Middle Jurassic stromatactis mud-mound in the Pieniny Klippen Belt (Western Carpathians)

Summary A stromatactis mud-mound has been found near Slavnické Podhorie in the Czorsztyn Unit of the Pieniny Klippen Belt (Western Carpathians, Slovakia). Its stratigraphic range is Bathonian to Callovian and it is one of the youngest known true stromatactis mud-mounds. The complete shape the mound is not visible since the klippe is a tectonic block encompassed by younger Cretaceous marls. The matrix is micritic to pelmicritic mudstone, wackestone to packstone with pelecypods, brachiopods, ammonites, and crinoids. An important component of the mound is stromatactis cavities that occur as low as the underlying Bajocian-Bathonian crinoidal limestones. The stromatactis cavities are filled by radiaxial fibrous calcite (RFC) as well as in some places by internal sediment and, finally, by clear blocky calcite. Some cavities remain open with empty voids in the centres. In some stromatactis cavities, tests of cavedwelling ostracodsPokornyopsis sp. were found, surrounded by the latest stages of the RFC. This indicates that stromatactis cavities formed an open network enabling migration of the ostracods and their larvae over a period of time. Except in the case of the stromatactis cavities, there are numerous examples of seeming recrystallizationsensu Black (1952) and Ross et al. (1975) and Bathurst (1977). The radiaxial fibrous calcite encloses patches of matrix and isolated allochems. The RFC crystals are oriented perpendicularly to the substrate whether it is a cavity wall or enclosed allochems. This means that the RFC crystals could not grow from the centre of the cavity outward as postulated by Ross et al. (1975). There are also numerous “floating” isolated allochems, which are much smaller than the surrounding RFC crystals. The explanation involving three-dimensional interconnection of allochems seems to be unlikely. In the discussed mud-mound there is a conflict between apparently empty cavities that had to exist in the sediment and seeming “recrystallization” related to the same RFC that forms the initial void filling. The authors favor an alternative explanation of the “recrystallization”. We presume that the allochems served as nucleation points on which the crystals started to grow. Obviously, the allochems and the micritic patches were different from the surrounding material. RFC crystals (either short-or long-bladed) of the “recrystallization” spar grew at the expense of decaying microbial mucillages. The mucus can enclose peloids, allochems, or whole micritic patches that “floated” in the cavity and served as nucleation sites for the RFC crystals. The entire mud-mound represents a microbially bound autochthonous micritic mass; the stromatactis and stromatactis-like cavities originated where purer mucillage patches occurred, giving rise to open spaces. Such features as the morphological variety of stromatactis fabrics, the pervasive penetration of the sparry calcite into matrix, and the enclosure of the “floated” allochems and mudstone patches by sparry calcite, seem to provide support for the presence of mucus aggregates within the mound body. The mucus might be related to protozoans rather than to sponges or other well organized metazoan organisms. Occurrence of the stromatactis cavities in the underlying Bajocian-Bathonian crinoidal limestones support the inference on biological origin of the stromatactis fabrics. The alternative inorganic models of stromatactis origin (e.g., internal erosion or water-escape) are hardly applicable to the sediment formed by crinoidal skeletal detritus.     

Quaternary dune carbonates from the Mediterranean Coast of Egypt: Petrography and diagenesis

Summary The Quaternary carbonates of the Mediterranean coast of Egypt between Alexandria and Salum appear as parallel limestone ridges rising up to 100 m above sea level. These ridges are dominated by dunal carbonates which differ not only in their primary composition but also by distinct grades of meteoric water diagenesis. Oolitic facies dominates the younger aeolianites of the first and second ridges. Bioclastic facies with abundant coralline algae, benthonic foraminifers, molluscs, echinoderms and intraclasts represents the major rock type in the older aeolianites. Features of meteoric water diagenesis include precipitation of increasing amounts of avoid-filling low Mg-calcite spar, dissolution of aragonite and stabilization of aragonite and high Mg-calcite to low Mg-calcite. Aeolianites below paleosol horizons contain abundant calcrete cements, micritized fossils and detrital grains which are commonly corroded and replaced by calcite. Three stages of progressive meteoric diagenesis are recognised in the Egyptian Quaternary aeolianites. In stage 1 minor precipitation of low Mg-calcite occurs at the grain boundaries. Stage 2 is marked by partial dissolution of aragonite, partial loss of high Mg-calcite and precipitation of low Mg-calcite in some pore spaces. In stage 3, most of the remaining pores are occluded by cementation. At the end of stage 3, Mg is removed from high Mg-calcite but some aragonite still persists. The early vadose cements are represented by miniscus, bridge and pendant cements. The phreatic cements were precipitated as bladed spar in the isopachous rims and equant spar in the intergranular and mouldic porosity. The late vadose cements are represented by micritic cements that were related to calcrete formation. Elemental behaviour during meteoric water diagenesis indicates that it leads to a gradual decrease in bulk Sr along with Na in progressively altered aeolianites. Mn distribution is controlled by the carbonate mineralogy (aragonite versus calcite) as well as the proximity of the aeolianites to the overlying paleosol horizons.     

