cave pearls the integrated product of abiogenic and biogenic processes abstract cave pearls up to 8 cm long collected from dried out rimstone pools in old man village cave on grand cayman british west indies have nuclei e g bone that are encased by laminated cortices these laminae are composed of trigonal prismatic calcite crystals calcite crystal bushes or particulate calcite that is formed of filamentous microbes four morphotypes spores seven morphotypes extracellular polymers eps disorganized and organized calcite fiber crystals and detrital calcite grains syntaxial overgrowths modified many of the fiber crystals and the trigonal calcite crystals thin laterally discontinuous growth lines are formed of micrite that may have been precipitated in a microbial mat growth of the cave pearls involved abiogenic and biogenic processes the trigonal calcite crystals and crystal bushes probably formed abiogenically whereas the particulate calcite laminae appear to have formed as microbes trapped and bound detrital grains to the surfaces of the cave pearls and indirectly mediated precipitation of the fiber crystals water flow into the rimstone pools which was linked to local rainfall controlled the influx of detrital grains water chemistry and microbe growth into the pools and hence the growth of the cave pearls periods of low water influx led to stagnation of pool waters that seemed to trigger periods of rapid abiogenic calcite precipitation and curtail microbial colonization high flow rates slowed abiogenic calcite precipitation promoted microbe growth and fed detrital materials into the pools despite the lightless environment of the cave the evidence clearly shows that the microbes and their associated eps played a critical role in the growth and development of the cave pearls

lamination of swampy rivulets rivularia haematites stromatolites in a temperate climate abstract swampy rivulets rivularia haematites d c agardh stromatolites were collected from the same site on the plateau de langres chaugey france over a seven year period specimens were studied in thin sections in culture media under varying conditions of temperature and illumination and by scanning electron microscope sem in order to investigate the processes of calcification and lamination new sem observations confirm the polycrystalline structure of the lamellae reported by the authors and show that the crystals composing these lamellae are elongate and aligned in the same direction as the filaments linked by them the lamellae were also isolated in transverse sections under sem and found to correspond to what under the light microscope appear to be honeycomb structures the diameter of alveoli can be the same as that of a false branching their walls formed by the outer sheath have a dense network of microfibrils these observations confirm the formation of calcitic lamellae at the false branching points and in contact with the microfibrils of the outer sheath confirmation of earlier hypotheses was sought through observation in thin section of the stromatolites collected over seven years micritic dark laminae are thought to form in three stages i formation of a dark lamina by formation of a false branching zone in the wet season ii initial calcification in this zone as microsparitic and sparitic lamellae during even a short dry spell and iii subsequent bacterial micritization of the lamellae during an extended warm dry season the sparitic light laminae form around the hair bearing regions lamination was first considered with regard to the annual radial growth rate of r haematites the thin section may display either a single dark lamina for two years growth or a dark lamina thicker than the annual growth rate examination of local meteorological data accounts for the first phenomenon by a wet spell of more than 13 months and the second by a warm dry period of one month it may therefore take several years for a couplet to form as growth of the cyanobacterial colony is influenced by climatic variations the different calcification phenomena were also correlated with dry spells these interpretations are consistent with results obtained by experiments

giant holocene freshwater microbialites laguna bacalar quintana roo mexico with more than 10 km of total length holocene microbialites in laguna bacalar mexico belong to the largest freshwater microbialite occurrences microbialites include domes ledges and oncolites domal forms can grow to diameters and heights of 3 m microbialites are composed of low magnesium calcite which is to a large extent precipitated due to the metabolic activity of the cyanobacteria homeothrix and leptolyngbya and associated diatoms photosynthesis removes carbon dioxide and triggers carbonate precipitation also an elevated carbonate concentration in lagoon waters derived from dissolution of cenozoic limestone in a karst system supports carbonate precipitation trapping and binding of detrital grains is also observed but is not as common as precipitation bacalar microbialites are largely thrombolitic however stromatolitic sections occur as well the bulk of bacalar microbialites probably formed in the late holocene ca 1 kyr bp until present according to 14c dating microbialites accreted 9 to 8 cal kyr bp however these ages may be too old as a result of a strong hard water effect this effect is seen in 14c ages of living bivalve and gastropod mollusc shells from bacalar lagoon which are 8 to 7 cal kyr bp the modern associated fauna of microbialites is characterized by low diversity and high abundance of the bivalve mollusc dreissena sp and the gastropod pomacea sp the abundant grazing gastropods presumably hamper modern microbialite formation a comparison of bacalar microbialites with other modern microbialite occurrences worldwide shows only a few patterns sizes shapes microbial taxa mineralogy type of accretion and settings including water properties of microbialite occurrences exhibit high variability a trend may be seen in the grazing metazoa which are rare to absent in the marine and brackish examples but apparently present in all the freshwater occurrences of microbialites also freshwater examples are usually characterized by elevated concentrations of carbonate and or calcium ions in the surrounding waters

