• Holocene Evolution of the Outer Lake of Hwajinpo Lagoon on the Eastern Coast of Korea; Environmental Changes with Holocene Sea-Level Fluctuation of the East Sea (Sea of Japan)

      Yum, Jong-Gwon; Yu, Kang-Min; Takemura, Keiji; Naruse, Toshiro; Kitamura, Akihisa; Kitagawa, Hiroyuki; Kim, Jong-Chan (Department of Geosciences, The University of Arizona, 2004-01-01)
      The evolution of the outer lake of Hwajinpo Lagoon in Korea has been reconstructed using environmental proxies (lithologic, geochemical, and fossil data) with a chronology established using 7 accelerator mass spectrometry (AMS) radiocarbon dates. Grain size, water content, and X-ray analyses from the core of outer coastal lakes (HJ99) were used to reconstruct sedimentary environments by using total organic carbon, C/N, S, and C/S chemical proxies. Assemblages of mollusc remains also provided paleoenvironmental information. The environmental changes of the outer lake of Hwajinpo Lagoon can be divided into 6 depositional phases. The basin of the Hwajinpo was exposed and underwent a weathering process before the Holocene period. The muddy sand layer on the weathered bedrock indicated an estuarine system about 6000 BP. The laminated layer implies that the lagoonal system was anoxic between about 5500-2800 BP. The marl layer implies a relatively oxic lagoonal condition with mollusc presence about 2500 BP. The layer of very low sulfur content indicates a freshwater lake system isolated by a sand barrier about 1700 BP. Beginning about 1000 BP, the river system deposits progress progradation on the marl layer. Two erosional landforms could be related with a high standing sea level span during Holocene. These high-stands are dated at 5700 BP and 2200 BP and are supposed to have formed erosional landforms of about 1.6 amsl and 0.8 amsl, respectively. Environmental changes of the outer lake of Hwajinpo Lagoon are considered due mainly to the lake-and sea-level fluctuation during Holocene.
    • 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 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.
    • Variation of the Radiocarbon Content in Tree Rings During the Spoerer Minimum

      Miyahara, Hiroko; Masuda, Kimiaki; Furuzawa, Hideki; Menjo, Hiroaki; Muraki, Yasushi; Kitagawa, Hiroyuki; Nakamura Toshio (Department of Geosciences, The University of Arizona, 2004-01-01)
      This paper presents the variation of radiocarbon content in annual tree rings for the period AD 1413-1553, which includes the Spoerer Minimum period (AD 1415-1534). Since the variation of the production rate of 14C is strongly related to solar activity, the variation of 14C content in annual tree rings gives us information on the characteristics of variation of solar activity. We have studied solar activity during the grand solar minima, focusing especially on the stability of the 11-yr cycle. The minima are determined to have been almost free of sunspots. Our results, however, have revealed quite remarkably the existence of the 11-yr cycle for most of the time during the Spoerer Minimum. The 11-yr variation weakened around AD 1460-1510, suggesting that solar activity might have been strongly suppressed during these 50 yr.