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

diversity of iron and silica precipitation by microbial mats in hydrothermal waters iceland implications for precambrian iron formations direct examination of microbial mats from icelandic hot springs with transmission electron microscopy and energy dispersive x ray spectroscopy revealed a consortium of bacterial cells in varying stages of mineralization differences in observed mineralogy largelyreflectdifferencesinthechemistryofthehydrothermalwaters silica richspheroids formedepicellularlyoncellwallsandsurroundingsheathsandcapsulesofmicroorganisms and insomecases intracellularlywhenpresumablythecell s hadlysed commonly these precipitateswereobservedcoalescingtoformamatrixofamorphoussilicathatcompletely encapsulated the cells and or replaced their cytoplasmic material however in other cells the precipitates were composed of amorphous granules made exclusively of iron and silica inapproximatelyequalproportions atonelocality thebacteriaformedseveralepicellular iron minerals ranging from iron mineralized capsules tofine grained spheroids of amorphous ferric hydroxide and acicular aggregates of goethite the complete encrustation of bacterial cells by silica iron or a combination of both may greatly enhance their preservation potential such that these mineralized microorganisms may conceivably represent future microfossils thus we may be witnessing contemporaneous biomineralization processes that are similar to those of the geologic past particularly with regard to the origin of some precambrian banded iron formations

depositional environment and mechanism of preservation of microfossils upper proterozoic bitter springs formation australia microfossils preserved in black chert nodules from the upper proterozoic bitter springs formation of australia represent the preserved remnants of cyanobacterial communities that inhabited a series of nonmarine saline lakes and ponds groundwaters of halite salinity pickled the microorganisms and thus inhibited their early bacterial degradation these same groundwaters were saturated in silica so that upon evaporation the silica and halite precipitated and formed a series of chert concretions nodules and bipyramidal quartz crystals where the chert nodules formed in areas of buried cyanobacterial mats further degradation of the buried sheaths and cells was arrested and the microorganisms were preserved

early middle triassic stromatolites from the luoping area yunnan province southwest china geobiologic features and environmental implications abstract an early middle triassic stromatolite deposit is documented from the guanling formation of the luoping area yunnan sw china the luoping stromatolite shows five types of constructional microbial forms in various magnifications 1 typical stratified columnar structures up to 20 cm high with crinkled laminae dark coloured laminae 1 mm thick are composed of upright filamentous tubes averagely 29 4 m in diameter showing a vertical growth fabric 2 laminoid fenestrae 0 5 1 mm wide and 3 prostrate filaments which are reflected by strong fluorescence in sharp contrast to dolomite cement in fluorescent images 4 rod like aggregates 4 6 18 m in diameter composed of minute dolomite rhombs are very common in stromatolitic laminae they resemble present day cyanobacterial trichome and thus may represent putative fossilized cyanobacteria 5 moreover small pits coccoid like spheroids calcified biofilms and fibrous structures are also common in stromatolitic laminae the last two may represent calcified extracellular polymeric substances eps that contributed to the development and lithification of stromatolites authigenic quartz grains are common and may also have involved biological processes in stromatolite formation of these microbial functional groups driving accretion and lithification processes of stromatolite documented in literature both lithified cyanobacteria oxygenic phototrophs and possible sulfate reducing bacteria srb which induced microbial formation of dolomite and contributed to the accretion of the luoping stromatolite are suggestive of biogenic origin the luoping stromatolite differs from the permian triassic boundary microbialites ptbms in having abundant filamentous structures and growing in an oxic marine environment both sedimentary facies analysis and abundant fossilized cyanobacteria may indicate proliferation of oxygenic phototrophs in a normal oxic habitat during the middle anisian early middle triassic a period when hospitable environments coupled with biotic diversification prevailed in south china and set an agenda for the full recovery of marine ecosystems in middle late anisian however the post extinction stromatolites and other anachronistic facies are not necessarily indicative of anoxic or oxic conditions and their environmental settings are much more complex than previously thought

metallic ion binding by bacillus subtilis implications for the fossilization of microorganisms the silicification of bacterial cells has been followed in a laboratory simulation by electron microscopy and energy dispersive x ray spectroscopy structural degradation of the bacteria was not influenced by silica availability but the remains of degraded cells did promote the precipitation of silica in contrast cells reacted with ferric iron before aging remained intact and structures conforming to the original morphology of the bacteria were preserved by silicification the binding of metallic ions by microbial cells in particular the retention of iron is therefore considered to be an important contributing factor to the fossilization of microorganisms

