2007 Annual Report - jamstec japan agency for marine-earth ...

Paleoclimate and Paleoceanography Group
a. Research activities in the field
To clarify changes in the Amur River discharge as related
to changes in past Asian summer monsoon activity, we participated
in a cruise of the R/V Khromov during August-September
2007 and collected sediment cores along the Sakhaline coast,
inside the Russian exclusive economic zone (EEZ). We also
participated in the Wakashio Maru cruise (one day in every
month) and collected living planktic foraminifera to understand
their ecology and to develop a new biological "thermometer" -a
proxy for sea surface temperature (SST)-by using the Mg/Ca
recorded in the planktic foraminiferal carbonate tests.
Fig.12 Sediment core sample locations.
Red circle, Okhotsk Sea; blue circle,
off Shimokita peninsula.
b. Research topic: Changes in intermediate-deep water circulation
in high latitudes of the North Pacific associated with the
B/Å warm event during the last deglaciation.
What will happen to the ocean environment in the near
future as global warming progresses? We can learn the oceanographic
responses to global warming from climate changes that
occurred in the past during the deglaciation from 1.9 ka BP to
1.1 ka BP, across a millennial time scale without the influence
of human activity.
We focused on the Heinrich 1 (H1) cold event (17.5-14.6
ka BP) and the Bølling/Ållerød (B/Å) warm event (14.6-12.9 ka
BP). We reconstructed past ventilation changes in intermediatedeep
water in the Okhotsk Sea (water depth, 1215 m; Fig.12),
which is a source region for North Pacific Intermediate Water
(NPIW), and off the Shimokita peninsula in the northwestern
North Pacific (water depth, 1366 m; Fig.12), which is a downstream
region of NPIW. We used 14 C age differences between
coexisting benthic and planktic foraminifers from sediment
cores to determine ventilation age (Fig.13). Because NPIW is
considered to be an important temporal carbon reservoir,
changes in its temperature and ventilation rate would change its
carbon capacity.
In the waters off Shimokita, the 14 C age differences
changed in response to the millennial time scale warm and cold
events; differences were relatively small during the H1 cold
period and large during the subsequent B/Å warm period
[Ahagon et al., 2003] (Fig.13). In the Okhotsk Sea, we could
not identify definite differences in ventilation ages corresponding
to the H1 and B/Å events because of the sparse data due to
poor preservation of foraminifer shells (Fig.13). However,
slower ventilation during the last glacial maximum (LGM) was
inferred from the older ventilation age in the Okhotsk Sea (Fig.
13), suggesting that the formation of Okhotsk Sea Intermediate
Water (OSIW), which is an important source of NPIW, was
reduced during the LGM [Okazaki et al., 2008]. One probable
cause of the slow ventilation at intermediate depths in the
Fig. 13 Ventilation ages at intermediate depths (1000 m) in the
Okhotsk Sea and off Shimokita peninsula. Pink fill, warm periods; gray
fill, cold period. See text for explanation of abbreviations.
Okhotsk Sea is the reduced formation of dense shelf water
(DSW) because of perennial sea-ice covering the source area of
DSW in the Okhotsk Sea.
The Mg/Ca ratio in benthic foraminiferal shells, which is a
proxy for bottom water temperatures, indicates that the bottom
water temperature at a depth of 1366 m off Shimokita during
the B/Å warm period was 2 to 3 C higher than that during H1
(data not presented; Kimoto et al., 2008). Furthermore, a high
CaCO3 content was found simultaneously during the B/Å warm
period in sediments from the Okhotsk Sea (data not presented),
which is a source area of NPIW, implying that good carbonate
preservation (deepening of the carbonate compensation depth)
was caused by a warming and reduced ventilation rate of intermediate-deep
water. It seems that the notable changes at the sea
surface propagated rapidly to intermediate-deep water, and the
temperature and ventilation age in intermediate-deep water and
the carbon cycle changed in response to climate changes across
the millennial time scale represented by the B/Å warm period.
References
Ahagon et al., 2003: Geophys. Res. Lett., 30,
doi:10.1029/2003GC000559.
Kimoto et al., 2008: Chikyu Monthly, 30(4), 182-188.
Okazaki et al., 2008: Geophys. Res. Lett., submitted.
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