Corals as climatic indicators

Corals have a long lifespan and record events that happened during their whole life in the incremental growth of their skeletons. Solitary rugose corals, such as the Silurian genus Kodono-phyllum, narrow and thicken along their length (Fig. 4.8). Wide patches represent times when the polyp was thriving. Narrow zones indicate times when the polyp was under stress and lost body mass, contracting towards the center of its calice, and only adding skeletal material to this central portion.

Corals with zooxanthellae are confined to a narrow range of environmental conditions. The presence of colonial corals in a rock sequence, especially scleractinian corals, is an indication that a region was within 30° of the equator when the rock was deposited. It is only within this region that the water temperature is between 23 and 29°C - the range in which

Direction of growth fliiSiliils

"Bench" representing an interval when the polyp was stressed and reduced its volume

Fig. 4.8 Kodonophyllum, a Silurian solitary rugose coral, showing periods of restricted growth and periods when the polyp was larger, representing favorable conditions.

reef corals thrive. The presence of these corals also indicates that deposition occurred in clear, shallow, nutrient-poor water (Fig. 4.9). Such a set of assumptions is less secure for rugose and tabulate corals as it is not known whether these forms had photosynthetic zooxanthellae. However, by using independent environmental indicators, studies on Lower Palaeozoic reefs such as the Wenlock Limestone of Shropshire confirm that they developed in shallow water depths and at low latitudes.

Reef corals of Pleistocene age or younger can be used to generate a much more detailed record of paleoclimatic change, preserved within a single colony, or in a series of colonies of overlapping ages. As the colony adds calcium carbonate (in the form of aragonite) to its skeleton, it removes carbon and oxygen from sea water. Each of these elements is stable with more than one atomic weight, and is thus said to have several stable isotopes. Heavier and lighter isotopes of the same element are fractionated in the oceans by a range of climatic events and these are recorded in the skeletons of growing corals. For instance, water evaporating from the oceans is enriched in the lighter isotope of oxygen. At times in the past when this water was locked up in ice on land, the oceans become isotopically heavy in oxygen, and this is a useful paleoclimatic signal. The ratios of stable isotopes of these elements in water, recorded as S13C and S18O, are hence good proxies for determining paleoclimate. In addition, corals incorporate traces of organic material into their skeleton, including humic acids from river outflows, derived from biological weathering. These show up in sections cut through the coral as thin bands that fluoresce in ultraviolet light. Bands within the coral where humic acid is abundant indicate that the colony was growing in periods of high run-off from nearby rivers.

"Bench" representing an interval when the polyp was stressed and reduced its volume

Carte Anset Continents
Fig. 4.9 Map of the modern world showing the current distribution of coral reefs within 30° of the equator. Note the abundance of open ocean sites that are areas of low nutrient availability.

Favosites

Tabulate coral

Upper Ordovician-Middle Devonian

A small (typically 3-5 cm diameter) colonial coral. Corallites are polygonal and cerioid; that is, each corallite retains its wall. Each is about 2 mm in diameter. Within the corallites short septal spines are developed. Tabulae are abundant and regular in their spacing. Mural pores connect the corallites. They are common in Lower Palaeozoic reefs.

A Silurian Favosites colony from Quebec preserves the calcified remain of polyps within their calices. This unusual soft-part preservation helps to confirm the position of tabulates as true corals.

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