary barite forms,it is nonetheless revealing that multiple precipitation mechanisms operate in the spring flow paths, producing morphologically distinct barite in overlapping phy-siochemical niches.In dysoxic marine sediments and around exhalative vents where sulphur and methane-metabolizing mi-crobes buffer both barite and carbonate solubility(Larkin and Henk,1996;Alfonso et al.,2005),biogenic and inorganic barite microcrystals are also likely to form in spatial proximity (Greinert et al.,2002).If microbially mediated barite crystals formed in bottom water environments are anhedral to subhedral in the5–10μm range,like those formed at Twitya Spring, Doughty Springs(Younger,1986),and Stinking Springs (Bonny and Jones,2007b),they may be difficult to distinguish from‘biogenic’barite crystals formed in ocean surface waters and deposited as particulate barite‘rain’(Dehairs et al.,1980; Bishop,1988;Paytan et al.,2002;Kiipli et al.,2004),and from small hydrothermal barite crystals(Bertine and Keene,1975;Fu et al.,1994;Shikazono,1994;Torres et al.,2003).Examination of substrate associations,and petrographic distributions should, however,allow primary,authigenic and diagenetic barite crys-tals to be distinguished(Arenas et al.,2000),suggesting a potential for microbially mediated barites to serve as biomar-kers in carbonates of low metamorphic grade.

As an example,Mesozoic carbonates in the Betic Cordillera, Southern Spain,have been found to contain diagenetic barite crystal clusters and veins,as well as dispersed poorly crystalline barites≤5μm long(Martin-Algarra and Sanchez-Navas, 2000).Scanning electron and electron microprobe analysis of the dispersed barite showed a consistent association with desiccated EPS structures,and more rarely with filamentous microfossils,indicating that they formed through‘bacterial’authigenesis,and can be considered biomarkers formed during very early diagenesis(Martin-Agarra and Sanchez-Navas,

S.M.Bonny,B.Jones/Sedimentary Geology203(2008)36–53

2000).Barite microcrystals found in Neoproterozoic carbonates are also commonly associated with preserved organic matter (Buick et al.,1981),and haveδ34S signatures indicative of precipitation from a microbially fractionated sulphur pool (Shields et al.,2007).Many of the Neoproteroizoic‘cap carbonates’contain barite-hosting calcite crystal‘fans’,com-monly interpreted as psuedomorphs after primary aragonite fans and/or dendrites precipitated on the seafloor(Grotzinger and James,2000).Interpretation of inorganic and biological influences on these crystal fans(which is important to assessing the biologic viability of Earth during one of its most intense glacial periods)has relied heavily upon modern analogues formed in microbially colonised hot spring flow paths(e.g. Frasier and Corsetti,2003).Interpretation of barite microcrys-tals hosted in these Neoproterozoic carbonates has not, however,incorporated nonthermodynamic variables found to affect the distribution of barite at subaerial springs(e.g.Walter and Bauld,1983;Hoffmann and Schrag,2002).

Limits placed on barite nucleation by the growth rate of co-deposited calcite at Twitya Spring and the importance of barium bioaccumulation at Stinking,Flybye and Doughty Spring (Younger,1986;Bonny and Jones,2007b)are not accommo-dated by thermodynamic or predictive adsorption precipitation models(e.g.SOLMINEQ88,1988;Langmuir,1997;Fein et al., 2001),and remain unconstrained for palaeoenvironmental reconstruction in modern(Finlay et al.,1983;Kiipli et al., 2004;Riedinger et al.,2006)and ancient(Hoffmann and Schrag,2002;Huston and Logan,2004;Shields et al.,2007) despositional environments.It is well established,however,that microbially mediated precipitates can record distinct isotopic signals(Paytan et al.,2002)and commonly have higher levels of co-precipitated trace elements than‘inorganic’precipitates formed in the same ambient depositional environment(Paytan et al.,2002;Wentworth et al.,2003).Distinguishing between abiotic and microbially mediated barite microcrystals may thus be important not only for establishing evidence of a biological presence in ancient rocks,but for ensuring accurate interpreta-tion of geochemical data retrieved from barite(cf.Jewell,2000; Bottrell and Newton,2006).Thermal springs like Twitya Spring,where barite is hosted in carbonate travertine,provide important modern analogues for examining barite formation and preservation in carbonate sediments;it is our hope that this paper will prompt further examination of barite associated with modern carbonates,to yield a more comprehensive dataset for the interpretation of barite microcrystals in the geologic record.

