HR: 09:30h
AN: B21A-07    [PDF]
TI: Biological Control of Uranium Incorporation Into Coralline Aragonite
AU: * Adkins, J F
EM: jess@gps.caltech.edu
AF: Caltech, 1200 E. California Blvd., Pasadena, CA 91125 United States
AU: Fernandez, D
EM: diego@gps.caltech.edu
AF: Caltech, 1200 E. California Blvd., Pasadena, CA 91125 United States
AU: Robinson, L F
EM: laurar@gps.caltech.edu
AF: Caltech, 1200 E. California Blvd., Pasadena, CA 91125 United States
AU: Wang, J
EM: jslw@gps.caltech.edu
AF: Caltech, 1200 E. California Blvd., Pasadena, CA 91125 United States
AB: Uranium in biogenic carbonates is an important tool in paleoclimate research. In closed systems, its alpha decay to thorium and protactinium provides one of our few independent chronometers. The U/Ca ratio has also been linked to both temperature and carbonate ion variations in the surrounding water. However, an understanding of Uranium's incorporation mechanism into coralline aragonite is still missing. We have used induced fission of U-235 in deep-sea corals, both modern and fossil, to measure the U distribution as it relates to the biogenic precipitation process. Modern deep-sea corals provide a growth environment with constant U isotopic ratio, constant U/Ca, constant temperature and constant carbonate ion concentration. Induced fission, as recorded in a mica mounted flush against the polished coral surface, provides unprecedented spatial resolution and whole sample maps of relative U concentration. We find that modern corals discriminate sharply (over a factor of 4) against U incorporation during initial precipitation. Secondary growth of aragonite around the coral has much higher concentrations with a distribution coefficient close to one. The shape of the U deficient region closely resembles the coral's banding pattern. Deep-sea corals clearly control their U concentration as they first lay down new aragonite. Fossil corals also show this pattern but with a much smaller difference between primary and secondary material. We will present transects of U isotopes in these two sample types to determine if the fossil corals are exchanging U with the surroundings or merely moving U within the coral lattice. These insights into the mechanisms behind U-transport will be invaluable in assessing whether carbonate systems have remained closed, and therefore whether U-series ages are reliable.
DE: 1035 Geochronology
DE: 1040 Isotopic composition/chemistry
DE: 1615 Biogeochemical processes (4805)
DE: 3344 Paleoclimatology
SC: Biogeosciences [B]
MN: 2003 Fall Meeting