Japan Geoscience Union Meeting 2019

Presentation information

[J] Poster

M (Multidisciplinary and Interdisciplinary) » M-IS Intersection

[M-IS19] Paleoclimatology and paleoceanography

Thu. May 30, 2019 5:15 PM - 6:30 PM Poster Hall (International Exhibition Hall8, Makuhari Messe)

convener:Yusuke Okazaki(Department of Earth and Planetary Sciences, Graduate School of Science, Kyushu University), Akira Oka(Atmosphere and Ocean Research Institute, The University of Tokyo), Michinobu Kuwae(Center for Marine Environmental Studies), Hitoshi Hasegawa(Faculty of Science and Technology, Kochi University)

[MIS19-P22] Application of the ramped pyrolysis 14C dating to postglacial sediments from the Alaskan margin, Arctic Ocean

*Kenta Suzuki1, Masanobu Yamamoto1,2, Brad Rosenheim3, Takayuki Omori4, Leonid Polyak5 (1.Graduate School of Environmental Science, Hokkaido University, 2.Faculty of Environmental Earth Science, Hokkaido University, 3.College of Marine Science, University of South Florida, 4.The University Museum, The University of Tokyo, 5.Bard Polar and Climate Research Center, Ohio State University)

Radiocarbon dating of biogenic carbonates, such as foraminiferal tests, is commonly used for dating marine sediments up to 50 ka old. However, in carbonate-poor environments, such as typical for polar seas, direct dating of carbonaceous material is difficult, while bulk organic matter (OM) may contain a mixture of carbon with different ages. Rosenheim et al. (2008) developed the ramped pyrolysis 14C dating of the bulk OM based on incremental generation of CO2 from a range of carbon fractions, from highly reactive to more thermochemically stable. Application of this approach to Southern Ocean sediments showed that ages of highly-reactive, lower temperature fractions were considerably younger than other fractions and the bulk 14C age. In this study, we explore ramped pyrolysis 14C dating in Arctic Ocean sediments from the Alaskan margin to determine if the same relationships hold true.
Samples were taken from two sediment cores collected on the 2005 HOTRAX expedition and comprising Holocene marine and older glaciomarine strata (Darby et al., 2009a). Several 14C ages have been generated on the Holocene biogenic carbonates (Darby et al., 2009b). New samples from both the Holocene and older sediments were pyrolyzed from room temperature to 900 °C, and the released, purified CO2 fractions were sealed in borosilicate glass ampoules at the University of South Florida. Measurement ofΔ14C was conducted at the University of Tokyo.
Pyrolysis followed by oxidation generated three peaks of CO2 at ~300 °C, ~400 °C and ~540 °C, which were deconvolved into three Gaussian peaks (G1 to G3). These peaks were interpreted to reflect the consecutive degradation of oxygen-containing OM functional groups, cleavage of carbon-carbon bonds, and cleavage of aromatic ring carbon, respectively. In this scenario G1 contains most of the autochthonous carbon from the sediment. We found that the slopes of the G1-G3 14C ages plotted against temperature of the Gaussian centroid are proportional to the offset between 14C ages of G1 and earlier analyzed biogenic carbonates from the same core levels. Assuming that this relationship is a constant function of the overall measured age spectra, we propose to use it for estimating sediment ages from the ramped pyrolysis 14C data. The estimated G1 age offsets range from 800 to 3,000 years in the Holocene and from 4,000 to possibly as high as 5,000 years in the older glaciomarine sediments.