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

organotemplate silica deposition in neoproterozoic deep marine environments evidence from the penganga group adilabad india silica in bedded cherts interstratified with manganese carbonates in a deep water carbonate ramp succession of the neoproterozoic penganga group india displays film like spherical to rod shaped and tubular branching microstructures the microstructures resemble mineralized extracellular polysaccharides biofilms and bacterial morphologies the microstructures suggest silicification by nucleation of silica on organic templates or indirectly as sorbed species accumulating on organic templates given that similar microstructures have also been documented in archean cherts it is proposed that organotemplates might have been an important sink for the deposition of silica in precambrian deep water marine environments

microbial carbonates the geological record of the calcified bacterial algal mats and biofilms summary deposits produced by microbial growth and metabolism have been important components of carbonate sediments since the archaean geologically best known in seas and lakes microbial carbonates are also important at the present day in fluviatile spring cave and soil environments the principal organisms involved are bacteria particularly cyanobacteria small algae and fungi that participate in the growth of microbial biofilms and mats grain trapping is locally important but the key process is precipitation producing reefal accumulations of calcified microbes and enhancing mat accretion and preservation various metabolic processes such as photosynthetic uptake of co2 and or hco3 by cyanobacteria and ammonification denitrification and sulphate reduction by other bacteria can increase alkalinity and stimulate carbonate precipitation extracellular polymeric substances widely produced by microbes for attachment and protection are important in providing nucleation sites and facilitating sediment trapping microbial carbonate microfabrics are heterogeneous they commonly incorporate trapped particles and in situ algae and invertebrates and crystals form around bacterial cells but the main component is dense clotted or peloidal micrite resulting from calcification of bacterial cells sheaths and biofilm and from phytoplankton stimulated whiting nucleation interpretation of these texturally convergent and often inscrutable fabrics is a challenge conspicuous accumulations are large domes and columns with laminated stromatolite clotted thrombolite and other macrofabrics which may be either agglutinated or mainly composed of calcified or spar encrusted microbes stromatolite lamination appears to be primary but clotted thrombolite fabrics can be primary or secondary microbial precipitation also contributes to hot spring travertine cold spring mound calcrete cave crust and coated grain deposits as well as influencing carbonate cementation recrystallization and replacement microbial carbonate is biologically stimulated but also requires favourable saturation state in ambient water and thus relies uniquely on a combination of biotic and abiotic factors this overriding environmental control is seen at the present day by the localization of microbial carbonates in calcareous streams and springs and in shallow tropical seas and in the past by temporal variation in abundance of marine microbial carbonates patterns of cyanobacterial calcification and microbial dome formation through time appear to reflect fluctuations in seawater chemistry stromatolites appeared at 3450 ma and were generally diverse and abundant from 2800 to 1000 ma inception of a proterozoic decline variously identified at 2000 1000 and 675 ma has been attributed to eukaryote competition and or reduced lithification thrombolites and dendrolites mainly formed by calcified cyanobacteria became important early in the palaeozoic and reappeared in the late devonian microbial carbonates retained importance through much of the mesozoic became scarcer in marine environments in the cenozoic but locally re emerged as large agglutinated domes possibly reflecting increased algal involvement and thick micritic reef crusts in the late neogene famous modern examples at shark bay and lee stocking island are composite coarse agglutinated domes and columns with complex bacterial algal mats occurring in environments that are both stressed and current swept products of mat evolution ecological refugia sites of enhanced early lithification or all three

a temperature tolerant interstitial worm with associated epibiotic bacteria from the shallow water fumaroles of deception island antarctica abstract a prominent not previously identified species of monocelidae platyhelminthes proseriata was found in the vicinity of fumarole activity at fumarole bay the distribution of this animal and the metazoan meiobenthos in the vicinity of this area suggests that this species constitutes the most abundant species and the bulk of the biomass at these shallow water fumaroles in contrast to the other metazoan meiofauna the distribution of this species is positively correlated with the water temperature and gas emissions indicating a preference for the areas around fumaroles the range of temperature tolerated by this animal was determined in in vivo experiments to be at least 30 40 c the outer surface the animals is colonized by apparently symbiotic bacteria which are usually rod like and approximately 0 68 m wide and 2 07 m long the results of this study revealed a remarkable difference between shallow water and deep sea hydrothermal vent meiobenthic communities generalists capable of tolerating extreme abiotic conditions appear to dominate shallow water vents whereas endemism seems to be the rule in the deep sea vents

