國際海洋古全球變遷研究─子計畫四：利用多種浮游有孔蟲氧碳同位素比值測定來重建古海洋表層水文結構(Ⅱ)：西太平洋暖池北緣及南沖繩海槽研究

We measured the oxygen and carbon isotopic ratios of several foraminiferal species to
reconstruct the hydrological structures of surface-waters during (1) the Marine Isotope
Stages (MIS) 10-12 of the South China Sea and made a comparison with the record of MIS
1-6, and (2) the Holocene of Southern Okinawa Trough. In addition, organic carbon content
and major elements in bulk sediment of the MIS 10-12 of two cores from the South China
Sea were analyzed to examine the surface-water productivity and bottom water conditions.
The fluctuation amplitudes of oxygen and carbon isotopic ratios of two studied species
and the isotopic offsets between them are both smaller during the MIS 12-10 compared to
that in MIS 1-6. It appears that the lower amplitude of solar insolation during MIS 10-12 is
responsible for the smaller isotopic fluctuation. The offsets in δ 18 O and δ 13 C between the
surface dwelling species (Globigerinoides ruber) and the thermohaline species
(Neogloboquadrina dutertrei) during the MIS 10-12 become smaller compared to that in the
MIS 1-6, signifying a less stratified surface-water column. The bottom waters are more
oxygenated and probably also better ventilated during the interglacials as evidenced by lower
content of total organic carbon and the elevated MnO/Al2O3 and P2O5/ Al2O3 ratios.
The δ 18 O values of four foraminiferal species (Globigerinoides ruber,
Neogloboquadrina dutertrei, Globorotalia menardii and Pulleniatina obliquiloculata) all
display a decreasing trend during the past 8 kyrs while the offsets among them remain to be
relatively constant. No sensible excursion can be identified to explain the drastic reduction of
Pulleniatina obliquiloculata, a Kuroshio characteristic species, during 4.5 – 3.0 Ka. This
result leaves the occurrence of the Pulleniatina obliquiloculata Minimum Event (PME) to be
an unsolved paleoecologic enigma. A major shift to heavier δ 13 C values occurred at ~8 ka as
manifested by all four species in the Southern Okinawa Trough. The offset in δ 13 C between
G. ruber and P. obliquiloculata increased after 8 ka compared to the earliest Holocene.
Integrating the various lines of evidence, we interpret that a stable hydrological condition in
the Okinawa Trough established at about 8 ka and lasted until 2 ka.