Dieter U. Schmid

Microbial crusts of the late jurassic: Composition, palaeoecological significance and importance in reef construction

SummaryUpper Jurassic reefs contain variable amounts of calcareous microbial crusts. In examples from Portugal, Spain and southern Germany they occur within coral biostromes and bioherms, mixed coral-siliceous sponge reefs, siliceous sponge meadows and mudmounds, and build up thrombolities with or without additional reef metazoans. The crusts are of paramount importance for the establishment and development of positive buildups. Commonly, reef growth starts with crusts which develop from a narrow base and rapidly expand laterally by rising above the sea floor. Reef associations with little or no microbial crust normally did not develop distinct relief.The basic microbial crust type is characterised by a dense to peloidal, mostly clotted, hence thrombolitic fabric which developed due to calcification triggered by microbes. Morphological evidence for this organic nature are positive relief, bridge-structures, and the shape and arrangement of peloids. The basic thrombolitic crust type is a eurytopic feature, equally occurring in settings of different bathymetry, waterenergy, salinity and oxygen/nutrient concentrations. However, the crusts also comprise additional micro-encrusters of variable abundance and diversity. The concurrent occurrence of these encrusters and diversity trends allows discrimination between crusts of different environments, particularly of different water depths. Microbial crusts from non-reefal marine oncoids show both similarities and differences with reefal crusts. For some of the mostly enigmatic micro-encrusters new clues to their nature could be detected. For instance, bubble-like structures, formerly interpreted as sporangia inLithocodium could be identified as the foraminiferBullopora aff.laevis, possibly living as a parasite or symbiont in theLithocodium algal tissue.Lithocodium andBacinella are regarded as different organisms.‘Tubiphytes’ morronensis clearly represents a symbiotic intergrowth between a nubeculinellid foraminifer and a microbe of unknown nature.The main prerequisite for the occurrence of microbial crusts is a cessation of background sedimentation which commonly can be tied to rises in sea level. This results in the development of crust-rich reefs. Fluctuations in oxygen and nutrient levels are indicated by dysaerobic bivalves and richness in authigenic glauconite, and led to the microbes outcompeting reefal metazoans, and to the development of thrombolites. Such thrombolites occur at very different depths which is interpreted to be related to a rise of dysaerobic waters due to climatic buffering and lowering of oceanic circulation during sea level rises.Microbial crusts in modern reefs are largely restricted to shaded, cryptic settings which contrasts with the wide distribution of crusts in Upper Jurassic reefs. Microbial crusts were increasingly replaced by coralline red algae since the Late Mesozoic, but despite their restricted modern habitat seem to still play an important, commonly overlooked role in the stabilisation of reef framework.

The origin of Jurassic reefs: Current research developments and results

SummaryIn order to elucidate the control of local, regional and global factors on occurrence, distribution and character of Jurassic reefs, reefal settings of Mid and Late Jurassic age from southwestern Germany, Iberia and Romania were compared in terms of their sedimentological (including diagenetic), palaeoecological, architectural, stratigraphic and sequential aspects. Upper Jurassic reefs of southern Germany are dominated by siliceous sponge—microbial crust automicritic to allomicritic mounds. During the Oxfordian these form small to large buildups, whereas during the Kimmeridgian they more frequently are but marginal parts of large grain-dominated massive buildups. Diagenesis of sponge facies is largely governed by the original composition and fabric of sediments. The latest Kimmeridgian and Tithonian spongiolite development is locally accompanied by coral facies, forming large reefs on spongiolitic topographic elevations or, more frequently, small meadows and patch reefs within bioclastic to oolitic shoal and apron sediments. New biostratigraphic results indicate a narrower time gap between Swabian and Franconian coral development than previously thought. Palynostratigraphy and mineralostratigraphy partly allow good stratigraphic resolution also in spongiolitic buildups, and even in dolomitised massive limestones.Spongiolite development of the Bajocian and Oxfordian of eastern Spain shares many similarities. They are both dominated by extensive biostromal development which is related to hardground formation during flooding events. The Upper Jurassic siliceous sponge facies from Portugal is more localised, though more differentiated, comprising biostromal, mudmound and sponge-thrombolite as well as frequent mixed coral-sponge facies. The Iberian Upper Jurassic coral facies includes a great variety of coral reef and platform types, a pattern which together with the analysis of coral associations reflects the great variability of reefal environments. Microbial reefs ranging from coralrich to siliceous sponge-bearing to pure thrombolites frequently developed at different water depths. Reef corals even thrived within terrigeneous settings.In eastern Romania, small coral reefs of various types as well as larger siliceous sponge-microbial crust mounds grew contemporaneously during the Oxfordian, occupying different bathymetric positions on a homoclinal ramp.Application of sequence stratigraphic concepts demonstrates that onset or, in other cases, maximum development of reef growth is related to sea level rise (transgressions and early highstand) which caused a reduction in allochthonous sedimentation. The connection of reef development with low background sedimentation is corroborated by the richness of reefs in encrusting organisms, borers and microbial crusts. Microbial crusts and other automicrites can largely contribute to the formation of reef rock during allosedimentary hiatuses. However, many reefs could cope with variable, though reduced, rates of background sedimentation. This is reflected by differences in faunal diversities and the partial dominance of morphologically adapted forms. Besides corals, some sponges and associated brachiopods show distinct morphologies reflecting sedimentation rate and substrate consistency. Bathymetry is another important factor in the determination of reefal composition. Not only a generally deeper position of siliceous sponge facies relative to coral facies, but also further bathymetric differentiation within both facies groups is reflected by changes in the composition, diversity and, partly, morphology of sponges, corals, cementing bivalves and microencrusters.Criteria such as authigenic glauconite, dysaerobic epibentic bivalves,Chondrites burrows or framboidal pyrite in the surrounding sediments of many Upper Jurassic thrombolitic buildups suggest that oxygen depletion excluded higher reefal metazoans in many of these reefs. Their position within shallowing-upwards successions and associated fauna from aerated settings show that thrombolitic reefs occurred over a broad bathymetric area, from moderately shallow to deep water. Increases in the alkalinity of sea water possibly enhanced calcification.Reefs were much more common during the Late Jurassic than during the older parts of this period. Particularly the differences between the Mid and Late Jurassic frequencies of reefs can be largely explained by a wider availability of suitable reef habitats provided by the general sea level rise, rather than by an evolutionary radiation of reef biota. The scarcity of siliceous sponge reefs on the tectonically more active southern Tethyan margin as well as in the Lusitanian Basin of west-central Portugal reflects the scarcity of suitable mid to outer ramp niches. Coral reefs occurred in a larger variety of structural settings.Upper Jurassic coral reefs partly grew in high latitudinal areas suggesting an equilibrated climate. This appears to be an effect of the buffering capacity of high sea level. These feedback effects of high sea level also may have reduced oceanic circulation particularly during flooding events of third and higher order, which gave rise to the development of black shales and dysaerobic thrombolite reefs. Hence, the interplay of local, regional and global factors caused Jurassic reefs to be more differentiated than modern ones, including near-actualistic coral reefs as well as non-actualistic sponge and microbial reefs.