porous sidewall and sloping floor crystallization experiments using a reactive mush implications for the self channelization of residual melts in cumulates abstract tank crystallization experiments with aqueous solutions were used to explore interactions between a buoyant residual fluid and a porous cumulate at a sloping floor and a vertical wall the fluid flow path in a non reactive mush glass beads was used as a control for a vertical non reactive mush the rise of the buoyant solute depleted fluid was confined within it fluid flow paths were curved in the case of a sloping mushy floor with the upper part of the mush being principally a plume discharge zone and the lower part being a recharge zone more realistic reactive mushes were created by supercooling a nh 4 cl solution cooling against a vertical surface produced an initial mat of dendrites that were subsequently overgrown by a vertically grooved dendrite structure the grooves contained upwardly flowing residual melt while the ridges extended beyond the buoyant boundary layer into the thermally convecting solute rich part of the tank with time the grooves were covered over by a dendrite mat but remained as open channels within the mush focussing and accelerating the upflow of residual melt a buoyant boundary layer on the outside of the mush was not observed in the experiment with a sloping floor a thin mat of homogeneous mush was succeeded by a more organized deposit characterized by chimneys residual melt escaped as plumes through the chimneys chimneys formed because buoyant residual melt dissolved the mush as it rose and warmed chimney spacing increased and the number of chimneys decreased as the mush thickened dissection of the mush showed that individual chimney channels were inclined upslope so that melt residual from crystallization at the foot of the slope emerged from chimneys near the top of the slope formation of channels in cumulates would greatly increase fractionation efficiency by increasing the effective volume from which residual melts could be extracted and by increasing the rate at which residual melts could be transferred to the upper part of the chamber

biofilms microbial mats and microbe particle interactions electron microscope observations from diatomaceous sediments sediments and diatoms from the mudflats of the bay of bourgneuf in western france were examined in an electron microscope study of biofilms and microbial mats the sediments were kept in an aquarium for study and a diatom culture was made of the benthic diatoms the sediment biofilm was composed of exopolymeric substances eps incorporated clay particles and rarely bacteria this film coated all particles at the sediment water interface its surface morphology reflected its composition and internal structure thin films were smooth whilst a lumpy structure or incorporated fibrils produced either a mammillated or ropy surface and clays in the structure gave rise to a flaky morphology at shallow depths in the sediment column 0 5 cm the biofilm was already degraded the biofilm coating degraded diatom frustules in the benthic diatom culture consisted of eps and bacteria and presented a ragged appearance microbial mats occurred on the surface of the fresh littoral sediments as well as those in the aquarium and on the wall of the aquarium the mat on the surface of the aquarium sediments had an open structure with webs of fibrils and bacteria in the pore space it formed in a relatively quiet environment pore space was more limited in the mat from the surface of the fresh littoral sediments in which direct contact between biofilm coated particles was common in the exposed environment of the aquarium wall there was a thick resistant coating of eps in addition to binding particles together the presence of mats and biofilms in sediments affects sediment physical properties such as porosity and permeability the flux of dissolved substances in pore waters and the dissolution of particles and can therefore influence early diagenesis mats and biofilms seem to be more readily preserved in the geological record than the micro organisms such as bacteria which produce them their identification in the sedimentary record would greatly aid interpretation of sediment genesis and evaluation of the microbial role in sediment formation

a magmatic hydrothermal transition in arkaroola northern flinders ranges south australia from diopside titanite pegmatites to hematite quartz growth a set of palaeozoic diopside titanite veins are present in mesoproterozoic metagranites and metasediments that constitute the basement mt painter inlier of the adelaide fold belt south aus tralia these massive veins up to 1 m of pegmatitic nature contain large crystals of diopside lree y enriched titanite up to 40 cm in length and minor amounts of quartz they can be used to trace the sys tem s development from a high temperature magmatic stage through to a massive hydrothermal event the pegmatitic origin of these veins is evident from a complex fluid melt inclusion assemblage consisting of a highly saline inhomogeneous fluid and relicts of melt immiscibility of melt and heterogeneous highly saline fluids exceeding 61 eq mass nacl is preserved in primary inclusions in diopside and secondary inclusions in titanite indicating relatively shallow conditions of formation 510 20 c and 130 10 mpa graphic intergrowth of diopside and albite occurs at the contact with granitic pegmatites the system evolved into hydrothermal conditions which can be deduced from a later population of only fluid inclusions homogeneous and less saline 40 eq mass nacl trapped around 350 20 c and 80 10 mpa during quartz crystallization the conditions moved across the halite liquidus resulting in a heterogeneous mixture of brine and halite crystals which were trapped at 200 20 c and 50 10 mpa brecciation and a palaeo geother mal system overprinted the pegmatitic veins with an epithermal hematite quartz assemblage and lesser amounts of bladed calcite and fluorite in an intermit tently boiling hydrothermal system of fairly pure h2o at 100 140 c and 1 5 mpa remobilization of lrees and y from titanite and or the granitic host rock is evidenced by precipitation of apatite allanite and wakefieldite in an intermediate stage occasional incorporation of radioactive elements or minerals presumably u rich in the fluorite is responsible for radiolysis of h2 ot o h 2

discussion of oxygen isotope and trace element compositions of platiniferous dunite pipes of the bushveld complex south africa signals from a recycled mantle component by gunther t haase k m junge m oberthur t woelki d and krumm s

