• Marine Reservoir Correction in the South of Vietnam Estimated from an Annually-Banded Coral

      Dang, Phong X.; Mitsuguchi, Takehiro; Kitagawa, Hiroyuki; Shibata, Yasuyuki; Kobayashi, Toshiyuki (Department of Geosciences, The University of Arizona, 2004-01-01)
      We measured radiocarbon in an annually-banded coral core collected from Con Dao Island, Vietnam, 90 km from the mouth of the Mekong River, and estimated the regional correction of the marine reservoir age (Delta-R value). Twelve samples were continuously taken from the annual bands (AD 1949-1960) which were clearly identified under UV light (approximately 352 nm) as well as by X-radiography. The 14C content of the samples was determined using an accelerator mass spectrometer at the National Institute for Environmental Studies, Tsukuba, Japan. Results provide a Delta-14C time series showing a relatively steady value of -48.6 +/4.6 per mil for the period of 1949-1955 and an abrupt increase starting from 1956 that indicates a quick response to the atmospheric testing of nuclear bombs. Using the prebomb 14C data, the Delta-R value in the south of Vietnam is estimated to be -74 +/39 yr.
    • Seasonal and Secular Variations of Atmospheric 14CO2 Over the Western Pacific Since 1994

      Kitagawa, H.; Mukai, Hitoshi; Nojiri, Yukihiro; Shibata, Yasuyuki; Kobayashi, Toshiyuki; Nojiri, Tomoko (Department of Geosciences, The University of Arizona, 2004-01-01)
      Air sample collections over the western Pacific have continued since 1992 as a part of Center for Global Environmental Research, National Institute for Environmental Studies (CGER-NIES) global environmental monitoring program. The air samples collected on the Japan-Australia transect made it possible to trace the seasonal and secular 14CO2 variations, as well as an increasing trend of greenhouse gases over the western Pacific. A subset of CO2 samples from latitudes of 10-15 degrees N and 23-28 degrees S were chosen for accelerator mass spectrometry (AMS) 14C analysis using a NIES-TERRA AMS with a 0.3-0.4% precision. These 14CO2 records in maritime air show seasonal variations superimposed on normal exponential decreasing trends with a time constant of about 16 yr. The Delta-14C values in the Northern Hemisphere are lower those in the Southern Hemisphere by 3-4 per mil during 1994-2002. The Northern Hemisphere record shows relatively high seasonality (2.3 +/1.5 per mil) as compared with the Southern Hemisphere (1.3 +/1.2 per mil). The maximum values of seasonal cycles appear in late autumn and early winter in the Northern and Southern Hemispheres, respectively. Oscillations of 1-10 yr over the western Pacific are found to correlate possibly with the El Nino/Southern Oscillation (ENSO) events.
    • Seasonal Radiocarbon Variation of Surface Seawater Recorded in a Coral from Kikai Island, Subtropical Northwestern Pacific

      Morimoto, Maki; Kitagawa, Hiroyuki; Shibata, Yasuyuki; Kayanne, Hajime (Department of Geosciences, The University of Arizona, 2004-01-01)
      A coral radiocarbon (Delta-14C) investigation with a high time-resolution is crucial for reconstructing secular and seasonal Delta-14C changes in the surface seawater which potentially reflect ocean circulations and dynamic ocean-atmosphere interactions. The Delta-14C values of a modern coral (Porites sp.) from Kikai Island, southern Japan, in the subtropical northwestern Pacific, were determined for the period of 1991-1998 at a monthly resolution. A coral Delta-14C time series for the 8 yr indicated seasonal cycles superimposed on a secular decreasing trend of 3.8 per mil per yr. The seasonal amplitude of the coral Delta-14C was about 18 per mil on the average and the minimum Delta-14C was observed in late spring and summer. The Delta-14C changes were tentatively explained by horizontal oceanic advections around Kikai Island or over the wide range of the equatorial and subequatorial Pacific.