mass occurrence of benthic coccoid cyanobacteria and their role in the production of neoarchean carbonates of south africa abstract the sparse archean fossil record is based almost entirely on carbonaceous remnants of microorganisms cellularly preserved due to their early post mortem silicification hitherto as an exception sedimentary carbonate rocks from the neoarchean nauga formation of south africa contain calcified microbial mats composed of microbiota closely resembling modern benthic colonial cyanobacteria chroococcales and pleurocapsales their remains visible under the scanning electron microscope sem after etching of polished rock samples comprise capsular envelopes mucilage sheaths and groups of cells mineralized by calcium carbonate with an admixture of al k mg fe silicates the capsular organization of the mucilaginous sheaths surrounding individual cells and cell clusters forming colonies and the mode of mineralization are the characteristic common features of the neoarchean microbiota described and their modern analogues the new findings indicate massive production of calcium carbonates by benthic coccoid cyanobacteria in the neoarchean and offer a solution to the problem of the origin of archean carbonate platforms stromatolites and microbial reefs

authigenic trioctahedral smectites controlling ph alkalinity silica and magnesium concentrations in alkaline lakes abstract the mineralogical composition of salts and the chemical composition of brines of lake yoa ounianga el kebir northern chad both reveal the neoformation of stevensite a trioctahedral smectite accompanied by aragonite the coprecipitation of these two minerals clearly controls the evolution of ph alkalinity and ca mg and silica concentrations in alkaline brines the geochemical evolution characterized by a strong depletion of ca and mg in solution and an increase in alkalinity and ph is confirmed as the dominant process governing the formation of soda alkaline lakes

fossilization modes in the chengjiang lagerstatte cambrian of china testing the roles of organic preservation and diagenetic alteration in exceptional preservation abstract one important and poorly understood issue concerning the taphonomy of nonmineralizing organisms in cambrian burgess shale type deposits is the diagenetic pathways by which organisms have become exceptionally preserved scanning electron microscopy sem analyses of exceptionally preserved fossils from the lower cambrian chengjiang deposit of china demonstrate that nonmineralized tissue is preserved in a variety of ways and further suggest that there was some taxonomic control over the precipitation of authigenic minerals during early diagenesis organic preservation is limited to certain decay resistant structures such as heavily sclerotized spines on the claws of anomalocaridids labile organic tissues of nearly all nonmineralizing animals however are preserved by thin films of diagenetic minerals microstructural studies energy dispersive x ray edx analyses and elemental mapping of these minerals indicate that mineral films include apatite pyrite and fe rich aluminosilicates the results support the assumption that early diagenetic mineralization such as phosphatization and pyritization played a key role in the preservation of nonmineralized organisms pyritization seems to be the most important process by which nonmineralizing chengjiang organisms are preserved in exceptional condition precipitation of fe rich aluminosilicates which occurred following pyritization also played a role in the preservation of some chengjiang fossils phosphatization in the chengjiang lagerstatte was evidently rare and limited taxonomically or to animals having large masses of organic material

