Tuatara: Volume 16, Issue 1, April 1968
Corals, Coral Reefs and Paleotemperatures
Corals, Coral Reefs and Paleotemperatures
Corals are frequently utilised as indicators of fossil marine temperatures, based, sometimes erroneously, on the understanding that coral reefs (and hence reef corals) are restricted to closely defined temperature and depth conditions. Many geologists overlook the fact that there are eurythermic non-reef building corals.
Coral reefs are found in marine temperatures of 16-36°C (18°C is the most often cited cited lower limit of reef growth) but most active reef building occurs in the range of 23-25°C. It is often difficult to precisely define a coral reef in such a fashion that presence or absence can be clearly demonstrated. Studies of reefs of the Gulf of California showed (Squires, 1959) that within a temperature gradation 20° through 14°C polyspecific coral reefs slowly gave way to monospecific reefs which were then replaced in the near-shore ecological niche by kelp beds with no sharp boundary marking the limit of reef occurrence.
Hermatypic (reef-building) corals may form coral reefs composed of as many as 60 genera, and over 30 genera are found at the physiographic southern terminus of the Great Barrier Reef. A diversified hermatypic fauna which has been only partially described (cf. Wells, 1962) exists as far south as Sydney, while several species of hermatypic corals occur completely around Australia (Squires, 1966). Hermatypic corals therefore do not respond as a group to a single minimal temperature requirement. It is stated that a range of 16° through 36°C is required with an optimal range between 25° and 29°C. Growth of hermatypic corals is restricted to the upper 90 meters of the sea where light is present in sufficient quantity to permit photosynthetic activity by the obligate symbiont dinoflagellates (Zooxanthellae).
Non-reef building corals are called ahermatypic and lack symbiont zooxanthellae. These forms, often solitary and lacking the intratentacular budding capability of the hermatypics, range from the intertidal zone to depths of 6000 meters, existing in temperatures from —3° to 30°C. Although such ecological factors as restricted temperature ranges may exist, these have not yet been defined for most individual species.page 17
Knowledge of deep water coral structures has developed considerably, (Teichert, 1958; Stetson, Squires and Pratt, 1962; Squires, 1964a) including the recognition of these structures as fossils (Squires, 1964b). Although absolute temperatures may not be restrictive in determining the occurrence of deep water coral structures, they are characteristically found just below the temporary thermocline in waters of relatively stable temperatures, and are associated with bottom currents.
Corals and Paleotemperatures
There are three general approaches which may be taken in application of coral data to problems of paleotemperature: (1) a classical comparative zoogeographic analysis; (2) application of the principle of latitudinal diversity gradients; and (3) consideration of the presence of constructions of coral origin.
Classical Comparative Zoogeographic Analysis
Comparison of modern ranges (and temperatures of occurrence) and fossil distributions is effective only when the organisms occur in sufficient diversity at a single locality to permit averaging of data, where ecological tolerances are such that assemblages are found in many types of sedimentary facies, and when modern ranges are sufficiently well known to permit detailed comparisons. When these conditions are met it is possible to draw paleogeographic conclusions with the considerable detail of Durham's (1950) classic analysis of the Californian Tertiary.
Corals in New Zealand have become better known in their distribution during the Tertiary, but comparative detail either from Australia. South America, or the Indo-Pacific are not of the necessary quality. Further, because of various ecological requirements, diversified faunas are not found, assemblages usually being restricted to a few species. Many facies are devoid of corals with the result that distributions are patchy. Finally, temperature data are not known for most modern species. Despite these handicaps, Mr. Keyes had attempted a general analysis of Tertiary paleotemperatures, averaging the data from New Zealand as a whole. For the present, this is the best possible analysis.
In the following discussion I shall attempt to refine selected points on the graph using data developed through the other two types of analysis.
Application of the Principle of Diversity Gradients
That tropical faunas and floras are composed of more diverse assemblages than their poleward counterparts has been known since Wallace (1878) wrote ‘… animal life is, on the whole, far more abundant and varied within the topics than in any other part of the page 18 globe …’ Fischer (1960) provided data for comparative studies in a diversity analysis of corals occurring along the Great Barrier Reef. Because this study is compiled at the generic level, it is possible to make broad comparisons with fossil forms of hermatypic corals, knowing that the data thus derived gives seasonal extremes for the shore communities in the upper portion of the water column.
From numerous localities in Northland of Otaian or Hutchinsonian age come worn fragments of reef corals. Although the full fauna has not been collected from any single locality, the diversity is such that in all probability the full faunas did exist at all localities. Though no reef has been found, these corals are clearly derived from a nearby reef, and although now enclosed in deeper water sediments, were clearly of shallow water origin. Comparison of the ranges of the 15 genera now represented in collections with the Great Barrier Reef (Wells, 1955) suggest that a reef composed of 53 genera was probably in existence, and that seasonal temperatures of 20° to 29°C existed. This is based upon the occurrence of one particular genus Leptoseris known from a single specimen. If this genus is disallowed, the key form becomes Alveopora, and temperatures are 19° to 28°C. Thus for Northland during the Otaian, one may conclude that truly tropical conditions existed with minimum near shore temperatures of 20°C.
Corals not collected from Waiheke Island and Cape Rodney are assemblages of corals of a peculiar facies. The growth form of these corals is typical of that found in the Leptoseris zone, one of the deepest of the coral reefs. Absence of any shallow water corals-from the association must indicate some unusual environmental condition and these corals are not included in the analysis. Originally interpreted as anomalous growth in a shallow water environment (Squires, 1962), these occurrences must still be regarded as problematical.
