ENVIRONMENTAL CONTROLS ON URANIUM IN REEF CORALS

A survey of corals from a variety of tropical settings reveals previou sly unseen seasonal variations in skeletal U/Ca. Based upon two corals from the Galapagos Islands, a comparison of U/Ca with delta(18)O sugg ests a possible temperature dependence of +3-4% per degree centigrade cooling. An overall range in U/Ca of 1.03-1.37 mu mol U/mol Ca (2.45-3 .25 ppm) between corals from warm and cool water settings supports thi s interpretation. An alternative control, however, cannot be ruled out . Changes in the carbonate ion content of surface waters are sufficien t to drive comparable variations, provided uranium is incorporated as UO22+:+ or a carbonate complex thereof. In addition to these possible controls, we identify a probable salinity influence on coral U/Ca whic h suggests that uranium uptake depends upon [U](seawater) and not [U/C a](seawater). Within individual corals, artifacts associated with ''vi tal'' influences appear minimal. Extension/calcification rate effects as assessed via comparisons of contemporaneous growth trajectories in individual colonies appear small relative to typical seasonal U/Ca var iations of 10-20%. Excluding corals from the Galapagos cool regime, fi ve species from warmer tropical settings exhibit remarkably little var iability in mean U/Ca(1.03-1.09 mu mol U/mol Ca). Our findings suggest that the range of U/Ca in modern corals defined by published data ref lects a combination of interspecific variability and environmental con trol. The possibility that coral U/Ca is associated with temperature, alkalinity, and salinity suggests many uses for this new paleotracer. Rapid and precise measurement of uranium by isotope dilution ICP-MS wi ll expedite future development and application. Key among the next ste ps must be studies to isolate the influences of the above mentioned en vironmental parameters. Additionally, potential microsampling artifact s (e.g., caused by drilling) and an apparent 6% ''leachable'' uranium fraction in a single test coral should be investigated through further pretreatment studies.