Marine Ecosystems Research Department - jamstec japan agency ...

JAMSTEC 2002 Annual Report
Mutsu Research Group
sediments on the order of to yr. In contrast,
methods such as δ O are available for dating carbonate-containing
sediments in this geological age range.
We focused on amino acid chronology as an alternative
dating method for siliceous sediments. We analyzed
the enantiomeric ratio (D-isomer/L-isomer) of
aspartic acid (Asp) in bulk diatom assemblages in
siliceous sediment cores collected at Station (St.)
(approx. lat ˚N, long ˚E) and St. (approx. lat
˚N, long ˚E) in the northwestern North Pacific.
Radiocarbon and paleomagnetic ages were also
obtained from both cores to use as reference ages. Asp
ages estimated by the parabolic model were consistent
with the reference ages (Figure ). However, although
generally the D/L ratio of Asp increased with increasing
depth in the core at St. , the ratio did not continue
to increase below about m depth. The D/L ratio of
Asp and the paleomagnetic age at that depth were .
and kyr BP, respectively. Therefore, the Asp
racemization reaction apparently does not continue to
progress in diatom frustules older than this age. This
finding implies that Asp chronology can be used to
determine ages up to about kyr BP in sediments
composed of diatom ooze. Although the Asp dating
method using the parabolic model has a limitation of
D/L ratio of Asp
0.4
0.3
0.2
0.1
D/L=a x square root (t) +C
a=4.45x10 -4 , C=0.12, r 2 =0.94
kyr BP for siliceous sediments, it is available for
the estimation of ages on the order of to yr BP,
which is beyond the time range (up to kyr BP) datable
by the C method (Harada et al., ).
Personal Research
1. The study of ecology and genetics of living calcareous
plankton
Planktonic foraminifera and coccolithophores, a
group of free-floating marine protista, secrete calcareous
hard skeletons. They are very sensitive to the
oceanic surface environmental changes and their
shells settle quickly and deposit in the sediment after
they died. Their calcareous tests provide considerable
information about the sea water when they lived and
are widely used for paleo-environmental reconstruction.
In spite of their importance, their ecology was
not well understood. In particular, there are few studies
on the ecology of living cold water species. We
then started () the collection of calcareous plankton
from the surface seawater, and () their cultivation in
the laboratory. Living specimens were collected
monthly from the Tsugaru Strait (˚.'N,
˚.'E; ca. ~m below surface), northernmost
part of Honshu, Japan. This area is located in the mixing
zone of the Tsugaru warm current and the cold
Oyashio component water, therefore it is suitable for
the study of the ecology of cold water species, as well
as the warm water species. The aims of this study are
) establishment of the cultivation protocols of calcareous
plankton in the laboratory, ) elucidation of
the ecology of the northern species, and ) clarification
of the relationships between skeletal microstructure
and gene.
0 100 200 300 400 500 600 700
Square root of 14 C and paleomagnetic ages
Fig. 5 Relationship between the D/L ratio of Asp obtained by the
parabolic model and the square root of the 14 C and paleomagnetic
ages from the sediment core from St. 5.
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