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

subtidal stromatolites in monterey formation and other organic rich rocks as suggested source contributors to petroleum formation the miocene monterey formation contains thin organic rich laminations that mimic benthic mats forming today in coastal upwelling regimes of the eastern pacific modern mats are comprised dominantly of filamentous sulfur oxidizing bacteria of the family beggiatoaceae the characteristic spongy texture imparted to the sediments by these bacterial mats is most readily recognized using scanning electron microscopy sem the spongy network becomes progressively compressed with increasing compaction and diagenesis but is still distinguishable rock eval pyrolysis of modern mats reveals high s1 peaks and unusually high oxygen indices both parameters decrease gradually with diagenesis as evidenced by mat laminated monterey formation samples of different thermal grades in addition anomalously light nitrogen isotopic signatures of modern mats may be retained the dgrn15 values of mat laminated monterey formation samples although diagenetically enriched in 15n are still lighter than would be expected for algally dominated organic matter in marine low oxygen environments on the basis of organic richness and high hydrogen indices of mat laminated samples and coincidence of mat laminated rocks and oil in some fields in the san joaquin valley of california the organic matter assemblage contained within filamentous bacterial mats is suggested to be a significant contributor to petroleum formation additional evidence for this conclusion is provided by similar investigations of subtidal cyanobacterial mats in carbonate rocks such as the green river formation oil shales

on the appearance of cyanobacterial calcification in modern stromatolites abstract in august 1991 calcified magnesian calcite filamentous cyanobacteria scytonema mirabile appeared on the surface of a modern lacustrine stromatolite lake r2 rangiroa atoll tuamotu archipelago french polynesia previously the topmost photosynthetic layer of this sediment was devoid of such microbial biomineralisers the calcified cyanobacteria are accompanied by magnesian calcite grains which are evocative of bacterial precipitates although the appearance of cyanobacterial calcification might be related to environmental changes in the lake in particular a decrease in water salinity the persistence of non calcified specimens of a filamentous cyanobacteria phormidium crossbyanum which was once a dominant species of the lake benthos shows that calcification and environmental conditions might be only indirectly related thus attempts to use the fossil record of calcareous cyanobacterial skeletons to infer changes in the chemical composition of past oceans cannot be made without due regard of biological processes like the appearance of new competitors

signatures of biologically influenced caco3 and mg fe silicate precipitation in hot springs case study from the ruidian geothermal area western yunnan province china hot springs at gongxiaoshe and zhuyuan maximum temperatures of 73 to 84 c respectively are characterized by deposits formed of calcite aragonite non crystalline si mg fe deposits and minor amounts of barite and gypsum the deposits at gongxiaoshe are formed largely of alternating calcite and aragonite laminae whereas those at zhuyuan are formed largely of calcite the calcite is in the form of i pseudodendrites that grew as sub crystals stacked upon each other and ii unattached euhedral and incompletely formed dodecahedral and rhombohedral crystals amorphous caco3 formed of nanoparticles 1 m long is common in some of the zhuyuan deposits but minor in the gongxiaoshe deposits the morphologically diverse arrays of aragonite crystals that lie parallel to bedding were not nucleated on a growth surface many substrates in these deposits are covered with reticulate coatings that are formed largely of si and mg with minor fe and micro granular coatings that are formed largely of si and fe biofilms with their extracellular polymeric substances and microbes are common at both springs the compositionally and crystallographically diverse precipitates at these two springs are attributed to a biologically influenced model with precipitation taking place in micro domains that developed in the extracellular polymeric substances according to this model precipitation varied at the micron scale influenced by the elemental concentrations that developed in the hydrogel of extracellular polymeric substances critically the very low preservation potential of the extracellular polymeric substance and its formative microbes means that the precipitates will rapidly lose evidence of their biotic origin the compositional diversity of the precipitates the crystallographic diversity of the calcite and aragonite with numerous incompletely formed crystals and local concentrations of si mg and fe may however serve as proxies of that biologically influenced precipitation

