PALEOCEANOGRAPHY DURING THE PAST 28 K.Y.

Both sea-surface temperatures (SSTs) reconstructed by U^k'₃₇ and ¹⁸O values of the surface-dwelling planktonic foraminifer Globigerinoides sacculifer show high-frequency and large-amplitude variations through the last 28 k.y. (Fig. F8) (Zhao et al., 2005). The high variability may be related to the fact that short-term episodic events can be recorded in rapidly deposited sediments. Because the analyses are not necessarily high resolution, a moving average profile best reflects the general history (Fig. F8). The SSTs during the latest Pleistocene (28–10 ka) remained at ~24°C, with the coldest occurring at the Last Glacial Maximum (LGM; 19–18 ka) and the Younger Dryas (13–12 ka), respectively. The SST rose from the LGM to the late Holocene in two steps: the first took place from 18 to 14 ka, before the reversal of the Younger Dryas (13–12 ka), and the second began at 11.5 ka and lasted until 7.0 ka (Fig. F8). The second rise of SST is conspicuous, as temperature rose quickly from 22° to 26°C in 3.5 k.y. The trend is also reflected by the parallel lowering in ¹⁸O values of the planktonic foraminifer G. sacculifer (Fig. F8). It appears that this second warming trend lasted longer here than in other areas. For instance, the Greenland ice core GISP 2 shows that air temperatures reached Holocene values at ~10 ka and decreased slightly during the Holocene (Stuiver et al., 1995).

Su and Wei (2005) examined calcareous nannofossil assemblages of the past 13 k.y. They found that a major change in floral composition took place at 9 ka. Before 9 ka the nannofossil assemblages were dominated by Emiliania huxleyi and Gephyrocapsa spp., whereas after 9 ka, deep-dwelling Florisphaera profunda became more abundant (Fig. F9A) at the expense of Gephyrocapsa. Su and Wei (2005) interpreted that such a change implies that the Kuroshio Current reentered into the southern Okinawa Trough at the same time. This interpretation is consistent with the granulometric data that the content of sortable silt (10–63 µm in grain size) reached a high stable value at 9.5 ka (Fig. F9C) (see next section for discussion). The timing of the floral change, however, predated the final rise of the SST to stable, high temperature at ~7 ka (Fig. F9D). This implies that the floral composition of the calcareous nannoplankton in the southern Okinawa Trough was affected mainly by the invasion of the Kuroshio Current to the trough rather than the local SST change.