biogenicity of silica precipitation around geysers and hot spring vents north island new zealand abstract before anthropogenic modifications ohaaki pool broadlands ohaaki and dragon s mouth geyser wairakei emitted waters at temperatures of 93 98 c the siliceous sinter that precipitated around their vents has the characteristics of geyserite a dense laminated deposit of presumed abiogenic origin that was precipitated from waters too hot 73 c to support microbes other than thermophilic bacteria petrographic and sem examinations of the sinters that surround the vent of dragon s mouth geyser and form the pool rim dam of ohaaki pool show that they incorporate columnar stromatolites and silicified laminated stromatolitic mats that contain well preserved filamentous microbes at both localities the microbes lack evidence of desiccation or shrinkage which implies that they were silicified rapidly at or shortly after their death microbe preservation is commonly spectacular even the trichome is recognizable in many specimens although replacement by silica preserved the cells of the trichome cell walls and septae were not preserved possibly because they were destroyed by autolysis before silicification the sheath that surrounded the trichome is normally well preserved however late silica precipitation produced thick external encrustations of amorphous silica around many sheaths although boiling and very hot 90 c waters were discharged temperatures at many sites surrounding the vents remained sufficiently low and moist to support a microbial community that included thermophilic bacteria and cyanobacteria in these cooler niches the microbes and their biofilms served as highly favorable templates for the nucleation and growth of amorphous silica and collectively provided a microbial framework for the laminated accretionary sinter silica was supplied from supersaturated spring and geyser fluids that were delivered to sites of sinter precipitation as air cooled spray as proximal runoff or as the result of oscillations in the surface level of the spring or geyser pool waters in the field and in hand specimen the resulting columnar sinter appear to be identical in morphology to that inferred to originate from abiotic silica precipitation in a sterile environment consequently some columnar spicular and stratiform geyserites are probably not abiotic precipitates but are true silica stromatolites

microstructure of high temperature 73 c siliceous sinter deposited around hot springs and geysers yellowstone national park the role of biological and abiological processes in sedimentation slightly alkaline hot springs and geysers in yellowstone national park exhibit distinctive assemblages of high temperature 73 c siliceous sinter reflecting local hydrodynamic conditions the main depositional zones include subaqueous pool and channel bottoms and intermittently wetted subaerial splash surge and overflow areas subaqueous deposits include particulate siliceous sediment and dendritic and microbial silica framework silica framework forms thin porous microbe rich films coating subaqueous surces spicules with intervening narrow crevices dominate in splash zones surge and overflow deposits include pool and channel rims columns and knobs in thin section subaerial sinter is composed of i dark brown nearly opaque laminated sinter deposited on surfaces that evaporate to dryness ii clear translucent silica deposited subaqueously through precipitation driven by supersaturation iii heterogeneous silica representing silica encrusted microbial filaments and detritus and iv sinter debris brownish laminations form the framework of most sinter deposited in surge and overflow zones pits and cavities are common architectural features of subaerial sinter and show concave upward pseudo cross laminations and micro unconformities developed through migration marked birefringence of silica deposited on surfaces that evaporate to dryness is probably a strain effect repeated wetting and evaporation often to dryness and capillary effects control the deposition morphology and microstructure of most high temperature sinter outside of the fully subaqueous zone microbial filaments are abundant on and within high temperature sinter but do not provide the main controls on morphology or structuring except in biofilms developed on subaqueous surfaces millimetre scale lamination cyclicity in much high temperature sinter represents annual layering and regular seasonal fluctuations in silica sedimentation

impact of cyclic freezing on precipitation of silica in me sio2 h2o systems and geochemical implications for cryosoils and sediments abstract cyclic freezing of aqueous solutions may cause a significant precipitation of amorphous silica where about 90 mol of the primary dissolved silicic acid can be fixed in analogy to evaporation h 2 o molecules are separated from aqueous solutions by freezing and supersaturation with respect to amorphous silica can be reached the results show that higher proportions of silica precipitated at lower input si oh 4 concentration and at increasing numbers of freeze thaw cycles additionally dissolved metal ions me favour the formation of amorphous silica in the sequence of na 2 2 2 2 this may be mostly related to a decrease of amorphous silica solubility in the high concentrated aqueous solutions which remained after ice formation co precipitation yields me si ratios of the depositions between 1 and 4 re dissolution of the primary amorphous precipitates is incomplete accordingly such solids are suitable precursor materials for neoformation of silicates as secondary authigenic silicates like zeolites may be formed from such precursors it is suggested that their occurrence and exposure in cryosoils and sediments might be used as a proxy indicator of past climate cyclic freezing can significantly reduce the geochemical and ecological availability of silica on the other side the decrease of dissolved silica induced by freezing provokes an increase of undersaturation with respect to silicates in the re thawed soil or interstitial solutions which stimulates the dissolution of siliceous minerals and rocks

