25th International Meeting on Organic Geochemistry IMOG 2011

P-202
Terrestrial higher plant biomarkers in hyperpicnite-like
sedimentary sequence in the Miocene Kawabata Formation,
central Hokkaido, Japan: Evidence for direct transport of
terrigenous matter by flood?
Satoshi Furota, Ken Sawada
Faculty of Science, Hokkaido University, Sapporo, Japan (corresponding
author:furota@mail.sci.hokudai.ac.jp)
Turbidity current is a main system for transport and
redeposition of organic matter from coastal to deepsea
provinces. The turbidite sediment provides us
multiple information for both marine and terrestrial
environments because it contains the autochthonous
matter derived from marine organisms and
terrigenous matter transported from land areas. It was
reported that biomarker compositions were quite
different between turbiditic and pelagic mudstone
layers in the Miocene turbiditic sequences of outcrops
in Hokkaido [1]. Studies on the turbiditic sequences
suggested that organic geochemical characteristics
were strongly controlled by types and phases of
transport, redeposition and postdepositional
diagenesis of turbidites. It, recently, has been known
that heavily sediment-laden effluent of flooding rivers
can form peculiar turbidity current, referred to as
hyperpycnal flow, in marine environment. The
hyperpycnal flow is thought to be direct transport
system of terrigenous matter from land to ocean. In
the present study, we focus terrigenous biomarker
compositions in the hyperpycnite-like sedimentary
sequence. Furthermore, we discuss the
biogeochemical interaction between land and ocean
such as higher marine primary production attributed to
efficient transport of terrigenous matter by flood
system.
We discovered a peculiar turbiditic sequence with
about 50 cm thick in an outcrop of the Miocene
Kawabata Formation in Yubari, central Hokkaido. The
Kawabata Formation is characterized by turbidite
facies that consist of alternations of conglomerate,
sand, and mudstones, and is thick sediments filled
with the Ishikari Trough during the Middle- Late
Miocene. This sequence consists of homogenous
mudstone, coarsening-upward sandstone, sandstone
intercalated thin layers of plant fragments, sandstone
randomly contained plant fragments, fining-upward
sandstone, sandstone with horizontal lamination, and
homogenous mudstone, which is resemble to that in
hyperpycnite sequence. The sediment samples were
crushed to fine powder. Carbonate-free samples were
analyzed for total organic carbon content (TOC; %) by
a J-Science Micro Corder. Lipids were ultrasonically
extracted from the powder samples as reported
previously [1]. These fractions were analysed by gas
chromatography / mass spectrometer (GC/MS).
TOC content was found to increase remarkably
from sandstone layers with plant fragments to
horizontal lamination layer, which is concordant with
hyperpycnite [2]. Organic geochemical characteristics
for biomarker were followings, 1) concentrations of
coniferous biomarker such as phyllocladane were
remarkably higher in homogenous mudstone below
the sandstones and coarsening-upward sandstone, 2)
concentrations of degraded oleanenes originated from
angiosperm were higher in sandstone layers with
plant fragments, and 3) maturity levels were
significantly lower in sandstone layers with plant
fragments. From these results, we conclude that this
sedimentary sequence was hyperpycnite that had
been formed by flooding river. Furthermore, we found
that concentrations of marine algal biomarkers such
as steranes and highly branched isoprenoid (HBI)
alkanes remarkably increased with increasing those
of terrigenous biomarkers in homogenous mudstones
above the sandstones. This fact suggests that marine
primary production was activated by efficient transport
of terrigenous materials by flood system in the Ishikari
Trough during Late Miocene. These results are
important for understanding material cycling and
biogeochemical processes with geological time scale.
References [1] Okano, K., Sawada, K. (2008)
Geochem. J. 42, 151-162. [2] Yoshida, M., Yoshiuchi,
Y., Hoyanagi, K. (2009) Island Arc 18, 320-332.
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