Jon N. Weber

CARBON AND OXYGEN ISOTOPIC COMPOSITION OF SELECTED LIMESTONES AND FOSSILS

Abstract Isotopic analyses are presented for 500 samples of marine and freshwater limestones and fossils. Mean carbon isotopic compositions, in the conventional δ-values, relative to the Chicago PDB standard, are +0.56 per mil (selected marine group) and −4.93 per mil (selected freshwater group); mean oxygen isotopic compositions are −5.25 per mil (marine) and −8.66 per mil (freshwater samples). The average carbon isotopic composition of marine limestones is nearly constant from the Cambrian age group to the Tertiary group and it is concluded that no great change has occurred in the carbon isotopic composition of marine bicarbonate over geologic time. Oxygen isotopic compositions of marine limestone exhibit a progressive increase in average oxygen-18 content from the oldest to the youngest age group, possibly due mainly to postdepositional recrystallization and oxygen exchange in the presence of waters different from those of the original environment. Mean δO18 is −9.7 per mil for the selected Cambrian group, − 1.2 per mil for the Quaternary sample group. Average carbon and oxygen isotopic compositions of freshwater limestones show less regular changes with time of formation. Carbon isotope ratios are significantly different from those of marine limestones in sample groups as old as Carboniferous or Devonian, whereas oxygen isotope ratios differ in marine and freshwater sample groups as old as Jurassic. A graphic plot of δC12 versus δO13 gives environmental separation of more than 80 per cent of the samples, and the percentage of correct diagnosis is considerably improved in Jurassic and younger samples. It is concluded that carbon and oxygen isotopic compositions of limestones and calcareous fossils provide significant but not infallible criteria of depositional environment for samples of a wide range of geologic age. The difference in C12 content between marine and fresh-water carbonates is attributed mainly to a variable contribution of C13-deficient carbon from land plants and humus, which is added to freshwater systems in several stages, but has a relatively minor influence in most marine environments.

Incorporation of strontium into reef coral skeletal carbonate

Abstract Two thousand and twenty well-characterized coral specimens from 17 localities have been analyzed for Sr. Seventy-three genera and subgenera, mostly hermatypic scleractinians, are represented. For some genera, specimens living in surface reef environments are compared with those from 18.3 m depths on the same reefs. Growth rates for some species have also been measured at these depths at one of the sampling sites. Skeletal strontium for a given genus decreases with increasing water temperature, a relationship which previously eluded detection. Aragonite deposited by corals living on the reef at a depth of 18.3 m contains more strontium than the skeletal aragonite of the same coral genera from shallow-water, surface environments. Quantitative treatment of the data for Acropora, one of the most abundant and widely distributed of the reef-building corals, suggests that the observed strontium variations may reflect variations in the rate of skeletal calcification, rather than direct dependence upon temperature or water depth. There is evidence for ‘species effects’, apparently unrelated to growth rate differences, in that certain coral genera are consistently enriched or depleted in skeletal strontium content relative to other genera living in the same reef environments under identical ambient conditions. Temperature, salinity, water depth, seawater composition, and/or other such parameters may in part determine the levels of trace element concentration in carbonates deposited by corals and other marine invertebrates, but it would appear that these variables more directly affect physiological processes which in turn control skeletal chemistry.

Carbon and oxygen isotope fractionation in the skeletal carbonate of reef-building corals

Abstract Important to the geochemistry of carbonate sediments and carbonate rocks is a detailed understanding of the isotopic composition of the various sediment contributors. The coelenterates are of particular interest in this respect as the stable isotope ratios of their skeletons are controlled by both vital and inorganic environmental factors. To determine the relative importance of these factors, 475 specimens referred to 50 genera were selected to represent the coral population of Heron Island on the Great Barrier Reef of Australia. With a few exceptions, the samples were taken from two depth environments: (1) surface, defined as 0–4.6 m; and (2) 18 m. The observed range of δ13C is 13‰ (max. +2.6‰; min. −10.0‰ with respect to PDB standard CO2); that of δ18O is 4‰ (max. −1.2‰; min. −5.4‰). A model incorporating both biological and environmental factors has been developed to account for isotope ratio variations in coelenterate carbonate. A depth effect predicted by the model provides an opportunity to test its validity in that a number of critical relationships must be observed. The analytical data are consistent with the model. At a specific reef locality, the most important factors causing isotope ratio variations in the skeletal carbonate of corals are zooxanthellar activity and temperature.

