Fossil Corals Research Articles

Uranium-series dating of fossil corals from marine sediments of southeastern United States Atlantic coastal plain : Szabo, B.J., 1985 Geol. Soc. Am. Bull., 96(3):398–406

Uranium-series dating of fossil corals from marine sediments of southeastern United States Atlantic coastal plain : Szabo, B.J., 1985 Geol. Soc. Am. Bull., 96(3):398–406

Uranium disequilibrium dating of phosphate deposits from the Lau Group, Fiji

Uranium-series methods of absolute age determinations were first applied to fossil coral reefs1 and more recently to fossil bone2, marine apatite3–6 and insular apatite7,8. Past sea level stands, climatic conditions, primary productivity, and other factors are thought to be influential in the deposition of avian guano and subsequent insular phosphorite formation9. Some deposits appear to have formed during high stands of the sea while others may be related to glacial epochs8. We report here an attempt to determine the absolute age of two phosphate deposits in the Lau Group, Fiji by uranium-series disequilibrium techniques, in order to assist in the evaluation of their origin in terms of known palaeoclimatic information.

Extracts from Agassiz's Lectures on Geology and Paleontology (1856-1861)

Selected passages, typical of Agassiz's teaching of geology and paleontology, are presented from his students' notes to illustrate what he was teaching at the Museum of Comparative Zoology during the period of 1856-61. Included are remarks on coral reefs, ancient rocks, geological periods, fossil records, mountain building, and river valleys. Personal comments are also included.

Stromatoporiods: Growth and Form

This summer in Washington one of the sessions of the Fossil Coral Symposium was devoted to the subject of stromatoporoids and their relatives; this fall a major part of this workshop on fossil sponges is being devoted to the same group. The closeness of these two meetings emphasizes the problematic nature of these fossils and focuses attention on a controversy over their affinity that is over 100 years old. Although these enigmatic fossils have many cnidarian features, I think you will agree that they are “in the right place” in this sponge workshop.

Late Holocene eustatic sea-level changes deduced from geomorphological features and their 14C dates in the Ryukyu Islands, Japan

This paper deduces late Holocene eustatic sea-level changes from 14C dates and present elevations of almost all Holocene raised coral reefs in the Ryukyu Islands (an active island arc), without the assumption of a constant uplift rate. Stable relative sea levels, are restored by using sea-level indicators, such as raised coral reef terraces, fossil coral, wave-cut notches, and beach rock. The oldest and the youngest 14C ages of calcium carbonate fossils, collected from a depositional terrace-surface, show their formative period. When all relative sea-level changes of thirteen islands are compared, the Ryukyu Islands form three groups according to the oldest age in the formative period of the Holocene raised reef terraces in each island, that is 6500 yr B.P., 5000 yr B.P., and 3500 yr B.P. The youngest of the terrace-formative periods in the three island groups correspond with each other. One eustasy explains the seismic upheaval and the formative process of the terrace-surfaces of the three island groups. The eustatic sea-level trend in the Ryukyu Islands can be summarized as follows. From 6500 to 3500 yr B.P., the sea level continued to rise generally. Around 6500 yr B.P., 5000 to 3500 yr B.P., and 3500 to 1700 yr B.P., the rates of sea-level rise became slower. The sea level between 3500 and 1700 yr B.P. was stable and it has been the highest in the Holocene. At that time its height was less than 1.0 m above the present sea level. The Holocene uplift rate of each island is estimated to be half to three times as great as that of the last 125 000 years. The uplift rates of the three island groups in the late Holocene are inferred to have been decreasing exponentially. The crustal movement and the distribution of raised coral reefs are further discussed here.

230Th, 231Pa, and open system dating of fossil corals and shells

Conventional uranium series and S‐R open system methods of age determination were applied to the dating of unrecrystallized fossil corals and molluscan shells. Two of the six corals have remained closed systems with respect to uranium and daughter isotopes 230Th and 231Pa; the other four coral samples were dated by the S‐R model. Analyses of coexistent mollusk shells revealed complex migrational patterns of uranium, thorium, and protactinium in these samples. Three of the six shells yielded acceptable conventional U series or S‐R model ages.