a jurassic counterpart for modern kopara of the pacific atolls abstract two types of laminated sediments are compared in this paper the kimmeridgian bituminous laminites of orbagnoux french southern jura mountains and the present day kopara of the french polynesia atolls or kiritimati christmas island from the pacific ocean the kopara is made of laminated sediments several tens of centimetres thick that cover the floors of most shallow 3 rich laminae the top few centimetres host a succession of bacterial populations from aerobic to strictly anoxic most carbonate grains are precipitated in situ due to bacterial activity s l the kimmeridgian bituminous laminae of orbagnoux are partly made of flat stromatolites that show many similarities with the kopara this analogy allows us to refine the interpretation of the southern jura platform where laminated organic matter rich carbonates were deposited in many places

christmas island lagoonal lakes models for the deposition of carbonate evaporite organic laminated sediments the atoll of christmas island now known as kiritimati in the kiribati republic central pacific lies at about 28n in the intertropical convergence zone much of the surface area of the atoll ca 360 km 2 is occupied by numerous lakes in which carbonate evaporite calcium sulfate halite and organic layers are deposited observations suggest that deposition of these different laminae is controlled by climatic and biologic factors it is thought that periodic climatic variations such as el nino southern oscillations enso events which bring heavy rainfall to the atoll result in the succession of the precipitation of carbonate minerals during periods after dilution of hypersaline waters by heavy rains followed by evaporitic minerals carbonate calcium sulfate halite when salinity increases through evaporation thick up to 5 cm microbial essentially cyanobacterial mats develop continuously on the lake bottom surfaces providing the sediment with an important total organic carbon 2 5 organic contribution in the form of an internal geometrically structured network in which the authigenic minerals precipitate the high bioproductivity of these microbial populations is reflected in low d 13 c values of sedimentary organic carbon 214 to 217 interpreted as being the result of high atmospheric co2 demand geochim cosmochim acta 56 1992 335 the well laminated organic layers present in the sediment profile result from the death and burial of microbial populations at the time of severe climatic events storms heavy rainfall these lagoonal lakes provide a model for the deposition of carbonate and organic matter in an evaporitic environment the high ratio of deposited carbonate vs sulfate 1 chloride when compared to low ratio in evaporitic salinas results from both a lack of limitation of calcium magnesium and carbonate ions in a carbonate reef environment and active processes of high mg calcite precipitation organomineralization q 2001 elsevier science b v all rights reserved

decoding the mechanism of formation in marine ooids a review abstract ooid shoals sedimentary structures consisting of carbonate grains are often used as proxies to explore changes in seawater chemistry over earth history despite their importance as contributors to the global carbonate budget and as paleo environmental indicators for water depth ph temperature and water energy the mechanism of formation of these concentric sub spherical concretions of tangentially or radially arranged aragonite or calcite crystals is still the subject of intense debate unlike oncoids whose microbial origins are well acknowledged ooids are traditionally viewed as abiotic grains mainly produced in suspension by agitated warm waters that are supersaturated with respect to caco3 yet there is considerable evidence including geochemical bio signatures and detection of viable microbes capable of mineralization and biofilm production and degradation to suggest otherwise the emergence of new and sophisticated technologies reveals that the level of microbial diversity of these sedimentary grains exceeds that of some microbialite systems these findings have refined our understanding of ooid genesis and ooid accretion processes and argue toward microbial mediation both as a direct result of metabolic activities inducing changes in the environment conducive to precipitation and by a passive mechanism in which mucilaginous extracellular polymeric substances eps act as a template for mineral nucleation furthermore new developments in the field indicate that carbonate mineralization in marine ooids occurs through sequential precipitations that involve an amorphous calcium carbonate phase acc which some argue are of biotic origins herein we discuss the most relevant theories of ooid formation including some of the early pioneering studies that laid the groundwork for new theories attempting to decode the origins of these controversial grains