Photoemission studies on Cu(In, Ga)Se2 thin films and related binary selenides

Abstract The aim of this work is to provide a data basis for both X-ray and UV photoelectron spectroscopy on chalcopyrite thin films. A model for the segregation behaviour at the surfaces of polycrystalline thin films of I–III–VI 2 -chalcopyrites (I = Cu; III = In, Ga; VI = Se) based on surface analysis data is presented. In situ photoemission measurements on a variety of Cu(In, Ga)Se 2 samples, as well as on Cu 2− x Se ( x ≈ 0.15), In 2 Se 3 and Ga 2 Se 3 thin film samples, clearly prove the existence of Cu 2− x Se on the surfaces of all Cu-rich thin films (i.e. Cu (In + Ga) ). The surface composition of Cu-poor thin films generally deviates strongly from the bulk stoichiometry. It has been found that the as grown surfaces of Cu-poor thin films are in most cases covered by In Ga-rich defect compounds. The surface of bulk Cu-poor CuInSe 2 has been identified as CuIn 3 Se 5 . For bulk Cu-poor CuGaSe 2 , the surface composition is determined by the conditions of film growth; it ranges between stoichiometric CuGaSe 2 and CuGa 5 Se 8 . This study is based on photo e lectron d istribution c urves (EDCs) which have been measured for photon energies of hv = 21.2, 40,8 and 1253.6 eV. The photoionization cross-sections of the atomic levels comprising the valence bands are strongly dependent on hv . This dependence is exploited to correctly interpret the observed valence band features of the different surface species investigated. The energy positions of the valence band maxima, which are of great practical importance considering heterojunction devices have been determined. For Cu-rich thin films these values could be extracted from the measurements by numerical subtraction of the EDC of the overlaying Cu 2− x Se from the EDCs of Cu-rich surfaces. The binding energies of the core levels and the kinetic energies of the main Auger structures are given for all the materials examined.

Influence of substrates on the electrical properties of Cu(In,Ga)Se2 thin films

Abstract Thin films based on Cu(In,Ga)Se 2 prepared on alkali free substrates are compared to films prepared on soda lime glass. On the latter, the presence of sodium species as detected with X-ray photoelectron spectroscopy is correlated with an enhanced formation of Se O, In O and Ga O bonds at the surface after several days air exposure. The electrical conductivity is also one order of magnitude higher for films on soda lime glass. Solar cells prepared on these substrates exhibit increased open circuit voltages. Capacitance-voltage characteristics on junctions prepared on alkali free substrates show an increased space charge width. The observations can be explained in terms of an increased net acceptor density in polycrystalline Cu(In,Ga)Se 2 when prepared on soda lime glass substrates. An alkali-metal-promoted oxidation of the surface is discussed.