Coral reefs have not yet been found in New Zealand. Were they, it would be possible to conclude that they were formed in temperatures above 18°C, and if well developed, probably in the range of 23-25 °C. The presence of a diversified fauna of reef corals in Northland suggests the presence of reefs, so one may conclude annual mean temperatures above 18° for the Otaian seas of that area. As has been shown, more definitive data is available from analysis of the faunal diversity.
How far back into the fossil record may the analogy between fossil and modern coral reefs be drawn? Unfortunately the answer is not known. Hermatypic corals of modern genera are not known before the Tertiary and as attempts to harvest remains of symbiont dinoflagellates from presumed hermatypic corals has as yet been unsuccessful, there is only the evidence of analogy. In general, page 19 large colonies seem to be dependent upon symbionts to provide the physiological framework for extensive growth. But how big is large? Recognition of the existence of deep water coral structures both in modern and fossil seas has added a new dimension which (potentially) further complicates the picture of older coral stuctures.
Deep water corals usually form colonies by extratenacular budding, often resulting in fragile, arborescent colonies. Although colonies are usually widely dispersed about the sea floor, occasional accumulations of corals may form the sequential development of thickets, coppices and finally coral banks. All occurrences of these structures have the commonality that they are formed in association with current activity whether it be surging as in Norwegian fjords, cascading on the margins of continental shelves, or in association with bottom currents. In the Atlantic, where deep water coral structures are best known, they are associated with the base of the seasonal thermocline, that is, at a depth at which water takes on a stable temperature. Above this depth the structures do not grow.
From these evidences of coral growth, it was suggested (Squires, 1964b) that the coral thickets in the Wairarapa were formed at temperatures between 6° and 10°C and that the depth of their growth (which unfortunately must be independently determined) they were at the base of the seasonal thermocline. Differences between the temperatures derived from analysis of the planktonic Foraminifera associated with these corals and the temperatures suggested for the bottom would indicate the magnitude of the seasonal fluctuation and the significance of the thermoclinal gradient.
Corals are potentially among the most informative of the macro-marine benthonic organisms in determining paleotemperatures. Until ahermatypic corals are better known in both modern and fossil seas, analysis of temperatures based upon fossil occurrences of these corals must be in terms of generalisations. Study of diversity gradients, particularly in the case of hermatypic corals, can be most informative. Analysis of the kind of coral involved and of the nature of the coral construction can result in important information when reefs or banks are present. In the latter case it is possible to derive much information regarding the structure of the water column as well as suggested temperatures. Caution must be exercised in interpretation of coral structures because of the ability of both shallow and deep water corals to form accumulations of calcium carbonate.
Dr. C. A. Fleming. I was interested to hear Dr Squires comments on the Rodney paleoecology. It seems as though he is holding out page 20 for a deep water environment, but I would emphasise that the other fossils and the conglomerates and coarse breccias are hard to reconcile with a deep water environment at the base of a cliff. Dr. D. F. SQUIRES. It is possible but not probable that the growth form of the coral at Rodney could be produced if the coral had grown in a cave. The growth form is a direct reflection of the amount of light it is getting and is independent of the systematics. Mr. J. GRANT MACKIE. I would like to comment on how far back we can project our ideas of the climatic conditions necessary for the growth of these corals containing symbionts. I am rather perturbed at the extrapolations back even to the Paleozoic for Rugose and Tabulate coral reefs and even Stromatoporoid reefs. Many people have said that these reefs have the same climatic requirements as modern Scleractinian corals. I think this is unreasonable.
Dr. D. F. Squires. I would agree, that projection of the hermatypic concept beyond the Mesozoic is problematical.
Durham, J. W., 1950. Cenozoic Marine Climates of the Pacific Coast. Bull. Amer. Geol. Soc. 61, 1243-64.
Fischer, A. G., 1960. Latitudinal Variations in Organic Diversity. Evolution 14: 64-81.
Squires, D. F., 1959. Result of the Puritan-American Museum of Natural History Expedition to Western Mexico. 7. Corals and Coral Reefs in the Gulf of California. Bull. American Mus. Nat. Hist., vol. 118, art, 7 pp. 367-432, pls. 28-34.
—— 1962. A Scleractinian Coral Faunule from Cape Rodney. New Zealand Jour. Geol. Geophys., vol. 5, no 3, pp. 508-514.
—— 1964a. Deep-water Coral Structure on the Campbell Plateau, New Zealand. Deep-sea Research, vol. 12, pp. 785-788.
—— 1964b. Fossil Coral Thickets in Wairarapa, New Zealand. Jour. Paleont. vol. 38, no. 5, pp. 805-1012.
—— 1966. Scleractinia in Port Phillip Survey 1957-1963. Mem. Nat. Mus. Melbourne, no. 27, pp. 167-384.
Stetson, T. R., Squires, D. F., Pratt, R. M., 1960. Coral Banks Occurring in Deep Water on the Blake Plateau. American Mus. Novitates, no. 2114, 39 pp.
Teichert, C., 1958. Cold- and Deep-water Coral Banks. Bull. American Assoc. Petrol. Geol., vol. 42, no. 5, pp. 1064-1082.
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Wells, J. W., 1955. A Survey of the Distribution of Reef Coral Genera in the Great Barrier Reef Region: Repts. Great Barrier Reef Comm., vol. 6, pp. 21-29, 1 table.
—— 1962. Two New Scleractinian Corals from Australia. Rec. Australian Mus., vol. 25, pp. 239-242, pls. 16-18.