occurrence and significance of microbialites in the uplifted tasmaloum reef sw espiritu santo sw pacific abstract in the sw pacific ocean subduction of the d entrecasteaux ridge system has caused rapid uplift of the central new hebrides island arc the maximum uplift rate of 6 mm yr 1 occurs along the southwest coast of espiritu santo island near tasmaloum the tasmaloum uplifted reef sequence internal structure which is strongly linked to its tectonic context was investigated through a series of drill holes to depths up to 42 m although a stable tropical coast would undergo approximately 120 m of post glacial sea level rise the net relative sea level rise on such a rapidly uplifting coast is only about 20 m colonization of the tasmaloum fringing reef occurred by 24 ka upon a pre reef substrate composed of a thick bioclastic sand formation accumulated during the last glacial period during the post glacial sea level rise the vertical succession of reef assemblages reflects environmental and bathymetric variations controlled by the interplay of rapid but variable rates of sea level rise and more or less constant uplift of 5 6 mm yr 1 microbialite crusts composed of high magnesian calcite laminae occur in the tasmaloum reef from 20 to 6 ka and are particularly abundant from 16 to 10 ka the development of microbialite crusts is related to nutrient enrichment of interstitial waters through mixing with meteoric groundwater after 6 ka when sea level ceased rising in the region and continuing uplift caused rapid emergence of the reef microbialites disappear within the subtidal assemblages several explanations can be put forward for their disappearance in particular nutrient input changes are a likely cause a new hydrologic and oceanographic regime was established when sea level ceased rising this change was accompanied by warming of tropical waters

patterns of biomediated caco3 crystal bushes in hot spring deposits in the eiyuan hot spring located in south china the vent pool is covered with crystal bushes up to 2 cm high 1 cm in diameter that grew in the biofilms that thrive in the spring waters that have a ph of 7 5 and a temperature of 88 degrees c the biofilms are formed largely of phototrophic purple bacteria and green bacteria growth of the crystal bushes which are formed of aragonite crystals wheat sheaves radiating clusters rhombohedral and dodecahedral calcite crystals amorphous caco3 acc and opal a is attributed to precipitation in the micro domains of the biofilms where physiochemical conditions can vary on the sub micron scale there is no evidence that the calcite was formed through recrystallization of the metastable aragonite and most of the calcite crystals developed as mesocrystals that are characterized by incomplete growth and porous crystal faces

microbialites in a modern lagoonal environment nature and distribution tikehau atoll french polynesia abstract microbialites including stromatolitic structures gelatinous masses and mats develop in tikehau lagoon tuamotu french polynesia on the flanks and slopes of numerous pinnacles and motu islets between 0 and 25 m deep on coral colonies algal tufts or on lagoonal sediments individual stromatolitic structures consist of hemispherical domes and are produced by monospecific populations of filamentous cyanobacteria belonging to three distinctive phormidium species symploca and possibly to one new species of schizothrix gelatinous masses hanging from the ceiling of caverns are produced by phormidium laysanense microbial mats are well developed at all depths on pinnacle slopes and are produced by either monospecific or mixed cyanobacterial populations the study of micro and ultrastructure of stromatolites reveals that they result both from trapping and binding of sediments and from carbonate precipitation in addition to phototrophic biomineralization pervasive carbonate precipitation occurred in association with decaying organic matter the basic constructional elements of the precipitated carbonate consist of 0 1 0 2 m rounded bodies that form very fine anhedral to subhedral micrite their surfaces comprise extremely small carbonate granules 20 50 nm gradual changes ageing in the ultrastructure of carbonate precipitates include the precipitation of successive euhedral calcite layers around these grains widespread development of microbialites in this pristine lagoonal environment results from rapid blooms of microbial benthic communities possibly induced by temporary and local nutrient pulses nutrients seem to undergo a restricted circulation between the sediment and pinnacle reef frameworks pinnacles and reef islets are repeatedly affected by catastrophic storm events which cause redistribution of sediments and nutrients