Infrared Spectroscopic Biosignatures from Hidden Cave,New Mexico: Possible Applications for Remote Life Detection

Subsurface environments are known to support and preserve diverse microbial communities. Giant pool fingers from Hidden Cave, New Mexico consist of mm-scale dark micritic calcite layers alternating with clear dogtooth spar crystals and contain morphological and geochemical evidence of past microbial communities. We used Fourier Transform infrared spectroscopy to identify fatty acids, proteins, PO₂-carrying compounds, and polysaccharides spatially related to morphological fossil filaments throughout the surface micritic laminations and central pool finger regions. These biomolecular signatures are important components that contribute to the biosignature suite under development that identify microbial involvement in carbonate precipitation on Earth and remotely.     

Carbonate organo-mineral micro- and ultrastructures in sub-fossil stromatolites: Marion lake, South Australia

Sub-fossil stromatolites (5000-3000 years old) occur on the marginal flat surrounding Marion Lake (South Australia). A micrite/microsparite crystal fabric characterises these fine-grained, well-laminated stromatolites, which lack trapped grains. The internal lamination is characterised by a sub-millimetric alternation of porous and dense laminae. The microfabric of the laminae is ubiquitously composed of a fine (10-20 μm) peloidal texture, with many thinner aphanitic layers. Aggregates of very fine, low-Mg calcite and aragonite constitute both peloidal and aphanitic micrite, which is coated, respectively, by spherulitic and fringing acicular microspar. Micrite, with a high organic matter content, is formed of coalescing nanospheres grading into small polyhedrons, probably composed mainly of aragonite, with less calcite enriched in Mg, Sr, Na and S. Bacteria-like microfossils and relics of extracellular polymeric substance (EPS) occur abundantly within this micritic framework. The former consist of empty moulds and mineralised bodies of coccoid forms, whereas EPS relics consist of sheet-like or filamentous structures that appear both mineralised and more often still preserved as a C-enriched dehydrated substance that represents the main organic matter component of the deposit. Acicular crystals, which show a prismatic elongate shape, are composed of Mg-depleted aragonite that lacks fossils or organic relicts. Degrading EPS and micro-organisms appear gradually to be replaced and entombed by the nanospherical precipitates, implying the existence of processes of organo-mineralisation within an original syn-sedimentary microbial community. Succeeding micron-scale crystals merge to form isolated or connected micritic aggregates (the peloids), followed by the gradual formation of the acicular crystals as purely inorganic precipitates.     

Characeae-derived carbonate deposits in Lake Ganau, Kurdistan Region, Iraq

Characeae, a family of calcifying green algae, are common in carbonate-rich freshwaters. The southwestern shoreline of Lake Ganau (Kurdistan Region, northeastern Iraq) harbors dense and thick mats of these algae (genus Chara). On the lake bottom and along the shore, carbonate sands and rocks rich in the remains of stems, branches, nodes, and whorls of Chara are deposited. These deposits show all stages of growth and degradation of characean algae, including replacement and lithification into limestone. The replacement of the fragments by fine-grained calcite preserved delicate microstructures of Chara, such as cortical walls, cell shape, inner and outer layers of the stems, and reproductive organs. Based on roundness, sorting, the degree of lithification, and preserved microstructures of the grains (fragments), three facies were recognized. The first is represented by a newly formed lime sand facies showing elongated grains, poor sorting, and reduced roundness, with pristine preservation of characean surface microstructures. The second is a weathered lime sand facies, which shows better sorting and good roundness, whereas internal structures of characean fragments are still well preserved. The third is comprised of a lithified lime sand facies (grainstone), with very well sorted and rounded grains, and poorly preserved external and internal structures of the characeans. As compared to the newly formed lime sand facies, the grainstone facies shows an increase in grain size by more than 30 %, owing to precipitation of micritic lamina of possible microbial origin. Eventually, the Characeae-derived lime sands are lithified into oolitic limestones with sparry calcite cement, forming a grainstone microfacies. The present study has important implications for the interpretation of pre-Quaternary environments, as it records all stages of the fossilization process of characean green algae and highlights the role of these algae in the formation of oolitic carbonate rocks.     