microfossils and possible microfossils from the early archean onverwacht group barberton mountain land south africa there is widespread textural evidence for microbial activity in the cherts of the early archean onverwacht group layers with fine carbonaceous laminations resembling fossil microbial mats are abundant in the cherty metasediments of the predominantly basaltic hooggenoeg and kromberg formations in rare cases filamentous microfossils are associated with the laminae the morphologies of the fossils as well as the texture of the encompassing laminae suggest an affinity to modern mat dwelling cyanobacteria or bacteria a variety of spheroidal and ellipsoidal structures present in cherts of the hooggenoeg and kromberg formations resemble modern coccoidal bacteria and bacterial structures including spores the development of spores may have enabled early microorganisms to survive the relatively harsh surficial conditions including the effects of very large meteorite impacts on the young earth

sublacustrine precipitation of hydrothermal silica in rift lakes evidence from lake baringo central kenya rift valley abstract many lakes in volcanic regions are fed by hot springs that in some basins can contribute a large percentage of the annual recharge especially during times of aridity it is important to recognize any contemporary hydrothermal contribution in paleoenvironmental reconstruction of lake basins because recharge from thermal waters can potentially confuse paleoclimatic signals preserved in the lacustrine sedimentary record hot spring deposits travertine sinter provide the most tangible evidence for thermal recharge to lakes although subaerial spring deposits have been widely studied lacustrine thermal spring deposits especially sublacustrine siliceous sinters remain poorly known detailed field petrographic and scanning electron microscope sem studies have been made of fossil sublacustrine sinter exposed at soro hot springs along the northeastern shoreline of ol kokwe a volcanic island in lake baringo kenya modern hot springs at soro which discharge na hco3 cl waters from a deep reservoir 180 c have thin 1 10 mm friable microbial silica crusts around their subaerial vents but thicker 1 cm sinter deposits are not forming the fossil sinter which is present as intergranular cements and crusts in littoral conglomerates and sandstones is composed mainly of opaline silica opal a three types of fossil sinter are recognized 1 massive structureless silica which fills intergranular pores and forms crusts up to 5 cm thick 2 pore lining silica some of which is isopachous and 3 laminated silica crusts which formed mainly on the upper surfaces of detrital particles all three types contain well preserved diatoms including lacustrine planktonic forms microbial remains mainly filamentous and coccoid bacteria including cyanobacteria and extracellular polymeric gels are locally abundant in the opaline silica together with detrital clays and thin laminae composed of authigenic chlorite most of the hydrothermal silica precipitated when the thermal springs were submerged by fresh lake water silica precipitated upon rapid cooling of thermal 90 c waters at and just below their interface with the overlying cooler 25 c lake waters microbial mats locally acted as a filter that limited mixing and rapid dilution of the thermal fluids some of the silica originally may have been soft and partly gelatinous planktonic diatoms and detrital clay rained down then became incorporated in the amorphous silica following a fall in lake level the opal a lithified and partly altered to cristobalite opal c and chalcedony the lowest fossil sinters were later encrusted by calcite stromatolites with calcite and quartz forming late pore filling cements the age of the sublacustrine sinters is unknown but some of the deposits could date back to the late pleistocene similar conglomerates cemented by hydrothermal silica are present along fault lines at neighbouring lake bogoria such rocks may provide evidence for deep hot fluid recharge to lakes when encountered in the geological record

origin of platy calcite crystals in hot spring deposits in the kenya rift valley abstract platy calcite crystals which have their c axis parallel to their shortest length axis are common components of travertine deposits found around some hot springs in the kenya rift valley they are composite crystals formed of numerous paper thin subcrystals individual plates allowed to grow without obstruction develop a hexagonal motif the kenyan crystals typically form in hot 75 c waters that have a low ca content 1 a high co2 content and a high rate of co2 degassing at chemurkeu aggregates of numerous small platy crystals collectively form lattice crystals that superficially resemble ray crystals the walls of the lattice crystals are formed of large platy crystals that have their long and intermediate length axes aligned parallel to the plane of the long axis of the lattice crystal internally the lattice crystals are formed of small platy calcite crystals arranged in a box like pattern that creates the appearance of a lattice when viewed in thin section lattice crystals are highly porous with each pore being enclosed by platy crystals at lorusio travertines are mainly formed of pseudodentrites that are constructed by numerous small platy crystals attached to a main stem which is a large platy crystal that commonly curves along its long axis the pseudodentrites are the main construction blocks in ledges and lilypads that form in the vent pool and spring outflow channels where the water is too hot for microbes other than hyperthermophiles the platy calcite crystals in the kenyan travertines are morphologically similar to platy calcite crystals that form as scale in pipes in the geothermal fields of new zealand and hydrothermal angel wing calcite from the la fe mine in mexico comparison of the kenyan and new zealand crystals indicates that platy calcite crystals form from waters with a low ca2 content and a high co3 ca ratio due to rapid rates of co2 degassing

