Below the photic zone within live microbial mats from tidal flats, the cyanobacteria are dead and bacteria are the dominant living biota. Field data indicate that precipitation of calcium carbonate occurs predominantly within the mats in the aphotic zone. In order to determine whether bacteria could be responsible for inducing the precipitation of calcium carbonate within the microbial mats, live microbial mats and bacteria were collected from modern tidal flats. Over 50 experiments were set up in the laboratory in which live, naturally dead, and sterilized (autoclaved) dead filamentous cyanobacteria were inoculated with bacterial cultures

Bacterially Induced Lithification of Microbial Mats

Influence of temperature on the composition of magnesian calcite overgrowths precipitated from seawater

Part I. Introduction: 1. Definitions, rationale and scope of the book 2. Overview of the Earth Part II. Production of Sediment at the Earth's Surface: 3. Weathering of rocks, production of terrigenous sediment, and soils 4. Biogenic and chemogenic sediment production Part III. Fundamentals of Fluid Flow, Sediment Transport, Erosion and Deposition: 5. Unidirectional turbulent water flow, sediment transport, erosion and deposition 6. Air flow, sediment transport, erosion and deposition 7. Multidirectional water flow, sediment transport, erosion and deposition 8. Movement of sediment by gravity 9. Generation and movement of volcaniclastic sediment 10. Ice flow, sediment transport, erosion and deposition 11. Biogenic and chemogenic depositional structures 12. Post-depositional deformation of soft sediment Part IV. Environments of Erosion and Deposition: 13. Rivers, alluvial plains and fans 14. Lakes 15. Coasts and shallow seas 16. Arid environments 17. Glacial and periglacial environments 18. Deep seas and oceans Part V. Sediment into Rock: Diagenesis: 19. Diagenesis Part VI. Long-term, Large-Scale Processes: Mountains and Sedimentary Basins: 20. Tectonic, climatic and eustatic controls on long-term, large-scale erosion and deposition References Index.

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Earth Surface Processes, Landforms and Sediment Deposits

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Paleolatitude distribution of Phanerozoic marine ooids and cements

The Bahamian Archipelago is one of the few locations in the world where ooid formation is actively occurring. Ooid cortices from six locations in the region were incrementally dissolved and analyzed for 14C, δ18O, δ13C, Mg/Ca, and Sr/Ca ratios. Ooids were examined under SEM after each step in the incremental analyses to characterize the nature of dissolution. Radiocarbon dating indicates that ooids presently at the sediment–water interface began forming across the Archipelago between 1000 and 2800 yr BP and continue to form today. The ooids have the same pattern of microboring alteration across the region. The surface and outer cortex of the ooids are punctuated with unfilled microborings, whereas the inner cortex contains two morphologies of aragonite cement filling the microborings. The two morphologies of cement form in association with two different species of cyanobacteria, one is Solentia sp. the other is interpreted to be Hyella sp. The chemistry of ooids from across the region is remarkably similar. δ18O and δ13C values for all samples vary directly, having a slope of approximately 1. The outer cortex has low δ18O and δ13C values of −3.4‰ and 0.2‰ respectively, whereas the δ18O and δ13C values of the inner cortex are high with values of 1.9‰ and 6.8‰ respectively. The presence of aragonite cement in microborings in the inner cortex increases the overall isotopic composition of both oxygen and carbon in the ooid, causing it to appear close to equilibrium with seawater. The isotopic variation in δ18O and δ13C within the cortex can be characterized as a mixing line between the low values in the unaltered ooid laminae and the aragonite cement in the microborings. The most exterior portion of the ooid has very high Mg/Ca values and is interpreted as an amorphous calcium carbonate (ACC) coating. There are two other phases in the cortex, both being aragonite. The outer cortex has a higher Mg/Ca ratio and lower Sr/Ca ratio than the inner cortex. This difference in chemistry is a result of the presence of aragonite cement in the inner cortex. Stable isotopic and trace element results coupled with SEM investigations indicate that microbes do not play a role in ooid formation, but instead alter the texture and chemistry of ooids after they have formed. A new model of ooid formation is proposed whereby a veneer of ACC precipitates on an ooid while it is at the sediment–water interface (the active phase). This veneer of ACC later recrystallizes to aragonite needles and a new cortex layer is formed (the stationary phase). Observations from this study lead to a deeper understanding of the chemical processes involved in ooid genesis, which allow a better understanding of paleoenvironments hosting ooid formation.

Microbes and Ooids

The radial coatings of Holocene ooids from Baffin Bay, Texas, are Mg-calcite, whereas the tangential and micritic coatings are aragonite. Centrifugally deposited ooid coatings are similar in many respects to centripetally deposited marine cements. The reason(s) for the mineralogical and textural variation in these apparently inorganic carbonate precipitates are not certainly known, but the rate of nucleation and precipitation, degree of agitation and presence of certain types of organic compounds appear to be the most likely explanations.

E. W. Behrens

Papers

The Ooids of Baffin Bay, Texas