Studies of Biominerals Relevant to the Search for Life on Mars

The evidence of the water erosion on Mars is particularly interesting since present climatic conditions are such that liquid water cannot exist at the surface. But, if water was present on the planet in the past, there may have been life, too. Since the discovery of carbonates on Mars also may have very important implications on the possibility that life developed there, we are studying minerals that can have biotic or abiotic origin: calcite (CaCO₃) and aragonite, a metastable state of calcite. We have analysed biomineral aragonite, in the form of recent sea shells, as well as crystals of mineral aragonite. Infrared spectroscopy in the 2–25 μm wavelength range reveals that, after thermal processing, the biotic samples have a different spectral behaviour from the abiotic ones. As a result, it is possible to distinguish abiotic mineral aragonite from aragonite of recent biological origin. Obviously, if life existed in the past on the Red Planet, we could expect to find “ancient” biotic carbonates, which should therefore be investigated, in order to search for a way of discriminating them from abiotic minerals. For this reason, at the beginning we have considered samples of crushed fossil shells of aragonite composition. Afterwards, in order to take into account that fossilization processes almost always produce a transformation of metastable form (aragonite) into more stable form (calcite), we also studied samples of mineral calcite and different types of fossils completely transformed into calcite. All these biotic fossil samples show the same spectral behaviour as the fresh biotic material after thermal annealing at 485°C. Instead, the calcite behaves like abiotic aragonite. Furthermore, it is known that seashells and other biominerals are formed through an intimate association of inorganic materials with organic macromolecules. The macromolecules control the nucleation, structure, morphology, crystal orientation and spatial confinement of the inorganic phase: this differentiates biominerals from minerals. Analysing the aragonite or calcite fossils with a Scanning Electron Microscope, we found that the fossilization process did not modify the structure of the biominerals which maintain their microscopic characteristics. Looking at the morphology of fossil biominerals, it is evident that the crystals are arranged in complex architectures compared with the compact structure of the mineral crystals. In conclusion, the properties and structure of the biominerals are different from those of the minerals. The rapid increase of the crystalline structure developed under biotic conditions makes these minerals less resistant to thermal treatments, compared with samples of abiotic origin. This result holds both for recent shells as well as all fossil samples. The spectroscopic behaviour of all analysed calcium carbonates of biotic origin is different from that of the abiotic one. Therefore, the infrared spectroscopy is a valid technique to discern the origin of the samples and a powerful tool for analysing in-situ and “sample-return” Mars missions specimens. Also Optical and Scanning Electron Microscopy can be useful to support this type of studies. ^*Presented at: National Workshop on Astrobiology: Search for Life in the Solar System, Capri, Italy, 26 to 28 October, 2005     

Evidence for Microbial Involvement in Pool Finger Precipitation,Hidden Cave,New Mexico

Although speleothems are usually considered inorganic precipitates, recent work has demonstrated hitherto unsuspected biogenic influence in some twilight areas. We have expanded this notion to the dark zone, examining pool fingers from Hidden Cave, New Mexico, to test for possible bacterial involvement. The pool fingers in Hidden Cave are pendant speleothems that formed subaqueously in paleo-pools. They are 1 to 4 cm in diameter and 5 to 50 cm long. A knobby, irregular external shape is underlaid by a layered interior on two scales, a 0.5 to 1.0 cm alternation between dense and porous layers and a mm-scale alternation between dark micritic calcite and clear dogtooth spar. The micrite is similar to microbialites identified in modern and ancient carbonates. Fossil bacteria were found in all layers. These include (1) calcified filaments 1 w m in diameter and 5–50 w m long and (2) micro-rods 0.1 w m by 1–2 w m. Most filaments are curved rods with a smooth surface but rare examples display a diamond crosshatch surface. The micro-rods occur as isolated crystals to dense meshes. We interpret the micro-rods as calcified bacilliform bacteria and the filaments as calcified filamentous bacteria. Carbon isotopic data are slightly more negative (by - 0.5 to - 1.0% in micritic layers than in dogtooth spar layers, suggesting a greater microbial influence in the micritic layers. Based on these similarities to known microbialites (e.g., petrographic fabrics, the presence of fossil bacteria, and the suggestive carbon isotopic data), we conclude that microbial activity was an intimate part of pool finger formation in Hidden Cave. The significance of such involvement goes beyond speleological contexts to wider questions of identification of biosignatures in rocks on earth and beyond.     