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

filamentous fossil bacteria from the archean of western australia abstract four morphotypes of structurally preserved filamentous fossil bacteria have been discovered in petrographic thin sections of laminated carbonaceous cherts from the 3500 ma old warrawoona group of northwestern australia these tubular and septate microfossils are interpreted here as being syngenetic with warrawoona sedimentation as such they are apparently the oldest such fossils now known in the geological record the diversity of this assemblage and the evident complexity of its individual components suggest that the beginnings of life on earth may have appreciably pre dated the deposition of the warrawoona sediments

hydrothermal alteration and microfossil artefacts of the 3 465 million year old apex chert the earliest fossils of cyanobacteria have been controversially reported from the 3 465 million year old apex chert in western australia mineralogical analyses of the rock near the fossil site indicate that the site experienced repeated alterations at low and medium temperatures and is therefore unlikely to have preserved any early forms of life the 3 465 million year old apex chert of the pilbara craton in western australia has been controversially reported to contain some of the oldest fossil evidence for life tiny carbonaceous filaments interpreted as cyanobacteria1 yet on the basis of the presence of barite and native metals it has been suggested that the chert formed from hydrothermal fluids at temperatures greater than 250 c refs 2 3 an unlikely environment for cyanobacteria here we use scanning electron microscopy to assess the mineralogy and depositional setting of the chert veins surrounding the nodes that contain the microfossils in addition to rare native metals we find an assemblage of iron oxides and clay minerals which we interpret to have formed during repeated pulses of hydrothermal alteration at low to medium temperatures we also find micrometre sized silica structures that resemble microbial exopolymers4 and textures formed by the partial dissolution of tubular minerals that look similar to fossilized microbial mats5 other branched microstructures suggest post depositional colonization of microcracks and fissures by microbes5 these observations are not directly applicable to the carbonaceous filaments purported to be the earliest fossil evidence of life as we did not recover any in our sample however because of the observed hydrothermal and groundwater alteration we conclude that the apex chert is unlikely to have preserved any early forms of life

primary silica precipitate at the precambrian cambrian boundary in the south oman salt basin sultanate of oman abstract an organic rich laminated chert silicilyte consisting of up to 90 microcrystalline quartz that formed at the precambrian cambrian boundary acts as a light oil reservoir in the subsurface of the south oman salt basin sultanate of oman fully encased in salt domes it was first discovered during the 1990 s hydrocarbon exploration activities of petroleum development oman because of the economic significance and the unconventional reservoir characteristics there is great interest in understanding the origin of the laminated chert its source of silica and mode of precipitation in an anoxic sulphur rich stagnant and highly saline basin the homogeneous distribution and high values of stable si isotope composition avg 30 si 0 83 0 28 coupled with a low molar ge si ratio 6 of the microcrystalline matrix quartz clearly reveal dissolved silica in the seawater as the si source whereas hydrothermal or biogenic e g sponge derived silica can be excluded silica precipitation from seawater was likely the result of a dramatic increase in salinity in response to salt dissolution atop and adjacent to the edges of transtensional depressions on the deep basin floor thus markedly reducing the solubility of amorphous silica in these brine filled seawater depressions this saturation triggered the formation of silica gel which accumulated at the basin floor forming a soft silica rich layer on bacterial mats giving rise to a laminated sediment the mean number of laminae is ca 32 per year suggesting that layering is non annual and controlled by processes such as fluctuations in nutrient supply lunar driven re mixing or diagenetic segregation the transformation of the silica gel to microcrystalline quartz occurred below 45 c indicating a less than 4 5 18 o composition of the pore water during microcrystalline quartz formation

geochemistry and mineralogy of the platreef and critical zone of the northern lobe of the bushveld complex south africa implications for bushveld stratigraphy and the development of pge mineralisation the northern lobe of the bushveld complex is currently a highly active area for platinum group element pge exploration this lobe hosts the platreef a 10 300 m thick package of pge rich pyroxenites and gabbros that crops out along the base of the lobe to the north of mokopane formerly potgietersrus and is amenable to large scale open pit mining along some portions of its strike an early account of the geology of the deposit was produced by percy wagner where he suggested that the platreef was an equivalent pge rich layer to the merensky reef that had already been traced throughout the eastern and western lobes of the bushveld complex wagner s opinion remains widely held and is central to current orthodoxy on the stratigraphy of the northern lobe this correlates the platreef and an associated cumulate sequence that includes a chromitite layer known as the grasvally norite pyroxenite anorthosite gnpa member directly with the sequence between the ug2 chromitite and the merensky reef as it is developed in the upper critical zone of the eastern and western bushveld implicit in this view of the magmatic stratigraphy is that similar critical zone magma was present in all three lobes prior to the development of the merensky reef and the platreef however when this assumed correlation is examined in detail it is obvious that there are significant differences in lithologies mineral textures and chemistries mg of orthopyroxene and olivine and the geochemistry of both rare earth elements ree and pge between the two sequences this suggests that the prevailing interpretation of the stratigraphy of the northern lobe is not correct the critical zone of the northern lobe cannot be correlated with the critical zone in the rest of the complex and the simplest explanation is that the gnpa platreef sequence formed from a separate magma or mixture of magmas chilled margins of the gnpa member match the estimated initial composition of tholeiitic main zone type magma rather than a critical zone magma composition where the gnpa member is developed over the ultramafic lower zone hybrid rocks preserve evidence for mixing between new tholeiitic magma and existing ultramafic liquid this style of interaction and the resulting rock sequences are unique to the northern lobe the gnpa member contains at least seven sulphide rich horizons with elevated pge concentrations some of these are hosted by pyroxenites with similar mineralogy crystallisation sequences and pd rich pge signatures to the platreef chill zones are preserved in the lowest main zone rocks above the gnpa member and the platreef and this suggests that both units were terminated by a new influx of main zone magma this opens the possibility that the platreef and gnpa member merge laterally into one another and that both formed in a series of mixing quenching events involving tholeiitic and ultramafic magmas prior to the main influx of tholeiitic magma that formed the main zone