silicon isotope fractionation during silica precipitation from hot spring waters evidence from the geysir geothermal field iceland this study aims to explore the extent and controls of silicon isotope fractionation in hot spring systems of the geysir geothermal area iceland a setting where sinter deposits are actively formed the 30si values of dissolved silica measured in the spring water and sampling sites along outflowing streams covering a temperature range between 20 and 100 c were relatively constant around 0 2 whereas the 30si signatures of associated opaline sinters from the streambeds were between 0 1 and 4 0 becoming progressively more negative in the downstream parts of the aprons here the deposited sinters represent some of the most 30si depleted abiotically produced terrestrial materials documented to date compared to the data reported for icelandic basalts considered to be the source of the silicon the 30si values of the fluids and sinter deposits are higher and lower respectively the resulting values for apparent solid water isotope fractionation 30sisolid water decreased with decreasing temperature from ca 0 7 at 80 c to 3 7 at 20 c locally down to 4 4 this temperature relationship was reproducible in each of the investigated hot spring systems and is qualitatively consistent with recent findings in laboratory experiments on kinetic fractionation for a flowing fluid however the apparent fractionation magnitudes observed in the field are ca 2 more negative and thus significantly larger we infer that solid water silicon isotope fractionation during deposition of amorphous silica from a flowing fluid correlates inversely with temperature but is essentially a function of the precipitation rate such that the fractionation factor decreases with increasing rate as an important corollary the effective fractionation behavior during precipitation of silica from saturated solutions is a system dependent feature which should be taken into account when using silicon isotopes for paleo environmental reconstructions

microstructural changes accompanying the opal a to opal ct transition new evidence from the siliceous sinters of geysir haukadalur iceland opaline silica opal a has formed in marine lacustrine and geothermal environments throughout geological time during diagenesis opal a normally changes to opal ct then opal c and finally to quartz such changes commonly destroy the original fabrics and any fossils that opal a contained the physical changes that accompany the opal a to opal ct transition however are known poorly x ray diffraction analyses electron microprobe analyses and high resolution high magnification scanning electron microscope imagery of siliceous sinters from the geysir geothermal area in iceland show that opal a is formed of heterometric arrays of randomly packed microspheres up to 5 m diameter with neighbouring spheres commonly being joined by small connection pads in contrast enlarged spheres lepispheres inverse opal two types and spindle frameworks with hexagonal motifs characterize opal ct the textures in opal ct which vary on a microscale reflect the complex interplay between dissolution e g inverse opal and precipitation e g enlarged spheres spindle frameworks that probably was mediated by groundwater in a near surface environment the processes deciphered from these young rocks should however be applicable to sedimentary opal a and opal ct of all ages irrespective of their origin

rapid precipitation of silica opal a disguises evidence of biogenicity in high temperature geothermal deposits case study from dagunguo hot spring china abstract dagunguo spring located in the tengchong geothermal area in the western part of yunnan province china is a very active spring with water temperatures of 78 to 97 c and ph of 7 7 to 8 8 the vent pool 5 6 m in diameter and up to 1 5 m deep is lined with opal a that was precipitated from the near boiling spring waters a glass suspended in the pool was coated with opal a in two months and two pvc pipes that drained water from the pool in late 2010 became lined with opal a precipitates in less than three months the opal a accumulated at rates of 0 5 to 0 75 mm month in the spring pool and 2 5 to 3 5 mm month in the pvc pipes the opal a precipitates irrespective of where they developed are formed primarily of silicified microbes and opal a spheres along with minor amounts of native sulfur detrital quartz and clay mainly kaolinite the fabrics in these opal a deposits were dictated largely by the growth patterns of the filamentous and rod shaped microbes that dominate this low diversity biota and the amount of opal a that was precipitated around them many of the microbes were preserved as rapid opal a was precipitated on and around them before the cells decayed with continued precipitation however the microbes became quickly engulfed in the opal a precipitates and morphological evidence of their presence was lost in essence the process that controls their preservation ultimately disguised them to the point where cannot be seen critically this loss of morphological identity takes places even before opal a starts its diagenetic transformation towards quartz

siliceous shrubs in hot springs from yellowstone national park wyoming u s a many of the siliceous hot springs in yellowstone national park contain subaqueous spinose siliceous precipitates up to 5 cm high that occupy shallow terracettes in siliceous terraced mound accumulations discharge channels etc these siliceous shrubs are composed of opal a with an arborescent or branching pattern and have strong morphological similarities to bacterial shrubs from carbonate precipitating hot springs siliceous shrubs constitute a major precipitate style associated with discharge channel flow path facies throughout most of the 20 m of flow path at cistern spring norris geyser basin they are found in siliceous spring waters ranging in temperature from 76 4 to 16 2 c and ph from 6 0 to 7 4 at every scale siliceous shrubs contain abundant evidence of microbial life in the form of bacterial body fossils and extracellular polymeric substances the presence of relict organic constituents and bacterial morphological fossils indicates that the shrub fabric and architecture are dominated by bacteria i e there is potentially a strong biotic effect on the precipitation process precipitation of opal in siliceous shrubs is very likely the result of either active bacterially induced precipitation or passive mediation through organic templates on a larger scale siliceous shrubs contain abundant evidence of former microbial activity in hot springs thus they are good microbial biomarkers resume plusieurs sources thermales siliceuses de yellowstone contiennent des precipites siliceux subaquatiques et en forme d aiguilles jusqu a 5 cm de haut qui occupent de petites terrasses peu profondes dans des accumulations en forme de monticules en terrasses dans les chenaux d ecoulement etc ces arbrisseaux siliceux sont composes d opale a avec un patron arborescent ou a branches et ont de fortes similitudes morphologiques aux arbrisseaux de bacteries de sources thermales precipitant du carbonate les arbrisseaux siliceux constituent un style commun de precipitation associe au facies de chenal de decharge chemin d ecoulement a travers la plus grande partie des 20 m du chemin d ecoulement a la source cistern du bassin du geyser norris ils se retrouvent dans les eaux de sources siliceuses dont la temperature varie de 76 4 a 16 2 oc avec un ph variant de 6 0 a 7 4 a toutes les echelles les arbrisseaux siliceux contiennent de nombreuses evidences de vie microbienne sous la forme de fossiles de corps bacteriens et de substances extracellulaires polymeriques la presence de constituants organiques reliques et de fossiles morphologiques bacteriens indique que la texture et l architecture des arbrisseaux sont dominees par les bacteries c a d qu i lyau n fortpotentiel d un effet biotique sur le processus de precipitation la precipitation de l opale dans les arbrisseaux siliceux est tres probablement le resultat d une precipitation active induite par les bacteries ou d une aide passive a travers des gabarits organiques a une plus grande echelle les arbrisseaux siliceux contiennent de nombreuses evidences d activite microbienne anterieure dans les sources thermales ils forment donc de bons marqueurs biologiques traduit par la redaction guidry and chafetz 1583

