Uranium thorium dating on corals
By measuring the oxygen isotopic (δ18O) content of the coral skeletal aragonite every 1mm down-core, we obtain timeseries that reflect changes in past ocean temperature and/or changes in the oxygen isotopic content of the seawater (the latter linearly related to salinity, and thereby used as a proxy for hydrological balance). Our long-term sites in the central equatorial Pacific are extremely sensitive to temperature and rainfall changes associated with the El Nino-Southern Oscillation (ENSO), so corals from these sites provide exceptional archives of past ENSO activity. We use so-called “modern” corals, drilled from large living colonies underwater, to calibrate our coral oxygen isotope proxy against instrumental climate data, and to reconstruct the evolution of 20th century climate. We use so-called “fossil corals”, drilled or collected from long-dead colonies scattered along beaches at our sites, to reconstruct climate of the more distant past (back 7,000 yrs so far). We core these heads and back in the lab, we date them with Uranium-Thorium dating and analyze their oxygen isotopic composition for climate reconstruction. Jess Conroy and Hussein Sayani comb a fossil coral rubble field on Christmas Island for Porites cobbles that we can use for a new reconstruction of tropical Pacific climate over the last several millennia. Charles, Highly variable El Nino-Southern Oscillation throughout the Holocene, Science DOI: 10.1126/science.1228246, 2013.
The average Th age of samples from beds of the Rappahannock River, Ponzer, and Ten Mile Hill localities is ∼212,000 yr, and they correlate with oxygen isotope stage 7.Pure thorium is very ductile and, as normal for metals, can be cold-rolled, swaged, and drawn.At room temperature, thorium metal has a face-centred cubic crystal structure; it has two other forms, one at high temperature (over 1360 °C; body-centred cubic) and one at high pressure (around 100 GPa; body-centred tetragonal).The amino acid values, however, ure unacceptably high in at least two groups of samples, those from localities near Charleston, South Carolina, and from central Virginia.
Thorium is a weakly radioactive metallic chemical element with symbol Th and atomic number 90.Thorium was formerly used as an alloying element in TIG welding electrodes, as a material in high-end optics and scientific instrumentation, and as the light source in gas mantles, but these have become marginal uses.