formation of aragonite cement by nannobacteria in the great salt lake utah brine shrimp egg cases in growth cavities in modern stromatolites in the great salt lake utah are replaced by aragonite and cemented together by aragonite cement the fabric of the cement changes dramatically as the distance from the egg case increases the cement within 50 to 70 m of the egg case exhibits a random fabric of 10 to 20 m equant crystals the surface of the cement is covered by bead like bumps 0 1 m in diameter interpreted as nannobacteria overlying the random beaded fabric with a relatively abrupt transition are epitaxial prismatic aragonite crystals that have smooth crystal surfaces lacking bead like bodies the smooth surfaced prismatic aragonite crystals are interpreted to be normal abiotic precipitates whereas the beaded microspar is interpreted to result from biotic processes where the nannobacteria serve as catalysts for creation of the cement a population explosion of bacteria occurs as the organic material of egg case rots which alters the microchemical environment and induces a rapid precipitation of aragonite enclosing tens of thousands of nannobacteria as the organic material is destroyed reproduction of bacteria slows and epitaxial prismatic aragonite crystals nucleate and grow abiotically on the structureless biotic layer

the distribution and main geologic features of discordant bodies of iron rich ultramafic pegmatite in the bushveld complex transgressive bodies of iron rich ultramafic pegmatite composed predominantly of varying proportions of hortonolite clinopyroxene ilmenite and ti magnetite are abundantly distributed throughout the layered mafic sequence of the bushveld complex new information from field and underground studies together with interpretations of detailed magnetic surveys is presented along with an overview of the available literature some whole rock and mineralogical chemical data are also included within the bushveld complex a number of distinct but possibly genetically related groups of postcumulus ultramafic rock are recognized these include besides the main group of iron rich ultramafic pegmatite bodies of magnesian dunite and the well known platiniferous ultramafic or dunitic pipes transgressive often pipelike bodies composed almost entirely of fe ti oxides occur in the main and upper zones of the complex which although part of the iron rich ultramafic pegmatite suite are described separately composite bodies in which massive fe ti oxides occur as a core within a peripheral shell of silicate rich pegmatite are also recognized the major occurrences of iron rich ultramafic pegmatite are documented and depicted in a series of maps their distribution is subject to two major stratigraphic controls iron rich ultramafic pegmatite occurs largely above the ultramafic cumulates in the lower and lower critical zones it is abundant in the upper critical and main zones where plagioclase is a major cumulus phase but is rarely observed above the highest cumulus ti magnetite layer in the upper zone it is also evident that these rocks become increasingly iron rich toward the top of the layered sequence so that in the upper zone fe ti oxide pegmatite is the dominant variety many bodies particularly the large ones are structurally controlled and occur in disturbed areas characterized by faulting and post bushveld dikes stratigraphic magnetic anomalies in the complex may be related to anorthositic layers containing small specks of iron rich ferromagnesian silicates and fe ti oxides this iron rich material is considered to represent the accumulated and trapped late stage residual and intercumulus fluid resulting from fractional crystallization within a cyclic unit lateral migration and channeling of these pegmatitic fluids into areas of structural weakness resulted in the formation of large pipelike bodies of iron rich ultramafic pegmatite these bodies may have resulted from metasomatic replacement of preexisting cumulates or they may have crystallized directly from the pegmatitic fluids the authors believe that the pegmatitic fluids have been derived from within the crystallizing cumulate pile and it is concluded that the formation of iron rich ultramafic pegmatite is an integral feature of the layered sequence

fabric development in proterozoic bedded chert penganga group adilabad india sedimentologic implications laterally persistent horizons of interstratified chert limestone manganese ores within a siliceous micritic limestone are interpreted to have been deposited on a distally steepened deep carbonate ramp in the proterozoic penganga group of the pranhita godavari valley south india the association of intraformational limeclast conglomerate turbidites and pelagic micritic limestone indicates deposition of the chert bearing horizons at the toe of a distally steepened ramp slope environment the chert is characterized by a wide variety of fabrics including cryptocrystalline and microcrystalline quartz equant megaquartz and chalcedony cryptocrystalline and microcrystalline quartz are most common they occur mainly as lenses of mosaic quartz and define laminae megaquartz in contrast occurs as irregular patches as fenestroids within cryptocrystalline or microcrystalline fabrics or in complex aggregates and disrupted mosaics the origin of the cryptocrystalline and microcrystalline quartz by maturation parallels the morphological evolution of many deep sea cherts that form by maturation of biogenic opal a to quartz chert through an opal ct stage within the quartz stage the microcrystalline fabric formed by pervasive space centered grain growth of cryptocrystalline crystals and porphyroid growth resulted in porphyrotopic fabric the megaquartz formed directly from pore water in continuity with maturation of opal ct the disrupted and curdled mosaics attest to high fluid activity within semilithified sediments the variation in fabric types at the microscale suggests fabric evolution at low temperature the presence of chert clasts in debris flow conglomerates and development of a plethora of water escape structures suggest maturation of silica at shallow burial depth maturation at shallow burial was favored by very low detrital content of these cherts and a mg rich pelagic carbonate depositional geochemical milieu estimated sedimentation rates from 2 to 10 mm ky super 1 closely match the average sedimentation rate from modern pelagic siliceous deposits