the use of mixing models and chemical geothermometers for estimating underground temperatures in geothermal systems abstract application of various chemical geothermometers and mixing models indicate underground temperatures of 260 c 280 c and 265 c in the geysir hveravellir and landmannalaugar geothermal fields in iceland respectively mixing of the hot water with cold water occurs in the upflow zones of all these geothermal systems linear relations between chloride boron and 18o constitute the main evidence for mixing which is further substantiated by chloride silica and sulphate relations in the geysir and hveravellir fields a new carbonate silica mixing model is proposed which is useful in distinguishing boiled and non boiled geothermal waters this model can also be used to estimate underground temperatures using data from warm springs this model as well as the chloride enthalpy model and the na li and co2 gas geothermometers invariably yield similar results as the quartz geothermometer sometimes also does by contrast the na k and the na k ca geothermometers yield low values in the case of boiling hot springs largely due to loss of potassium from solution in the upflow the results of these geothermometers are unreliable for mixed waters due to leaching subsequent to mixing

co2 clathrate destabilization a new model of formation for molar tooth structures abstract molar tooth structures mt are unusual and enigmatic temporally restricted calcite bleb and sheet like structures hosted almost exclusively in mesoproterozoic and neoproterozoic continental ramp and shelf carbonates characteristically mt are sharply bounded interconnected often ptygmatically folded or broken structures filled with uniform fine blocky calcite but with a distinct lack of detrital infill collectively their sedimentological characteristics indicate that the calcite filled channels were not open to the sediment water interface and that they were an early possibly pre diagenetic feature several models have been proposed for their formation involving various physical chemical and biological processes two models are presently favoured gas expansion j sediment res 68 1998 104 and seismic shaking geol soc am bull 110 1998 1028 but the temporal restriction of these sedimentological features to the pre phanerozoic suggests that unique atmospheric or biospheric conditions at this time in the earth s evolution may be responsible for mt generation and or preservation this paper proposes a new hypothesis for the origin of deeper water mt that invokes the decomposition of co2 clathrates in the ramp carbonate sediment column we suggest that the higher co2 partial pressures thought to be prevalent in the proterozoic atmosphere allow for the stabilization of co2 clathrates in the sediment column not unlike the methane clathrates observed in the seafloor environment today it is further postulated that the dramatic increase in molar volume associated with clathrate breakdown opens cracks and voids in the host sediment and the precipitation of calcite associated with the breakdown of the co2 clathrates results in rapid but not instantaneous infill and therefore subsequent preservation of mt in this model the decomposition of the co2 clathrates may have been triggered by or in turn have triggered seismic shock waves the collapse of the co2 clathrates could have expelled co2 gas forming pressurized voids in the sediment which minimized clastic infill the released co2 could have combined with seawater to precipitate caco3 thus this model complements both the gas bubble expansion and seismic shaking models and can explain some but not all of the many diagnostic characteristics and occurrences of these intensely studied and oft debated structures especially those formed in deeper cooler waters 13 c and 18 o depletions in mt relative to the immediate host rock are consistent with mt formation at lower temperature than the host rock deposition the host rocks have previously been interpreted to form above storm wave base outside the stability field of clathrate however the exact water depth at which the mt were formed is ambiguous the cooler temperatures of mt formation indicated by the stable isotope data are consistent with deeper water and higher pressure or lower temperature conditions which may bring the sediments to within the co2 clathrate stability field