Subaerial Diagenesis of Axial Corallite of Acropora Cervicornis: ABSTRACT

In the subaerial environment the axial corallite of Acropora cervicornis is micritized. The micritization starts in the biologically secreted aragonite; later, the pore-infilling cement (recrystallized or not) is micritized. If the micritized fibers are leached out, the nonmicritized fibers are left without support and the corallite becomes crumbly (chalky). As more fibers are micritized, the corallite may be leached out entirely. If the pores of the corallite are infilled with minerals resistant to leaching, a reticular three-dimensional frame will be left in the space occupied by the corallite. If the new voids are infilled by drusy calcite a cast of the corallite will be created. The aragonitic fossil corals located in the splash zone may be replaced by calcite crystal , with much of the texture of the original cement, septa, septothecae, and costae reappearing and being preserved as ghosts in the calcitic crystals. The spherulitic texture of septa, septothecae, and costae produced by a submarine recrystallization process is not preserved. End_of_Article - Last_Page 515------------

Sea level change in the Holocene on the northern Great Barrier Reef

Detailed studies, utilizing a range of both well controlled sea level criteria and dates, are required if Holocene time-sea level curves are to be established with any degree of confidence. This paper is restricted to an interpretation of Expedition results from the northern Great Barrier Reef, excluding those from the drill core. Extensive colonies of emergent fossil corals in growth position indicate that present sea level was first reached about 6000 a b. p. Elevations of cay surfaces, cemented rubble platforms, microatolls, coral shingle ridges, reef flats and mangrove swamps, referenced to present sea level show an array of heights. However, levels of particular features are accordant on many reefs: it is believed that these can be related to particular sea levels. Radiometric dating provides the time framework. Ages of samples from similar deposits on different reefs are surprisingly consistent. Oscillations in sea level since 6000 a b.p ., relative to present sea level, are identified with varying degrees of confidence. This history of relative sea level does not separate eustatic from noneustatic components.

Uranium-series dating of insular phosphorite from Ebon atoll, Micronesia

THE uranium-series method of absolute age determination has been widely applied to the radiometric geochronology of fossil coral reefs and associated problems of palaeoclimatology1–3. Attempts have also been made to extend this method to marine phosphorites, leading to a better definition of the environmental conditions prevailing at the time of their formation4–8. Although the occurrence of phosphorite deposits on many islands in the central Pacific and Indian Ocean is well documented9, no absolute ages for any of these deposits have been available. This is regrettable because the widely accepted hypothesis9 for the origin of these insular phosphorites by chemical interaction of bird guano with the underlying reef limestones is based largely on circumstantial rather than direct evidence. We report here the first attempt to determine the age of both the apatite component as well as that of the associated coral material in a phosphorite deposit from Ebon atoll in Micronesia, in an effort to explore the feasibility of dating insular phosphorite deposits by the uranium series method and thus to relate the time of phosphate deposition to known environmental conditions in a given area.

HOLOCENE-EMERGED CORAL REEFS AND SEA-LEVEL CHANGES IN THE RYUKYU ISLANDS

The numerous coral reefs in the Ryukyu Islands provide detailed evidence of the relationship between reef development and late Holocene sea-level changes. The islands belong to the tectonically active island arc and have experienced many destructive earthquakes in the historical period. It is, therefore, assumed that the Holocene reefs have been terraced at various elevations, due to sudden uplift associated with major earthquakes, at various sites during the period of the post-glacial eustatic sea-level changes. In this paper, the authors have attempted to restore the late Holocene sea-level curve as follows. First, cross-profiles of various emerged reefs were constructed, and fossil corals were collected for 14C dating (Figs. 2, 3, 5, 7, 8, 10 and 11). Secondly, 58 fossil corals were dated by the 14C method (Table 1), and relative sea-level curves were constructed for Kodakarajima, Takarajima and Kikaijima islands (Figs. 13, 14 and 15). An attempt was then made to restore the late Holocene sea-level curve, corrected for intermittent seismic uplifts, using the Kikaijima relative sea-level curve (Fig. 15). The amounts of intermittent seismic uplift were calculated using a constant long-term up-lift rate (1.87m/1, 000yrs.) which minimized the difference between pre- and post-uplift sea-levels, the rate being calculated by least squares methods. The restored sea-level curve (Fig. 15) resembles the Shepard curve, which shows a gradual rise of the sea-level towards the present sea-level, rather than the Fairbridge curve, which oscillates considerably during the past 6, 000 years including several stands higher than the present.