the silica cycle in the precambrian abstract whereas the modem silica cycle is heavily dependent on the biology of silica secreting organisms such as the diatoms the precambrian silica cycle was dominated by inorganic reactions among dissolved silica clay and zeolite minerals and organic matter there is no evidence for deposition of a layered amorphous silica but abundant evidence for diagenetic silicification in the neoproterozoic the time period chosen for this analysis the influx of silica to the neoproterozoic ocean would have been governed as today by the balance among tectonism weathering and hydrothermal input the evidence of neoproterozoic rocks favors tectonic and weathering regimes not very dissimilar to those of the early phanerozoic and hydrothermal inputs significantly altered at certain periods one flux would have been very different the present diffusional influx of silica from interstitial waters into the oceans would have been altered to an efflux from the oceans to interstitial waters reactions of dissolved silica with inorganic phases would have controlled silica concentrations at a level of about 60 ppm this figure is based on experimental determinations of solubility and silica sorption on clay minerals and zeolites as silica in interstitial waters was lowered by opal ct precipitation a diffusional efflux from the ocean into the sediment was set up silicification would have started in early diagenesis as opal ct precipitated from interstitial waters diffusion of silica from overlying seawater declined as the sediment was buried a few hundred meters and the diffusion path lengthened during this still relatively shallow burial silicification continued but this time powered by hydrologie transport of dissolved silica from coastal plain sediments during transgressions and regressions thus the major removal of silica from the neoproterozoic ocean took place by diagenetic reactions

proterozoic microfossils from manganese orebody india the presence of carbonaceous matter and stromatolitic structures in manganese deposits of proterozoic 600 myr age1 3 provides indirect evidence for the activity of living organisms during their formation here i report the presence of microfossils from the proterozoic manganese orebody of the penganga group india these microfossils occur commonly in chert and are present within manganese oxide sediments the microfossils probably have affinities with the cyanobacteria and may have been planktonic both coccoid and filamentous forms are present the presence of these microfossils suggests a possible genetic relationship between microbiota and manganese deposition and provides further information about proterozoic ecology

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

chemistry morphology and origin of magmatic reaction chromite stringers associated with anorthosite in the upper critical zone at winnaarshoek eastern limb of the bushveld complex

metal sorption and mineral precipitation by bacteria in two amazonian river systems rio solimoes and rio negro brazil the surfaces of epiphytic and episammic bacteria from the solute rich waters of the rio solimoes brazil interacted with available cations in solution and provided major sites for iron deposition once bound to the bacteria the iron served as nucleation sites for the formation and growth of fine grained authigenic mineral phases because of progressive mineralization the precipitates exhibited a wide range of morphologies from amorphous gel like to crystalline structures complete encrustation of some bacterial cells was observed the most abundant mineral phase associated with the bacteria was a complex fe al silicate with a variable composition energy dispersive x ray spectroscopy suggested that the gel like structures were similar in composition to a chamositic clay whereas the crystalline phases were increasingly kaolinitic bacteria collected from the solute deficient waters of the rio negro remained conspicuously unmineralized suggesting that metal sorption and biomineralization largely reflect the availability of dissolved metals in the water column in a solute rich river system the fate of the metal loaded bacteria has profound implications for the transfer of metals from the hydrosphere to the sediment through diagenesis the bound metals may either be recycled to the overlying water column or become immobilized as stable mineral phases if the latter occurs microorganisms will have played an important role in metal deposition low temperature clay formation and invariably mudstone diagenesis this implies that in the geologic record microorganisms may have been instrumental in the genesis of many sedimentary rocks of fluvial origin