production and cycling of natural microbial exopolymers eps within a marine stromatolite extracellular polymeric secretions eps that are produced by cyanobacteria represent potential structuring agents in the formation of marine stromatolites the abundance production and degradation of eps in the upper layers of a microbial mat forming shallow subtidal stromatolites at highborne cay bahamas were determined using 14 c tracer experiments and were integrated with measurements of other microbial community parameters the upper regions of a type 2 reid r p visscher p t decho a w stolz j bebout b macintyre i g dupraz c pinckney j paerl h prufert bebout l steppe t des marais d 2000 the role of microbes in accretion lamination and early lithification of modern marine stromatolites nature london 406 989 992 stromatolite mat exhibited a distinct layering of alternating bgreenq cyanobacteria rich layers layers 1 and 3 and bwhiteq layers layers 2 and 4 and the natural abundance of eps varied significantly depending on the mat layer the highest eps abundance occurred in layer 2 the production of new eps as estimated by the incorporation of 14 c bicarbonate into eps occurred in all layers examined with the highest production in layer 1 and during periods of photosynthesis i e daylight hours a large pool i e up to 49 of the total 14 cbicarbonate uptake was released as low molecular weight mw dissolved organic carbon doc this doc was rapidly mineralized to co2 by heterotrophic bacteria eps degradation as determined by the conversion of 14 c eps to 14 co2 was slowest in layer 2 results of slurry experiments examining o2 uptake following additions of organic substrates including eps supported this degradation trend and further demonstrated selective utilization by heterotrophs of specific monomers such as acetate ethanol and uronic acids results indicated that natural eps may be rapidly transformed post secretion by heterotrophic degradation specifically by sulfate reducing bacteria to a more refractory remnant polymer that is relatively slow to accumulate a mass balance analysis suggested that a layer specific pattern in eps and low mw doc turnover may contribute to major carbonate precipitation events within stromatolites our findings represent the first estimate of eps

mg silica precipitation in fossilization mechanisms of sand tufa endolithic microbial community mono lake california abstract the detection of mg silica associated with endolithic biofilms of cyanobacteria and bacterial communities living in lake shore sand tufa formations in mono lake california has been observed using scanning electron microscopy with backscattered electron imaging sem bse and energy dispersive x ray spectroscopy eds mg accumulated in cell walls of living cyanobacteria is interpreted to promote mg silicification with structural preservation while non cyanobacterial prokaryotes are not fossilized however extracellular polymeric substances around the living communities were prominent as nucleation sites for authigenic mineral precipitation a mg silicate served to stabilize the biofilm remains further diagenesis of these textures resulted in occlusion of the cytoplasm pore spaces by carbonate precipitates this precipitate damaged the microfossil structures but the mg silica fabric was still present the paleontological significance of mg silica precipitates as a possible biosignature of the former presence of endolithic biofilms is addressed

authigenic phyllosilicates in modern acid saline lake sediments and implications for mars 1 aluminum and fe mg phyllosilicates are considered important geochemical indicators in terrestrial and martian sedimentary systems traditionally al phyllosilicates are characterized as forming and remaining stable under conditions of low to moderate ph while fe mg phyllosilicates are considered representative of only dilute and moderate to high ph conditions however we have observed al and fe mg phyllosilicates in acid saline lake sediments in western australia phyllosilicate formation mechanisms in these lake systems include direct precipitation from lake waters early diagenetic precipitation from shallow groundwaters and deposition alteration of detrital grains x ray diffraction analysis of silt and clay size sediments from two acid saline lakes in western australia indicates the presence of several complex mineral assemblages with extreme spatial heterogeneity that reflects the complex geochemistry of these lakes these assemblages include unique combinations of authigenic and or detrital phyllosilicates e g kaolinite smectite and palygorskite sepiolite sulfates e g alunite jarosite and gypsum fe oxides e g hematite and goethite and other silicates e g mullite and heulandite clinoptilolite observations of fe mg phyllosilicates found in acid saline sediments in southern western australia suggest their degradation under conditions of low ph 2 5 5 4 is slowed by the high salinity 5 25 of the lake and shallow groundwaters the occurrence of both al and fe mg phyllosilicates in these acid saline lake sediments suggests that environmental interpretations based on the occurrence of phyllosilicates require additional consideration of their spatial distribution and association with other minerals moreover the similarity between the diverse mineral assemblages in these terrestrial acid saline systems and those on mars indicates similar conditions may have existed on mars