cryogenic opal a deposition from yellowstone hot springs sub zero winter temperatures on the yellowstone plateau alter the opal a precipitation pathway offluid erupting from hot springs and geysers frozen fluid often only meters from boiling pools contains abundant opal a particles comprising sheet and filamentlike aggregations of opal a microspheres which are formed by opal a precipitation in brine pockets channels and veins by natural cryogelling unconsolidated cryogenic opal a sediment accumulates in and below water ice where it is locked until spring thaw conditions sediment is then either remobilized contributing large volumes of opal a particulate to geothermally influenced wetlands or becomes adhered in situ by dehydration and cementation this strongly seasonal opal a precipitation regime has been overlooked in investigations of sinter deposition accretion rates and microbe mineral interactions natural opal a textures recorded fromyellowstonemaybereplicatedsimplybyfreezingandthawingsyntheticsilica saltsolutioninthelaboratory cryogenicprocess may have influenced mineral precipitation and sediment accumulation in many other geothermal areas particularly active terrestrial springs located at high altitude latitude fossil systems influenced by ancient glaciations plus potential astrobiological targets e g mars and europa 2007 elsevier b v all rights reserved

cryogenic silicification of microorganisms in hydrothermal fluids silica rich hydrothermal fluids that experience freezing temperatures precipitate cryogenic opal a coa within ice bound brine channels we investigated cryogenic silicification as a novel preservation pathway for chemo and photo lithotrophic bacteria and archaea we find that the co partitioning of microbial cells and silica into brine channels causes microorganisms to become fossilised in coa rod and coccoidal form bacteria and archaea produce numerous cell casts on coa particle surfaces while chloroflexus filaments are preserved inside particle interiors coa particles precipitated from natural icelandic hot spring fluids possess similar biomorphic casts including those containing intact microbial cells biomolecules and inorganic metabolic products are also captured by coa precipitation and are detectable with a combination of visible shortwave infrared reflectance ftir and raman spectroscopy we identify cryogenic silicification as a newly described mechanism by which microbial biosignatures can be preserved within silica rich hydrothermal environments this work has implications for the interpretation of biosignatures in relic hydrothermal settings and for life detection on mars and enceladus where opaline silica indicative of hydrothermal activity has been detected and freezing surface conditions predominate

effect of diagenetic phase transformation on the silicon isotope composition of opaline sinter deposits of geysir iceland detectible 30si variations in present day chemical silica deposits have stimulated the application of silicon isotopes to infer environmental conditions from ancient equivalents interpretations of 30si signatures remain problematic in view of potential post depositional changes of which magnitudes and underlying mechanisms are largely unknown a critical issue in the interpretation of isotope data from cherts concerns the extent to which early diagenetic processes modify original 30si signatures here we report 30si variations in opal a opal a ct and opal ct from fossil sinter deposits in an active discharge apron in the geysir geothermal field iceland opal a samples show an average 30si of 0 7 0 2 while opal ct samples are isotopically lighter with an average 30si of 2 0 0 4 implying a sizable shift of approximately 1 3 between the different phases this shift can be explained by repetitive dissolution re precipitation processes diffusive transport or temperature differences during phase transitions on average the fossil opal a tends to be less negative in 30si than amorphous silica that recently precipitated from the hydrothermal water the difference can be attributed to primary variability in isotopic fractionation that accompanies precipitation out of spring water at the surface or to a post depositional release of surface 28si at the onset of diagenetic formation our results corroborate the perception that original silicon isotope signatures of silica acquired during chemical precipitation from a saturated solution may not be preserved in the geological record and that post depositional changes must be taken into account when interpreting data from ancient chert deposits

life cycle of a geyser discharge apron evidence from waikite geyser whakarewarewa geothermal area north island new zealand abstract waikite a geyser located in the whakarewarewa geothermal area on the north island new zealand has a history of eruptive dormancy cycles that have been attributed to natural and anthropogenic causes the last cycle involved an active period from 1932 to 1968 69 that was followed by a period of dormancy that continues today such cycles are important because they control the temporal development of the discharge apron when the geyser is active growth of the discharge apron is dictated by the precipitation of opal a which is controlled by factors such as discharge patterns water chemistry ph temperature rate of cooling and the resident microbiota with dormancy conditions change radically because water no longer flows down the discharge apron instead the discharge apron lapses into a phase of degradation that on waikite is evident from 1 deflation of the apron surface 2 blocks splaying off the apron margins along margin parallel fractures 3 tension fractures 4 saucer shaped collapse zones 5 increasingly unstable surfaces resulting from subsurface opal a dissolution 6 fractures from which steam and other gases emanate and 7 incursion of native vegetation around the edge of the apron and on the distal parts of the discharge apron when the geyser becomes active again silica precipitation will resume and the discharge apron will once again accrete vertically and expand laterally analysis of the waikite system shows that successions that develop on geyser discharge aprons are formed of unconformity bounded packages of sinter that reflect the eruptive dormancy history of the parent geyser

impact of seasonal changes on the formation and accumulation of soft siliceous sediments on the discharge apron of geysir iceland abstract unconsolidated highly porous permeable siliceous sediments found in shallow water pools on the middle and distal parts of geysir s discharge apron are formed of cryogenic opal a coa opal a spheres opal a cement silicified microbes and detrital lithic grains silicified leaves and silica coated grass stems and twigs are locally present with these pool sediments and microbial mats commonly cover the sediment surface partially consolidated layers 0 5 cm thick form when the constituent grains are bound together by filamentous microbes exopolysaccharides and or opal a cement most components of the sediment can be related to processes that are controlled largely by seasonal variations low temperatures in winter promote freezing of the medial to distal discharge waters and lead to formation of coa in ice when the ice melts the released coa grains become part of the pool sediments microbial mats trap and bind sediment and provide templates for silica precipitation during spring and summer but become semi dormant during the constant darkness that iceland experiences during winter when the waters are sufficiently supersaturated opal a precipitates as spheres in the water column and cement in pool deposits in some pools seasonal changes are evident from the alternation of soft and hard partly cemented laminae elsewhere however the distinction between the different time dependent components is lost because they become superimposed on each other or as aperiodic changes in flow patterns affect different pools

