The interaction of magnesium with calcite during crystal growth at 25–90°C and one atmosphere

Upper 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.

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

Beachrock formation and degradation along the shores of the Gulf of Aqaba (Sinai) were studied in the field and at the laboratory. Different types of beachrock were found, including laminated rocks. Rates of photosynthesis and respiration were studied under high‐and low‐tide conditions. Epilithic, chasmolithic, and endolithic bacteria, cyanobacteria, and algae were studied quantitatively. Bacterial contribution to carbonate precipitation and cementation was estimated in thin sections and by laboratory experiments. Precipitation of aragonite and monohydrocalcite was observed at salinities between 28‰ and 45‰. Concentrations of dissolved organic matter (DOM) and particulate organic matter (POM) were high at initial stages of beachrock formation and extremely low in final rock. Anaerobic and later aerobic decay processes initiate carbonate precipitation. A model is suggested that places beachrock as an intermediate between cyanobacterial influenced subtidal and su‐pratidal rocks. Formation, cementation, and ...

Photolithotropic and chemoorganotrophic activity of bacteria and algae as related to beachrock formation and degradation (gulf of Aqaba, Sinai)

A variety of precipitative synsedimentary CaCO3 cements lines or fills cavities and surrounds sediment particles within Recent algal cup reefs which dot the rim of the Bermuda platform. These include aragonite needle, spherulitic, and lath cements as well as calcite micrite, palisade, scale, and blocky cements, listed according to respective relative abundances. No distribution pattern of cements is apparent; various combinations of three or four cements are found in almost every sample or thin section. From the occurrence of various cements, factors determining composition and fabric are deduced: Various sequences of two or three precipitative cements result from changes in microenvironment; the only cause of such change recognized is the entrance of endolithic algae into partly cemented cavities. This study provides some insight into the origin of submarine cements and a basis for comparative interpretation of cements in fossil marine environments, especially fossil reefs.

Fabrics and sequences of submarine carbonate cements in Holocene Bermuda cup reefs

Abiotic marine carbonates have been precipitated as ooids and synsedimentary cement within shallow marine carbonate sequences throughout the Phanerozoic. These components exhibit variable abundances and compositions that temporally correlate with estimates of global eustasy. With rising sea level, aragonite and acicular magnesium calcite precipitation gave way to the formation of equant calcite and possibly dolomite. Similarly, the abundance of ooids was limited by positions of global sea level and associated changes in ocean chemistry. Modern abiotic marine carbonates also exhibit variable abundances and compositions as a function of changing fluid chemistry with latitude and depth. Chemical data on ambient marine waters suggest that carbonate ion concentrations and rates of precipitation presently control the nature of marine cement and ooids. Using data from modern oceans to calculate the requisite atmospheric $$pCO_{2}$$ levels necessary to form this range of phases in ancient seas suggests that atmos...

Secular variation in abiotic marine carbonates: constraints on Phanerozoic atmospheric carbon dioxide contents and oceanic Mg/Ca ratios

The carbonate cements in a specimen from the living cup-shaped algal reefs of Bermuda were studied with scanning electron and light microscopes. These small cup reefs positioned on the margins of the platform are built by encrusting organisms; principally coralline algae, secondaril hydrocorals, gastropods and worm tubes; tire cavities of this framework are partly to completely filled with internal sediment. The principal cement is a mosaic of anhedral high-magnesium calcite, 15 tool % MgCO3 that lines and fills pores of skeletons, borings, and interstices of internal sediments. The size frequency maxima of the crystals is 2-4µ, the range of micrite. The other much less common cement is fibrous aragonite whose preferred habitat is as linings and fillings of gastropod shells. The SEM micrographs show that the micrite cement crystallizes directly within voids and is not the recrystallization product of earlier carbonate cement. The micrographs also show all stages from initial linings to complete fillings of voids as well as multiple generations of micrite. The crystallization of micrite within voids offers an alternative explanation to the accepted view that fossil micrites result from recrystallization. Because the cup reefs are extremely permeable and are located on the seaward margins of an oceanic platform where there is strong and continual wave surge, the cement must form from seawater of oceanic position. The predominance of repeated generations of micrite-sized crystals indicates that the mechanism and chemistry of precipitation must be studied within voids and on the scale of microns.

D. S. Marszalek

Papers

Ultrastructure of Carbonate Cements in a Holocene Algal Reef of Bermuda