HR: 0800h
AN: PP31B-1533    [Abstracts]
TI: Apportioning carbon sources of authigenic carbonate of extremely ¹³C-depleted foraminifera from the western North Pacific sediments: Implication from the coupled ¹³C and ¹⁴C isotopic mass balance approach
AU: * Uchida, M
EM: uchidama@jamstec.go.jp
AF: JAMSTEC, 2-15 Natsushima, Yokosuka, 237-0061 Japan
AU: Ohkushi, K
EM:
AF: AIST, 1-1-1 Higashi, Tsukuba, 305-8567 Japan
AU: Ahagon, N
EM: ahagon@ep.sci.hokudai.ac.jp
AF: Hokkaido Univeristy, Kita10 Nishi8 Kitaku, Sapporo, 060-0810 Japan
AU: Kimoto, K
EM: kimopy@jamstec.go.jp
AF: JAMSTEC, 2-15 Natsushima, Yokosuka, 237-0061 Japan
AU: Inagaki, F
EM: inagaki@jamstec.go.jp
AF: JAMSTEC, 2-15 Natsushima, Yokosuka, 237-0061 Japan
AU: Shibata, Y
EM: yshibata@nies.go.jp
AF: National Institute for Environmenatal Studies, 16-2 Onogawa, Tsukuba, 305-8567 Japan
AB: Recently, Uchida et al. (G-cubed, 2004) and Ohkushi et al. (G-cubed, 2005) interprete /delta ¹³C variations of planktonic and benthic foraminifera found in Last Glacial sediments in off Shimokita Peninsula and Tokachi as evidence for periodic releases of methane, arising from the dissociation of methane hydrate, and its subsequent oxidation in bottom- and/or surface-water environments. According to recent observations of anomalous bottom-simulating reflections, northwest Pacific marginal sediments around Japan main islands bear large abundances of methane hydrate. In this study, analyzed piston cores (42° 21.42' N, 144° 13.36' E) at a water depth 1066-m was retrieved from the off Tokachi continental slope in the Oyashio current region, where recently is found to bear immense amounts of methane hydrate. The piston core covered past 22 ka with high-resolution. Here we showed that carbon isotope signals indicated that planktonic and benthic foraminifera in several glacial sediment layers in the core were highly depleted in¹³ C; both the planktonic and benthic foraminiferal /delta ¹³C values ranged from about -10/permil to -2/permil. Most foraminiferal tests in these horizons were brown as a result of postdepositional alteration. Foraminiferal oxygen isotopes fluctuated abnormally in the glacial sediment layers, showing small (about 0.5/permil) positive shifts relative to normal glacial values. We attributed the positive shifts to authigenic carbonate formation in the foraminiferal tests. In order to decipher the relation between foraminifera carbon isotopic signal and methane release from the seafloor, we have apportioned carbon sources (methane from methane hydrate or not) of foraminiferal carbon isotopic anomalies using dual mass balance isotopic model (¹⁴C/ ¹²C and ¹³C/ ¹²C). It has been suggested that sulfate-dependent anaerobic methane oxidation (AOM) dominates carbon oxidation and attendant authigenic carbonate precipitation to foraminifera. To this assumption, we have quantified the relative contributions of dissolved carbon dioxide (/SigmaCO/_2) from oxidation of methane in anomaly foraminifera. At the layer of 17,840 years cal. age with planktonic foraminifera of dual isotopic data (/delta ¹³C: -8.1/permil and /Delta ¹⁴C: -847/permil) , relative contribution of carbon from authigenic carbonate was 17 percent of total carbonate and its /delta ¹³C was -48.1/permil, suggesting indirect records of enhanced incorporation of ¹³C-depleted CO₂ formed by anaerobic methane oxidation process that use ¹²C-enriched methane as their main source of carbon. Moreover, biomarker and phylogenetic compositions were investigated in the light of the past activity of methanotrophic bacteria in the oxic-anoxic interface in the overlying water column and/or surface sediment. Mg/Ca ratios were also analyzed to evaluate foraminiferal ¹³C depleted carbonate precipitation in comparison with authigenic carbonate produced in the cold seep environment. In the conference, we discuss about what mechanism contribute to authigenic carbon precipitation in terms with carbon source with ¹³C-depleted foraminifera.
DE: 4901 Abrupt/rapid climate change (1605)
SC: Paleoceanography and Paleoclimatology [PP]
MN: Fall Meeting 2005