the effect of solution chemistries and freezing temperatures on the morphology of cryogenic opal a coa implications for past climates on mars abstract cryogenic opal a coa is a type of hydrated amorphous silica that forms when silica rich geothermal fluids erupt sub aerially and are exposed to air temperatures below 0 c as the fluid cools and water ice forms the solution increases in ionic strength and reaches saturation of opal a this results in the precipitation of opal a between water ice crystals natural coa is found embedded within frozen fluids near hydrothermal sources and has been shown to preserve signs of biological activity in this study two solution chemistries hydrochloric acid hcl and sulfuric acid h2so4 and three freezing temperatures 20 80 196 c were used to synthesize coa in the lab sem analyses showed that coa frozen at 20 c in hcl solutions produced smooth silica branches similar to what was observed in previous studies coa precipitated at lower freezing temperatures demonstrated progressively smaller particle sizes and unique morphologies such as halite dendrites within silica desiccation cracks and pores coa made in h2so4 solutions was more angular and fragmented than coa made in hcl solutions particle size also decreased with decreasing freezing temperature coa textures have implications for terrestrial samples as well as future samples returned from mars because they can provide information about past climate and fluid chemistry plain language summary cryogenic opal a coa is a hydrous silicate material that can form when geysers and hot springs erupt and are exposed to air temperatures below 0 c coa forms in brine veins that are between water ice crystals and takes the shape of the veins in which they form coa has been shown to preserve signs of biological activity in this study two solution chemistries hydrochloric acid hcl and sulfuric acid h2so4 and three freezing temperatures 20 80 196 c were used to form coa in the lab scanning electron microscope analyses showed that in general coa particle size decreased with decreasing freezing temperature additionally coa made in hcl solutions were smooth and tubular whereas coa made in h2so4 solution were angular and fragmented coa textures have implications for terrestrial samples as well as future samples returned from mars because they can provide information about past climate and fluid chemistry

increased continental margin slumping frequency during sea level lowstands above gas hydrate bearing sediments we present sup 14 c data on sediment samples from cores of the upper 7 m of the sediment column overlying a major continental rise gas hydrate field on the southern carolina rise and inner blake ridge offshore the southeastern united states the data show that glacial age sediments are underrepresented in the cores the observation is consistent with a previously predicted association between sea level lowstands and increased frequency of sea floor slumping on continental margins containing gas hydrates 26 refs 3 figs

genesis of large siliceous stromatolites at frying pan lake waimangu geothermal field north island new zealand lilypad stromatolites up to 3 m long and 1ae5 m wide were found to be actively growing in the shallow marginal waters of frying pan lake and its outflow channel these stromatolites composed of phormidium 90 fischerella and a variety of other microbes develop through a series of distinct growth stages dark green microbial mats cover the floor of the outflow channel and give rise to columns of various sizes and shapes in the shallower marginal waters once the columns reach the water level the mats spread laterally to form a lilypad stromatolite the lilypads are characterized by a raised dark green rim 4 5 mm high that encircles a flat interior covered with a distinctive orange red mat the microbes forming the columns and lilypad plate are being actively silicified the stromatolites are formed of i flat lying phormidium filaments p laminae ii upright filaments of phormidium that are commonly associated with fischerella u laminae and iii mucus diatoms and pyrite framboids m laminae p laminae dominate most of the columns with tripartite cycles of p u to m laminae being found mostly in the upper parts of the stromatolites the transition from the p to u laminae is marked by a change in the growth pattern of the phormidium and branching of fischerella which was probably triggered by a change in environmental conditions in the frying pan lake outflow channel this change may be related to fluctuations in water level and flow rates that are caused by periods of heavy rain seasonal changes long term variations in rainfall and or the unique 40 day hydrological cycle that exists between frying pan lake and inferno crater which is a nearby hydrothermal crater lake

the geological history of geysir iceland a tephrochronological approach to the dating of sinter the vertical succession through the discharge apron of geysir up to 3 5 m thick is formed of five lithological units a e units a b c and e are formed of siliceous sinters whereas unit d is a brown ash bed tephra grains in units c d and e came from katla hekla and vei ivotn katla tephra in unit d has been largely altered to palagonite integration of tephrochronological ages historical records and geological and geomorphological relationships shows that the geysir succession developed in four main phases during phase i c 10 000 4000 years ago laminated sinters formed from hot spring waters phase ii initiated c 3300 years ago saw the termination of sinter formation and blanketing of the area with tephra from katla and hekla phase iii c 3000 900 years ago represented by the boundary between units d and e was characterized by weathering and little sinter deposition phase iv was initiated with the re birth of geysir 800 900 years ago sinter deposited from the discharged waters has however been largely confined to the southern part of the discharge apron only limited opal a precipitation has occurred since 1916 because of a decline in geysir9s eruptions

