DEVELOPING A TEPHRA DATABASE FOR IODP SITES U1417 & U1418: LATE MIOCENE TO PRESENT EVOLUTION OF ERUPTIVE VOLCANISM ALONG THE GULF OF ALASKA
Our methods include light microscopy and magnetic susceptibility identification of tephra deposits; grain-specific major and trace element geochemical fingerprinting using electron probe microanalysis and laser-ablation inductively-coupled plasma mass spectrometry; and 40Ar/39Ar radiometric dating of glass separates. Our results will be integrated with the paleomagnetic, stable d18O isotope, and microfossil chronostratigraphies developed for Sites U1417 & U1418. Further cross-verification will come from 40Ar/39Ar radiometric dating of basalt and tephra samples from nearby DSDP Site 178, as well as from published terrestrial Alaskan and Canadian Yukon tephra data. These results will provide a unique tool for constraining the true age of southern Alaska’s terrestrial tephra deposits, and will be made available to the paleoclimate and tectonic communities through online databases.
An early key finding is the identification of 129 distinct macroscopic tephras recovered during the last ~26 Ma at Site U1417, with the oldest tephra dated to ~17 Ma. There also appears to be an increase in explosive volcanism beginning around ~6 Ma, which is corroborated in the terrestrial volcanic record onshore. Following the Mid-Pleistocene Transition after ~1.2 Ma, when glacial/interglacial cycles changed from a 41-kyr to 100-kyr periodicity, the rate of explosive volcanism in the Gulf of Alaska increased by >500 %, which may reflect loading and unloading of regional magma chambers by overlying glacial icesheet(s). This last result adds to the growing body of evidence that links glacial cycles with variations in volcanic eruptive frequency.