USING MG/CA RATIOS TO INVESTIGATE GROWTH AND LONGEVITY IN CRANIID BRACHIOPODS

To what extent is biomineralization influenced by ambient Mg/Ca ratios in the ocean and the temperature of the water (affecting the rate at which Mg may be incorporated into skeletal calcite), and to what extent can organisms control the incorporation of Mg? If the latter is significant, Mg/Ca ratios might be used as a geochemical proxy to measure longevity and ontogenetic variation in growth rate, both critically important pieces of difficult to determine life-history information, that can be used to investigate a host of ecological and evolutionary questions. Craniida is an order of inarticulated brachiopods possessing a low-Mg calcitic shell; they are cosmopolitan in distribution, well-represented from the Cambrian to Recent. Despite their ubiquity, little is known of craniid longevity or growth rates. Because they possess a calcitic shell, craniids are amenable to geochemical sclerochronological investigation. We used a laser ablation inductively-coupled mass spectrometer (LA-ICPMS) to measure the ontogenetic variation in Mg/Ca ratio of Novocrania dorsal valves from Bamfield, B.C. We sectioned the shells sagitally, and used the LA-ICPMS to sample and analyze a series of 30 micron spots, approximately 90 microns apart, avoiding punctae, from the commissure to well past the apex, following the exterior edge of the shell. A single peak and valley composed of several data points was interpreted as one year. Fluctuations of Mg/Ca ratio appear to be caused by seasonal temperature changes, as Mg substitution rate increases with temperature, presumably affecting shell growth rate. Both shells, ~ 1 cm in diameter, showed approximately 10-12 years of growth. Strong correspondence in results exists among these craniid, and thecideide and terebratulide brachiopods microsampled and analyzed for δ18O and Mg/Ca. LA-ICPMS investigations of growth and longevity may resolve time more precisely, particularly in small specimens. LA-ICPMS has a much higher resolution (12 micron spot size), costs are lower, and sample preparation and analysis is faster; all are advantages over combined microsampling and δ18O analysis. The main disadvantage is that the δ18O system is much better understood than the Mg/Ca system. We hope that future empirical contributions will help us understand both craniid evolution and the Mg/Ca system.