a geothermally influenced wetland containing unconsolidated geochemical sediments hot spring waters flowing from porkchop geyser norris geyser basin yellowstone national park usa enter a shallow wetland basin and precipitate opal a silica particulate particulate formation by chemical rather than biochemical and colloidal mechanisms is suggested by floc and shard like particle morphologies comprising opal a silica nanospheres and microspheres of colloidal dimensions and precipitation from waters with opaque milky blue colouration indicative of aqueous silica sol conditions sediment accumulates in the wetland at a rate of ca 20 25 mm year is unconsolidated and massive to diffusely bedded to laminated post depositional features include soft sediment deformation and scouring and in drying conditions relatively deep desiccation establishment of geochemically dominated wetland sedimentation is favoured where alkali chloride hot spring fluids of circum neutral to basic ph and high silica concentration discharge to and cool to ca 35 c within topographic depressions that receive only small volumes of non hot spring water local wetland vegetation which is composed of hydrophytes halophytes and alkali tolerant species more typical of coastal wetlands colonizes the soft wetland substrate and may be relatively quickly buried by rapid sediment accumulation prior to the evolution of the diatom silica sink geothermal wetlands containing geochemically precipitated silica sediments may have been much more common and widespread rhizoliths chert nodules with organic cores scour fabrics soft sediment deformation desiccation cracks and massive to diffuse bedding preserved in palaeozoic geothermal environments may all be evidence of ancient unconsolidated geochemical sediments and geothermal wetland conditions

relationship between spring and geyser activity and the deposition and morphology of high temperature 73 c siliceous sinter yellowstone national park wyoming u s a abstract six classes of alkaline hot springs and geysers are here distinguished in yellowstone national park based on activity and eruptive style 1 non surging non boiling springs 2 gently surging non boiling springs 3 non surging to gently surging boiling springs 4 strongly surging springs 5 fountain geysers and 6 cone geysers each displays morphologically distinctive siliceous sinter deposits in the high temperature 73 c parts of the vent and outflow systems non surging and gently surging non boiling springs that do not overflow their rims develop thin digitate to lilypad like sinter rims that grow horizontally over the pool surfaces similar springs with rim overflow have rims that accrete vertically boiling springs often have nearly circular steep sided pools and rims dominated by splash and rim overflow and composed mainly of spicular and columnar sinter fountain geysers characterized by broad fountain like eruption bursts have a central vent surrounded by a zone of spicular sinter beyond which is a broad flat bottomed moat but lack steep sinter cones cone geysers have narrow vents jet like discharges that cool rapidly and fall close to the vent and steep sided spicule dominated sinter cones sinter deposition takes place within three main zones 1 fully subaqueous sides and bottoms of vent pools outflow basins and outflow channels 2 periodically wetted splash zones surrounding hot springs and geysers and 3 largely subaerial parts of the rim and outflow areas wetted mainly by overflow and surging and that dry quickly fully subaqueous surfaces are covered by fine earthy particulate siliceous sediment pss composed largely of amorphous silica spherules and spherule aggregates that appear to have precipitated around heterogeneous nuclei including microbes splash zones of hot springs and geysers are characterized by spicular sinter largely subaerial zones wetted mainly by rim overflow and surging with little or no splash show complex varieties of terraced columnar and knobby sinter the highest silica deposition rates occur where surfaces are wetted frequently and evaporate to dryness such as the tops of spicules columns and knobs associated lower areas and crevices that remain wet or moist show slow accumulation of pss most continuously wet surfaces in high temperature 73 c hot spring and geyser vent and outflow systems are probably covered by microbial biofilms although microbes may locally influence rates of silica deposition and sinter microstructure the results of this study suggest that the depositional sites morphology and macrostructuring of high temperature sinter are controlled largely by the processes of surface wetting evaporation and drying

geothermal mineralization i the mechanism of formation of the beowawe nevada siliceous sinter deposit the siliceous sinter deposit at beowawe nev has a volume of 7 71 x 10 sup 7 m sup 3 and contains 1 28 x 10 sup 11 kg silica this silica precipitated from geothermal solutions that welled up along the malpais fault a graben bounding fault sinter formation involved three steps 1 geothermal solutions saturated with quartz in the reservoir moved to the surface where they cooled and became supersaturated with amorphous silica 2 amorphous silica particles nucleated heterogeneously to produce a colloidal suspension 3 the amorphous silica particles agglomerated and were cemented together by amorphous silica precipitating in the embayments between the particles at least 5 58 x 10 sup 14 kg of geothermal solution must have been produced over time to account for the mass of silica in the beowawe deposit this fluid would have carried at least 4 95 x 10 sup 17 kj of heat to the surface 21 references 6 figures 1 table

