Article: The use of Late Jurassic coral growth bands as palaeoenvironmental indicators

A detailed Pan-European sclerochronological study was carried out on two Jurassic corals, Thamnasteria concinna (Goldfuss) and Isastraea explanata (Goldfuss), the aim of which was firstly, to identify the controls on the deposition of growth bands, on both a regional and local scale, and secondly, to assess the potential value of Mesozoic sclerochronology as a tool for palaeoenvironmental and palaeoclimatic interpretations.The results indicate that Isastraea explanata was the faster growing of the two species with growth rates reaching 4.0 mm/yr where conditions were optimal for coral growth (compared with 2.8 mm/yr for Thamnasteria concinna in the same environment). In deep-water environments, growth rates of both corals were considerably lower. This reduction in growth rate is accompanied by a change in growth form from dome-shaped colonies in shallow water to plate-shaped forms in deeper water. Both the reduction in growth rate and change in growth form are interpreted as a response to a reduction in light availability in the deep-water reefs. However, although Thamnasteria concinna had the lower growth rate of the two corals it showed a substantially greater variability in its growth rate between different reef environments (up to 50 per cent, of its optimal growth rate).Data from similar reef types have been compared from different palaeolatitudes in an attempt to identify a palaeoclimatic gradient. No systematic palaeolatitudinal changes in the nature of the growth banding or the growth rate are seen. This is inferred to be a consequence of a Jurassic climate more equable than at present. Reefs from high palaeolatitudes have low growth rates and L/H (low/high) density band ratios. It is suggested that this is a response to low solar radiation. The study therefore shows that these corals can provide useful data for palaeoenvironmental and palaeoclimatic work and should be included, where possible, in such investigations.