kiloyear scale climate events and evolution during the last interglacial mu us desert china the fifth segment of the milanggouwan stratigraphical section mgs 5 in the mu us desert provides high resolution geological information on environmental variations during the last interglacial the analysis of grain content 50 mu m organic content sio2 al2o3 tofe and sio2 al2o3 tofe ratios in the mgs 5 suggest that there were 17 kiloyear scale climate fluctuations in the last interglacial including 9 warm events w1 w9 and 8 cold events c1 c8 dominated by the east asian summer monsoon and winter monsoon respectively the analysis also suggests that the eemian interglacial was unstable with 3 warm events w7 w9 and 2 cold events c7 c8 indicating that climate fluctuations affected the east asian monsoon in the mu us desert during the last interglacial the change cycles and the nature of the kiloyear scale climate events have a close temporal relationship with the greenland ice core oxygen isotope data suggesting that the climate forming mechanism was affected by polar weather north atlantic sea ice range of the eurasian ice front and movement of the arctic frontal all of which affect the intensity of the siberian mongolian high pressure region through the movement of the cold air mass c 2012 elsevier ltd and inqua all rights reserved

taphonomic study of ediacaran organic walled fossils confirms the importance of clay minerals and pyrite in burgess shale type preservation burgess shale type bst macrofossils and organic walled microfossils are preserved as carbonaceous compressions and may share similar taphonomic processes previous taphonomic investigations of carbonaceous compressions have primarily focused on the microchemistry of cambrian bst fossils but comparative data from organic walled microfossils were not available to address these issues we analyzed two organic walled taxa from the yangtze gorges area of the south china block chuaria an acritarch and vendotaenia a ribbon shaped fossil their abundance offers the opportunity for destructive microanalysis including petrography electron microscopy and elemental mapping on both bedding planes and in cross sections our data suggest that chuaria preservation is remarkably similar to bst fossils in that its vesicle walls are closely associated with clay minerals in addition like many bst macrofossils chuaria and vendotaenia are also closely associated with pyrite chuaria vesicles are often filled with framboidal pyrite and vendotaenia fossils are associated with sulfate partly derived from pyrite oxidation the comparative taphonomy of chuaria and vendotaenia and bst macrofossils indicates that the preservation of organic walled acritarchs may be aided by clay and pyrite mineralization and that these abundant microfossils may serve as proxies for uncovering in more detail the taphonomic histories of bst preservation

formation of fe silicates and fe oxides on bacterial surfaces in samples collected near hydrothermal vents on the southern explorer ridge in the northeast pacific ocean samples collected in low temperature 2 50 degrees c waters near hydrothermal vents of the southern explorer ridge in the northeast pacific ocean contained fine 500 nm fe and mn oxide and fe silicate particles coating bacterial surfaces partially to totally mineralized bacteria along with bacterial exopolymers were covered with a mixture of poorly ordered si rich fe oxides possibly ferrihydrite mn oxides and fe silicates possibly nontronite minerals occur as very fine 2 20 nm granular material fine 20 100 nm needles and sheets small 200 500 nm nodules and filaments i e mineralized exopolymers under saturation conditions we infer that bacterial surfaces provided nucleation sites for poorly ordered oxides and silicates the formation of fe and mn oxides was likely initiated by the direct binding of soluble fe and mn species to reactive sites like carboxyl phosphate and hydroxyl groups present within the bacterial cell wall and the exopolymers fe silicate formation involved a more complex binding mechanism whereas metal ions such as fe possibly bridged reactive sites within the cell walls to silicate anions to initiate silicate nucleation

remarkable preservation of microbial mats in neoproterozoic siliciclastic settings implications for ediacaran taphonomic models abstract it is beyond doubt that the appearance of infaunal bioturbation and metazoan biomineralization across the ediacaran cambrian transition irreversibly affected the nature of marine sediment architecture and biogeochemistry here we review those changes in relation to their likely effect upon the processes of fossil preservation especially within siliciclastic sediments processes of soft tissue preservation in siliciclastic settings from the ediacaran period including microbes and microbial mats as well as ediacaran macrofossils are here reviewed within this context highlighted examples include the exceptional preservation of microbes found in association with wrinkle structures and ediacaran macrofossils in england and newfoundland replicated by silicate minerals and in the white sea region of russia replicated by iron sulphide these occurrences show that soft tissue preservation in siliciclastic settings went well beyond that typical for ediacaran macrofossils alone and also extended to similar modes of preservation in associated microbes using these new observations it can be argued that several existing explanations for ediacaran fossil preservation can be united within a biogeochemical model that involves evolution of the sediment mixed layer across this transition