Microbialite morphology, structure and growth: a model of the Upper Jurassic reefs of the Chay Peninsula (Western France)

Abstract During the Early Kimmeridgian, the northern margin of the Aquitaine Basin (Western France) is characterised by a significant development of coral reefs. The reef formation of the Chay Peninsula comprises two main reefal units, in which the microbial structures can contribute up to 70% of framework. The microbial crusts, which played an important role in the stabilisation and growth of the reef body, show the characteristic clotted aspect of thrombolitic microbialites. Corals are the main skeletal components of the build-ups. The bioconstructions of the Chay area are thus classified as coral-thrombolite reefs. Four main morpho-structural types of microbial crusts are distinguished: (1) pseudostalactitic microbialites on the roof of intra-reef palaeocaves; (2) mamillated microbialites, found either on the undersides or on the flanks of the bioherms; (3) reticular microbialites in marginal parts of the reefs and between adjacent bioconstructed units; and (4) interstitial microbialites in voids of bioclastic deposits. Thrombolitic crusts developed on various substrates such as corals, bivalves, or bioclasts. The thrombolites formed a dense, clotted and/or micropeloidal microbial framework, in which macro- and micro-encrusters also occur. Variations in accumulation rate strongly influenced the reef morphology, in particular its relief above the sediment surface. The coalescence of the coral-microbialite patches created numerous intra-reef cavities of metre-scale dimensions. The direction of microbial growth, which defined the macroscopic microbialite forms, strongly depended on the position within the reef framework but was also controlled by water energy, accumulation rate and light availability.

Growth dynamics and ecology of Upper Jurassic mounds, with comparisons to Mid-Palaeozoic mounds

The Mid-Palaeozoic, including the Late Jurassic, was a time of both widespread coral reef growth and pronounced mound formation. A comparison of mound features and their general setting highlights, despite all differences, general similarities in overall growth dynamics. Mound formation was frequently driven by discontinuous patterns, particularly by background sedimentation. In many examples, episodes of mound stabilisation by early lithification, growth of microbolite crusts and winnowing of fines was followed by growth episodes of benthic fauna under reduced to negligible background sedimentation. This pattern of variable sedimentation and organic buildups may have occurred in different orders and magnitudes, inducing a fractal pattern in some mound complexes. A composite approach in estimating growth rates of mounds demonstrates that highfrequency oscillations necessary for growth of most mounds might have ranged from a few thousand years to 4th and 5th order Milankovich cycles that were superimposed by autocyclic factors. D 2001 Elsevier Science B.V. All rights reserved.

Influence of substrates on the electrical properties of Cu(In,Ga)Se/sub 2/ thin films

Thin films based on Cu(In,Ga)Se/sub 2/ prepared on alkali free substrates are compared to films prepared on soda lime glass. On the latter, the presence of sodium species as detected with X-ray photoelectron spectroscopy is correlated with an enhanced formation of Se-O, In-O and Ga-O bonds at the surface after several days air exposure. The electrical conductivity is also one order of magnitude higher on soda lime glass and solar cells prepared on molybdenum-coated substrates exhibit increased open circuit voltages. Capacitance-voltage characteristics on junctions prepared on alkali free substrates show an increased space charge width. The observations can be explained in terms of an increased net acceptor density in polycrystalline Cu(In,Ga)Se/sub 2/ when prepared on soda lime glass substrates. An alkali-metal-promoted oxidation of the surface is discussed.

Mesozoic Reefal Thrombolites and Other Microbolites

Calcareous microbolites are widespread in the Mesozoic. They play a paramount role in reef-building and often contribute to the reef framework. In the Early Triassic, stromatolites took over the vacant reef habitats. During the Middle Triassic and Late Jurassic, microbolites reached their peak development in the Mesozoic, often forming reefs together with different groups of metazoans. No major break in microbolite development appeared from the Late Jurassic to the Early Cretaceous. In the course of the Cretaceous, microbolites in shallow water reefs were for the most part replaced by encrusting corallinaceans.

Growth of bulk Cu0.85In1.05Se2 and characterization on a micro scale

Crystals of the semiconductor Cu0.85In1.05Se2 were grown from the melt. The crystals were characterized by scanning electron microscopy as well as transmission electron microscopy. A coexistence of two phases was found in the entire boules which was detected by energy dispersive spectroscopy and by electron diffraction. High resolution electron microscopy showed a direct intergrowth of both phases.

Studies of the growth mechanism of polycrystalline CuInSe2 thin films prepared by a sequential process

Abstract CuInSe 2 (CIS) is a semiconductor compatible with almost all criteria to be considered for thin film solar cells. However, due to the fact that the material is quite complex, the success of process development depends on the knowledge about the mechanism of film growth. In this contribution we present a new process to deposit CuInSe 2 thin films on molybdenum-coated soda lime glass. The specific growth path involved is studied by energy dispersive X-ray spectroscopy (EDX), secondary electron microscopy (SEM), X-ray powder diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS). We show that a Cu-rich growth front is necessary for large crystallites and that the orientation depends on the selenization process.