authigenic chabazite and implications for weathering in sirius group diamictite table mountain dry valleys antarctica abstract petrographic examination shows authigenic chabazite and calcite in the ice free or weathering horizon of glacially deposited diamictites in the sirius group on table mountain in the dry valleys area of antarctica some samples contain as much as 18 chabazite which fills pores in matrix clays these pores are thought to have formed by repeated cycles of freezing and thawing during a warmer and wetter climate than the present the chabazite is sodium rich and has an average composition of ca0 9na1 9k0 2 al3 9si8o24 6h2o the presence of authigenic minerals in the sirius suggests that chemical weathering is active and may take place in frozen ground along interfacial films of brine precipitation of the minerals is likely to occur at the boundary between the ice free and ice cemented horizons as the ice cement sublimates the film of brine becomes more concentrated until precipitation occurs in the process of efflorescence precipitation is kinetically driven by sublimation allowing the crystals to grow in the open pore space of the ice free horizon just above the ice cement because the depth to the ice cement boundary fluctuates in response to major climatic changes authigenic minerals that mark the position of this boundary may be a proxy indicator of past climates

guide to gas hydrate stability in various geological settings abstract the presence of clathrates gas hydrates is well established observationally in the offshore region of the south caspian basin as well as in many other regions of the world hydrates of fixed composition can exist under particular pressure temperature conditions however under the impact of neotectonic processes those conditions can change part or all of the mass of hydrates can then be dissociated and released as gas and water the dissociation can take place gradually or explosively depending on how fast the pressure drops or temperature increases five major variations of hydrate evolution are considered to illustrate possible patterns of behavior caused by variation of geological conditions 1 variations in hydrate existence conditions due to sediment and mud redeposition earthquakes volcanic eruptions and other processes can release part of the sediments from a sloping sea bottom and transport them to be redeposited in other places thereby changing pressure conditions at the top of a hydrate layer 2 variations in hydrate existence conditions due to glacial interglacial conditions removal of ice cover not only decreases overlying pressure but also allows water temperatures overlying the sediments to increase 3 variations in hydrates due to sea level rise and fall 4 enrichment of ethane in hydrates as a consequence of varying neotectonic conditions 5 evaporation and reformation of hydrates in aeolian conditions due to winter cooling and summer heating hydrates can be a potential explosive hazard for exploration rigs production platforms and pipelines especially in deep water conditions these geological patterns of instability should therefore be considered when potential hazards are assessed 1998 elsevier science ltd all rights reserved

hot spring and geyser sinters the integrated product of precipitation replacement and deposition complex ornate sinter deposits are found in many hot spring and geysers systems throughout the world including those located in the taupo volcanic zone on the north island of new zealand those sinters are formed of opal a that replaced microbes opal a precipitated as cement accessory minerals e g kaolinite jarosite calcite biological detritus e g leaves wood pollen grains and lithic detritus the opal a is compositionally variable because of the amount of water oh and h2o and in some cases accessory elements e g au ag bound into its structure the composition and fabric of the siliceous sinter found at any locality reflect the relative balance among the processes of replacement precipitation and deposition the microbes that inhabit these systems are of critical importance because they are commonly replaced by and or encrusted with opal a in many settings copious amounts of opal a are precipitated as cement around the frameworks of silicified filaments the cementation pro

actively growing siliceous oncoids in the waiotapu geothermal area north island new zealand oncoids that are actively growing in some of the shallow water pools around champagne pool waiotapu new zealand are formed of amorphous silica opal a with minor amounts of native sulphur the growth of these oncoids is being mediated by a high diversity microbial biota that achieves optimal growth in waters that have a temperature of 35 42 c and ph of 6 6 6 9 although this biota is dominated by coccoid bacilliform and filamentous cyanobacteria sulphur oxidizing bacteria are common in the cortical laminae of some oncoids in addition diatoms and silicified pollen grains are present in some cortical laminae the silicified microbes are superbly preserved with their three dimensional form and some of their internal structures being apparent including the sites of sulphur globules in the sulphur oxidizing bacteria the microbes probably acted as templates for silica precipitation and thereby mediated the growth of the oncoids some cortical laminae are formed entirely of silicified pollen grains that came from the pinus radiata that grow in the exotic pine plantations around champagne pool by using these laminae as a record of the annual pollination event typically in september november it can be shown that many of the oncoids are

intrinsic versus extrinsic controls on the development of calcite dendrite bushes shuzhishi spring rehai geothermal area tengchong yunnan province china abstract in the rehai geothermal area located near tengchong there is an old succession of crystalline calcite that formed from a spring that is no longer active the thin bedded succession exposed on the south bank of zaotang river is formed of three dimensional dendrite bushes that are up to 6 cm high and 3 cm in diameter with multiple levels of branching bedding is defined by color which ranges from white to gray to almost black and locally accentuated by differential weathering that highlights the branching motif of the dendrites the succession developed through repeated tripartite growth cycles that involved phase i that was characterized by rapid vertical growth of the dendrite bushes with ever increasing branching phase ii that developed once growth of the dendrites had almost or totally ceased and involved an initial phase of etching that was followed by the precipitation of various secondary minerals sheet calcite trigonal calcite crystals hexagonal calcite crystals hexagonal plates formed of ca and p mn precipitates si mg reticulate coatings opal ct lepispheres on the branches of the calcite dendrites and phase iii that involved deposition of detrital quartz feldspar clay and calcite on top of the dendrite bushes the tripartite growth cycle is attributed primarily to aperiodic cycles in the co 2 content of the spring water that was controlled by subsurface igneous activity rather than climatic controls high co 2 coupled with rapid co 2 degassing triggered growth of the dendrite bushes as co 2 levels waned saturation levels in the spring water decreased and calcite dendrite growth ceased and precipitation of the secondary minerals took place possibly in the microcosms of microbial mats deposition of the detrital sediment was probably related to surface runoff that was triggered by periods of high rainfall critically this study shows that intrinsic factors rather than extrinsic factors e g climate were the prime control on the development of the tripartite growth cycle