early diagenesis of biogenic silica in the amazon delta alteration authigenic clay formation and storage deltaic environments are commonly assumed to be relatively minor sites of biogenic silica burial because of the small quantities of opaline silica detected by most operational analytical techniques rapid conversion of biogenic silica into authigenic silicates is also often discounted as a significant control on oceanic silica budgets a variety of evidence for extensive early diagenetic alteration of biogenic silica in rapidly accumulating amazon delta sediments indicates that both of these general assumptions are unjustified apparent lack of significant biogenic silica storage in deltaic environments particularly in the tropics may be largely an artifact of operational definitions that do not include early diagenetic products of biogenic silica biogenic silica particles buried in suboxic amazon delta deposits can be unaltered partially dissolved covered with aluminosilicate or metal rich coatings or completely reconstituted into authigenic k fe rich aluminosilicate minerals pore water k mg f si and solid phase distributions direct observations of particles laboratory experiments and depositional context indicate that authigenic clays form rapidly 1 yr in the seasonally reworked surface layer 0 5 2 m of the delta topset and are disseminated during sediment remobilization fe al oxide rich debris derived from the tropical drainage basin is an abundant reactant and thus the supply of biogenic silica is a major control on the amount of clay formed the mild 1 na2co3 alkaline leach procedure commonly used to estimate biogenic silica was modified to include an initial mild leach step with 0 1n hcl to remove metal oxide coatings and to activate poorly crystalline authigenic phases for alkaline dissolution well crystallized clays are not significantly affected by this modification nor is bulk amazon river bed sediment the two step procedure indicates that 90 of the biogenic silica originally present in deposits is converted to clay or otherwise altered raising the effective quantity of biogenic silica stored from 33 to 296 mol si g 1 1 8 sio2 biogenic si stored in the delta increases away from the river mouth across shelf and along the dispersal system where primary production is highest the k si ratio of labile authigenic material is 0 19 mol mol 1 far higher than amazon river suspended matter 0 07 mol mol 1 diagenetic models indicate formation rates in the mobile sediment layer of 2 8 mol k g 1 yr 1 16 mol si g 1 yr 1 inclusion of authigenic alteration products of biogenic silica in estimates of reactive si burial increases the deltaic storage of riverine si to 22 of the amazon river input the rapid formation of aluminosilicates from biogenic sio2 seawater solutes and remobilized fe al oxides represents a form of reverse weathering rapid reverse weathering reactions in tropical muds and deltaic deposits the largest sediment depocenters on earth confirms the general importance of these processes in oceanic elemental cycles

cyanobacterial mats from silurian black radiolarian cherts phototrophic life at the edge of darkness abstract remains of silicified microbial mats composed of benthic colonial coccoid cyanobacteria similar to modern entophysalidaceans and or pleurocapsaleans have been identified in lower silurian black radiolarian cherts from central and southwestern poland contrary to widespread views ascribing the genesis of such deposits to permanently anoxic deep water marine environments the abundance of benthic mats of phototrophic cyanobacteria suggests that the water mat interface must have been located at moderate depth most probably close to the limit of light penetration dysphotic zone depending on ambient sulfide levels the mats could intermittently perform anoxygenic psi or oxygenic psii photosynthesis thriving under anoxic oxic or dysoxic microaerophilic conditions the open marine offshore character of these cherts is consistent with their paleooceanographic location and with the presence of remains of such planktonic organisms as acritarchs radiolarians chitinozoans and graptolites entrapped by the cyanobacterial mats