microbial carbonates as indicators of environmental change and biotic crises in carbonate systems examples from the late devonian alberta basin canada microbial precipitation of calcium carbonate has played a vital role in the development of carbonate platforms since their initiation in the proterozoic we report here the varied roles that microbial carbonates played in late devonian carbonate platforms in the alberta basin canada we recognize microbial carbonates as important contributors within the carbonate system during times of major environmental change including transgressive events in platform environments and the recovery interval following the frasnian famennian mass extinction detailed sequence stratigraphic analysis of two isolated platforms in the canadian rockies was used to document their evolution from a regional ramp to isolated platforms with phases of progradation aggradation and backstepping and renewed progradation related to rates of second order and third order sea level change and basin infill the carbonate system was reorganized following annihilation of many carbonate producing biota during the frasnian famennian mass extinction microbial carbonates figure prominently in both frasnian platform development and the famennian recovery of the carbonate system following the frasnian famennian mass extinction during transgressive platform phases deeper water facies with a crinoid rugose coral and microbe dominated biota abruptly overlie prograding stromatoporoid framestones microbial carbonates consist of abundant girvinellid oncoids that nucleated around other fossil clasts the temporary replacement of stromatoporoid communities during transgression implies a shift from oligotrophic to mesotrophic conditions and the microbes likely capitalized on the change in nutrient supply microbes played a significant role in highstand reef margin facies where renalcis was a binder and cementer in stromatoporoid framestones and downslope rugose coral stromatoporoid mounds microbially laminated carbonates also form stromatolitic mats in peritidal shallowing upward cycles and repetitive stromatolitic intervals commonly indicate sea level lowstands associated with the development of sequence boundaries stromatoporoid reefs and their associated reef interior facies indicate an oligotrophic ecosystem where microbial carbonates were relegated mainly to cryptic or stressed marine habitats an end frasnian sea level lowstand exposed the isolated carbonate platforms transgression and mass extinction characterize the frasnian famennian boundary event and microbial carbonates occur at this horizon in western canada stromatolites oncoids and large scale microbial thrombolites appear to be

kopara in polynesian atolls early stages of formation of calcareous stromatolites widespread modern calcareous stromatolites known as kopara in the vernacular language on the tuamotu archipelago have been found accreting on the bottom of shallow generally 1 m depth lakes located on the rims of these french polynesian atolls they are flat gelatinous sediments several tens of centimetres thick with lamination comprised of vertically alternating red organic rich and white carbonate rich millimetre thick laminae they originate from benthic microbial communities mainly composed of cyanobacteria generally dominated by the genus phormidium the living cyanobacteria occupy only the top 1 3 cm of the deposit the red organic matter below being made of remnants of the dead microbes essentially of polysaccharide microfibrils inherited from the sheaths and arranged into a three dimensional network enclosing pores ranging from a few tens of angstroms to a few micrometres wide the carbonates are predominantly high mg calcite 9 19 mole mgco3 which precipitated as micron size bunches within the pores on walls of the organic network it is sometimes accompanied by aragonite less than 22 of the total mineral fraction when present and lesser high mg calcite allochthonous bioclasts which occasionally form up to few centimetres thick detrital intercalations the pore waters are variable mixtures of freshwater rain or ground water and sea water from the ocean or lagoon which are feeding waters of the lakes their chemistry deviate from a straight mixture gradient with respect to ca2 mg2 and alkalinity under the influence of local calcification and bacterial processes e g ammonia production sulphate reduction within the sediments it is hypothesized that the calcification is initiated at carboxylic sites on the walls of the polysaccharide network the pores acting as confining organic compartments with increased internal supersaturation lamination is suggested to be due to the combined effects of the stratification of the microbial activity of the deposits and alternations of fresh and saline periods within the lakes lacustrine environments on oceanic atolls are found to be important habitats of modern microbialite formation

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

organic matter in anoxic sediments off central peru relations of porosity microbial decomposition and deformation properties abstract microstructure of central peru continental margin sediments suggests that in environments of rapid hemipelagic sedimentation organic matter decomposition has a significant effect on geotechnical properties in highly porous surface sediments particles resembling the remains of fecal pellets appear to be coated by bacterial sheaths but after burial sheaths disappear as an increasingly uniform and closed structure of organo mineral aggregates develops interstitial water nutrient and solid phase c n and p profiles support the interpretation that this progression corresponds to decomposition and metabolite regeneration from labile organic matter by microbial sulfate reduction it is proposed that early aggregation has a strengthening effect and that this combined with the low density of near surface sediments results in an effective overburden pressure lower than experimentally determined preconsolidation pressures more basic knowledge of the lifetime of organo mineral aggregates and the influence of gross and local microstructure on the rates and products of diagenetic reactions is needed in order to verify the relationship between organic matter accumulation and sediment stability in continental margin environments

