The supply and accumulation of silica in the marine environment

Matter which is exposed to cosmic radiation undergoes characteristic changes in its chemical and isotopic composition. Such irradiated material can be found in many places on earth and belongs to either of two categories:

Cosmic ray produced radioactivity on the earth.

A detailed curve of ice volume versus time is needed in order to test the validity of the hypothesis that changes in the earth's orbital parameters are the cause of oscillations in Pleistocene climate. Although absolute ages available for glacial moraines and raised coral reefs provide a number of key points, they by no means allow a continuous curve to be drawn. Those points that exist, however, are entirely consistent with the hypothesis that the O18/O16 curves from deep-sea cores provide good approximations to the ice volume record. If so, then the primary glacial cycle must be sawtoothed in character; gradual glacial buildups over periods averaging 90,000 years in length are terminated by deglaciations completed in less than one tenth this time. Modulating this primary cycle are secondary oscillations. Those recognized during glacial growth phases average 20,000 years in length and those during the retreats about one thousand years in length. When the ice volume curve obtained in this way is compared with the summer insolation curve for the northern hemisphere, it is seen that the rapid deglaciations occur during times of unusually great seasonal contrast and that the secondary cycles modulating the glacial buildups closely parallel the insolation variations. Although these findings provide convincing evidence for the influence of orbital changes on climate, the cause of the primary sawtoothed cycle is still an open question. In conjunction with this study, we have determined the O18/O16 record for Caribbean core V12-122 and find it to be compatible with those given by Emiliani for cores P6304-8 and P6304-9. Our dating of this core by Pa231-Th230 and by magnetic reversals, however, strongly suggests that the absolute time scale adopted by Emiliani for deep-sea cores must be increased by 25%.

Insolation changes, ice volumes, and the O18 record in deep‐sea cores

Different techniques for preparing organic carbonates for mass spectrometric analysis may give slightly different results depending upon the material used and the grain size to which it is reduced. Two deep-sea cores from the central Caribbean have been analyzed. The samples from one core have been treated with vacuum roasting, and the samples from the other core have not been roasted. The two sets of results are similar and differ 0.5‰ from samples that have been ground to powder and roasted in a stream of helium. When using pelagic Foraminifera, no roasting and vacuum roasting appear to yield isotopic results slightly closer to the true temperature than the results obtained with helium roasting. The two cores analyzed and others previously described are correlated by means of several different criteria. This correlation yields a generalized temperature curve which extends from the present to 425,000 years ago. The high (.997), direct correlation observed between the stratigraphic positions of the isotop...

Paleotemperature Analysis of Caribbean Cores P6304-8 and P6304-9 and a Generalized Temperature Curve for the past 425,000 Years

Summary
1. The classical ‘coral reef problem’ concerned the geological relationships of reefs as major topographical features; modern coral studies consider reefs both as complex biological systems of high productivity and as geological structures forming a framework for and being modified by coral growth.

2. Deep borings in reefs have conclusively confirmed the general arguments of Darwin, that oceanic reefs developed by progressive subsidence of their foundations. Darwin failed to take account of Pleistocene changes in sea level and their effect on the present surface features of reefs. Daly's alternative ‘glacial control theory’ was based on false assumptions concerning marine erosion rates during glacial periods, but if sea level during the Holocene was higher than at present, as Daly also supposed, the effects on reef features would be profound.

3. Reefs are complex biological systems in tropical seas, dominated by scleractinian corals. Coral faunas are larger and more diverse in the Indo-Pacific than in the Atlantic. Hermatypic corals are restricted to shallow water by the light requirements of their symbiotic algae, but temperature is a major control of worldwide distributions. Temperature, salinity and sediment tolerances of corals are wider than formerly supposed, and corals can survive brief emersion except when it coincides with heavy rainfall. Water turbulence is an important ecological control, but difficult to measure.

4. The trophic status of corals is still unclear, but in spite of their anatomical and physiological specialization as carnivores it is likely that they derive some nutrient substances from zooxanthellae. Suggestions that filamentous algae in coral heads play a major part in the economy of the corals have not been supported by later work, but biomass pyramids constructed on the basis by Odum and Odum remain the only ones available. Most reefs are apparently autotrophic, with 1500–3500 g. Carbon being fixed per m.2 per year.

5. Few animals eat corals, which may account for their success. Important predators are fish and the echinoderm Acanthaster. Quantitative estimates of biogenic erosion of organic skeletons on reefs are high. Fish affect not only corals but other invertebrates, algae and marine phanerogams.

6. Corals may be killed by ‘dark water’, intense rain or river floodwaters, earth movements, human interference and especially hurricanes. Reef recovery after hurricanes may take 10–20 years.

7. In addition to fringing, barrier and atoll reefs, intermediate types are recognised. The main types may consist of linear reefs or faros. Smaller lagoon reefs include pinnacles, patches and platforms, and submerged knolls. Complex cellular or mesh reef patterns are also found.