Age and magnitude of the late Pleistocene sea-level rise on the eastern Yucatan Peninsula

230 Th ages of fossil corals from upper Pleistocene limestones on the eastern Yucatan Peninsula and Cozumel Island are about 122,000 yr. Similar ages for corals at about the same elevations at other areas in the Caribbean region indicate that eastern Yucatan has been tectonically stable since late Pleistocene time. Stratigraphic relationships in Yucatan suggest that maximum sea level during the late Pleistocene period was 5 to 6 m above present sea level.

The mangal of Sinai: Limits of an ecosystem

The northernmost mangrove forests of the Red Sea are described. Temperature and salinity conditions were found to be extreme, obviously reducing the diversity of the ecosystem and presumably setting the geographical limits of its distribution. The Sinai mangal grows on hard fossil coral bottom, without any influence of estuarine sedimentation. Primary production of the open Red Sea waters is extremely low, therefore, the rich biomass of the Sinai mangal is built up by an autarkic nutrient recycling system. Several biotic zones within the Sinai mangal are described following an inshore-offshore transect. Vertical zonation of algae and animals on the aerial roots ofAvicennia is discussed. The Sinai mangal, “Shura” by its local name, is proposed as an ideal model for the study of trophic relations within the mangrove biome.

Oxygen isotope stratigraphy of Late Pleistocene coral terraces in Barbados

DISPUTE over the correlation of the last interglacial event in deep-sea cores with the corresponding shoreline feature on islands preserving fossil coral terraces has become a fundamental stumbling-block to progress in Pleistocene geochronology. If polled independently most scientists working in this area would agree that the last interglacial event is clearly recorded in their own data, yet the problem of precise correlation among the various types of data has remained the subject of controversy for the past eight years1–8. By making oxygen isotope determinations in molluscs collected from the Late Pleistocene coral terraces in Barbados, we have directly correlated these terraces with the oxygen isotope stratigraphic record of deep-sea cores, and have established that Barbados terraces I, II, and III (ages 82,000, 105,000 and 125,000 yr BP) were all formed during stage 5 of the standard oxygen isotope stratigraphy.

Amino acids in fossil corals: racemization (epimerization) reactions and their implications for diagenetic models and geochronological studies

Thirty-eight fossil coral samples of known age from late Pleistocene uplifted reef terraces have been analyzed for amino acid composition and for the degree of racemization of selected amino acids. Particular attention has been given to the epimerization (racemization about the α-carbon) of isoleucine. The D L ratios observed in many of these samples do not conform to the concept of increasing racemization being associated with increasing fossil age. It is shown that sixteen of the samples demonstrate concordance between their known age and the age estimated from racemization data. Racemization ages are based upon a kinetic model derived from the kinetics observed in foraminifera in marine sediments and published estimates of the temperature dependence of the isoleucine epimerization reaction. Lack of concordance for the remaining samples is explained by either of two separate diagenetic phenomena: extensive leaching of free amino acids from the fossil, or contamination by ‘young’ amino acids. Ambiguities observed in many of the results may be due to the fact that coralline organic matter can have a complex history because of variations in its source and in its relation to the mineral phase. Neither of these factors has been observed to exert such a significant effect on racemization kinetics observed in fossil foraminifera or molluscs. Fossil corals are of only modest value as specimens for amino acid geochronological studies.

