Marine Ecosystems Research Department - jamstec japan agency ...

Japan Marine Science and Technology Center
Institute for Frontier Research on Earth Evolution (IFREE)
the OAEs. Lake Kaiike in Kamikoshiki Island,
Kagoshima Prefecture, is a saline meromictic lake
with a O
/H
S interface at m depth. In the lake, photosynthetic
bacteria including Chromatiaceae and
Chlorobiaceae and chemosynthetic bacteria are densely
populated at the interface, forming a bacterial plate.
We have been investigating geochemical, geological,
and biological features of the lake to understand the
biogeochemical and sedimentary processes in the lake.
We performed pigment analyses using a HPLC/APCI-
MS and found a layered distribution of chlorophyll a
from cyanobacteria, bacteriochlorophyll a from purple
sulfur bacteria, and bacteriochlorophylls e from green
sulfur bacteria in the water column (Nakajima et al.,
submitted). Furthermore, nitrogen isotopic analyses of
particulate organic matter strongly suggest that nitrogen
fixation is a major pathway for assimilating nitrogen
in these microbes. This project is continued in
FY and we plan to conduct several more field
observations in the lake.
3. Late Cenozoic Icehouse Earth
3.1. Okhotsk Sea: Toward understanding an ocean
sensitive to climatic change
In the northwest Pacific Ocean, a water mass called
the North Pacific Intermediate Water (NPIW) occupies
the water depth ranging from to m. The NPIW
originateds from intermediate water formed in the
Okhotsk Sea, in which the source of excess salt is transported
though the high salinity Soya Warm Current
from the Japan Sea. Therefore, the supply of salinity to
the Okhotsk Sea by the Soya Current may strongly
influence the quantity of Okhotsk Sea Intermediate
Water (OSIW) formation. Based on this consideration,
we estimated the contribution of the Japan Sea water to
the OSIW by using isotope ratios of dissolved inorganic
carbon as a water mass tracer. Our results suggest
that the contribution of the Japan Sea water to the
OSIW reaches up to % (Itou et al., ).
In the climate system, sea ice has a positive feedback
function due to high albedo. It plays a crucial
role in the redistribution of solar energy between the
ocean and atmosphere in the high latitudinal regions.
Since the Okhotsk Sea is the southernmost oceanic
region in the Northern Hemisphere widely covered by
sea ice during the winter-spring season, it has been
suggested that the Okhotsk Sea is potentially very sensitive
to global climatic change. With our colleagues,
we performed sediment trap experiments in the
Okhotsk Sea over two years, and found that () terrestrial
particles of silt-sand size held in the sea ice were
released and settled on the seafloor during the sea ice
retreating period, and () coarse grains in the sediments
originate from ice rafted debris (IRD). By quantifying
the IRD in well dated sediment cores, we
found as many as sea ice expanding events during
the past kyr with several hundred to several thousand
year cycles. They appear to have occurred when
atmospheric circulation in the Northern Hemisphere
was strengthened. To reconstruct the history of sea ice
formation in the Okhotsk Sea throughout Late
Cenozoic, an IODP proposal is currently submitted in
collaboration with Prof. Kozo Takahashi at Kyushu
University and a number of both domestic and foreign
scientists.
4. Sedimentation and degradation processes of
organic matter in the continental margin
4.1. Monitoring of sedimentation process at mass
sedimentation area
We are monitoring the biological, chemical, and
sedimentological processes at the sediment/water
interface at a station in Sagami Bay (m water
depth). The Sagami Bay area is characterized by
a high sedimentation rate with hemipelagic clays
being provided from a horizontal flux of recycled
terrestrial material and resuspended particles (Soh,
). Generally, the major fraction of the sedimentary
organic matter is consumed and then remineralized
by benthic organisms, whereas some part is transferred
to a deeper area or buried in the sediments.
To evaluate the processes of organic matter concern-
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