Coral fossil

Figure 10. SEM photographs of polished, etched thin sections of fossil Acropora palmata coral (after Edwards 1988). The scale bar in a is 10 microns, a depicts sample AFS-12, a last interglacial coral from Barbados. The crystal morphology in this well-preserved sample is indistingnishable from that of a modem sample (see Fig. 9b). The scale bar in b is 100 microns, b depicts sample PB-5B, a fossil coral collected from North Point Shelf on Barbados. The crystal morphology of this sample shows clear evidence of alteration, inclnding a large calcite crystal filling in a macroscopic pore (dark area in npper right portion of photograph).

The above analysis of Barbados corals also places constraints on the history of the marine value. Empirical evidence suggests that diagenesis generally acts to raise the value of Barbados fossil corals, which makes the lowest initial value for each terrace the best estimate of the marine value at the time the terrace was formed. This makes it possible to extend evidence for a stable marine value beyond the last... 

Initial U concentration. If uranium concentration has changed as a result of diagenetic reactions, one may, in principle detect this by comparison between uranium concentrations in modem corals and their fossil counterparts. Early work documenting and studying uranium concentrations in corals is extensive (e g., Barnes et al. 1956 Tatsumoto and Goldberg 1959 Veeh and Turekian 1968 Schroeder et al. 1970 Thompson and Livingston 1970 Gvirtzman et al 1973 Amiel et al. 1973 Swart 1980 Swart and Hubbard 1982 Cross and Cross 1983). This broad body of data shows that primary surface coral uranium concentrations lie between 1.5 and 4 ppm (see Fig. 1). Concentrations appear to be species dependent (Cross and Cross 1983). Furthermore, uranium concentrations vary within individual coral skeletons (Schroeder et al. 1970 Shen and Dunbar 1995 Min et al. 1995). 

Chen JH, Curran HA, White B, Wasserbmg GJ (1991) Precise chronology of the last interglacial period 238 j/23o-pjj fossil coral reefs in the Bahamas. Geol Soc Amer Bull 103 82-97... 

Min GR, Edwards RL, Taylor FW, Recy J, Gallup CD, Beck JW (1995) Annual cycles of U/Ca in corals and U/Ca thermometry. Geochim Cosmochim Acta. 59 2025-2042 Moore WS (1981) The thorium isotope content of ocean water. Earth Planet Sci Lett 53 419-426 Moran SB, Hoff JA, Edwards RL, Landing WM (1997) Distribution of Th-230 in the Laborador Sea and its relation to ventilation. Earth Planet Sci Lett 150 151-160 Muhs DR, Simmons KR, Steinke B (2002) Timing and warmth of the Last Interglacial period new U-series evidence from Hawaii and Bermuda and a new fossil compilation for North America. Qrrat Sci Rev 21 1355-1383... 

This is exploited in the technique of uranium-thorium dating, which involves measuring the amount of thorium-230 that has accumulated in a substance by decay of uranium. If the object contained no thorium at all when it was formed, the ratio of remaining U to accumulated °Th is a measure of the age. The object being dated must not have had access to sources of fresh uranium that could reset the clock. This is true, for example, of coral left stranded on fossil beaches when sea levels recede, or of... 

The amino acid racemization method is based on the measurement of the degree of racemization acquired by a material expressed as the ratio dextro amino acid (D) enantiomeric form/levo amino acid (L) enantiomeric form, which is an indicator of the age of the object. This method has been applied in fossil shells, bones, teeth, wood, plant remains, and coral [68]. 

Just between 1960 and 2000, C02 in the atmosphere increased by 17% (Box 20-1)—mostly from our burning of fossil fuel. This increase drives Reaction A to the right and threatens the existence of coral reefs,9 which are huge, living structures consisting laigely of CaCOj. Coral reefs are a unique habitat for many aquatic species. 

A significant aspect of skeletal growth in corals is the existence of an internal calendar where daily, seasonal, and annual records are kept on file in the form of individual or series of bands. The band thicknesses within a species are environmentally controlled. The number of bands per year in fossil corals has been used in studies on the Earth-Moon system, i.e. effect of tidal friction on number of days per year336. ... 

Bellwood, D. R., Reef fish biogeography habitat associations, fossils and phylogenies. Proc. 8th Int. Coral Reef Symp., 1, 379, 1997. 

When forming, limestone may contain both the polymorphs of calcium carbonate, calcite and aragonite. But aragonite is the least stable of the two minerals, and most older carbonate deposits will contain only calcite. Since limestone is a marine deposit that forms in quiet seas, it often contains visible fossils of marine organisms such as coral, mollusks, and foraminifera. 

The storage of C02 in the deep ocean is an area of active modeling and some experiments. At issue is the retention time for various sites as well as the biological impacts. Advocates of deep injection point out that even in the absence of deliberate injection, the oceans already receive a portion of the carbon released from fossil fuels because of the continuous exchange of C02 between the atmosphere and the oceans. Add C02 to the atmosphere and some of it will move naturally to the ocean, as equilibrium is sought at the ocean surface.18 Also under study are the biological impacts of additional C02 in the near-surface ocean—for example, the impacts on coral reefs. 

Fossil corals preserved in limestone are often seen in polished marbles used for ornamental or decorative ptirposes, but being relatively soft this material is less suitable for jewellery. Silicified fossil corals, being harder, are sometimes seen in jewellery. This type of material mostly comes from Antigua, India and China. Some fossilised material has earlier been found in Wiltshire, England, but this is now only seen in items such as old jewellery. 

Thompson W. G., Spiegelman M. W., Goldstein S. L., and Speed R. C. (2003) An open-system model for U-series age determinations of fossil corals. Earth Planet. Sci. Lett. 210, 365-381. 

One of the most important applications of uranium-series methods of age determination has been the dating of fossil corals and other carbonate materials. In contrast to deep-sea sediments, which accumulate excess h and Pa that decay over time, carbonates accumulate uranium by co-precipitation from seawater that is essentially free of °Th and Pa. The radioactive ingrowth of °Th and Pa over time toward secular equilibrium with and is the basis of the two methods. 

Bar-Matthews M., Wasserburg G. J., and Chen J. H. (1993) Diagenesis of fossil coral skeletons—correlation between trace-elements, textures, and U-234/U-238. Geochim. Cos-mochim. Acta 57, 257-276. 

Szabo B. J. (1985) Uranium-series dating of fossil corals from marine sediments of southeastern United States of America coastal plain. Geol. Soc. Am. Bull. 96, 398—406. 

Because aragonite is more susceptible to dissolution than calcite, especially under the influence of meteoric waters, and because most fossil corals are recovered from uplifted terrestrial deposits, diagenesis is an especially important limiting factor in recovering older coral records. This problem can be circumvented by drilling into submerged fossil deposits, but because of logistical difficulties, so far this has been accomplished in only a few key spots such as Barbados and Tahiti (Eairbanks, 1989 Bard et al., 1996). 

Because aragonite reverts to calcite when it interacts with meteoric water, subaerial exposure of fossil corals has the potential to change the of the coral. Generally, diagenetically altered corals can be avoided by using X-ray... 

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