8. Reefs are conspicuously zoned, both laterally in response to changing exposure to waves to form windward and leeward reefs, and transversely, as a result of steep environmental gradients across reef flats from sea to lagoon. Topographic and ecological zones may be characterized by particular coral species, but these vary widely from reef to reef. A major distinction can be made between reefs with and without algal ridges, which are common on open-ocean trade-wind reefs, in the Indo-Pacific, but are absent on Caribbean reefs and on Indo-Pacific reefs in more sheltered waters. gorgonians are common on Caribbean reefs, alcyonaceans in the Indo-Pacific.

9. Much of the difficulty in comparing reefs stems from the lack of uniformity in surveying methods. Problems of describing the complex three-dimensional patterns of organisms on reefs have yet to be solved, and hence little progress has been made in explanation of these patterns. Explanation in terms of simple environmental controls is inadequate.

10. Understanding the distribution of corals is made more difficult both by taxo-nomic problems and by the plasticity of growth form in different situations.

11. Growth of corals and reefs may be estimated by measuring the growth of individual colonies, measuring rates of calcium carbonate deposition in the skeleton, measuring topographic change on the reef and deducing net rates of reef growth from geological evidence. Massive corals may increase in diameter by 1 cm./year, branches of branching corals may increase in length by 10 cm./year. Study of deposition rates shows variation within colonies, between species, in light and dark, and seasonally. Rates of reef growth extrapolated from colony measurements reach 2–5 cm./year, and contrast with figures as low as 0–02 cm/year averaged over 70 million years from borehole data. Both colony growth rates and geological data suggest worldwide variations in rates of reef growth.

12. In spite of clear evidence of long-continued subsidence, present surface features of reefs, often only thinly veneered by modern corals, have been much affected by recent sea level fluctuations. Many slightly raised reefs at 2–10 m. above sea level date at 90–160 thousand years B.P.; there is evidence for a sea level at about the present level at 30–35 thousand years B.P.; and controversy continues over whether sea level has stood higher than the present at any time since the last sea level rise began about 20,000 years ago. Evidence from many reefs suggests a slightly higher sea level in the last 4000 years, but on other reefs such evidence is lacking.

13. Several reef features (submerged terraces, groove-spur systems, algal ridge, reef flat, reef blocks and reef islands) have been interpreted either as relict features dating from a higher sea level in the last 5000 years, or contemporary features developed in response to present processes. In some cases the evidence is equivocal; in others it is clear that diverse features are being grouped together under the same name. If such features are referable to a higher sea level, this may have been of last Interglacial or even Interstadial age rather than Holocene.

14. A reef consists of a rigid framework defining several major depositional environments within and around it. Sediments are of biological, mainly skeletal origin, except in unusual environments such as the Bahama Banks. The characteristics of sediments derived from organisms depend partly on the breakdown patterns of particular skeletons, partly on transportation and sorting processes. Fine sediments may be either detrital, or physicochemical precipitates.

15. Organisms affect sediments after deposition, by disturbance, transportation and probably comminution. Fish and holothurians have been studied in detail.

16. While new theories of coral reefs are proposed from time to time, the need is less for new theories than for standardised procedures to ensure comparability of reef studies and the identification of variations in reefs both on local and regional scales. While reefs as biological systems adjust relatively rapidly to changes, reefs as geological systems adjust much more slowly. Because of the magnitude and recency of Pleistocene fluctuations in sea level, many biological features of reefs are out of phase with inherited geological features, and this had led to much controversy.

Ecology and morphology of recent coral reefs

Oxygen isotopic analysis of Globigerina-ooze cores from the Atlantic and adjacent seas showed that surface ocean temperatures underwent numerous, apparently periodical, variations during the past few hundred thousand years. C14 dating showed that the last temperature minimum of the deep-sea cores was synchronous with the last major glaciation, the Main Wurm. Previous attempts to date deep-sea cores were based on the decay of uranium-unsupported Th230 (ionium). This method requires, among other conditions, that the supply of uranium-supported Th230 in sea water and the rate of non-carbonate sedimentation remained essentially constant over the time interval to be dated. Attempts to correct for possible variations in the non-carbonate sedimentation rate have been made by using such ratios as Th230/Th232 or Th230/Fe2O3. The validity of these corrections is questionable because Th230 produced in sea water by the decay of U238 and U234 has a geochemical history different from that of Th232 and Fe2O3. The requir...

Absolute Dating of Deep-Sea Cores by the Pa(231}/Th({230)) Method

O18/O16 paleotemperature analysis of core A254-BR-C from the central Caribbean Sea has yielded a temperature curve similar to the temperature curves previously obtained from other cores from the same area. Pa231/Th230 dating of significant temperature events has yielded a set of dates that are identical, within the limits of error of the analytical methods and of the stratigraphic correlations, to the set of dates previously obtained from the Caribbean deep-sea cores A240-M1 and A179-4.

P. J. Wangersky

Papers

Absolute Dating of Deep-Sea Cores by the Pa(231}/Th({230)) Method

Pa231/Th230 dating and O18/O16 temperature analysis of core A254‐BR‐C

"Absolute Dating of Deep-Sea Cores by the Pa(231}/Th({230)) Method" and Accumulation Rates: A Reply