ENVIRONMENTAL CONTROLS ON URANIUM IN REEF CORALS

A survey of corals from a variety of tropical settings reveals previously unseen seasonal variations in skeletal U/Ca. Based upon two corals from the Galapagos Islands, a comparison of U/Ca with delta(18)O suggests 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 this interpretation. An alternative control, however, cannot be ruled out. Changes in the carbonate ion content of surface waters are sufficient 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 which suggests that uranium uptake depends upon [U](seawater) and not [U/Ca](seawater). Within individual corals, artifacts associated with ''vital'' 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 variations of 10-20%. Excluding corals from the Galapagos cool regime, five species from warmer tropical settings exhibit remarkably little variability 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 reflects a combination of interspecific variability and environmental control. 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 will expedite future development and application. Key among the next steps must be studies to isolate the influences of the above mentioned environmental parameters. Additionally, potential microsampling artifacts (e.g., caused by drilling) and an apparent 6% ''leachable'' uranium fraction in a single test coral should be investigated through further pretreatment studies.