7.Conclusions

Controls on the distribution and habit of barite crystals precipitated in the flow path of Twitya Spring include(1)spring water oxidation state,(2)rate of CO2-degassing,(3)availability of adsorptive substrates,and(4)abundance of microbes and associated EPS:

1.T1platy barite crystals precipitate in solution in the upper

spring flow path as a result of atmospheric oxidation of dissolved sulphide to sulphate.2.CO2-degassing drives precipitation of calcite in the lower

Twitya Spring flow path,producing laminated travertine.

The travertine is dominantly composed of rapidly precipi-tated calcite dendrites that are barium-enriched relative to infilling calcite cements.Incorporation of barium into calcite may inhibit barite precipitation during springtime surges in calcite dendrite growth.

3.Precipitation of barite in solution is inhibited in the lower

Twitya Spring flow path,such that barite nucleation requires adsorption of barium to a substrate.T2tabular and rhombohedral barite crystals nucleate on calcite crystal faces, whereas T3barite forms in association with microbial biomass.

4.T3barite crystals formed among EPS-rich microbes are

anhedral,whereas subhedral T3barite crystals form on and around EPS-free filaments.

At Twitya Spring,T1,T2and T3barite are morphologically distinct,but form and/or are deposited in overlapping physiochemical niches whose boundaries are continually adjusted by seasonal and episodic variations in spring water flow,microbial growth,and calcite precipitation rate.Although this study confirms that specific precipitation mechanisms produce distinct barite morphologies,it also reveals that similar-sized microbially mediated and inorganic barites can be co-deposited in microbially colonized sediments,indicating that close attention to petrographic textures,and precipitation substrates,will be required to resolve barite microcrystals of distinct origin in the geologic record. Acknowledgements

This project was funded by the Department of Indian and Northern Affairs Northern Science Training Program,the Alberta Ingenuity Fund(studentship200300215to Sandy Bonny),and the National Science and Engineering Research Council of Canada(grant A6090to Brian Jones;CGS PhD Scholarship to Sandy Bonny),and conducted under a permit issued by the Aurora Research Institute(License#13590N). Special thanks are extended to Richard Popko of the Sahtu Department of Environment and Natural Resources for guiding us to the Twitya Spring site,Robin Renaut for his assistance with SOLEMINEQ88,George Braybrook and Sergei Matveev for providing invaluable assistance on the SEM and EMP, respectively,and Dustin Rainey for his help in the field.Drs. Chafetz and Fielding provided valuable feedback on an earlier version of this manuscript.Fig.1A is adapted from an image provided by the Aurora Research Institute,Aurora College, Inuvik,NWT,Canada.

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on the contrary的解析

On the contrary Onthecontrary, I have not yet begun. 正好相反，我还没有开始。 https://www.wendangku.net/doc/164951266.html, Onthecontrary, the instructions have been damaged. 反之，则说明已经损坏。 https://www.wendangku.net/doc/164951266.html, Onthecontrary, I understand all too well. 恰恰相反，我很清楚 https://www.wendangku.net/doc/164951266.html, Onthecontrary, I think this is good. ⑴我反而觉得这是好事。 https://www.wendangku.net/doc/164951266.html, Onthecontrary, I have tons of things to do 正相反，我有一大堆事要做 Provided by jukuu Is likely onthecontrary I in works for you 反倒像是我在为你们工作 https://www.wendangku.net/doc/164951266.html, Onthecontrary, or to buy the first good. 反之还是先买的好。 https://www.wendangku.net/doc/164951266.html, Onthecontrary, it is typically american. 相反，这正是典型的美国风格。 222.35.143.196 Onthecontrary, very exciting.

恰恰相反，非常刺激。 https://www.wendangku.net/doc/164951266.html, But onthecontrary, lazy. 却恰恰相反，懒洋洋的。 https://www.wendangku.net/doc/164951266.html, Onthecontrary, I hate it! 恰恰相反，我不喜欢！ https://www.wendangku.net/doc/164951266.html, Onthecontrary, the club gathers every month. 相反，俱乐部每个月都聚会。 https://www.wendangku.net/doc/164951266.html, Onthecontrary, I'm going to work harder. 我反而将更努力工作。 https://www.wendangku.net/doc/164951266.html, Onthecontrary, his demeanor is easy and nonchalant. 相反，他的举止轻松而无动于衷。 https://www.wendangku.net/doc/164951266.html, Too much nutrition onthecontrary can not be absorbed through skin. 太过营养了反而皮肤吸收不了. https://www.wendangku.net/doc/164951266.html, Onthecontrary, I would wish for it no other way. 正相反，我正希望这样 Provided by jukuu Onthecontrary most likely pathological. 反之很有可能是病理性的。 https://www.wendangku.net/doc/164951266.html, Onthecontrary, it will appear clumsy. 反之，就会显得粗笨。 https://www.wendangku.net/doc/164951266.html,