rapid in situ silicification of microbes at loburu hot springs lake bogoria kenya rift valley microbial mats located along the margins of hot spring pools and outflow channels at lake bogoria kenya are commonly silicified forming friable laminated crusts columnar microstromatolites composed of silica and calcite are also forming at several springs in sites of oscillating water level or spray silicification of the microbes involves impregnation of organic tissue by very fine amorphous silica particles and encrustation by small 2 m silica spheroids rapid silicification of the microbes which may begin while some are still alive can preserve sheaths and in some examples the filaments capsules and cells although this provides evidence of their general morphology the biological features that are required for taxonomic identifications are commonly poorly preserved the silica precipitation results mainly from evaporative concentration and rapid cooling of spring waters that have been drawn upward through the mats and microstromatolites by capillary processes almost all the silica at the loburu springs nucleates on microbial substrates this affinity of silica for functional groups on microbial surfaces contributes to the rapid silicification of the microbes and their preservation in modern and ancient cherts

lake bogoria hot springs kenya geochemical features and geothermal implications abstract many boiling springs and fumaroles are present along the shores of lake bogoria which is a closed basin alkaline saline lake typical of african rifts two different geothermal waters both of na hco3 type have been recognized the first discharged by the boiling springs located along the western shores of the lake comes from a shallow steam heated thermal aquifer its temperature is close to 100 c as indicated by chalcedony solubility while the chloride content of these waters is slightly higher than 200 mg l 1 the second recognizable in the southernmost boiling springs is representative of a deeper and hotter geothermal reservoir the occurrence of mixing and boiling processes complicates the interpretation of geochemical data nevertheless a chloride content of about 660 mg l 1 an equilibrium temperature close to 170 c and a high carbon dioxide partial pressure at least 10 20 bar have been estimated for the deep geothermal reservoir

the role of microorganisms in phytokarst development on dolostones and limestones grand cayman british west indies phytokarst is spectacularly developed on the hard finely crystalline dolostones of the oligocene miocene bluff formation and to a lesser extent on the soft limestones of the pleistocene ironshore formation on grand cayman the black weathered surfaces which are a reflection of an organic coating contrast sharply with the white colour of the unaltered host rocks which lack an organic coating on the dolostones this organic coating is formed principally of fungal sporangia and spores associated with mucilage along with lesser numbers of algae and bacteria endolithic and epilithic filamentous microorganisms are rare on the limestones the organic coating is formed principally of endolithic and epilithic filaments and mucilage fungal spores are rare and no sporangia were found the limestones of the ironshore formation are characterized by an intensely bored zone beneath the weathering surface in contrast the dolostones of the bluff formation have relatively few endoliths penetrating from the w

evidence for amazonian northern mid latitude regional glacial landsystems on mars glacial flow models using gcm driven climate results and comparisons to geological observations a fretted valley system on mars located at the northern mid latitude dichotomy boundary contains lineated valley fill lvf with extensive flow like features interpreted to be glacial in origin we have modeled this deposit using glacial flow models linked to atmospheric general circulation models gcm for conditions consistent with the deposition of snow and ice in amounts sufficient to explain the interpreted glaciation in the first glacial flow model simulation sources were modeled in the alcoves only and were found to be consistent with the alpine valley glaciation interpretation for various environments of flow in the system these results supported the interpretation of the observed lvf deposits as resulting from initial ice accumulation in the alcoves accompanied by debris cover that led to advancing alpine glacial landsystems to the extent observed today with preservation of their flow texture and the underlying ice during downwasting in the waning stages of glaciation in the second glacial flow model simulation the regional accumulation patterns predicted by a gcm linked to simulation of a glacial period were used this glacial flow model simulation produced a much wider region of thick ice accumulation and significant glaciation on the plateaus and in the regional plains surrounding the dichotomy boundary deglaciation produced decreasing ice thicknesses with flow centered on the fretted valleys as plateaus lost ice scarps and cliffs of the valley and dichotomy boundary walls were exposed providing considerable potential for the production of a rock debris cover that could preserve the underlying ice and the surface flow patterns seen today in this model the lineated valley fill and lobate debris aprons were the product of final retreat and downwasting of a much larger regional glacial landsystem rather than representing the maximum extent of an alpine valley glacial landsystem these results favor the interpretation that periods of mid latitude glaciation were characterized by extensive plateau and plains ice cover rather than being restricted to alcoves and adjacent valleys and that the observed lineated valley fill and lobate debris aprons represent debris covered residual remnants of a once more extensive glaciation 2011 elsevier inc

