(Table 1) Age determination of ODP Hole 134-831B

We have determined convergence rates of the Australia plate with the New Hebrides Island Arc using the chronostratigraphy of Bougainville Guyot, drilled at Site 831, Ocean Drilling Program Leg 134. The convergence rate at the New Hebrides Island Arc is the vectorial sum of convergence rates between the Australia and Pacific plates (8.8 cm/yr at Espiritu Santo Island) and the opening rate of the North Fiji Basin. We assume that the relative motion of the Australia and Pacific plates is unchanging on the 1.5 m.y. time scale and that any changes of rate occurred in the North Fiji Basin. Convergence rates can be calculated because we know the distances at which carbonate sedimentation would cease and resume as the Bougainville Guyot emerged and submerged during its crossing of the outer rise flexure west of the New Hebrides Island Arc. From 1.42 to 0.393 Ma, Bougainville Guyot was subaerially exposed as it moved approximately 177 km across the outer rise and no sediment was deposited. The mean convergence rate during this time interval was 17.2 +/- 7 cm/yr, as determined from strontium-isotope and uranium-series ages of the last carbonates before emergence and the first carbonates deposited after submergence. The Australia plate has converged approximately 52 km with the New Hebrides Island Arc at a mean rate of 13.2 +/- 1 cm/yr since 0.393 +/- .011 Ma when Bougainville Guyot re-submerged and carbonate sedimentation resumed. This age is based on a precise mass-spectrometric 230Th age measurement and is reliable because the uranium isotopic composition of the sample indicates no diagenetic alteration. The change in convergence rates from 17.2 to 13.2 cm/yr indicates a significant change in the opening rate of the North Fiji Basin. However, this conclusion depends on the age of initial opening of the North Fiji Basin. If the North Fiji Basin began to open at 10 Ma, then the average opening rate at Espiritu Santo Island has been 6 cm/yr. If opening began at 12 Ma, then the average rate had to be 5 cm/yr. Because the relative motion between the Australia and Pacific plates is 8.8 cm/yr, the net convergence rate at the central New Hebrides Island Arc must have averaged 13.8 to 14.8 cm/yr. Younger dates of initial opening would require higher average convergence rates. If the convergence rate of 13.2 cm/yr at Espiritu Santo Island had prevailed for the entire opening of the North Fiji Basin, then the basin would have taken 13-14 m.y. to open at a mean rate of 4.4 cm/yr. This is contrary to hypotheses for the time of origin of the North Fiji Basin

Surface Area as a Predictor of Sample Preservation in Fossil Coral Species

Undergraduate Research Opportunities Program (UROP), Earth & Environmental Sciences, University of Minnesota DuluthRecords of sea level are important for scientists studying climate and geology in the past. Sea level is correlated with how much water is stored in glaciers and ice sheets, which indicates what the climate was like at that time. Past climate records are the key to understanding the future as Earth's climate changes. One method of documenting past sea level is with radiometric uranium-thorium (U-Th) dating of fossil corals. Corals make a good proxy for sea level because they need to grow near the surface of the water, where light is most abundant. Coral concentrate 238U from seawater in their skeleton (Gallup, 2015). Over time, 238U decays to 234U and then to 230Th at a predictable rate. When they first form, corals do not contain any 230Th, because it is insoluble. Thus, the concentration of 230Th in fossil coral skeletons allows us to determine their ages (Gallup, 2015). The elevation that the sample was taken from is then presumably the sea level at the time that coral was alive. In some cases, the coral is at a higher elevation due to tectonic uplift, but this can be corrected for. It's important for the sea level record to be accurate, since it serves as a reference for other research. Thus, it's crucial that U-Th dating methods be as accurate as possible. However, coral fossils are susceptible to alteration after their deposition, which can cause remobilization of 238U and 230Th (Gallup, 2015). The result is a shift in the apparent age of the sample. The quality of the sample can be inferred by looking at the ratio of 234U to its parent, 238U. The ratio at the time of deposition, the initial 234U value, can be calculated from the ratio at the time of analysis. How far it is from the accepted value for seawater at the time of deposition is an indicator of sample quality. Corals with a larger surface area are more susceptible to alteration, since they have more area for chemical and physical weathering. Five different types of coral, most notably Porites, Favids, and Leptids, were studied. Out of these, Porites has highest surface area (El-Sorogy, et. al, 2013), and is therefore more susceptible to alteration over time

Records of cosmogenic radionuclides <SUP>10</SUP>Be, <SUP>26</SUP>Al and <SUP>36</SUP>Cl in corals: first studies on coral erosion rates and potential of dating very old corals

We present results of measurements of cosmogenic 10Be, 26Al and 36Cl, and the indigenous (intrinsic) concentrations of the stable elements Be, Al and Cl in 120-200 kyr old corals from Barbados and Puerto Rico. The concentration levels of these radionuclides in the corals lie in the range 104 to 108 atoms/g. A comparison of the measured nuclide concentrations with those expected to be produced in the corals by nuclear interactions of energetic cosmic radiation shows that (i) the radionuclides 26Al and 36Cl are derived from in situ cosmic ray interactions in the corals after their formation, but that (ii) the radionuclide 10Be owes its provenance in the coralline lattice primarily due to incorporation of dissolved beryllium in seawater in the lattice structure of the corals