microbial silica interactions in icelandic hot spring sinter possible analogues for some precambrian siliceous stromatolites silicified deposits such as sinters occur in several modern geothermal environments but the mechanisms of silicification and crucially the role of microorganisms in their construction are still largely unresolved detailed examination of siliceous sinter in particular sections of microstromatolites growing at the krisuvik hot spring iceland reveals that biomineralization contributes a major component to the overall structure with approximately half the sinter thickness attributed to silicified microorganisms almost all microorganisms observed under the scanning electron microscope sem are mineralized with epicellular silica ranging in thickness from 5 lm coatings on individual cells to regions where entire colonies are cemented together in an amorphous silica matrix tens of micrometres thick within the overall profile there appears to be two very distinct types of laminae that alternate repeatedly throughout the microstromatolite microbial layers are predominantly consisting of filamentous intact vertically aligned biomineralized cyanobacteria identified as calothrix and fischerella sp and weakly laminated silica layers which appear to be devoid of any microbial component the microbial layers commonly have a sharply defined base overlying the weakly laminated silica and a gradational upper surface merging into the weakly laminated silica these cyclic laminations are probably explained by variations in microbial activity active growth during spring summer allows the microorganisms to keep pace with silicification with the cell surfaces facilitating silicification while during their natural slow growth phase in the dark autumn winter months silicification exceeds the bacteria s ability to compensate i e grow upwards at this stage the microbial colony is probably not essential to microstromatolite formation with silicification presumably occurring abiogenically when conditions once again become favourable for growth recolonization of the solid silica surface by free living bacteria occurs cell motility is not responsible for the laminations we have also observed that microbial populations within the microstromatolite some several mm in depth appear viable i e they still have their pigmentation the trichomes are not collapsed cell walls are unbroken cytoplasm is still present and they proved culturable this suggests that the bulk of silicification occurred rapidly probably while the cells were still alive surprisingly however measurements of light transmittance through sections of the microstromatolite revealed that photosynthetically active light pal only transmitted through the uppermost 2 mm therefore the deeper microbial

experimental study of iron and silica immobilization by bacteria in mixed fe si systems implications for microbial silicification in hot springs the immobilization of silica and iron by the bacteria bacillus subtilis was monitored in controlled microcosms to elucidate the role iron may play in aiding bacterial silicification in hot springs silica and iron immobilization was monitored as a function of bacterial concentration iron concentration and silica concentration both undersaturated and oversaturated with respect to amorphous silica results demonstrate that bacterial cells do immobilize more fe than bacteria free systems in solutions with iron concentrations 50 ppm fe however as iron concentrations increase the difference between fe immobilization in bacterial and bacteria free systems decreases as non bacterially mediated precipitation processes dominate additionally bacterial systems that had immobilized more fe compared with bacteria free systems did not immobilize more silica than bacteria free systems by comparing molar ratios of silica in solution bacterially bound fe it is evident that insufficient iron is bound to the bacterial surface to act as an effective salt bridge for silica sorption this appears to be because much of the iron is immobilized by non bacterially mediated precipitation of phases such as fe oh 3 and poorly ordered hydrous iron silicates it follows that in silica enriched hot springs silica and iron immobilization processes are significantly dominated by non bacterially mediated precipitation any bacterially mediated processes are exceedingly small and outside the resolution of these experiments

seasonal changes in silica deposition in hot spring systems abstract monthly monitoring of water chemistry at octopus spring yellowstone national park reveals no changes in the chemical composition over this eight month study differences in silica deposition are attributed to flow dynamics and seasonal air temperatures colder air temperatures in winter promote visible accumulation of siliceous sinter probably as a result of decreased solubility periodic drying promotes accumulation and subsequent dehydration of the silica this combination of intermittent supply and periodic dehydration may account for the laminated siliceous deposits observed in the geological record the processes described herein are important in the siliceous preservation of microbial fossils but the following observations underscore the difficulty in preserving microbial fabrics in the geological record pools populated by cyanobacteria are bounded by margins built of silicified microbial mats silicification of the pool margins occurs through an evaporative process called wicking silica rich water is transported upward through the microbial mats by capillary action depositing silica at the surface of the mat as water evaporates the upper surface of such wicking deposits is extremely resistant to brecciation but the underlying support is highly friable and would be difficult to preserve intact in winter months temporary isolation of some pools from active flow causes fragile silica deposition around biological and other debris film coated gas bubbles on the surfaces of microbial mats are observed during all seasons and probably are filled with oxygen released during photosynthesis although this has not been confirmed they become coated with silica during winter months presumably because of increased cooling possibly forming thin discontinuous silica sheets as observed in the geological record