diverse microstructures from archaean chert from the mount goldsworthy mount grant area pilbara craton western australia microfossils dubiofossils or pseudofossils abstract a diverse assemblage of indigenous carbonaceous microstructures classified here as highly probable microfossils to pseudomicrofossils is present in the ca 2 97 ga farrel quartzite gorge creek group at mount grant and mount goldsworthy pilbara craton western australia the microstructures are an integral part of the primary sedimentary fabrics preserved in black chert beds the interbedding of chert with layers of large silicified crystal pseudomorphs and fine to coarse grained volcaniclastic clastic beds indicate deposition in a partially evaporitic basin with terrigenous clastic and volcaniclastic input similar associations of microstructures are present at the same stratigraphic level in outcrops more than 2 km apart four major microstructural types are present thread like film like spheroidal and lenticular to spindle like each of which can be further subdivided into several sub types most of the microstructures were deposited as part of an assemblage of chemical and clastic sediments although there are some thread like microstructures present for which a synsedimentary origin cannot be confirmed many specimens appear to have originally had flexible but breakable walls and some occur in colony like aggregations size distributions for the four major types are generally narrow a feature typical of biogenic structures the microstructures are composed of disordered carbon as revealed by raman spectroscopy and the bulk isotopic composition of the carbon is 13 c the combined morphological and geological evidence suggests that the film like structures small spheres associated with films large spheroids and spindle like structures are probable to highly probable fossil remains of microorganisms the morphological variety among the microstructures suggests that a diverse microbial ecosystem flourished in the pilbara region during the archaean

formation of silica oncoids around geysers and hot springs at el tatio northern chile siliceous oncoids up to 4 cm in diameter are common on the laterally extensive sinter aprons that surround the spectacular geysers and hot springs at el tatio in northern chile many of these complex oncoids developed close to geyser and spring vents that discharge boiling water internally the oncoids which are composed of precipitated amorphous silica are formed of complex arrays of spicules and concentric laminae as well as detrital volcanic grains spicular growth is dominant in most examples the formation and growth of the spicules and concentric laminae were mediated by a microbial community which included filamentous microbes mucus and possibly bacteria the microbes and mucus were silicified by replacement and encrustation in some laminae the filamentous microbes lay parallel to the growth surface in other laminae most filaments forming the thin mats were suberect amorphous silica precipitated between the filaments occluded porosity and commonly disguised the microbial fabric the oncoids grew on the proximal sinter aprons around the geyser vents and hot spring pools most growth took place subaerially with the silica delivered to the precipitation sites by splashing water from the geysers and or periodic shallow flooding of the discharge aprons unlike silica oncoids at other geothermal sites vertical growth of oncoids that formed in some rimstone pools was not limited by water depth

source of silica and silicification of the lowermost cambrian yanjiahe formation in the three gorges area south china abstract the rise of siliceous organisms is generally considered to have driven the abrupt decrease of silica in the ocean during the ediacaran cambrian e c interval even if the exact timing and biological triggers of this event are still debated although ediacaran cambrian chert depositions are widely distributed multiple evidences converge to support their hydrothermal origin in this study we provide petrographic study supplemented by elemental concentration sem and raman spectral data on the siliceous rocks from the yanjiahe formation the three gorges area south china in order to ascertain the origin of included silica and discuss the involved processes of silica deposition contrary to most previous studies our analyses suggest that studied cherts mainly resulted from biogenic silica and subsidiary from abiotic precipitation influenced by surrounding hydrothermal activity and from detrital input the deposition model of silica includes direct deposition of siliceous bioclasts secondary replacement of carbonate by dissolved biosilica and microbial induced silicification from seawater the sink of silica recorded in the yanjiahe formation was characterized of a transition from typical precambrian to phanerozoic process in addition micropalaeontological and taphonomical analyses of the yanjiahe formation revealed two pulses of microfossil abundance and diversity in the fortunian and stage 2 respectively they can be correlated with a regression and an enrichment in dissolved phosphate that probably derived from enhanced detrital flux

the hot spring and geyser sinters of el tatio northern chile abstract the siliceous sinter deposits of el tatio geothermal field in northern chile have been examined petrographically and mineralogically these sinters consist of amorphous silica opal a deposited around hot springs and geysers from nearly neutral silica saturated sodium chloride waters water cooling and evaporation to dryness are the main processes that control the opal a deposition in both subaqueous and subaerial settings in close spatial relation to microbial communities all fingerprints of organisms observed in the studied sinter samples represent microbes and suggest that the microbial community is moderately diverse cyanobacteria green bacteria and diatoms the most important ecological parameter is the temperature gradient which is closely related to the observed depositional settings 1 geyser setting water temperature 70 86 c boiling point at el tatio 4200 m a s l coarse laminated sinter macrostructure with rapid local variations biota comprises non photosynthetic hyperthermophilic bacteria 2 splash areas around geysers water temperature 60 75 c laminated spicule and column macrostructure locally forming cupolas

controls on aragonite and calcite precipitation in hot spring travertines at chemurkeu lake bogoria kenya subfossil travertines precipitated around hot spring orifices at chemurkeu on the western shore of lake bogoria kenya are composed of calcite aragonite and minor dolomite aragonite crystal beds which form 5 10 of the travertine by volume are formed of large pseudohexagonal prisms whereas the calcite crystal beds are composed mainly of feather dendrites dolomite is only found between aragonite crystals boundaries between aragonite and calcite crystals commonly display evidence of dissolution but there is no evidence to indicate that calcite formed by inversion of aragonite or that the dolomite replaced the aragonite thus the aragonite calcite and dolomite are each treated as primary precipitates reticulate gelatinous coatings with a high si and mg content cover most external and internal surfaces of the aragonite and calcite crystals the travertines may have formed under more humid conditions than today when the spring waters contained more ca2 the physiochemical conditions at the mo