英语造句

一般过去式时间状语：yesterday just now (刚刚) the day before three days ag0 a week ago in 1880 last month last year 1. I was in the classroom yesterday. I was not in the classroom yesterday. Were you in the classroom yesterday. 2. They went to see the film the day before. Did they go to see the film the day before. They did go to see the film the day before. 3. The man beat his wife yesterday. The man didn’t beat his wife yesterday. 4. I was a high student three years ago. 5. She became a teacher in 2009. 6. They began to study english a week ago 7. My mother brought a book from Canada last year. 8.My parents build a house to me four years ago . 9.He was husband ago. She was a cooker last mouth. My father was in the Xinjiang half a year ago. 10.My grandfather was a famer six years ago. 11.He burned in 1991

学生造句--Unit 1

●I wonder if it’s because I have been at school for so long that I’ve grown so crazy about going home. ●It is because she wasn’t well that she fell far behind her classmates this semester. ●I can well remember that there was a time when I took it for granted that friends should do everything for me. ●In order to make a difference to society, they spent almost all of their spare time in raising money for the charity. ●It’s no pleasure eating at school any longer because the food is not so tasty as that at home. ●He happened to be hit by a new idea when he was walking along the riverbank. ●I wonder if I can cope with stressful situations in life independently. ●It is because I take things for granted that I make so many mistakes. ●The treasure is so rare that a growing number of people are looking for it. ●He picks on the weak mn in order that we may pay attention to him. ●It’s no pleasure being disturbed whena I settle down to my work. ●I can well remember that when I was a child, I always made mistakes on purpose for fun. ●It’s no pleasure accompany her hanging out on the street on such a rainy day. ●I can well remember that there was a time when I threw my whole self into study in order to live up to my parents’ expectation and enter my dream university. ●I can well remember that she stuck with me all the time and helped me regain my confidence during my tough time five years ago. ●It is because he makes it a priority to study that he always gets good grades. ●I wonder if we should abandon this idea because there is no point in doing so. ●I wonder if it was because I ate ice-cream that I had an upset student this morning. ●It is because she refused to die that she became incredibly successful. ●She is so considerate that many of us turn to her for comfort. ●I can well remember that once I underestimated the power of words and hurt my friend. ●He works extremely hard in order to live up to his expectations. ●I happened to see a butterfly settle on the beautiful flower. ●It’s no pleasure making fun of others. ●It was the first time in the new semester that I had burned the midnight oil to study. ●It’s no pleasure taking everything into account when you long to have the relaxing life. ●I wonder if it was because he abandoned himself to despair that he was killed in a car accident when he was driving. ●Jack is always picking on younger children in order to show off his power. ●It is because he always burns the midnight oil that he oversleeps sometimes. ●I happened to find some pictures to do with my grandfather when I was going through the drawer. ●It was because I didn’t dare look at the failure face to face that I failed again. ●I tell my friend that failure is not scary in order that she can rebound from failure. ●I throw my whole self to study in order to pass the final exam. ●It was the first time that I had made a speech in public and enjoyed the thunder of applause. ●Alice happened to be on the street when a UFO landed right in front of her. ●It was the first time that I had kept myself open and talked sincerely with my parents. ●It was a beautiful sunny day. The weather was so comfortable that I settled myself into the

英语句子结构和造句

高中英语~词性~句子成分~语法构成第一章节：英语句子中的词性 1.名词：n. 名词是指事物的名称，在句子中主要作主语.宾语.表语.同位语。 2.形容词;adj. 形容词是指对名词进行修饰~限定~描述~的成份，主要作定语.表语.。形容词在汉语中是（的）.其标志是: ous. Al .ful .ive。. 3.动词：vt. 动词是指主语发出的一个动作，一般用来作谓语。 4.副词：adv. 副词是指表示动作发生的地点. 时间. 条件. 方式. 原因. 目的. 结果.伴随让步. 一般用来修饰动词. 形容词。副词在汉语中是（地）.其标志是：ly。 5.代词：pron. 代词是指用来代替名词的词，名词所能担任的作用，代词也同样.代词主要用来作主语. 宾语. 表语. 同位语。 6.介词：prep.介词是指表示动词和名次关系的词，例如：in on at of about with for to。其特征：