Petrography and biosedimentology of the Rosso Ammonitico Veronese (middle-upper Jurassic, north-eastern Italy)

A petrographic and biosedimentological study of the Rosso Ammonitico Veronese from the Trento Plateau (north-eastern Italy) shows that diagenetic (neomorphism, recrystallization) and biological processes (microbial content and pigmentation) influenced the formation and alteration of the carbonate matrix. The subject of this article is the interaction of early diagenetic processes and an attempt to explain the different colors of the matrix (red, pink, grey). Nearly 200 samples derived from 14 sections (Callovian to Tithonian) located in the Verona area have been studied by means of classical, cathodoluminescence and SEM microscopy. Calcite and ferruginous microfilaments of different shapes and sizes are present and tentatively attributed to fungi and iron bacteria. These micro-organisms precipitated iron oxy-hydroxides at poorly dysoxic-anoxic sediment–water interfaces. Further liberation of submicronic hydroxides (now hematite) was responsible for the red pigmentation of the carbonate matrices, originally composed of less than 1 μm-sized micrite. Controversial smaller nanograins (0.1–0.5 μm) attributed to nanobacteria or planktonic picoeukaryotes have been observed in the reddish samples. Recrystallization of the micrite leads to the formation of new micritic crystals, between 2 and 4 μm in size, then to microspar crystals. Micritic textures are linked to the different colours of the samples. The intensity of the red colour is correlated with the presence of hematite (former iron hydroxides) and the presence of planar subhedral micritic grains. In contrast, pink and greyish samples are linked to the increasingly coalescent structure of anhedral micritic and microsparitic crystals.     

Carbonate fabrics in the modern microbialites of Pavilion Lake: two suites of microfabrics that reflect variation in microbial community morphology,growth habit,and lithification

Modern microbialites in Pavilion Lake, BC, provide an analog for ancient non‐stromatolitic microbialites that formed from in situ mineralization. Because Pavilion microbialites are mineralizing under the influence of microbial communities, they provide insights into how biological processes influence microbialite microfabrics and mesostructures. Hemispherical nodules and micrite–microbial crusts are two mesostructures within Pavilion microbialites that are directly associated with photosynthetic communities. Both filamentous cyanobacteria in hemispherical nodules and branching filamentous green algae in micrite–microbial crusts were associated with calcite precipitation at microbialite surfaces and with characteristic microfabrics in the lithified microbialite. Hemispherical nodules formed at microbialite surfaces when calcite precipitated around filamentous cyanobacteria with a radial growth habit. The radial filament pattern was preserved within the microbialite to varying degrees. Some subsurface nodules contained well‐defined filaments, whereas others contained only dispersed organic inclusions. Variation in filament preservation is interpreted to reflect differences in timing and amount of carbonate precipitation relative to heterotrophic decay, with more defined filaments reflecting greater lithification prior to degradation than more diffuse filaments. Micrite–microbial crusts produce the second suite of microfabrics and form in association with filamentous green algae oriented perpendicular to the microbialite surface. Some crusts include calcified filaments, whereas others contained voids that reflect the filamentous community in shape, size, and distribution. Pavilion microbialites demonstrate that microfabric variation can reflect differences in lithification processes and microbial metabolisms as well as microbial community morphology and organization. Even when the morphology of individual filaments or cells is not well preserved, the microbial growth habit can be captured in mesoscale microbialite structures. These results suggest that when petrographic preservation is extremely good, ancient microbialite growth structures and microfabrics can be interpreted in the context of variation in community organization, community composition, and lithification history. Even in the absence of distinct microbial microfabrics, mesostructures can capture microbial community morphology.     