shape and distribution analysis of merensky reef potholing northam platinum mine western bushveld complex implications for pothole formation and growth syn magmatic removal of the cumulate pile during the formation of the bushveld complex resulted in potholes erosion progressed downward in the cumulate pile resulting in a series of steep transgressive contacts between locally conformable potholed reefs in the regional pothole sub facies of the swartklip facies in the western limb of the bushveld complex the deepest of these potholes third order or fwp2 potholing occurs where the base of the merensky cyclic unit transgresses the upper pseudo reef chromitite marker horizon the base of a fwp2 pothole on northam platinum mine consists of an unconformable stringer merensky chromitite overlain by a medium grained poikilitic orthopyroxenite and underlain by either a pegmatitic harzburgite or the medium grained lower pseudo reef anorthosite detailed shape and distribution analysis of fwp2 potholes reveals underlying patterns in their shape and distribution which in turn suggest a structural control the ratio between pothole short vs long axes is 0 624 n 1 385 although the ratio increases from 0 48 to 0 61 in the long axis range 10 to 60 m then decreases from 0 61 to 0 57 from 61 to 100 m increasing again from 0 57 to 0 61 from 101 to 400 m suggesting that there is not a simple relationship between pothole shape and size shape circularity eccentricity and dendricity analysis of a subset of 638 potholes indicates that potholes with long axes 100 m have an average normalized shape that is bilobate and elongate on a 120 orientation the average aspect ratio short axis length divided by long axis length of potholes is highest for potholes with long axis lengths 100 m and lowest for potholes with long axis lengths between 35 and 60 m the most common long axis orientation for potholes with long axis lengths 100 m fractal analysis indicates that the distribution of pothole centers is controlled neither by a single nor several interacting fractal dimensions autocorrelation fry analysis of the distribution of pothole centers shows recurring pothole distribution trends at 038 070 and 110 for potholes over the full range of long axis lengths while the trends of 008 and 152 occur in potholes with long axes lengths between 60 and 100 m chi squared x2 analysis of the locations of pothole centers suggests that the distribution of small potholes is highly non uniform but becomes exponentially more uniform with increasing pothole size the model which best fits the observed shape and distribution analysis is a combination of protracted independent growth and nearest neighbor merging along specific orientations for instance the clustered distribution of original pothole centers resulted in merged potholes with long axes lengths of up to 60 m exhibiting short vs long axes ratios of 0 61 preferred orientations of 150 and alignment along 010 and 150 trends further independent growth allowed for merging of similar sized and smaller neighboring potholes generating potholes with long axes of up to 100 m in length a preferred long axis orientation of 150 and alignment along 010 040 075 and 150 subsequent preferential merging occurred along a 120 trend thereby preserving a bilobate form this implies that while pothole initiation and enlargement may be driven by a top down i e possibly thermomechanical process an underlying linear or structural catalyst control is revealed in changes in pothole shape during enlargement and furthermore in the preferred trends along which potholes merged over a considerable period possibly concomitant with adjustment of major structures in the footwall to the bushveld complex and pulses into the magma chamber

palaeocene recent plate boundaries in the ne atlantic and the formation of the jan mayen microcontinent abstract breakup and sea floor spreading between greenland and eurasia established a series of new plate boundaries in the north atlantic region since the late palaeocene a conventional kinematic model from pre breakup to the present day assumes that eurasia and greenland moved apart as a two plate system however new regional geophysical datasets and quantitative kinematic parameters indicate that this system underwent several adjustments since its inception and suggest that additional short lived plate boundaries existed in the ne atlantic among the consequences of numerous plate boundary relocations is the formation of a highly extended or even fragmented jan mayen microcontinent and subsequent deformation of its margins and surrounding regions the major oligocene plate boundary reorganization and microcontinent formation might have been precluded by various ridge propagations and or short lived triple junctions ne and possibly sw of the jan mayen microcontinent from the inception of sea floor spreading 54 ma to c18 40 ma our model implies a series of failed ridges offshore the faeroe islands a northern propagation of the aegir ridge ne of the jan mayen microcontinent and a series of triple junctions and or propagators in the southern greenland basin

nanometre scale spheroids on sands vulcano sicily possible nannobacterial alteration spheroids in the size range of 30 200 nm coat sand grains on sediments around the island of vulcano they occur as single individuals as ball shaped clumps resembling biological colonies with radiating chains of bodies and eventually merge into sheets in which the individual nannobacteria are no longer evident rarely they appear to bore into the glass grains older rocks are composed mainly of closely packed nanospheroids these spheroids are believed to be nannobacterial cells based on their resemblance to nannobacteria found in other surficial environments and their occurrence in colony like groups if so nannobacteria play a very important role in the destruction of volcanic materials on vulcano

exploration geophysics of the bushveld complex in south africa the bushveld complex consists of three roughly coeval components namely the lebowa granite suite the rashoop granophyre and the rustenburrg layered suite rls the latter being the only economically important unit the rls underlies an area of 65 000 km2 in the northeast sector of south africa and is the oldest and largest mafic layered complex in the world figure 1 it is also the largest repository of magmatic ore deposits and hosts inter alia 80 of the world s platinum group metal pgm resources and most of the chromitite resources over an east west distance of roughly 350 km the rls outcrops as three apparently discrete crescent shaped lobes the western eastern and northern limbs and lies below cover rocks along the southeastern limb the far western limb is currently not of economic significance lithologies range across mafic and ultramafic cumulates plus anorthosites pseudo stratigraphic layering exhibits dips of 8 20 toward the center of the complex the ug2 and merensky reefs constitute the major pgm bearing horizons of mining interest and fall close to the top of the 1500 m thick critical zone an assemblage of alternating leuconorite anorthosite and pyroxene rocks outcropping midway through the rls succession

the 190pt 186os decay system applied to dating platinum group element mineralization of the bushveld complex south africa abstract we evaluate the 190pt 186os decay system measured by laser ablation mc icpms as a useful geochronometric tool for direct dating of platinum group minerals pgm in large mafic ultra mafic intrusions by analysing mineralised rocks from the geochronologically well constrained bushveld complex s africa mixed pgm laurite cooperite laurite platarsite sperrylite and pt fe alloy from the merensky reef yield a pt os isochron age of 1995 50 ma 2 n 48 mswd 1 16 initial 186os 188os 0 119819 0 000006 while this is 59 myr younger than the u pb zircon age for the merensky reef scoates and friedman 2008 it is consistent with recent re os ages for the platreef considering the younger age recorded by both re os and pt os systems we propose that the pt os isochron age records a late low temperature hydrothermal event affecting pgm and sulphides in the platreef and merensky reef pt rich phases may also yield single grain 190pt 186os model ages since initial 186os 188os can be well constrained a pt os model age of 2024 ma 101 ma was calculated for a merensky cooperite a pt fe alloy from the onverwacht pipe and a tweefontein hill sperrylite give pt os model ages of 2113 106 ma and 2042 102 ma respectively a pt os isochron derived from all bushveld units gives an age of 2012 47 ma 2 186os 188osi 0 119818 0 000006 with an mswd of 1 19 hence pge mineralisation in different stratigraphic horizons of the complex cannot be distinguished temporally using the pt os chronometer nonetheless the potential for the pt os system in dating events in pge mineralised systems is evident and the agreement of single grain model ages in high pt os phases with the pt os and re os isochron ages indicates their usefulness in the geochronology of these systems