介词后的动词要用—ing形式。介词加代词时，代词要用宾格。例如：give up her（him）这种形式是正确的，而give up she（he）这种形式是错误的。 7.冠词：冠词是指修饰名词，表名词泛指或特指。冠词有a an the 。 8.叹词：叹词表示一种语气。例如：OH. Ya 等 9.连词：连词是指连接两个并列的成分，这两个并列的成分可以是两个词也可以是两个句子。例如：and but or so 。 10.数词：数词是指表示数量关系词，一般分为基数词和序数词第二章节：英语句子成分主语：动作的发出者，一般放在动词前或句首。由名词. 代词. 数词. 不定时. 动名词. 或从句充当。谓语：指主语发出来的动作，只能由动词充当，一般紧跟在主语后面。宾语：指动作的承受着，一般由代词. 名词. 数词. 不定时. 动名词. 或从句充当. 介词后面的成分也叫介词宾语。定语：只对名词起限定修饰的成分，一般由形容

六级单词解析造句记忆MNO

M A: Has the case been closed yet? B: No, the magistrate still needs to decide the outcome. magistrate n.地方行政官，地方法官，治安官 A: I am unable to read the small print in the book. B: It seems you need to magnify it. magnify vt.１．放大，扩大；２．夸大，夸张 A: That was a terrible storm. B: Indeed, but it is too early to determine the magnitude of the damage. magnitude n.１．重要性，重大；２．巨大，广大 A: A young fair maiden like you shouldn’t be single. B: That is because I am a young fair independent maiden. maiden n.少女，年轻姑娘，未婚女子 a.首次的，初次的 A: You look majestic sitting on that high chair. B: Yes, I am pretending to be the king! majestic a.雄伟的，壮丽的，庄严的，高贵的 A: Please cook me dinner now. B: Yes, your majesty, I’m at your service. majesty n.１．［Ｍ－］陛下（对帝王，王后的尊称）；２．雄伟，壮丽，庄严 A: Doctor, I traveled to Africa and I think I caught malaria. B: Did you take any medicine as a precaution? malaria n.疟疾 A: I hate you! B: Why are you so full of malice? malice n.恶意，怨恨 A: I’m afraid that the test results have come back and your lump is malignant. B: That means it’s serious, doesn’t it, doctor? malignant a.１．恶性的，致命的；２．恶意的，恶毒的 A: I’m going shopping in the mall this afternoon, want to join me? B: No, thanks, I have plans already. mall n.（由许多商店组成的）购物中心 A: That child looks very unhealthy. B: Yes, he does not have enough to eat. He is suffering from malnutrition.

base on的例句

意见应以事实为根据. 3 来自辞典例句 192. The bombers swooped ( down ) onthe air base. 轰炸机突袭空军基地. 来自辞典例句 193. He mounted their engines on a rubber base. 他把他们的发动机装在一个橡胶垫座上. 14 来自辞典例句 194. The column stands on a narrow base. 柱子竖立在狭窄的地基上. 14 来自辞典例句 195. When one stretched it, it looked like grey flakes on the carvas base. 你要是把它摊直, 看上去就象好一些灰色的粉片落在帆布底子上. 18 来自辞典例句 196. Economic growth and human well - being depend on the natural resource base that supports all living systems. 经济增长和人类的福利依赖于支持所有生命系统的自然资源. 12 1 来自辞典例句 197. The base was just a smudge onthe untouched hundred - mile coast of Manila Bay. 那基地只是马尼拉湾一百英里长安然无恙的海岸线上一个硝烟滚滚的污点. 6 来自辞典例句 198. You can't base an operation on the presumption that miracles are going to happen. 你不能把行动计划建筑在可能出现奇迹的假想基础上.

英语造句大全

英语造句大全English sentence 在句子中，更好的记忆单词！ 1、(1)、able adj. 能句子：We are able to live under the sea in the future. (2)、ability n. 能力句子：Most school care for children of different abilities. (3)、enable v. 使。。。能句子：This pass enables me to travel half-price on trains. 2、(1)、accurate adj. 精确的句子：We must have the accurate calculation. (2)、accurately adv. 精确地句子：His calculation is accurately. 3、(1)、act v. 扮演句子：He act the interesting character. （2）、actor n. 演员句子：He was a famous actor. (3)、actress n. 女演员句子：She was a famous actress. (4)、active adj. 积极的句子：He is an active boy. 4、add v. 加句子：He adds a little sugar in the milk. 5、advantage n. 优势句子：His advantage is fight. 6、age 年龄n. 句子：His age is 15. 7、amusing 娱人的adj. 句子：This story is amusing. 8、angry 生气的adj. 句子：He is angry. 9、America 美国n.