taphonomy of very ancient microfossils from the 3400ma strelley pool formation and 1900ma gunflint formation new insights using a focused ion beam abstract focused ion beam fib milling permits the accurate extraction of ultrathin c 100 nm cross sectional lamellae from microfossils found in geological thin sections subsequent tem analysis of these lamellae can provide unique insights into the ultrastructure chemistry and taphonomy of precambrian microfossils at the micrometer to nanometer scale combining serial fib milling with sem imaging extends this capability to three dimensional 3d tomographic reconstruction and visualization of precambrian microfossils revealing information not available in light microscopy here we apply these techniques to two iconic silicified microfossil assemblages from the 3400 ma strelley pool formation of western australia and the 1900 ma gunflint formation of canada all the examined microfossils have carbonaceous walls surrounded by pure silica impregnation of microfossil walls by nano grains of silica is common together with variable degrees of wall displacement and replacement by silica all microfossils are rigidly preserved in 3d and show little or no folding or compression however there are also notable differences in taphonomic preservation our examples of the spheroidal gunflint microfossil huroniospora showed the highest fidelity of preservation with a continuous carbonaceous wall fossilized by spheroidal nano silica grains that resemble those found on bacterial surfaces in modern silicifying hot spring environments the nucleation of these silica nano spheres on the microfossil walls has induced an artificial saw tooth like ridged wall texture that may subsequently hinder species level identification the strelley pool microfossils in comparison show a lower fidelity of preservation with small parts of the microfossil walls completely replaced by silica plus extensive recrystallization of spheroidal silica nano grains to angular micro quartz our examples of the sheath like filamentous gunflint microfossil siphonophycus showed the lowest fidelity of preservation with many gaps in the carbonaceous walls and significant redistribution of carbon by recrystallizing silica grains a model is presented to explain these observations criteria for distinguishing highly probable microfossils from non cellular carbonaceous microstructures e g botryoids and grain coatings using fib based imaging are put forward for the first time here using examples drawn from the strelley pool formation and comparisons with younger gunflint material the combined in situ techniques of fib tem and fib sem nano tomography potentially provide a wealth of new nano scale information regarding the biogenicity antiquity and taphonomy of precambrian microfossils however the destructive nature of both techniques makes their application to unique palaeontological specimens problematical

diversity and origin of quartz cements in continental carbonates example from the lower cretaceous rift deposits of the south atlantic margin abstract silica precipitation in continental carbonates is a common process occurring during sedimentation and diagenesis the lower cretaceous rift deposits of the south atlantic equatorial margin which are intensively explored by petroleum companies provide good examples of such silicifications in carbonates exhibiting a wide diversity of petrographic habit of early to late quartz cements in order to understand the palaeoenvironmental and diagenetic conditions leading to this diversity we integrated detailed petrography of diagenetic sequences and quartz habit with 18oquartz measurements by sims of individual cements observed in samples from the offshore and onshore basins of the west african margin the petrographic description highlights the omnipresence of early fibrous microquartz cements exhibiting either length fast or length slow habit in addition to laminated microquartz and micro or mega quartz forms amongst the isotopic analysis the 18oquartz data show that length slow cements are generally strongly enriched in 18o 18oquartz ranging from 31 to 37 smow whereas length fast forms show less elevated values 25 c and

textural preservation in siliceous hot spring deposits during early diagenesis examples from yellowstone national park and nevada u s a abstract a petrographic mineralogical and geochemical investigation reveals evidence for selective preservation of microfacies in siliceous sinters plio pleistocene sinters from steamboat springs nevada contain identical but fewer petrographic textures than pleistocene sinters from artist point yellowstone national park wyoming the latter are preserved in altered sediments of unknown origin a sequence of pore filling and mineralogical changes explains the excellent preservation of nine microfacies and two petrographic textures plio pleistocene steamboat springs sinters are preserved in successions of andesitic basalt flows and were likely subjected to a more extreme thermal and a different hydrological environment than those from yellowstone national park most microfacies were obliterated during postdepositional heating of steamboat spring sinters differences in diagenetic histories related to depositional environment water chemistry and or subsequent burial must account for the loss of textural evidence between the diagenetic stage represented by artist point sinters and that of steamboat springs sinters hence early postdepositional history affects both the likelihood and quality of microfossil preservation

