HR: 13:55h
AN: B53C-02    [Abstracts]
TI: Origin of sedimentary organic matter at the Northern Cascadia Margin
AU: Kaneko, M
EM: dns19110@cc.okayama-u.ac.jp
AF: Department of Earth Sciences, Okayama University, 3-1-1Tsushima-Naka, Okayama, 7008530, Japan
AU: * Naraoka, H
EM: naraoka@cc.okayama-u.ac.jp
AB: Gas hydrate in marine sediments may have important roles on global carbon cycle and climatic change. We examined origins of sedimentary organic matter and bacterial activity in deep and hydrate-bearing sediment cored in Site U1327 and U1328 at northern Cascadia Margin by IODP Exp311, using σ¹³C of total organic carbon (TOC), σ¹⁵N of total nitrogen (TN), σ³⁴S of total sulfur (TS), and σ¹³C of biomarkers in hydrocarbon fraction.
In both sites, TOC/TN ratios and σ¹³C of TOC values ranged from 5.5 to 18.0 and –25.7 to –21.5 ‰, respectively, suggesting that sedimentary organic matter is a mixture of terrestrial and marine sources. Long chain (n)-alkanes (C₂₇, C₂₉, and C₃₀), known as biomarkers of terrestrial higher plant were most abundant components (up to ~50 μg/gC_org) through down to 300 mbsf, and their σ¹³C values (-34.3 to –28.7 ‰) reveal their C3 plant origin. In addition, very long-chain alkene (C₃₇) occurred in some sediments, which suggests the blooming by coccolithophore in the past. σ³⁴S of TS values at both sites show large variation between –30 to +20 ‰. Most of σ³⁴S of TS values were less than present σ³⁴S value of seawater sulfate (+20.3 ‰). This is attributable to isotope fractionation during microbial sulfate reduction. Crocetenes including one double bond occurred in deep sediments with higher σ¹³C values (–23 ‰) than the reported σ¹³C values (\< ~ -100 ‰, Elvert et al, 2000), providing possibility of heterotrophic archaea using marine organic matter as a carbon source. Pentamethylicosane (PMI) was detected in relatively high concentrations at 249 mbsf at Site U1328 and its σ¹³C value was –46.4 ‰. This PMI could be chemoautotrophic archaea in origin such as methanogen. Diploptene was also detected in most sediments with the σ¹³C value of –37 to –35 ‰, probably being characteristic of chemoautotrophic bacteria.
DE: 1055 Organic and biogenic geochemistry
DE: 4805 Biogeochemical cycles, processes, and modeling (0412, 0414, 0793, 1615, 4912)
DE: 4870 Stable isotopes (0454, 1041)
SC: Biogeosciences [B]
MN: 2007 Fall Meeting