Conservation of Ornamental Stone by Myxococcus xanthus-Induced Carbonate Biomineralization

Increasing environmental pollution in urban areas has been endangering the survival of carbonate stones in monuments and statuary for many decades. Numerous conservation treatments have been applied for the protection and consolidation of these works of art. Most of them, however, either release dangerous gases during curing or show very little efficacy. Bacterially induced carbonate mineralization has been proposed as a novel and environmentally friendly strategy for the conservation of deteriorated ornamental stone. However, the method appeared to display insufficient consolidation and plugging of pores. Here we report that Myxococcus xanthus-induced calcium carbonate precipitation efficiently protects and consolidates porous ornamental limestone. The newly formed carbonate cements calcite grains by depositing on the walls of the pores without plugging them. Sonication tests demonstrate that these new carbonate crystals are strongly attached to the substratum, mostly due to epitaxial growth on preexisting calcite grains. The new crystals are more stress resistant than the calcite grains of the original stone because they are organic-inorganic composites. Variations in the phosphate concentrations of the culture medium lead to changes in local pH and bacterial productivity. These affect the structure of the new cement and the type of precipitated CaCO₃ polymorph (vaterite or calcite). The manipulation of culture medium composition creates new ways of controlling bacterial biomineralization that in the future could be applied to the conservation of ornamental stone.     

Studies of crystal growth mechanisms of proteins by electron microscopy

We have used electron microscopy to examine the surfaces of lysozyme crystals and deduce mechanisms of crystal growth. We find that growth occurs by a lattice defect mechanism at low supersaturation and by two-dimensional nucleation at high supersaturation. Step velocities and two-dimensional nucleation rates are obtained, and their dependence on supersaturation is compared with theory. Some features of the observed surface structure can be related to the specific topology and strengths of the bonds in the P4(3)2(1)2 lattice. Preliminary results on the early stages of nucleation and the phenomenon of cessation of growth are presented.     

Conservation of ornamental stone by Myxococcus xanthus-induced carbonate biomineralization

Increasing environmental pollution in urban areas has been endangering the survival of carbonate stones in monuments and statuary for many decades. Numerous conservation treatments have been applied for the protection and consolidation of these works of art. Most of them, however, either release dangerous gases during curing or show very little efficacy. Bacterially induced carbonate mineralization has been proposed as a novel and environmentally friendly strategy for the conservation of deteriorated ornamental stone. However, the method appeared to display insufficient consolidation and plugging of pores. Here we report that Myxococcus xanthus-induced calcium carbonate precipitation efficiently protects and consolidates porous ornamental limestone. The newly formed carbonate cements calcite grains by depositing on the walls of the pores without plugging them. Sonication tests demonstrate that these new carbonate crystals are strongly attached to the substratum, mostly due to epitaxial growth on preexisting calcite grains. The new crystals are more stress resistant than the calcite grains of the original stone because they are organic-inorganic composites. Variations in the phosphate concentrations of the culture medium lead to changes in local pH and bacterial productivity. These affect the structure of the new cement and the type of precipitated CaCO(3) polymorph (vaterite or calcite). The manipulation of culture medium composition creates new ways of controlling bacterial biomineralization that in the future could be applied to the conservation of ornamental stone.     

Calcification processes of siliceous sponges in Viséan Limestones (Counties Sligo and Leitrim,Northwestern Ireland)

Summary In the Lower Carboniferous limestones and shales of the Benbulben Range, Counties Sligo and Leitrim, northwestern Ireland, a suite of carbonate nodules, about 1 to 4 cm in diameter, has been sampled and investigated by thin sectioning. The nodules consist of micritic, peloidal and fenestral fabrics. Many of them contain relics of desma bearing demosponges and hexactinellid sponge skeletons. The nodules are interpreted as calcified siliceous sponges. Micrite and peloids have been formed via microbial activity during the decay of the soft sponge tissue. The actual processes are deduced from Recent examples investigated at Lizard Island, Autralia, byReitner (1993). The skeletal opal was dissolved very early. In places where the skeleton was already embedded in micrite the spicules are preserved as molds cemented by granular ferroan calcite. The nodules were extensively inhabited by agglutinating polychaetes and bored by sponges. Micrite clasts have been exported to the surrounding seafloor before the sponges were completely covered by sediments. Fenestral fabrics represent primary sponge cavities, that may be enlarged due to volume reduction of the soft tissue during calcification. Some originated from non-calcification of decaying tissue. The granular calcite cement, filling the fenestral fabrics, contains relics of spicules and faintly visible peloids floating unsupportedly in the cement. These peloids were probably produced in situ by calcification of organic mucilages that filled the cavities almost entirely. It is evident that most diagenetic processes occurring within the sponges happened on the seafloor, most likely within the still living individuals. Possibly the nodules represent a precursor stage of mud mound development.     

