OXYGEN ISOTOPE SEASONAL PROFILING AS A TOOL FOR MEASURING GROWTH RATES AND INFERRING CARBONATE PRODUCTION RATES IN FOSSIL BRYOZOANS

This is a preliminary report on a methodology to quantify growth rates and carbonate production rates in fossil bryozoans. It uses seasonal water temperature fluctuations as recorded in profiles of skeletal oxygen isotopic ratios to determine annual growth rates, rather like counting geochemical tree rings. A pilot study was performed on living bryozoans from the coastal waters (65 to 110 m deep) of the South Island, New Zealand. The five dominant carbonate producing bryozoans were included: Adeonellopsis sp., Celleporaria agglutinans, Celleporina grandis, Cinctipora elegans, and Hippomenella vellicata. Growth rates were independently determined from mark and recapture, C-14 dating, and seasonal oxygen isotopic ratio profiles. Carbonate volume was determined morphometrically. Growth rates varied from 1 to 7 mm/yr while carbonate production varied from 0.1 to 6.4 g/yr. This variation is attributed to interspecific and environmental differences. The methodology is now being applied to fossil bryozoans from the Middle Ordovician Courtown and Tramore Limestone Formations which crop out in Counties Wexford and Waterford, respectively, in southeastern Ireland. These cool-water carbonates were chosen as they were deposted at mid palaeolatitudes (roughly 45^oS), and Global Climate Models for this time and place predict at least a 5^oC seasonal water temperature variation. Several species of trepostome bryozoans, with either hemispherical or ramose growth forms have been collected. Their mineralogy is being tested with cathodoluminescence for diagenetic alteration before isotopic analyses are performed with laser ablation. Obviously the mark and recapture as well as the C-14 dating methods are not applicable to these Ordovician fossils. It is hoped that this methodology will permit the quantification of carbonate production rates in fossils that lived in environments with strong seasonal water temperature fluctuations and whose original oxygen isotopic signatures are preserved.