• AMS Radiocarbon Dating of Holocene Tephra Layers on Ulleung Island, South Korea

      Toyota, Ayu; Tayasu, Ichiro; Fujimaki, Reiji; Kaneko, Nobuhiro; Uchida, Masao; Shibata, Yasuyuki; Hiura, Tsutom (Department of Geosciences, The University of Arizona, 2010-01-01)
      Ulleung Island, a large stratovolcano, is located in the western part of the Japan Sea (East Sea), 130 km off the eastern coast of the Korean Peninsula. The Ulleung-Oki (U-Oki) is a widely distributed tephra in and around the Japan Sea, and has an age of 10.7 cal ka BP obtained from the Lake Suigetsu data set (central Japan). Of the 7 tephra layers (U-7 to -1) on the island, the pumiceous U-4, U-3, and U-2 tephra layers are petrochemically and petrographically similar to the U-Oki tephra. To determine the eruption ages of 3 tephra layers on Ulleung Island, we conducted radiocarbon dating for 5 soil and 2 charcoal samples. Although the soil samples have the C/N ratios from 5 to 10, the obtained 14C dates are still consistent with the tephra stratigraphy of the island. The calibrated 14C dates for the U-4, U-3, and U-2 tephras are 11 cal ka BP, 8.3 or 9 cal ka BP, and 5.6 cal ka BP, respectively, indicating that the explosive eruptions occurred in the island with a time interval of 2000 to 3000 yr during the period of the early to middle Holocene. Based on our chronology, the U-4 tephra is most likely correlated with the U-Oki tephra.
    • Effects of Vegetation Switch and Subsequent Change in Soil Invertebrate Composition on Soil Carbon Accumulation Patterns, Revealed by Radiocarbon Concentrations

      Toyota, Ayu; Tayasu, Ichiro; Fujimaki, Reiji; Kaneko, Nobuhiro; Uchida, Masao; Shibata, Yasuyuki; Hiura, Tsutom (Department of Geosciences, The University of Arizona, 2010-01-01)
      Vegetation types strongly affect soil organic carbon (SOC) accumulation in the terrestrial ecosystem through multiple factors such as litter quality and soil biodiversity. However, the roles of soil fauna in SOC accumulation remain unclear. The objectives of this study were to (1) examine how changes in litter types and soil animal communities affect SOC accumulation in continuously forested or vegetation-switched forest areas; and (2) discuss the role of soil animals in SOC accumulation in forest ecosystems. We focused on soils that have accumulated on top of a volcanic ash layer in the 268 yr since a volcanic eruption in 1739. The radiocarbon "bomb spike" in the late 1950s and early 1960s provides a unique isotopic signature of soil carbon age. We investigated the combined effects of litter quality and soil invertebrate function on soil 14C accumulation patterns. To determine the effects of vegetation types on SOC accumulation, we selected 4 types of cool temperate forests, 2 of which had undergone a vegetation switch in about 1960 (conifer to broadleaved forest, CB; broadleaved forest to conifer, BC), and 2 that had continuous forests (conifer forest, CC; broadleaved forest, BB). The ∆14C values at depth intervals in CC were consistent with the expected bomb-14C profile. In contrast, ∆14C patterns in BB, BC, and CB differed from that of CC. Compared to CC, ∆14C values of the other sites showed relatively high 14C concentrations even in deeper soil layers, which suggests the bomb-induced 14C has been transported to a greater depth by soil animals. Current broadleaved forests (BB and CB) had higher biomass of litter-feeding invertebrates than in current coniferous forests (CC and BC). These results suggest that carbon from leaf litter was vertically translocated to deeper soil layers by the abundant soil invertebrates in broad-leaved forests. Disagreement with the expected soil profile in BC suggests that past vegetation (broadleaved forest) has affected the present SOC accumulation pattern.
    • High Contribution of Recalcitrant Organic Matter to DOC in a Japanese Oligotrophic Lake Revealed by 14C Measurements

      Nara, Fumiko Watanabe; Imai, Akio; Uchida, Masao; Matsushige, Kazuo; Komatsu, Kazuhiro; Kawasaki, Nobuyuki; Shibata, Yasuyuki; Amano, Kunihiko; Mikami, Hajime; Hanaishi, Ryuji (Department of Geosciences, The University of Arizona, 2010-01-01)
      Carbon isotopes (14C and 13C) of dissolved organic carbon (DOC) in a Japanese oligotrophic lake (Lake Towada) were measured to study the origin and cycling of dissolved organic matter (DOM) in Lake Towada. Lake water samples were collected at 3 depths (0, 30, and 80 or 85 m) during 4 months (April, June, August, and October) in 2006. 14C measurements of DOC were performed by accelerator mass spectrometry (AMS) at the National Institute for Environmental Studies (NIES-TERRA) in Japan. ∆14C and delta-13C values of DOC in Lake Towada showed light carbon isotopic values ranging from -750 to -514v and -29.0 to -27.8‰, respectively. These values are similar to those of humic substances reported. The very low carbon isotopic values of DOC in Lake Towada suggest a very small contribution of DOC derived from fresh phytoplankton to the lake DOC. There is an extremely high linear relationship between the ∆14C and delta-13C of DOC in Lake Towada when all data points are plotted (r2 = 0.818, p < 0.01), suggesting that the DOC in Lake Towada has 2 specific sources contributing heavy and light carbon isotopes. Although the freshly produced DOC of phytoplankton origin can be decomposed easily, the variation in the autochthonous DOC should influence the carbon isotopic values of DOC in Lake Towada.
    • Spatial Distribution of ∆14C Values of Organic Matter in Surface Sediments off Saru River in Northern Japan, One Year after a Flood Event in 2006

      Nagao, Seiya; Irino, Tomohisa; Aramaki, Takafumi; Ikehara, Ken; Katayama, Hajime; Otosaka, Shigeyoshi; Uchida, Masao; Shibata, Yasuyuki (Department of Geosciences, The University of Arizona, 2010-01-01)
      Dispersion and deposition of terrestrial organic matter by flooding on the inner shelf were studied using C/N ratios, 13C, and ∆14C values of sedimentary organic matter. Surface sediment samples (top 2 cm) were collected from coastal areas near the Saru River in southwestern Hokkaido, northern Japan, 1 yr after a flood event in 2006. Riverine suspended solids were also collected at a fixed station downstream during 2006-2008. Sandy sediments were located at the front of the river mouth and the western part of the sampling area, with the 13C of organic matter ranging from -23.8 to -22.0, ∆14C of -655 to -388, and an organic carbon/total nitrogen (C/N) ratio of 5.9-7.7. On the other hand, silt and clay sediments were distributed in a restricted area 11-16 km from the river mouth, with lighter 13C (-26.7 to -24.1) and higher ∆14C (-240 to -77) of organic matter and C/N ratio (7.8-13.3). From end-member analysis, the apparently younger and less degraded organic matter in the silt and clay sediments consists mainly of terrestrial organic matter released by flood events. They remain in the depression, although most flood deposits were moved to deep-sea environments.