Overview 363 3.6.2. Indirect Evidence of the Influences of Organic Matter on Calcium Carbonate 363 3.6.3. Direct Evidence for Organic Matter Adsorption on Calcium Carbonate 365 4. Oceanic Sources of Marine Carbonates 366 4.1. Overview 366 4.2. Primary Carbonate Sources for Deep Sea Sediments 366 4.3. Shallow Water Biogenic Carbonate Sources 367 4.3.1. General Considerations 367 4.3.2. Sources of Fine-Grained Carbonate Sediments 367 4.3.3. Sources of Carbonate Sands 368 4.3.4. Carbonate Cements 370 4.3.5. Dolomite Formation 371 4.3.6. Carbonate Minerals Formed in Siliciclastic Sediments 371 5. Dissolution of Calcium Carbonate 372 5.1. Dissolution of Carbonate Minerals in the Pelagic Environment 372 5.1.1. General Considerations 372 5.1.2. Dissolution in the Water Column 372 5.1.3. Dissolution on the Deep Sea Floor 372 5.2. Dissolution of Carbonate Minerals in Carbonate-Rich Shallow Water Sediments 373 6. Response of Carbonate-Rich Sediments to the Acidification of the Ocean Due to Rising Atmospheric pCO 2 374 6.1. General Principles and Considerations 374 6.2. The Major Ocean Basins 374 6.3. Shallow Water Carbonate-Rich Sediments 375 7. Summary and Thoughts on Possible Future Research Directions 375 7.1. Summary 375 7.2. Thoughts on Possible Future Research Directions 377 8. Acknowledgements 378 9. References 378

No abstract

ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract, please click on HTML or PDF.

No abstract

The mineral dolomite and the uncertainties surrounding its origin have attracted the attention of earth scientists for over a century. The core of the dolomite ''problem'' is the apparent paradox posed by the paucity of dolomite in modern marine depositional environments versus its relative abundance in the sedimentary rock record. Solving this problem requires knowledge of the conditions under which the mineral forms and the rate of precipitation under those conditions. As a working hypothesis, it is suggested that the precipitation rate of dolomite may be quantified and modeled in a manner similar to other carbonate minerals through application of a rate law that represents the rate as a simple function of saturation index, r ‫؍‬ k(⍀ ؊ 1) n. This hypothesis is tested in a series of experiments by measuring the steady state rate of dolomite precipitation in a dolomite-seeded flow reactor through analysis of reacted fluid chemistry. By varying temperature from approx 100°to 200°C and saturation index (⍀) from near saturation to D100, sufficient data were collected to solve for the reaction order and Arrhenius rate constant (k ‫؍‬ A exp 5؊(⑀ A /RT)6) of this rate law. The dolomite produced in these experiments was variable in composition but typically a calcium-rich protodolomite, forming syntaxial overgrowths on the seed material. At the highest supersaturations obtained, formation of distinct nucleation centers was observed. These experiments do confirm a strong temperature dependence for the precipitation reaction (activation energy ⑀ A ‫؍‬ 31.9 kcal mol ؊1) and moderate dependency on saturation index (n ‫؍‬ 2.26, log A ‫؍‬ 1.05). The experimental findings of this paper suggest that the abundance of dolomite in the sedimentary rock record reflects, at least in part, environmental changes in temperature and seawater chemistry over geologic time.

No abstract

Plasmon resonance is expected to occur in metallic and doped semiconducting carbon nanotubes in the terahertz frequency range, but its convincing identification has so far been elusive. The origin of the terahertz conductivity peak commonly observed for carbon nanotube ensembles remains controversial. Here we present results of optical, terahertz, and DC transport measurements on highly enriched metallic and semiconducting nanotube films. A broad and strong terahertz conductivity peak appears in both types of films, whose behaviors are consistent with the plasmon resonance explanation, firmly ruling out other alternative explanations such as absorption due to curvature-induced gaps.

No abstract