(完整版)主谓造句

主语+谓语 1. 理解主谓结构 1) The students arrived. The students arrived at the park. 2) They are listening. They are listening to the music. 3) The disaster happened. 2．体会状语的位置 1) Tom always works hard. 2) Sometimes I go to the park at weekends.. 3) The girl cries very often. 4) We seldom come here. The disaster happened to the poor family. 3. 多个状语的排列次序 1) He works. 2) He works hard. 3) He always works hard. 4) He always works hard in the company. 5) He always works hard in the company recently. 6) He always works hard in the company recently because he wants to get promoted. 4. 写作常用不及物动词 1. ache My head aches. I’m aching all over. 2. agree agree with sb. about sth. agree to do sth. 3. apologize to sb. for sth. 4. appear (at the meeting, on the screen) 5. arrive at / in 6. belong to 7. chat with sb. about sth. 8. come (to …) 9. cry 10. dance 11. depend on /upon 12. die 13. fall 14. go to … 15. graduate from 16. … happen 17. laugh 18. listen to... 19. live 20. rise 21. sit 22. smile 23. swim 24. stay (at home / in a hotel) 25. work 26. wait for 汉译英： 1.昨天我去了电影院。 2.我能用英语跟外国人自由交谈。 3.晚上7点我们到达了机场。 4.暑假就要到了。 5.现在很多老人独自居住。 6.老师同意了。 7.刚才发生了一场车祸。 8.课上我们应该认真听讲。9. 我们的态度很重要。 10. 能否成功取决于你的态度。 11. 能取得多大进步取决于你付出多少努力。 12. 这个木桶能盛多少水取决于最短的一块板子的长度。

初中英语造句

【it's time to和it's time for】 ——————这其实是一个句型,只不过后面要跟不同的东西. ——————It's time to跟的是不定式（to do）.也就是说,要跟一个动词,意思是“到做某事的时候了”.如： It's time to go home. It's time to tell him the truth. ——————It's time for 跟的是名词.也就是说,不能跟动词.如： It's time for lunch.(没必要说It's time to have lunch) It's time for class.(没必要说It's time to begin the class.) They can't wait to see you Please ask liming to study tonight. Please ask liming not to play computer games tonight. Don’t make/let me to smoke I can hear/see you dance at the stage You had better go to bed early. You had better not watch tv It’s better to go to bed early It’s best to run in the morning I am enjoy running with music. With 表伴随听音乐 I already finish studying You should keep working. You should keep on studying English Keep calm and carry on 保持冷静继续前行二战开始前英国皇家政府制造的海报名字 I have to go on studying I feel like I am flying I have to stop playing computer games and stop to go home now I forget/remember to finish my homework. I forget/remember cleaning the classroom We keep/percent/stop him from eating more chips I prefer orange to apple I prefer to walk rather than run I used to sing when I was young What’s wrong with you There have nothing to do with you I am so busy studying You are too young to na?ve I am so tired that I have to go to bed early

The Kite Runner-美句摘抄及造句

《The Kite Runner》追风筝的人--------------------------------美句摘抄 1.I can still see Hassan up on that tree, sunlight flickering through the leaves on his almost perfectly round face, a face like a Chinese doll chiseled from hardwood: his flat, broad nose and slanting, narrow eyes like bamboo leaves, eyes that looked, depending on the light, gold, green even sapphire 翻译：我依然能记得哈桑坐在树上的样子，阳光穿过叶子，照着他那浑圆的脸庞。他的脸很像木头刻成的中国娃娃，鼻子大而扁平，双眼眯斜如同竹叶，在不同光线下会显现出金色、绿色，甚至是宝石蓝。 E.g.: A shadow of disquiet flickering over his face. 2.Never told that the mirror, like shooting walnuts at the neighbor's dog, was always my idea. 翻译：从来不提镜子、用胡桃射狗其实都是我的鬼主意。E.g.:His secret died with him, for he never told anyone. 3.We would sit across from each other on a pair of high