microbial fossilization in carbonate sediments a result of the bacterial surface involvement in dolomite precipitation recent dolomitic sediment samples from lagoa vermelha brazil were examined microscopically to study the process of bacterial fossilization in carbonate sediments bacteria like bodies were intimately associated with carbonate mineral surfaces and coatings on the former demonstrate the calcification of single bacterial cells the bacterial fossilization process in lagoa vermelha sediments was simulated in the laboratory by cultivation of mixed and pure cultures of sulphate reducing bacteria which were isolated from the lagoa vermelha sediments these cultures produced carbonate minerals that were studied to provide insight into the initiation of the fossilization process in mixed culture experiments bacterial colonies became calcified whereas in pure culture experiments single bacterial cells were associated with dolomite surfaces dolomite nucleated exclusively in bacterial colonies intimately associated with extracellular organic matter and bacterial cells electrophoretic mobility measurements of the bacterial cells in electrolyte solutions demonstrated the specific adsorption of ca 2 and mg 2 onto the cell surfaces indicating the role of the bacterial surface in carbonate nucleation and bacterial fossilization the affinity of the cells for mg 2 was related to the capability of the strains to mediate dolomite formation combined with sulphate uptake which dissociates the mgso 4 o ion pair and increases the mg 2 availability the concentration of mg 2 ions in the microenvironment around the cells where the conditions are favourable for dolomite precipitation may be the key to overcome the kinetic barrier to dolomite formation these results demonstrate that bacterial fossilization is a consequence of the cell surface involvement in carbonate precipitation implying that fossilized bacterial bodies can be used as a tool to recognize microbially mediated carbonates

polymeric substances and biofilms as biomarkers in terrestrial materials implications for extraterrestrial samples organic polymeric substances are a fundamental component of microbial biofilms microorganisms especially bacteria secrete extracellular polymeric substances eps to form slime layers in which they reproduce in the sedimentary environment biofilms commonly contain the products of degraded bacteria as well as allochthonous and autochthonous mineral components they are complex structures which serve as protection for the colonies of microorganisms living in them and also act as nutrient traps biofilms are almost ubiquitous wherever there is an interface and moisture liquid liquid liquid solid liquid gas solid gas in sedimentary rocks they are commonly recognized as stromatolites we also discuss the distinction between bacterial biofilms and prebiotic films the eps and cell components of the microbial biofilms contain many cation chelation sites which are implicated in the mineralization of the films eps biofilms and their related components thus have strong preservation potential in the rock record fossilized microbial polymeric substances fps and biofilms appear to retain the same morphological characteristics as the unfossilized material and have been recognized in rock formations dating back to the early archaean 3 5 b y we describe fps and biofilms from hot spring deep sea volcanic lake and shallow marine littoral environments ranging up to 3 5 b y in age fps and biofilms are more commonly observed than fossil bacteria themselves especially in the older part of the terrestrial record the widespread distribution of microbial biofilms and their great survival potential makes their fossilized remains a useful biomarker as a proxy for life with obvious application to the search for life in extraterrestrial materials

early archean fossil bacteria and biofilms in hydrothermally influenced sediments from the barberton greenstone belt south africa abstract sem imaging of hf etched 3 3 3 5 ga cherts from the onverwacht group south africa reveals small spherical 1 m diameter and rod shaped structures 2 3 8 m in length which are interpreted as probable fossil coccoid and bacillar bacteria prokaryotes respectively preserved by mineral replacement other possibly biogenic structures include smaller rod shaped bacteriomorphs 900 m textures in the sediments in which these biogenic structures occur suggest that they were probably deposited in a shallow water environment which was subjected to intermittent subaerial exposure pervasive hydrothermal activity is evidenced by oxygen isotope studies as well as the penecontemporaneous silicification of all rock types by low temperature 220 c hydrothermal solutions

