Marine Ecosystems Research Department - jamstec japan agency ...

JAMSTEC 2002 Annual Report
Institute for Frontier Research on Earth Evolution (IFREE)
Fig. 29 Benthic foraminifera, Uvigerina akitaensis Asano with
green colored cytoplasm. The green color is caused by
the assimilation of algae with chlorophyll.
ing benthic organisms, we operated in situ culture
experiments measuring uptake rate of organic
carbon by benthic organisms with C labeled food
(cultured algae) using the manned research submersible
"Shinkai ". Shallow infaunal benthic
foraminifera assimilated a high concentration of
labeled carbon within two days on the deep-sea floor.
Deep infaunal species of benthic foraminifera, which
mainly live a few centimeters below the sedimentwater
interface, also assimilated labeled carbon within
six days. However, some foraminiferal species
including Chilostmela ovoidea showed quite low
amounts of assimilation of labeled carbon, suggesting
that the food preferences of benthic foraminifera are
variable between species. By contrast, all the metazoan
meiobenthos assimilated only a small amount
of labeled carbon. These results suggest that fresh
organic matter reaching the sea floor is primarily
consumed by protozoan such as foraminifera, and
then consumed by metazoan meiobenthos of higher
trophic level. We are currently analyzing lipid compounds
of foraminiferal cells to determine more
details of the food preferences.
4.2. Sagara Drilling Project: Microbiological activity
and diagenesis of organic matter in a plate
convergence margin
We have drilled and cored rock samples in the onland
Sagara oil field, Shizuoka Prefecture during
FY. The aims of this drilling survey are to understand
the processes concerning buried organic matter
in forearc regions of high sedimentation rate at plate
convergence margins, and to examine bacterial activity
on the synthesis and decomposition of hydrocarbons
in the subground anaerobic environments. In
FY, we performed many kinds of physical and
chemical measurements of the core samples, including
sedimentary petrology, microstructural analyses, physical
properties, fluid geochemistry, and microbiology.
Based on these investigations, we found that lithology
and physical properties are closely related with presence
or absence of oil. Oil commonly occurs in
unlithified porous sandstone overlain by a high-density
conglomerate with a mainly carbonate-cemented
matrix (Fig.).
Microbiological analyses showed that sulfate
reducers are absent throughout the core, and that the
oil-bearing layers have a microbial population density
of approximately times higher than that of the oilabsent
intervals. The absence of microrganisms
depending on reductive components suggests that we
have a significantly low flux of reductive gasses and
oil from the source rock of the Sagara oil reservoir.
The carbon isotopic ratio of methane and ethane in
fluid samples suggests that the origin of hydrocarbon
is organic matter in a deeper part of the field, supporting
above results. We will examine the relationship
and interactions of hydrocarbon in Sagara oil field
and the Nankai accretionary prism in terms of
methane hydrate formation off Cape Omaezaki by
using preexisting data from borehole/logging and
crustal structure.
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