Replamineform: A New Process for Preparing Porous Ceramic, Metal, and Polymer Prosthetic Materials

The replamineform process (meaning replicated life forms) is a technique for duplicating the microstructure of carbonate skeletal components in ceramic, metal, or polymer materials. The special pore structures of marine invertebrate skeletal materials such as echinoid spines and corals, which are difficult or impossible to create artificially, can thus be copied in useful materials. Of immediate interest is the possibility of using these replicated microstructures in the fabrication of orthopedic prosthetic devices. By means of this technique, prosthetic materials having a controlled pore microstructure for optimum strength and tissue ingrowth may be obtained.

Depth related changes in the 13C12C ratio of skeletal carbonate deposited by the Caribbean reef-frame building coral Montastrea annularis: further implications of a model for stable isotope fractionation by scleractinian corals

Systematic variations in the isotopic composition of skeletal carbonate deposited by the Caribbean reef-frame building coral Montastrea annularis are correlated with water depth, location of the corallites within the corallum, and polyp packing density, as is demonstrated by isotope ratio measurements for 426 samples collected at 4.6 m depth intervals between 0 and 27.4 m at St. Croix, U.S. Virgin Islands. These data support a model, based on a study of Indo-Pacific scleractinians, proposed earlier for stable isotope fractionation by corals. Of particular interest is the fact that, within this species, ecotypic differentiation into shallow-water and deep-water subpopulations, with a boundary close to 20m, is reflected by changes in skeletal °13C. Stable isotope geochemical studies of both modern and fossil coral-derived carbonate may contribute to the solution of several problems having geologic and paleontologic significance.

Correlation of density banding in reef coral skeletons with environmental parameters; the basis for interpretation of chronological records preserved in the coralla of corals

Many species of scleractinian reef corals deposit aragonite skeletons with cyclic bands of higher and lower density whose periodicity is annual. As these growth bands are frequently preserved in fossil corals, attention has been focused on their possible use as en- vironmental indicators in as much as density variations may reflect changes in water tem- perature and/or light intensity. Relationships between skeletal growth banding and environ- mental parameters have been investigated by X-radiographic examination of 1488 specimens of modern reef corals representing 31 reef localities widely distributed over the Indo-Pacific and Caribbean regions. Average monthly seawater temperature and solar radiation data are available for each locality, and the date of collection is known for each sample. Of the 47 genera and subgenera included in the study, density variations were most pronounced in skeletons of Astreopora, Coscinarea, Cyphdstrea, Favia (especially species with small corallites such as F. stelligera), Goniastrea, Hydnophora (massive forms only, e.g. M. microconos), Leptoria, Montastrea (especially M. annularis, with small corallites), Pavona, Polyastra, and Plesiastrea. Less useful are Diploastrea, Diploria, and Favites (which tend to have large corallite diameters), and Goniopora, Alveopora, Porites, Siderastrea, and Stephanaria (which frequently exhibit numerous secondary density variations within the annual cycle). Despite considerable variability among different individual corals from the same population, the average thickness of the skeletal growth bands is positively correlated with mean annual wa- ter temperature. By comparing characteristics of the outermost growth increment and the date of collection with monthly records of water temperature and solar radiation, it appears that maximum skeletal density is associated with those periods of the year when seawater

Systematic Relationships between Carbon and Oxygen Isotopes in Carbonates Deposited by Modern Corals and Algae