Surface analytical techniques applied to calcite: evidence of solid-state diffusion and implications for isotope methods

Surface analytical techniques have become common tools for physicists and materials scientists for making direct observations at the molecular scale but they can also provide Earth Scientists with new information about mineral/fluid interfaces. High resolution surface analysis compliments our traditional methods for analysing the bulk of solids and solutions which can give us, for example, a more complete picture of the geochemical processes that affect the mineral grains in sediments. Stable isotope methods have become widely used for dating and palaeoclimate studies but they rely on the assumption of a closed system. Artifacts resulting from carbonate mineral recrystallization can be avoided by careful sampling but ion by ion replacement can significantly alter the composition of a solid without leaving visible traces. Calcite has long been known to take up substituting ions into its atomic structure. Recent evidence gained from surface analysis shows Cd²⁺ and Zn²⁺, that had been adsorbed or precipitated on calcite surfaces, moves into calcite at the rate of nanometers per month, apparently by solid-state diffusion. No fractures that could serve as conduits from surface to bulk have ever been observed at the micrometer to nanometer scale on the single crystals used for these experiments. Mixing within the top few surface layers by reprecipitation during exposure to the humidity in air can account for some incorporation, but the evidence collected so far does not explain the exchange of position for ions beyond about ten calcite monolayers. With similar rates of movement, other trace components, such as K⁺, Na⁺, Cl^- and F^-, have been observed to move out of bulk calcite and to accumulate in discrete crystallites on surfaces exposed to air. Such mobility may be particular to these near-perfect, Iceland spar crystals and the ions investigated, but if O and C also move into and out of bulk calcite at similar rates, the integrity of isotope ratios from carbonate minerals, even from non-diagenetic environments, may be questionable.     

Origin of peloids in Early Cretaceous deposits, Dorset, South England

Peloids are ubiquitous components in modern and fossil carbonates. The term peloid is non-genetic because the origin of these grains and the pathways of their formation are not fully understood. Based on Berriasian material originating from Dorset, southern England, we report here on peloids that result from the more or less in-place breakdown of previously micritized bivalve shells. The continuum from shell breakdown to peloids is documented by petrography and observation by scanning electron microscopy. The identical elemental composition of peloids and micritized shells confirms the petrographic observation and interpretation. Bivalve shells that were previously entirely micritized appear to be the preferential source for the formation of peloids. Obviously, the micritization weakened the shells, facilitating their breakdown and abrasion. This result identifies the fragmentation of micritized shells as a process leading to the formation of distinct peloids, adding to the categories of peloids recognized to date. Mold, mud, and microbial peloids observed in the studied sections and documented herein are distinct from peloids derived from bivalve shells.     

Common crystal nucleation mechanism in shell formation of two morphologically distinct calcite brachiopods

Closely related mineral-producing organisms share common biomineralisation processes. We demonstrate that, in cases of disparate mineral structures where crystal growth mechanisms are necessarily diverse, nucleation processes are the common underlying mechanism during shell formation. Detailed crystallography in the context of shell microstructure in two morphologically distinct calcite brachiopods indicates that, despite differences in shell growth and fabric, at the centre of growth, calcite crystals nucleate with the c-axis 0001 parallel to the shell surface. Such detailed contextual crystallography of biomineralisation using electron backscatter diffraction (EBSD) will have significant applications for future research in biological and medical sciences.     