microbialite formation in seawater of increased alkalinity satonda crater lake indonesia abstract the crater lake of the small volcanic island satonda indonesia is unique for its red algal microbial reefs thriving in marine derived water of increased alkalinity the lake is a potential analogue for ancient oceans sustaining microbialites under open marine conditions current reef surfaces are dominated by living red algae covered by non calcified biofilms with scattered cyanobacteria and diatoms minor caco3 precipitates are restricted to the seasonally flooded reef tops which develop biofilms up to 500 m thick dominated by the cyanobacteria pleurocapsa calothrix phormidium and hyella microcrystalline aragonite patches form within the biofilm mucilage and fibrous aragonite cements grow in exopolymer poor spaces such as the inside of dead lysed green algal cells and reef framework voids cementation of lysed hadromerid sponge resting bodies results in the formation of wetheredella like structures hydrochemistry data and model calculations indicate that co2 degassing after seasonal mixis can shift the carbonate equilibrium to cause caco3 precipitation increased concentrations of dissolved inorganic carbon limit the ability of autotrophic biofilm microorganisms to shift the carbonate equilibrium therefore photosynthesis induced cyanobacterial calcification does not occur instead passive diffusion controlled eps mediated permineralization of biofilm mucus at contact with the considerably supersaturated open lake water takes place in contrast to extreme soda lakes the release of ca2 from aerobic degradation of extracellular polymeric substances does not support caco3 precipitation in satonda because the simultaneously released co2 is insufficiently buffered subfossil reef parts comprise green algal tufts encrusted by microstromatolites with layers of fibrous aragonite and an amorphous unidentified mg si phase the microstromatolites probably formed when lake satonda evolved from seawater to ca2 depleted raised alkalinity conditions because of sulfate reduction in bottom sediments and pronounced seasonality with deep mixing events and strong co2 degassing the latter effect caused rapid growth of fibrous aragonite while mg si layers replaced the initially mg calcite impregnated biofilms this could be explained by dissolution of siliceous diatoms and sponge spicules at high ph followed by mg calcite dissolution and mg silica precipitation at low ph due to heterotrophic activity within the entombed biofilms

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

preservational modes in the ediacaran gaojiashan lagerstatte pyritization aluminosilicification and carbonaceous compression abstract burgess shale type bst fossils often are preserved as two dimensional carbonaceous compressions sometimes aided by two mineralization processes pyritization and aluminosilicification defined by a thin and sometimes localized coating of authigenic pyrite or aluminosilicate minerals on the carbonaceous materials here we report similar mineralization modes within the late ediacaran gaojiashan lagerstatte of southern shaanxi province south china examination of two common gaojiashan fossils conotubus hemiannulatus and gaojiashania cyclus hereafter referred by generic names only indicates that pervasive pyritization is the primary taphonomic pathway in this deposit responsible for the preservation of 80 of the collected fossils pervasive pyritization of gaojiashan fossils results in their three dimensional preservation as opposed to two dimensional carbonaceous compression with thin pyrite coatings seen in some bst lagerstatte however microscale investigation using environmental scanning electron microscopy esem and energy dispersive x ray spectroscopy eds shows that some flattened specimens of conotubus and gaojiashania are preserved as carbonaceous compressions which are in some cases associated with aluminosilicate minerals rare compressed specimens of conotubus are replicated by greenish clay minerals which are interpreted as diagenetic products of aluminosilicate precursors our taphonomic analysis shows that pervasive authigenic pyritization tends to replicate more labile tissues than aluminosilicate aided carbonaceous compression but both pyritization and aluminosilicification processes can facilitate the replication and preservation of soft bodied organisms in siliciclastic sediments building upon this conclusion we propose that multiple taphonomic processes including kerogenization or polymerization of organic molecules pyritization and aluminosilicification can contribute to the mode of bst preservation viewed in this light various siliciclastic hosted lagerstatten that are facilitated by kerogenization pyritization and aluminosilicification can be plotted on a ternary diagram based on the relative importance of these taphonomic processes