preferential soft tissue preservation in the hot creek carbonate spring deposit british columbia canada abstract the relict holocene hot creek carbonate spring deposit in southeast british columbia is characterized by excellent preservation of soft tissue organisms e g cyanobacteria but poor preservation of organisms with hard tissue e g wood diatoms the deposit is formed mainly of calcified cyanobacteria with fewer mineralized macrophytes plants bryophytes mosses wood and diatoms cyanobacteria grew as solitary filaments lyngbya and as radiating hemispherical colonies rivularia both were preserved by encrustation and encapsulation while alive and as casts after filament death and decay sheath impregnation was rare to absent filament encrustation whereby calcite crystals nucleated on and grew away from the sheath exterior produced moulds that replicated external filament morphology but hastened filament decay filament encapsulation whereby calcite nucleated in the vicinity of and grew towards the encapsulated filament promoted sheath preservation even after trichome decay subsequent calcite precipitation inside the hollow sheath generated sheath casts the inability of mineralizing spring water to penetrate durable cell walls meant that bryophytes macrophytes and most wood was preserved by encrustation some wood resisted complete decay for several thousand years and its lignified cell walls allowed rare permineralizations diatoms were not preserved in the relict deposit because the frustules were dissolved by the basic spring water amorphous calcium carbonate produced by photosynthetic co 2 removal may have acted as nucleation sites for physicochemically precipitated calcite thus metabolic activities of floral organisms probably initiated biotic mineralization but continuous inorganic calcite precipitation on and in flora ensured that soft tissues were preserved

sulfate reducing bacteria and silica solubility a possible mechanism for evaporite diagenesis and silica precipitation in banded iron formations selective replacement of sulfate evaporite minerals by silica and the precipitation of silica in association with sulfide mineral phases in banded iron formations may be mediated by the metabolic activities of sulfate reducing bacteria hydrogen sulfide is known to be a product of this metabolism and is often called upon as a source of sulfur for metallic sulfides in sedimentary rocks we report here on the influence that chemical changes induced by bacterial sulfate reduction have on silica solubility controlled in vitro growth experiments with desulfovibrio desulfuricans and silica show 1 this organism can grow in silica concentrations as great as 400 ppm with no inhibition and 2 growth in the presence of silica yields a decrease in dissolved silica growth experiments with 80 ppm silica produced a lowering in dissolved silica from 80 ppm to 60 ppm a 25 decrease in just 30 h control experiments in the absence of cells resulted in no effective decrease in dissolved silica the ability of sulfate r

diagenetic transformations opal a to quartz of low and mid temperature microbial textures in siliceous hot spring deposits taupo volcanic zone new zealand silica sinter is a subaerial hot spring deposit formed upon cooling 100 c of discharging alkali chloride waters silica deposition traps and fossilizes living microbes in low temperature 35 c to mid temperature 35 59 c apron terrace outflow channels and pools which record distinctive macrotextures and microtextures along a thermal gradient sinters from four geothermal fields orakei korako northern waiotapu te kopia and umukuri within the taupo volcanic zone new zealand were sampled from two common microbe rich microfacies low temperature palisade mid temperature bubble mat through a range of ages modern to 40 000 years bp we observed morphologic changes in microbial silicification and stepwise transitions in silica phase mineralogy throughout diagenesis opal a to quartz x ray powder diffractometry analysis of taupo volcanic zone sinter samples revealed that mode of microbial fossilization is controlled by silica phase mineralogy which also determines the preservation potent

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

trigonal dendritic calcite crystals forming from hot spring waters at waikite north island new zealand amorphous silica and calcite form the deposits in the vent and on the discharge apron of waikite spring 100 ws 100 which is located in the waikite geothermal area on north island new zealand these precipitates formed from spring water that has a temperature of 90 c and a ph of 8 1 8 8 the opaline silica is restricted to areas around the vent where cooling and evaporation of the spring water triggered precipitation the calcite deposits in the spring vent and on the discharge apron are formed of large up to 15 cm long asymmetrical dendrite crystals that are characterized by multiple levels of branching branches grew preferentially from the downflow side of their parent branch all branches have a trigonal transverse cross section except in areas where competition for growth space induced merger of neighboring crystals the primary branches of the dendrite crystals are sub perpendicular to the substrate even in areas where the discharge apron slopes at a high angle up to 80 on the steeper parts of the discharge apron the plate like primary branches form the floors of the small terrace pools whereas their distal edges form the rims of the pools growth of these dendrite crystals is attributed to abiotic processes high levels of saturation with respect to calcite were caused by rapid co2 degassing of the sheets of spring water that flowed down the steep discharge apron calcite crystals with different crystal morphologies characterize other springs near this spring the variation in crystal morphologies from spring to spring is attributed to different levels of saturation that are related to the initial pco2 of the spring water upon discharge and the rate of co2 degassing at each spring

