25th International Meeting on Organic Geochemistry IMOG 2011

O-81
Hydrogen isotopic compositions of archaeal tetraethers
Masanori Kaneko 1 , Hiroshi Naraoka 2
1 University of Nevada-Reno, Reno, United States of America, 2 Kyushu University, Fukuoka, Japan
(corresponding author: m_kaneko@jamstec.go.jp)
During the past decade, compound-specific
hydrogen isotope (δD) analysis has been conducted
on eukaryotic and bacterial lipid biomarkers, which
provides us information about paleoclimate,
paleosalinity, and metabolic pathways (Pagani et al.,
2006; Zhang et al., 2009). However, the hydrogen
isotopic composition of archaeal membrane lipids has
not been explored because of analytical difficulty.
Recently we have developed δD analysis of
archaeal tetraether lipid in which biphytane iodide
produced by HI treatment of the tetraether lipid is
quantitatively hydrogenated with H2 bubble on a
catalyst PtO2. Using the developed method, we will
present the δD values of archaeal membrane lipids in
sediment cores (Sites U1327 and U1328) collected
from the northern Cascadia margin accretionary prism
during IODP Expedition 311.
Lipids were extracted using organic solvents, and
tetraether lipids were separated using silica gel
column chromatography. The tetraether lipids were
reacted with HI at 110°C for 3h to cleave ether bonds.
Resulted biphytane iodides were recovered with
hexane and were subsequently hydrogenated as
descrived above. δ 13 C and δD analyses of BPs were
performed by GC/IRMS. δD value of incorporated
hydrogen to BPs was determined by hydrogenation of
δD–known iodeoctadecane, which allowed to correct
δD value of BPs.
In the sediment cores, BPs with the valuable
number of cycloalkane ring (BP[0] to BP[3], [n] is the
ring number) were existed as components of
tetraether lipids. BP[3] contained a cyclohexane ring,
suggesting a marine crenarchaeotic origin. From an
aspect of δ 13 C values of BPs (Fig. 1), archaeal
sources in this study were divided into two groups,
crenarchaeotic origin (BP[3], BP[0] and BP[2] with
−19.9±1.8‰) and other (BP[1] with −16.1 to −48.8‰).
The BPs derived from Crenarchaeota are a little
enriched in 13 C relative to bulk organic carbon
(−23.7‰), which indicates the Crenarchaeota are
heterotroph. On the other hand, the 13 C-depletion of
BP[1] suggests an additional mixing of archaeal
source (e.g. methaogen).
δD variation of BPs ranges from −324 to −181‰
with a unique δD distribution pattern: δDBP[0]< δDBP[2]<
δDBP[3]< δDBP[1]. This distribution pattern is consistent
with a relationship between δD value and the ring
number of BPs in Sulfolobus culture (Kaneko et al, in
press), except for BP[1]. This observation also
suggests the two archaeal sources. Considered large
hydrogen isotope fractionation during lipid synthesis
by heterotrophic archaea (−213 to −161‰, Kaneko et
al., in press), it is possible that BP[3], BP[2], and
BP[0] is derived from heterotrophic Crenarchaeota.
On the other hand, contribution of methanogen
accounts for D-enrichment of BP[1] because more
than 60% of hydrogen of BP synthesized by
methanogen can be same to that of methane (−180‰,
Pohlman et al., 2010). Thus, δD value of BPs should
reflect archaeal metabolic pathways.
Fig. 1. Carbon and hydrogen isotopic composition
of biphytanes from this study.
REFERENCES
Kaneko et al, Organic Geochemistry in press.
Pohlman et al. (2009) Earth Planet. Sci. Lett. 287,
504–512.
Pagani et al. (2006) Nature 442, 671–675.
Zhang et al. (2009) Proc. Natl. Acad. Sci. U.S.A.
106,12580–12586.
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