Reworked marine sandstone concretions: a record of high-frequency shallow burial to exhumation cycles

Concretions, with abundant calcite-dolomite cement-replacement textures originally hosted in shallow-marine sandstones, were reworked into Lower Cretaceous fluvio-deltaic conglomerates and shoreface sandstones (External Zones, Betic Cordillera). A cycle of host sand deposition, early diagenetic concretion formation and concretion reworking is documented: (1) Well-sorted shoreface sandstone deposited. (2) Spherical to ovoid, non-ferroan calcite-cemented concretions formed below flooding surfaces at shallow-burial depths during early eodiagenesis. Non-ferroan calcite cements were precipitated from the bicarbonate derived from seawater and from dissolution of marine bioclasts, as shown by isotope analyses. (3) Concretions were reworked and exposed on the seafloor in a high-energy setting as indicated by the presence of numerous bivalve borings (Entobia ichnofacies), laminated micritic microbial crusts around the concretions, and epilithobionts (oysters, barnacles and corals) on the concretion surface. Concretions also appear as erosional remnants on the floor of channels which were incised into the shoreline sandstone when sea-level fell. (4) The fluvio–deltaic channels were filled with sediment during flooding in the late lowstand of sea-level. (5) The concretions are partly dolomitized, and the presence of siderite, pyrite and barite in the outer part of the concretions precipitated before the dolomite, suggests that the latter formed during shallow burial.     

Effects of Organic Matter on Solubilities and Crystal Form of Carbonates

The results of a study of the role of organic compounds in theformation of carlxmate crystals in marine biological systemsare reported. In an increasing concentration of certain organiccompounds which complex calcium ions, the proportion of aragonitedecreases and that of calcite increases. In increasing concentrationsof magnesium ions the proportion of aragonite increases andthat of calcite and vaterite decreases. When the influence oforganic compounds is greater or smaller than that of magnesiumions, only calcite or only aragonite is formed, respectively.Organic compounds forming a strong complex with calcium ionscause the formation of magnesium-rich calcite, and with an increasein temperature and the concentration of magnesium ions, themagnesium carbonate content of precipitated magnesian calciteincreases. When the influence of organic compounds is almostequivalent to that of magnesium ions, in increasing or decreasingtemperatures, the proportion of calcite decreases or increases,respectively, and the proportion of aragonite increases or decreases,respectively. The concentration of magnesium ions in the bodyfluids of marine calcareous organisms seems to differ littlefrom that of other organisms, and seems to be similar to thatof sea water. Only the presence of certain organic compoundsbrings about the formation of the carbonate crystals observedin marine biological systems. The very important role of organicmatter in the formation of crystals found in skeletal carbonatesis emphasized.     

Studies on shell formation. VIII. Electron microscopy of crystal growth of the nacreous layer of the oyster Crassostrea virginica

Electron microscope observations have been made by means of the replica method on growth processes of calcite crystals of the nacreous layer of the shell of the oyster, Crassostrea virginica. Layer formation is initiated by the secretion of a conchiolin matrix and the deposition of rounded crystal seeds on or in this material. In some areas crystal seeds are elongate and within a given area show a similar orientation, probably due to slower deposition. The seeds appear to increase in size by dendritic growth, and smaller seeds become incorporated into larger ones which come into contact to form a single layer. With further growth, crystals overlap, forming a step-like arrangement. The direction of growth is frequently different in neighboring regions. Crystal seeds deposited on crystal surfaces are usually elongate and oriented. Well developed crystals have a tabular idiomorphic form and are parallel in their growth. Rounded and irregular crystals were also observed. The crystals show reticular structure with units of the order of 100 A and striations corresponding with the rhombohedral axes of the crystals. The role of the mantle is discussed in relation to the growth patterns of crystals and shell structure.     

The Role of a Green Alga in the Precipitation of Calcite and the Coprecipitation of Phosphate in Freshwater

The precipitation of calcite from a calcium bicarbonate solution, similar in ionic strength to natural hardwaters, was observed in a series of experiments utilizing an automated culture apparatus. Seeded growth experiments, using calcite seed crystals, were performed at a range of phosphate concentrations to observe inhibitory effects. These experiments demonstrated a linear relationship of increasing inhibition with increasing initial phosphate concentration. A further series of experiments was performed in which an actively photosynthesizing culture of a unicellular green alga (Chlorococcum sp.) was added to the culture vessel in order to initiate precipitation. Experiments to observe spontaneous precipitation, occurring in the absence of both seed and alga additions, were carried out to compare with precipitation rates in the algal experiments. A control experiment was also performed to investigate whether precipitation occurrred in algal cultures maintained in darkness. The carbonate site mechanistic model, developed for calcite precipitation in abiotic conditions, was used to analyse the results of the algal experiments and found to be applicable.