Sediments in Hole 1020B consist of biogenic and terrigenous components, of which biogenic components occupy ~10 wt% of bulk sediments. The biogenic component includes ~1 wt% TOC, 0-5 wt% biogenic carbonate, and ~3 wt% biogenic silica (Table 1).
In the interval from 400 to 600 ka, high values of biogenic carbonate and silica indicate only limited dilution by terrigenous materials; this is also evident from low quartz abundance and low values for calculated terrigenous input (Fig. 3). To normalize the data with respect to the dilution effect, terrigenous-free percentages of biogenic carbonate, biogenic silica, and TOC have been calculated by using the following equation from Gardner et al. (1997):
where %biogenic silica, %biogenic carbonate, and %TOC are substituted for X to calculate the respective terrigenous-free values.
The resulting variations of terrigenous-free biogenic silica and carbonate are depicted in Figure 4 compared with the oxygen isotope ratio of benthic foraminifers at Site 849 in the Eastern Equatorial Pacific, a representation of global ice-volume change (Mix et al., 1995). The terrigenous-free biogenic silica concentration record correlates with global ice-volume change and shows high concentrations during most interglacial isotope stages and low concentrations during most glacial stages. The temporal pattern of biogenic carbonate, in contrast, shows high concentrations in glacial periods over the last 800 k.y. Variations in terrigenous-free organic carbon concentrations are less obviously tied to glacial-interglacial changes but have a more similar variation than terrigenous-free biogenic silica.
The observed temporal variations of biogenic components may be caused by changes in the character of deep water. In general, variations in carbonate and silica concentrations are influenced by production, dissolution, and dilution. Carbonate productivity in this region has been reported to have been high in glacial periods and low in the interglacial periods (Karlin et al., 1992). In addition, these authors pointed out that carbonate dissolved because the carbonate compensate depth (CCD) was shallower during the interglacial periods. In this region, the CCD has varied significantly on glacial-interglacial time scales: its depth has been estimated at a water depth of 2700 m during the last interglacial period and 4400-4500 m during last glacial period (Karlin et al., 1992). These authors suggested a change in deep-water characteristics as one factor of this remarkable CCD change. During glacial periods, CO2-depleted North Pacific Deep Water (NPDW) would have formed in the North Pacific region and would have caused a deepening of the CCD. On the other hand, the CCD would have been shallower during interglacial times because of the inflow of CO2-rich deep water derived from North Atlantic Deep Water (NADW). Our results on the variation of biogenic carbonate during the last 800 k.y. agree with this model developed for the last glacial-interglacial cycle.
The variation in terrigenous-free biogenic silica in this study may be an indication for NPDW production during glacial periods. Young and Si-unsaturated NPDW would rapidly dissolve biogenic silica during glacial times; on the other hand, old and Si-saturated NADW should enhance preservation of biogenic silica during interglacial times. Moreover, the productivity of biogenic silica in this high-latitude region would increase during strong upwelling comparable to the present summer situation during interglacial times and would decrease during weakened upwelling under glacial conditions.