cambrian burgess shale animals replicated in clay minerals although the evolutionary importance of the burgess shale is universally acknowledged there is disagreement on the mode of preservation of the fossils after burial elemental mapping demonstrates that the relative abundance of elements varies between different anatomical features of the specimens these differences reflect the compositions of the minerals that replicated the decaying organism which were controlled by contrasts in tissue chemistry delicate morphological details are replicated in the elemental maps showing that authigenic mineralization was fundamental to preserving these fossils even though some organic remains are also present

climatically induced lake level changes at lake van turkey during the pleistocene holocene transition sediment core k10 from lake van eastern turkey provides a continuous varve record back to 14 570 calendar years b p before present 1950 the longest unbroken and non floating lake varve sequence yet described the underlying sediment is unvarved and hard changes in the aragonite calcite ratio the presence of protodolomite and magnesite in certain profile sections the annual record of the sedimentation rate the water content of the sediment the concentrations of organic carbon and opal and the texture of the sediments from this core provide a record of the lake level history the new chronology enabled us to redate the old pollen profile van zeist and woldring 1978a b and to establish an accurate timescale for the reconstructed lake level change carbon 14 dates show that the highest lake terrace corresponds to high lake level at around 19 000 years b p during the last glacial 70 m above its present level before 15 000 years b p the lake must have been completely dry marking a reduction of lake level by 500 m in maximum 4000 years beginning at 14 600 years b p and ending at 12 040 years b p the lake level recovered by 250 m to fall again during the next 1400 years by 10 600 years b p the lake began to rise and reached following another regression between 9000 and 8100 years b p the holocene highstand by about 7500 years b p dropping to today s level at about 3000 years b p

authigenic smectite on diatom frustules in bolivian saline lakes abstract observation under the electron microscope of diatom frustules from bolivian altiplano saline lakes shows that many of these are coated with particles occurring as tiny sheets the frustules can be found to be almost completely replaced by these sheets isolated sheet aggregates seem to have resulted from completely transformed frustules section observations of altered frustules bear out that the sheets have grown from biogenic silica through replacement selected area diffraction dark field observation microdiffraction and elemental microanalysis show that the particles on the diatom frustules consist of a poorly crystallized mg smectite the unambiguous localisation of this authigenesis allows us to reconstruct its hydrochemical and sedimentological environment observation of the most recent lake sediments has pointed out that at least two main conditions are required for this authigenesis at 5 c saturation with respect to amorphous silica and a ph above 8 2 variations in the mg concentration have no significant effect

a geological study of lake van eastern turkey lake van is a lake with a volume of 607 km3 and a maximum depth of 451 m in a tectonically active zone in eastern anatolia it may be divided into three physiographic provinces lacustrine shelf sublacustrine slope and lake basin the shelf is largely made up of submerged fluvial valleys and alluvial plains as evidenced by the local occurrence of karst like topography buried leveed river valleys and sediment wedges interpretable as relict deltas the shelf and basinal deposits are characterized by an alternating succession of well stratified and chaotically reflecting layers consisting of turbidites and slump deposits respectively this thick sequence of unconsolidated to consolidated sediments is underlain by a paleozoic metamorphic or upper cretaceous limestone basement

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

bacteria as nucleation sites for authigenic minerals in a metal contaminated lake sediment abstract sediment samples contaminated with metals arising from mine tailings drainage were obtained from lower moose lake in the onaping region near sudbury ontario canada the samples were examined by electron microscopy selected area electron diffraction and energy dispersive x ray spectroscopy individual bacterial cells and their remains were prominent as nucleation sites for both metal sulfides and a complex polymorphic fe al silicate the principal metal sulfide species associated with the bacteria were amorphous mackinawite fes 1 x and microcrystalline millerite nis trace amounts of cu and zu were also detected in some of the sulfide precipitates at least two structural forms of the fe al silicate were present and energy dispersive x ray spectroscopy point analyses revealed corresponding differences in chemical composition poorly ordered limonitic clay type phases had a granular morphology and contained less fe than well developed crystalline material which generated hexagonal diffraction patterns with reflections d 4 60 and 2 55 characteristic of an interstratified chamositic clay