Seasonal high and low density bands in reef coral skeletons

THE skeletal aragonite deposited by scleractinian reef corals is characterised by alternating bands of high and low density carbonate that are concordant with the growth surface of the colony. These density variations, which are readily observed by X-radiographic examination of 5–10 mm thick sections of the corallum cut parallel to the axis of growth, are annual in periodicity, such that one high density band plus one adjacent low density band represent the skeletal increment formed over one calendar year1–4. Skeletal X radiography is a powerful tool for studying both modern and fossil corals, for in addition to providing a wealth of growth rate data from specimens that grew undisturbed for many years in their natural habitat on the reef, valuable information about the coral itself and about the environment in which the coral lived can be obtained. As Buddemeier5 points out, density banding in reef corals can not only yield physiological and ecological information about these marine organisms, but also help to unlock the record of long term environmental conditions that is embodied in a coral skeleton. Reef corals, for example, are effective samplers of certain important radionuclides6.

Sea level events and Pleistocene coral ages in the northern Bahamas

Emergent Pleistocene sea level indicators in the northern Bahamas include: a bioerosional notch at +5.3 to 5.9 m; sea caves, notches, and marine terraces at about +4.3 m; and lithified coral rubble and reef deposits between 0 and 3 m. Thorium 230 dates of the fossil corals, which were deposited as these features were being produced, span the age range from 100 to 145 thousand years BP with a majority falling between 115 and 130 thousand years BP. The notch at about +5.6 m is interpreted to be the product of a sea level stand 125 thousand years BP, while the features at +4.3 m are believed to be formed sometime later as sea level fell from the higher position. Part of the age span is inherent in the dating technique and possible sample alteration. Another cause of the spread may be mixing of corals of different ages into a single deposit.

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 environmental indicators in as much as density variations may reflect changes in water temperature and/or light intensity. Relationships between skeletal growth banding and environmental 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, Cyphastrea, 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 water 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 temperature is above average.

Age determinations of fossil corals using230Th/234Th and230Th/227Th

The growth toward radioactive equilibrium of 230Th with 238U and of 231Pa with 235U has successfully been used to determine the ages of fossil corals. The chemical procedures employed in these measurements can be simplified dramatically if only thorium isotopes need be measured. Our method uses 234Th as a measure of 238U and uses 227Th as a measure of 231Pa to calculate the age of the coral. In addition to reducing the length of time required for an analysis by more than one half, our method does not rely on spikes but on a calibration standard that is counted to determine relative alpha and beta counting efficiencies. Ages of fossil corals from Jamaica correlate with the 120,000- to 135,000-yr-high sea stand reported by other workers.

Evolution of the Moon's orbit and the origin of life

PERIODICITIES in fossil corals may be used to determine the number of days in a year1; both daily and seasonal variations are visible in the growth patterns. Scrutton2 used similar data to determine the number of days in a lunar month; monthly and fortnightly periodicities are present because of tides. The available data3–6 through the Palaeozoic are given in Fig. 1a and b. We conclude, from studies of living crustaceans and recent fossils, that the growth patterns are sensitive to the solar day. So, the data represent the number of solar days in a year and in a lunar month; the present values are 365.25 and 29.53 d respectively. Panella3 has also used periodicities in stromatolites to determine the number of solar days in a lunar month during the Precambrian. His results are given in Fig. 1c.

Io/U ages of corals from Saurashtra coast

Un-recrystallized fossil corals occurring at 2–6 m above the present mean low-tide level on the Saurashtra coast (Western India) have been dated by 230Th/ 234U and 234U/ 236U radiometric methods. The measured ages of the corals, which are in agreement with radiocarbon ages, fall in three groups, 6,000, 30,000 and 120,000 years before present. This observation, coinciding with the high sea-level stands reported in several locations in the world, in the absence of evidence in support of instability of Saurashtra coast, suggests that the coral reefs are remnants of one Holocene and two Quaternary high sea-level stands.