in situ silicification of an icelandic hot spring microbial mat implications for microfossil formation transmission electron microscopy and energy dispersive x ray analysis revealed that filamentous phototrophic bacteria resembling chlorojlexus aurantiacus underwent rapid silicification in an icelandic hot spring microbial mat the mineralization associated with the cells occurred both extracellularly within and on the external sheaths of the bacteria and intracellularly within the cytoplasm the exceptional preservation of the bacterial sheaths is due to the presence of distinct mineral nucleation sites this results in the production of silica casts of the bacteria which bear a striking resemblance to microbial remains in ancient microfossil assemblages rcsumc les analyses par microscopie clectronique i transmission et par 1 6nergie dispersive des rayons x ont rcvcl6 que les bactkries filamenteuses phototrophes ressemblant b chlorojlexus aurantiacus ont subi une silicification rapide au sein d un tapis microbien de source therrnale en islande la mineralisation associee aux cellules se presente sous forme extracellulaire b l interieur de et sur la membrane externe des bacteries et sous forrne intracellulaire i l interieur du cytoplasme la preservation exceptionnelle de la membrane des bacteries est due b la presence de sites de nucleation minerale disperses le resultat est la production des moulages siliceux des bactkries affichant une resemblance remarquable avec les vestiges microbiens d anciens assemblages microfossiles traduit par la raaction fossil evidence of the earth s earliest biosphere shows that complex communities of microorganisms were abundant and well distributed throughout the precambrian era among the richest beds of microbial fossils are those found in the gunflint iron formation north of lake superior in canada barghoorn and tyler 1965 the early archaean apex chert of australia schopf and packer 1987 schopf 1993 and the riphean section of the anabar uplift in russia sergeev 1993 all ranging in age from 0 8 to 3 5 ga the distribution and types of microorganisms preserved as cellularly intact microfossils are remarkably similar to those in modern microbial mats in which organic rich layers formed by cyanobacteria and or other types of filamentous bacteria alternate with layers that are comprised of detrital inorganic sediment and authigenic mineral precipitates morse and mackenzie 1990 these benthic microbial communities are considered to be analogous and possibly homologous with those that were preserved billions of years ago as stromato

environmental significance of upper miocene phosphorites at hominid sites in the lukeino formation tugen hills kenya the lukeino formation contains an important sedimentary and fossiliferous record of the late miocene 6 09 5 68 ma which has in particular yielded the fossil remains of the oldest east african bipedal hominid called orrorin tugenensis this fluvio lacustrine sedimentary succession crops out in the kenyan part of the east african rift it is mainly composed of clay to sandy clay deposits intercalated with volcanic ash horizons and localized layers of carbonates and diatomites a detailed sedimentological and mineralogical study of the lukeino formation was conducted to throw light on the environmental conditions in which the hominids lived several centimetric relatively continuous and indurated phosphatic horizons of sedimentary origin were identified at two sites sunbarua and kapcheberek mineralogical xrd and geochemical analyses as well as observations by sem which was coupled with an energy dispersive spectroscopy eds microprobe indicate that the autochthonous phosphate layers are composed of a micritic matrix of francolite 38 93 with incorporation of silicates in variable proportions from one layer to another the phosphate matrix contains very well preserved and abundant diatom frustules in the basal phosphate layer these diatoms are identified as aulacoseira granulata implying a ph of 7 8 8 2 for freshwaters of the palaeolake lukeino calcitic tubular structures linked to a possible bacterial origin are also observed locally phosphate layers occur abruptly within a thick clay sandy series associated with an intense runoff phase during the deposition of this interval of the lukeino formation the massive and cyclic input of phosphorus to the lake promoted productivity to the stage where it caused a diatom bloom the establishment of several phosphate horizons testifies to successive phases of eutrophication of palaeolake lukeino the diatom cells provided some of the organic matter which was decomposed by bacterial activity at the bottom of the lake in suboxic conditions but in insufficient quantities to fully form the phosphatic materials the rest of the organic matter needed for phosphogenesis came from terrigenous supply plant debris suggesting the presence of dense vegetation in the catchment of the palaeolake lukeino during this well drained interval of deposition of the lukeino formation

formation of fine grained metal and silicate precipitates on a bacterial surface bacillus subtilis abstract the ability of the gram positive bacterium b subtilis to bind and nucleate precipitates from silicate anions has been studied over 24 weeks in the presence of fe and a1 at concentrations close to those levels in soils and at slightly acid 5 5 and basic 8 0 ph in all cases formation of silicate crystallites quasi crystalline precipitates on the bacterial surfaces was observed bacterially mediated minerals were more diverse in composition and morphology less crystalline smaller and sometimes more abundant than those that were abiotically formed fe pretreatment of the bacterial cells enhanced the binding of silicate at ph 8 0 walls which were not pretreated with fe bound silicate more favourably at acid values when heavy metals pb cd zn cr ni cu were added to the mixture at ph 4 5 silicate retention was greatly favoured giving greater retention of either si or metals than was seen in abiotic controls experiments with only heavy metals showed a high affinity of the bacterial walls for the metals even at low temperatures 4 c it is postulated that a cationic bridging mechanism is involved in the binding of silicate anions by bacterial cell walls