microbialite formation in seawater of increased alkalinity satonda crater lake indonesia discussion arp et al 2003 present data which confirm and refine the results of our long time studies field campaigns of 1986 1993 1996 on satonda crater lake summarized in kempe and kazmierczak 1993 they reiterate the importance of marine stratified water bodies in generating through sulfate reduction excess alkalinity which in turns causes very high caco3 supersaturation levels this geochemical process called the alkalinity pump kempe 1990 was conceptually refined using the quasi marine satonda lake as a geochemical model for some epicratonic marine basins from the geological past kempe and kazmierczak 1994 the concept of an alkalinity pump is important for understanding the abundance of calcified benthic cyanobacterial mats in ancient shallow seas leading to the formation of a variety of marine calcareous microbialitic structures and fine grained calcareous sediments accumulated on the seafloor kempe and kazmierczak 1990 kazmierczak et al 1996 while principally agreeing with arp et al s results we strongly disagree with two of their major conclusions the first major objection concerns arp et al s scepticism with respect to the calcification potential of benthic cyanobacterial mats presently living in the lake on the basis of samples gathered in 1993 and 1996 arp et al conclude that these mats calcify very weakly or in some cases do not calcify at all this sharply contradicts our observations showing strong permineralization of the mats both with caco3 and silica e g kempe and kazmierczak 1993 pl 6 figs 3 6 mat samples we took at the end of dry season 1986 demonstrate that calcification of the cyanobacteria occurred in vivo masses of minute grains of caco3 predominantly mg calcite were observed not only on the mat surface but also within the common mucilage sheaths glycocalyx surrounding aggregates of living cyanobacterial coccoid cells classified as representatives of pleurocapsales minute caco3 grains

stevensite in the modern thrombolites of lake clifton western australia a missing link in microbialite mineralization microbialites form the earliest macroscopic evidence of life and have always been important in particular aquatic ecosystems they demonstrate the remarkable ability of microorganisms to provide the foundation for structures that can rival coral reefs in size microbialites are generally assumed to form by microbial trapping and binding of detrital grains by carbonate organomineralization of microbial biofi lms or by inorganic mineralization around microbial templates here we present a signifi cant discovery that modern thrombolitic microbialites in lake clifton western australia gain their initial structural rigidity from biofi lm mineralization by the trioctahedral smectite mineral stevensite this nucleates in and around microbial fi lament walls when biological processes suppress carbon and ca activities leaving mg to bind with silica and form a microporous framework that replaces and infi lls the fi lament web after microbial materials are entombed local carbon and ca activities rise suffi ciently for aragonite microcrystals to grow within the stevensite matrix and perhaps replace it entirely with eradication of biogenic textural features this may explain why many ancient microbialite carbonates lack clear evidence for biogenicity stevensite may provide the missing link between microbial organomineralization and subsequent abiotic calcifi cation

calving processes at a grounded ice cliff repeat photographs and field survey reveal the mechanism of short term ice cliff evolution at maud glacier a temperate lake calving glacier in new zea land calving is cyclic each cycle involving four stages 1 waterline melting and collapse of the roof of a sub horizontal notch at the cliff foot 2 calving of ice flakes from the cliff face leading to a growing overhang from the waterline upwards and crack propagation from the glacier surface 3 large but infrequent calving of slabs in response to the devel oping overhang returning the cliff to an initial vertical profile 4 rare subaqueous cal ving of a submerged ice foot results indicate that the rate controlling process is the speed of waterline melting and that calving rate is independent of water depth at least at time scales of weeks to months slowly calving lake terminating glaciers have mass balances more negative than land terminating glaciers but nevertheless advance and retreat in response to mass balance driven changes in ice velocity