the geology of the giant nyankanga gold deposit geita greenstone belt tanzania nyankanga is the largest gold deposit in the geita greenstone belt of the northern tanzania craton the deposit is hosted within an archean volcano sedimentary package dominated by ironstones and intruded by a large diorite complex the nyankanga intrusive complex the supracrustal package is now included within the intrusive complex as roof pendants the ironstone fragments contain evidence of multiple folding events that occurred prior to intrusion the supracrustal package and nyankanga intrusive complex are cut by a series of ne sw trending moderately nw dipping fault zones with a dominant reverse component of movement but showing multiple reactivation events with both oblique and normal movement components the deposit is cut by a series of nw trending strike slip faults and e w trending late normal faults the nyankanga fault zone is a major nw dipping deformation zone developed mainly along the ironstone diorite contacts that is mineralised over its entire length the gold mineralization is hosted within the damage zone associated with nyankanga fault zone by both diorite and ironstone with higher grades typically occurring in ironstone ore shoots dip more steeply than the nyankanga fault zone the mineralization is associated with sulfidation fronts and replacement textures in ironstones and is mostly contained as disseminated sulphides in diorite the close spatial relationship between gold mineralization and the ironstone diorite contact suggests that the reaction between the mineralising fluid and iron rich lithotypes played an important role in precipitating gold intense brecciation and veining mainly in the footwall of nyankanga fault zone indicates that the fault zone increased permeability and allowed the access of mineralising fluids the steeper dip of the ore shoots is consistent with mineralization during normal reactivation of the nyankanga fault zone

pathways of the north pacific intermediate water identified through the tangent linear and adjoint models of an ocean general circulation model 1 this study develops a strategy for tracing a target water mass and applies it to analyzing the pathway of the north pacific intermediate water npiw from the subarctic gyre to the northwestern part of the subtropical gyre south of japan in a simulation of an ocean general circulation model this strategy estimates the pathway of the water mass that travels from an origin to a destination area during a specific period using a conservation property concerning tangent linear and adjoint models in our analysis a large fraction of the low salinity origin water mass of npiw initially comes from the okhotsk or bering sea flows through the southeastern side of the kuril islands and is advected to the mixed water region mwr by the oyashio current it then enters the kuroshio extension ke at the first ke ridge and is advected eastward by the ke current however it deviates southward from the ke axis around 158 e over the shatsky rise or around 170 e on the western side of the emperor seamount chain and enters the subtropical gyre it is finally transported westward by the recirculation flow this pathway corresponds well to the shortcut route of npiw from mwr to the region south of japan inferred from analysis of the long term freshening trend of npiw observation copyright 2013 john wiley sons ltd

recent stromatolites in landlocked pools on aldabra western indian ocean abstract stromatolites occurring in three areas of landlocked pools on aldabra show distinctive morphologies petrography and biological characteristics related both to water chemistry and the physical characteristics of sites they are formed apparently by indirect precipitation of high magnesium calcite in association with growths of cyanobacteria and cladophora the three contrasted areas are dominated respectively by phormidium lyngbya and pleurocapsa by entophysalis lyngbya and pleurocapsa and by pleurocapsa dichothrix gypsophila and schizothrix reflecting an increasing concentration and increasing dissolved carbonate activity of pool waters centimeters to decimeters of carbonate encrustations are made up of irregular laminate columns porous layered coatings of planar or cerebroid form including oncolites and finely laminate branching columns laminae are typically alternations of dense and porous micrite with in one locality radiating fibrous sparry crystal growths although exclusively associated with cyanobacteria calcification typically begins several millimeters beneath living cells with few examples of carbonate coated unicells or filaments micrite consists of randomly arranged particles of varying size products of sparse homogeneous nucleation followed by both overgrowth and dissolution coarse cements of contrasted morphologies develop within internal cavities resembling the endostromatolites described by others chemical data indicate that waters in all three pools are supersaturated with respect to caco 3 calcite this is adequate but is not in itself sufficient to bring about precipitation of carbonate which is site specific cyanobacteria are not directly calcified but metabolic activities may promote precipitation through mechanisms such as extraction of co 2 or by the generation of specific organic molecules which promote nucleation or colonies may provide a closed passive environment in which other bacteria or inorganic processes bring about crystallization the precise path is not known but the reduction of possibilities carries implications both for the accretion of other stromatolites and the crystallization of carbonate in deposits such as tufas