| 氏名 |
相沢 千恵子 |
| 論文題目 |
鹿島沖コア（IMAGES MD01-2421)の石灰質ナノ化石群集に基づく過去１５万年間の海洋環境変動
" Late Quaternary paleoceanography deduced from coccolith assemblage in a piston core recovered from off the coast of central Japan." |
| 論文要旨 |
Nannofossil assemblage was studied for the last 150,000 yrs (~MIS6c) in a
piston core MD01-2421 from the northwestern Pacific (36° 01.4’N, 141°
46.8’E, WD: 2,224m) off the coast of central Japan. The site is under
influence of both subtropical Kuroshio and subarctic Oyashio Currents and
locates just north of the Kuroshio Front. Emiliania huxleyi, Gephyrocapsa
oceanica, very small Gephyrocapsa (< 2μm) and very small placolith (< 2μm)
are the major components comprising approximately 90% on average of the
upper-photic assemblage in all samples. Coccolithus pelagicus, Calcidiscus
leptoporus, Helicosphaera spp. and Umbilicosphaera spp., occupied
apporoximately 90% of the upper-photic subordinate species assemblage
throughout the studied interval. The limitation factor for Florisphaera
profunda was considered to be the lower-photic temperature in this studied
area rather than the nutricline dynamics or the water turbidity. The
transition from “very small placolith” to E. huxleyi was remarkable at
around 85 ky and the local environmental changes might not be reflected in
this phenomenon. In order to reconstruct the paleoceanographic changes,
“Kuroshio Species”, “Oyashio Species”, “Subtropical Species” and “Coastal
Species” were suggested. Tn value was calculated using the “Kuroshio
Species” and “ Oyashio Sepcies” and expresses the influence of the
Kuroshio Current quantitatively. Tn values fluctuated broadly raging from
0.7 (141.4ky) to 84.2 (112.5 ky). The ratio between Tn and Tp (index of the
influence of the Kuroshio Current based on the planktonic foraminifers)
drastically increased at near 20ka in the MIS2, this suggests that thin warm
water might be covered the studied area during the interval. The estimated
annual mean surface temperatures and salinities were obtained by transfer
function method (Tanaka, 1991) and core-top values were coincidence with the
modern values off Kashima-nada within error. The lowest temperature (12.9°C)
(13.1ky) was equivalent to the modern surface water temperature off
Hachinohe (40.5°N) and the highest one (22.7°C) (108.6ky) is off Miyazaki
(31°N). The profile of Ta was similar to that of the alkenone-derive
temperature (Yamamoto, submitted). Although the signals like the Younger
Dryas, Heinrich event 1 (HE1) and HE2 were detectable in some proxies, the
events of HE3 to HE5 were not correlated well with the oceanographic
proxies. There is no consistent relation to all HEs within one proxy. This
might be the temporal resolution was still not enough or the signals of the
D-O cycles and Heinrich events might be buffered in surface water due to the
influence both of atmosphere and ocean. The ratio between CMAR (coccolith
Mass Accumulation Rate: coccolith/cm-2kyr-1) and DMAR (diatom mass
accumulation rate: valve/cm-2kyr-1) drastically increased during the glacial
periods (especially the MIS2). This phenomenon might be brought by the
southward shift of the Oyashio Current, which has high carbonate/opal ratio
in the modern ocean. Although weakening of upwelling might be the critical
factor of the suppressed productivity in the LGM, increasing of preservation
efficiency of organic matter due to the high sedimentation rate of detritus
or/and the reliability of the CaCO3 presevation index also have to be taken
into account for interpretation of the profile of the CMAR.
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