Analyses of organic carbonates from Jamaican coral reefs show a positive correlation between the O18: O16 ratio and the C13 C12 ratio in some taxonomic groups of corals and algae, but essentially no correlation (nearly constant O18) in one suborder of reef-building corals. The strontium and magnesium contents apparently are controlled mainly by skeletal mineralogy and show no correlation with carbon or oxygen isotopic composition. The observed positive correlations between C13 and O18 content may be due to calcification processes utilizing carbon-oxygen compounds from two isotopically different sources or utilizing selected portions of a wide spectrum of carbon-oxygen compounds in which there is a positive correlation between C13 C12 and O18;O16 ratios. Coral and algal carbonates from Jamaican waters, with an annual temperature range of only about 4�C, exhibit a total δC13 range of more than 13 per mil and a σO18 range of more than 4 per mil. The wide isotopic variability resulting from vital effects of calcifying organisms must be taken into account in applying isotopic analysis to the study of sedimentary carbonate rocks which may include reef-derived carbonates.

Unusual strength properties of echinoderm calcite related to structure.

Echinoderm skeletons are composed of high-magnesium calcite having a fenestrate structure. Each skeletal ossicle appears to be a single crystal of calcium carbonate rather than a polycrystalline aggregate as, for example, are mollusc shells. Measurements of crushing strength for spines of three species of sea urchin demonstrate that the strength to weight ratio of these spines is equivalent to or greater than that of mollusc skeletons and most calcareous rocks. Echinoderms utilize calcium carbonate with unusually high efficiency, fulfilling the strength and volumetric requirements of a suitable skeleton with a minimum amount of material. Comparative dimensional analysis of structural and mechanical properties of these skeletal materials may provide further insight into the evolution of echinoderms.

Coral growth rate: Variation with depth

Light and temperature are two of the most important physical factors affecting rates of growth of reef corals. The effect of light has been determined by X-radiographic measurement of long-term growth rates for 89 colonies of the coralMontastrea annularis collected over a 27.5-m depth range from St. Croix, U.S. Virgin Islands. These measurements, in conjunction with measurements of skeletal density, have established thatM. annularis calcifies most rapidly at intermediate depths, and they have confirmed the identification of two distinct populations within this important frame-building species.

Trace element composition of dolostones and dolomites and its bearing on the dolomite problem

Abstract 300 specimens of “primary” and “secondary” dolostones and 150 specimens of dolomite quantitatively analyzed in triplicate for 20 trace and minor elements statistically yield separate populations of the two major lithologic varieties of dolomitic carbonate rocks for certain trace elements. Histograms, probability plots and measure of the coefficient of variation suggest a lognormal distribution for the trace element data, which when tested statistically at the 1 per cent level of significance, indicate higher concentrations of Al, Ba, Fe, K, Li, Zn and Na in the “primary” dolostones. Sr is significantly concentrated in the secondary dolomites separated from the dolostones. On the basis of carbon-oxygen isotope ratios and crystallo-chemical considerations, the dolomite is considered to be a replacement mineral after calcite or aragonite and the abundance of Sr in “secondary” dolomite is interpreted as a minor impurity originally derived from metastable aragonite and entrapped in the dolomite structure. For the samples studied, trace element patterns appear to suggest that “primary” dolostones (characterized by very fine and uniform grain size; complete absence of fauna, relict textures and phantom structures; lack of appreciable porosity; relatively light color; frequent fine lamination; conchoidal fracture; and association with evaporitic sequences) may represent the early replacement of predominantly calcitic limestones under conditions of somewhat above normal salinity, whereas “secondary” dolostones (characterized by relatively coarse and non-uniform grain size; euhedral dolomite rhombs; frequent oolith, pellet and fossil textures; or organic fossil remains) may represent the early replacement of predominantly aragonitic limestones under normal marine conditions. A specific example of Ordovician Nittany dolostone comprising alternating zones of “primary” and “secondary” dolostones confirms the relationship evident in samples differing widely in age and geographic location.