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

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

geomicrobiology of carbonate silicate microbialites from hawaiian basaltic sea caves abstract modern microbial mats and microbialites are described from basaltic sea caves on the island of kauai hi the mats grow on the ceilings and walls in the photic zone of several open caves where fresh water seeps out of the rock scanning sem and transmission electron microscopy tem showed that the active mats are dominated by filamentous and nonfilamentous cyanobacteria in the surface layers and heterotrophic bacteria in deeper layers energy dispersive x ray analysis revealed that copious amounts of extracellular polymeric substances eps are rich in mg si o and ca likely concentrated from solution petrographic microscopy and electron microprobe analysis of the mineralized microbialites showed textures reminiscent of stromatolitic laminations consisting mainly of alternating calcium carbonate calcite and aragonite and magnesium rich silicate kerolite thin coatings rich in magnesite hydromagnesite and monohydrocalcite surround the microbialites on the rock surfaces and are likely inorganic in origin within the mats minerals tend to form and concentrate within or around dense matrices of eps microenvironments with geochemical conditions favorable for mineral crystallization likely develop in the mats as a result of the mucilaginous extracellular material and the development of bacterial microcolonies in addition copious amounts of extracellular polymers bind ions from solution and provide nucleation sites for mineral crystallization and growth this combination of biological and inorganic processes can explain the occurrence of the secondary minerals in these caves as well as the stromatolitic textures of the microbialites

character of the ug2 chromitite and host rocks and petrogenesis of its pegmatoidal footwall northeastern bushveld complex the ug2 chromitite and its immediate footwall have been mapped at the centimeter scale in a characteristic section exposed in a wall in the middelpunt mine northeast bushveld complex in this region the ug2 chromitite and its coarse grained footwall rocks are underlain and overlain by plagioclase pyroxenite the ug2 chromitite itself is a massive 70 cm thick layer composed of 75 to 90 percent chromite with plagioclase as the dominant interstitial mineral immediately beneath the chromitite is the so called mixed layer a previously undescribed but distinct mappable layer 50 to 70 cm thick consisting of chromite bearing pegmatoidal pyroxenite and chromitite intimately mixed at scales of centimeters to decimeters the mixture is chaotic with no megascopic fabric and contains in bulk about 15 percent chromite the contact between the mixed layer and overlying chromitite is well defined but sinuous at scales of centimeters to decimeters based on observations of drill core and outcrop the mixed layer appears to exist throughout the central sector of the eastern bushveld and may be ubiquitous throughout the entire complex the mixed layer usually rests directly on the ug2 pegmatoid which is a well known and widespread pegmatoidal plagioclase pyroxenite the pegmatoid is much coarser grained than the underlying pyroxenite but the two lithologic units are similar in terms of mineral modes and compositions the contact between them is irregular at scales of decimeters to meters with vertical fingers of pegmatoid extending into the pyroxenite the mixed layer and ug2 pegmatoid contain rare lenses of massive anorthosite encased by continuous centimeter thick rims of chromitite a model is proposed to account for the footwall sequence of the ug2 chromitite in this model the ug2 pegmatoid formed when hydrous interstitial melt percolated up through the partially molten crystal pile accumulated beneath the preexisting compacted massive chromitite and metasomatized and partially melted the pyroxenite protolith which upon cooling recrystallized to a coarse grained rock on the basis of published experiments illustrating the effect of water on melting in mafic systems it is shown how the hydrous melt was able to penetrate the pyroxene bearing assemblages below the chromitite but unable to partially melt the chromitite itself which in consequence remained relatively impermeable the anorthosite lenses in the pegmatoid mark local areas of relatively high degrees of hydration partial melting with the anorthosite representing the crystalline residue the mixed layer formed by gradual collapse of the base of the ug2 chromitite into the less dense relatively fluid footwall the mixed layer and ug2 pegmatoid are diagnostic of the ug2 chromitite and the three lithologic units should be considered a single petrologic unit

secular distribution of biogenic silica through the phanerozoic comparison of silica replaced fossils and bedded cherts at the series level abstract the modern marine silica cycle is dominated by silica secreting phytoplankton principally diatoms but this cycle has evolved considerably during the phanerozoic we analyzed the temporal distribution of silica replaced fossils and bedded chert to determine the influence of factors such as extinctions and climate change on siliceous facies trends in silica replacement of fossils match faunal radiations during the ordovician and siluro devonian with peaks in silica replacement in the late ordovician and middle devonian corresponding to peaks in the abundance and diversity of siliceous sponges sharp drops in the abundance of silica replaced fossils and or bedded cherts coincide with four of the five major mass extinctions no discernible decrease marks the extinction at the end of the triassic we expected peaks in bedded chert deposition during glacial episodes because enhanced ocean circulation should favor more and stronger upwelling orogenic episodes which may trigger continental glaciati

geomicrobiology its significance for geology abstract some microorganisms including various kinds of bacteria fungi algae and protozoa serve as geochemical agents in the uppermost lithosphere and in the hydrosphere some promote rock weathering by mobilizing mineral constituents with inorganic or organic acids or ligands that they excrete others promote rock weathering by redox attack of mineral constituents such as fe and mn still others cause active or passive mineral formation by precipitation and subsequent nucleation of crystal formation on or in the cell or in the bulk phase they play a role in some phases of fossil fuel formation and accumulation they modulate the terrestrial and marine cycles of c n s and p and some other elements and they influence the composition of the atmosphere in respect to o 2 co 2 and ch 4 some discriminate between stable isotopes of h c o n and s

nannobacteria and the precipitation of carbonate in unusual environments abstract nannobacteria are abundant not only in normal marine limestones but also in other environments such as continental caliches fresh water streams springs water pipes caves etc cells are typically 50 to 200 nm in diameter

reply to discussion by r n scoon and a a mitchell on oxygen isotope and trace element compositions of platiniferous dunite pipes of the bushveld complex south africa signals from a recycled mantle component by t gunther k m haase m junge t oberthur d woelki s krumm