翻译加造句

一、翻译 1. The idea of consciously seeking out a special title was new to me., but not without appeal. 让我自己挑选自己最喜欢的书籍这个有意思的想法真的对我具有吸引力。 2.I was plunged into the aching tragedy of the Holocaust, the extraordinary clash of good, represented by the one decent man, and evil. 我陷入到大屠杀悲剧的痛苦之中，一个体面的人所代表的善与恶的猛烈冲击之中。 3.I was astonished by the the great power a novel could contain. I lacked the vocabulary to translate my feelings into words. 我被这部小说所包含的巨大能量感到震惊。我无法用语言来表达我的感情（心情）。 4，make sth. long to short长话短说 5.I learned that summer that reading was not the innocent(简单的) pastime(消遣) I have assumed it to be., not a breezy, instantly forgettable escape in the hammock(吊床)，( though I’ ve enjoyed many of those too ). I discovered that a book, if it arrives at the right moment, in the proper season, will change the course of all that follows. 那年夏天，我懂得了读书不是我认为的简单的娱乐消遣，也不只是躺在吊床上，一阵风吹过就忘记的消遣。我发现如果在适宜的时间、合适的季节读一本书的话，他将能改变一个人以后的人生道路。二、词组造句 1. on purpose 特意，故意 This is especially true here, and it was ~. (这一点在这里尤其准确，并且他是故意的) 2.think up 虚构，编造，想出 She has thought up a good idea. 她想出了一个好的主意。 His story was thought up. 他的故事是编出来的。 3. in the meantime 与此同时助记：in advance 事前in the meantime 与此同时in place 适当地... In the meantime, what can you do? 在这期间您能做什么呢？ In the meantime, we may not know how it works, but we know that it works. 在此期间，我们不知道它是如何工作的，但我们知道，它的确在发挥作用。 4.as though 好像，仿佛 It sounds as though you enjoyed Great wall. 这听起来好像你喜欢长城。 5. plunge into 使陷入 He plunged the room into darkness by switching off the light. 他把灯一关，房

改写句子练习2标准答案

The effective sentences:(improve the sentences!) 1.She hopes to spend this holiday either in Shanghai or in Suzhou. 2.Showing/to show sincerity and to keep/keeping promises are the basic requirements of a real friend. 3.I want to know the space of this house and when it was built. I want to know how big this house is and when it was built. I want to know the space of this house and the building time of the house. 4.In the past ten years,Mr.Smith has been a waiter,a tour guide,and taught English. In the past ten years,Mr.Smith has been a waiter,a tour guide,and an English teacher. 5.They are sweeping the floor wearing masks. They are sweeping the floor by wearing masks. wearing masks,They are sweeping the floor. 6.the drivers are told to drive carefully on the radio. the drivers are told on the radio to drive carefully 7.I almost spent two hours on this exercises. I spent almost two hours on this exercises. 8.Checking carefully,a serious mistake was found in the design. Checking carefully,I found a serious mistake in the design.

用以下短语造句

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英语造句

English sentence 1、(1)、able adj. 能句子：We are able to live under the sea in the future. (2)、ability n. 能力句子：Most school care for children of different abilities. (3)、enable v. 使。。。能句子：This pass enables me to travel half-price on trains. 2、(1)、accurate adj. 精确的句子：We must have the accurate calculation. (2)、accurately adv. 精确地句子：His calculation is accurately. 3、(1)、act v. 扮演句子：He act the interesting character.（2）、actor n. 演员句子：He was a famous actor. (3)、actress n. 女演员句子：She was a famous actress. (4)、active adj. 积极的句子：He is an active boy. 4、add v. 加句子：He adds a little sugar in the milk. 5、advantage n. 优势句子：His advantage is fight. 6、age 年龄n. 句子：His age is 15. 7、amusing 娱人的adj. 句子：This story is amusing. 8、angry 生气的adj. 句子：He is angry. 9、America 美国n. 句子：He is in America. 10、appear 出现v. He appears in this place. 11. artist 艺术家n. He is an artist. 12. attract 吸引 He attracts the dog. 13. Australia 澳大利亚 He is in Australia. 14.base 基地 She is in the base now. 15.basket 篮子 His basket is nice. 16.beautiful 美丽的 She is very beautiful. 17.begin 开始 He begins writing. 18.black 黑色的 He is black. 19.bright 明亮的 His eyes are bright. 20.good 好的 He is good at basketball. 21.British 英国人 He is British. 22.building 建造物 The building is highest in this city 23.busy 忙的 He is busy now. 24.calculate 计算 He calculates this test well. 25.Canada 加拿大 He borns in Canada. 26.care 照顾 He cared she yesterday. 27.certain 无疑的 They are certain to succeed. 28.change 改变 He changes the system. 29.chemical 化学药品