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

is pore water oxygen content decoupled from productivity on the california margin trace element results from ocean drilling program hole 1017e san lucia slope california 1 trace element evidence from the california margin ocean drilling program hole 1017e shows significant changes in sediment redox chemistry at intermediate water depths over the last 60 kyr the influence of export production on intermediate water oxygen content can be distinguished from lateral ventilation through comparisons among proxies representing export production flux percent corg and sediment surface i br and pore water mo ag cd re and u oxygenation proxies sulfate reduction within the uppermost sediment column occurred during late quaternary interstadials and the holocene indicating reduced intermediate water ventilation however sulfate reduction was most intense during major interstadials when productivity was enhanced decoupling between export production and pore water oxygen content is demonstrated during deglaciation when the sediment pore water oxygen content increases without a corresponding reduction in export productivity thus both export production and intermediate water ventilation play a role in oxygen minimum zone processes on the california margin

dolomite silica stromatolites in miocene lacustrine deposits from the duero basin spain the role of organotemplates in the precipitation of dolomite this research provides an ancient analogue for biologically mediated dolomite precipitation in microbial mats and biofilms and describes the involvement of highly structured extracellular polymeric secretion eps templates in dolomite nucleation the structure of eps is shown to match the hexagonal trigonal lattice geometry of dolomite which favoured the epitaxial crystallization of dolomite on the organic substrate this structure of eps also matches the arrangement of silica nanospheres in opal which further accounts for the organically templated formation of opal enabling the non replacive co existence of dolomite and silica the study is focused on a 50 m thick dolomite succession that is exposed in central areas of the tertiary duero basin and was deposited in a mudflat saline lake sedimentary complex during the middle to late miocene 9 to 15 ma in the intermediate intervals of the succession poorly indurated dolomite beds pass gradually into silica beds on the basis of sedimentological compositional geochemical and petrographic data silica and dolomite beds have been interpreted as mineralized microbial mats the silica beds formed in marginal areas of the lake in response to intense evaporative concentrations this resulted in the rapid and early precipitation of opal silicification accounted for the exceptional preservation of the microbial mat structure including biofilms filamentous and coccoid microbes and eps extracellular polymeric secretions have a layered structure each layer being composed of fibres which are arranged in accordance with a reticular pattern with frequent intersection angles at 120 and 60 therefore the structure of eps matches the lattice geometry of dolomite and the arrangement of silica nanospheres in opal additionally eps binds different elements with preference to si and mg the concurrence of suitable composition and surface lattice morphologies in the eps favoured the crystallization of dolomite on the substrate in some cases dolomite nucleation took place epicellularly on coccoid micro organisms which gave way to spheroid crystals organic surfaces enable the inorganic mineral precipitation by lowering the free energy barrier to nucleation most of the microbial mats probably developed on the lake floor under sub aqueous conditions where the decomposition of organic matter took place the subsequent formation of openly packed dolomite crystals with inter related si enriched fibrils throughout is evidence for the pre existence of fibrillar structures in the mats miocene dolomite crystals are poorly ordered and non stoichiometric with a slight ca excess up to 5 which is indicative of the low diagenetic potential the microbial dolomite has towards a more ordered and stoichiometric structure this confirms that microbial imprints can be preserved in the geological record and validates their use as biosignatures

impacts of flamingos on saline lake margin and shallow lacustrine sediments in the kenya rift valley abstract studies of modern holocene and pleistocene sediments around saline to hypersaline alkaline lake bogoria and lake magadi show that evidence of flamingo activity in marginal areas of these lakes is nearly ubiquitous flamingos produce discrete structures such as webbed footprints 9 cm long 11 cm wide and nest mounds 30 cm wide 20 cm high and they also extensively rework sediments in delta front delta plain and shoreline areas large 0 5 2 cm in diameter pinched bubble pores and ped like mud clumps are formed by the trampling and churning of wet clay rich sediments in these settings flamingo nest mounds although superficially similar to some thrombolite mounds are typically internally structureless unless formed on pre existing sediments that preserve internal structures the flamingo mounds consist of a dense packed oval shaped core a surrounding body of packed sediment and an external layer with a ped like texture of clumped mud the nests may contain open holes from roots or feather shafts incorporated into the nest and or burrows produced once the nests are abandoned in areas with high densities of flamingos lake margin sediments may be preferentially compacted particularly at breeding sites and become resistant to subaerial erosion and the effects of transgressive ravinement on time scales ranging from seasons to tens of thousands of years the relatively well compacted nest mounds and associated sediments also contribute to the stability of delta distributary channels during regressive transgressive cycles and can lead to the minor channelization of unconfined flows where currents are diverted around nest mounds pleistocene exhumed surfaces of relatively well indurated lake margin sediments at lake bogoria and lake magadi that are interpreted as combined regressive and transgressive surfaces flooding surface sequence boundary preserve evidence of flamingo activities and are overlain by younger porous lacustrine silts that preserve large bubble pores produced by flamingos

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