THE USE OF LATE JURASSIC CORAL GROWTH BANDS AS PALEOENVIRONMENTAL INDICATORS

A detailed Pan-European sclerochronological study was carried out on t wo Jurassic corals, Thamnasteria concinna (Goldfuss) and Isastraea exp lanta (Goldfuss), the aim of which was firstly, to identify the contro ls on the deposition of growth bands, on both a regional and local sca le, and secondly, to assess the potential value of Mesozoic sclerochro nology as a tool for palaeoenvironmental and palaeoclimatic interpreta tions. The results indicate that Isastraea explanata was the faster gr owing of the two species with growth rates reaching 4.0 mm/yr where co nditions were optimal for coral growth (compared with 2.8 mm/yr for Th amnasteria concinna in the same environment). In deep-water environmen ts, growth rates of both corals were considerably lower. This reductio n in growth rate is accompanied by a change in growth form from dome-s haped colonies in shallow water to plate-shaped forms in deeper water. Both the reduction in growth rate and change in growth form are inter preted as a response to a reduction in light availability in the deep- water reefs. However, although Thamnasteria concinna had the lower gro wth rate of the two corals it showed a substantially greater variabili ty in its growth rate between different reef environments (up to 50 pe r cent. of its optimal growth rate). Data from similar reef types have been compared from different palaeolatitudes in an attempt to identif y a palaeoclimatic gradient. No systematic palaeolatitudinal changes i n the nature of the growth banding or the growth rate are seen. This i s 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 re sponse to low solar radiation. The study therefore shows that these co rals can provide useful data for palaeoenvironmental and palaeoclimati c work and should be included, where possible, in such investigations.