Our study has shown that k.y.-scale CaCO3 and Corg events are a common feature of sediment records from the northern California margin for the late Pleistocene and that these events can be used with care for chronostratigraphic control. One should be aware that events do have different amplitudes at different drill sites. However, using both Corg and CaCO3 for stratigraphic control alleviates most of the ambiguities.
We also conclude that the k.y.-scale CaCO3 events are production driven, not caused by inorganic dissolution processes. The magnitude of the events are simply too large to be caused purely by changes in calcite saturation in North Pacific waters. We are less certain of the oceanographic processes that cause CaCO3 events, but can hypothesize about changes in the structure of the North Pacific Ocean during typical glacial conditions as well as during the k.y.-scale events.
The CaCO3 events are linked in some manner to the small interstadial Dansgaard/Oeschger events of the north Atlantic. Because the CaCO3 events are driven by changes in production, there must have been significant changes on short time scales in the surface structure of the North Pacific. The lack of strong similarity between the CaCO3 events and the Dansgaard/Oeschger events means that the climate of the North Pacific has been influenced by more than oscillations of the North Atlantic, however.
By comparison to the sediment trap studies we can at least present some hypotheses about surface oceanographic conditions along the California margin during average glacial conditions and during the k.y.-scale CaCO3 events. High CaCO3 burial implies that high CCO3/Corg conditions must have prevailed in the surface waters. In turn this must mean that coastal upwelling conditions along the California margin were severely weakened, as is also suggested by the lack of Sequoia in the glacial forests of northern California (Fig. 7; Sancetta et al., 1992). Sequoia require foggy conditions associated with cool summer upwelling for optimal growth.
The increase in CCO3/Corg implied by average glacial conditions as well as the k.y.-scale events could be explained in a scenario in which surface conditions block nutrients from reaching the uppermost euphotic zone, and in particular obligate nutrients favorable to the growth of large diatoms (Si, Dugdale, and Wilkerson, 1998; Berger and Lange, 1998; or Fe, Martin, and Fitzwater, 1988; Martin et al., 1991). Large diatoms are important exporters of organic matter from the euphotic zone. Removal of these species from the community significantly reduces the Corg rain and loss of nutrients from the euphotic zone while also increasing nutrient availability to other small phytoplankton like coccolithophorids. Increases in coccolithophorid community should increase the absolute flux of CaCO3. The modern subarctic Pacific Ocean is dominated by small phytoplankton (Miller et al., 1991) and fits with this scenario.
Ortiz et al. (1997) suggested one means of limiting nutrients along the California margin, by slowing upwelling. Moving from a coastal upwelling regime to a curl-of-windstress upwelling regime will lower the net rate of upwelling and replacement of Si in the surface ocean. A second way to limit nutrients is to increase the stability of surface waters. Increasing the stability implies making the surface water fresher, because other evidence strongly favors significant glacial cooling of surface waters in the region of the northern California margin (Prahl et al., 1995; Doose et al., 1997; Kreitz et al., Chap. 10, this volume). Zahn et al. (1991) suggested that the glacial subarctic northeastern Pacific surface ocean was indeed fresher than the modern surface ocean and more stable, from oxygen isotope measurements upon planktic foraminifers. These waters, when advected into the northern California region and when the surface layer was warmed somewhat, would tend to be even more stable and conditioned for high CaCO3 production.
A third way to limit critical nutrients is if the Fe flux to surface waters were reduced by limiting the aeolian atmospheric source or by weakening the local river source. The atmospheric source could be weakened either by moving the depocenter of Asian aeolian dust to the south of its present 40°N axis or by making the dust flux smaller. However, studies in the western Pacific indicate that the depocenter was stable in its latitudinal position and that glacial dust fluxes from Asia were significantly larger during glacials (Rea, 1994). The local riverine source of Fe was probably larger as well during glacials. Hovan et al. (Chap. 18, this volume) found that terrigenous deposition from a hemipelagic source was significantly higher at both Sites 1018 and 1020 during glacial periods.
Finally, one can decrease Si in upwelled waters by replacing the 100-200 m deep source water with a more Si-poor source (Berger and Lange, 1998). However, such a depletion should also affect the other nutrients, albeit to a lesser extent. One should observe both lower Corg rain and CaCO3 rain in this scenario.
Based upon these observations, we believe that average glacial conditions could be governed by some combination of (1) an "offshore" scenario (Ortiz et al., 1997) in which the rate of ekman pumping of nutrients were diminished with (2) a "subarctic" scenario, in which a cold but relatively fresh surface layer existed. The k.y.-scale events were probably caused by slight improvements in conditions during minor northern hemisphere warmings—that is, injection of somewhat more nutrients into the euphotic zone but slightly less than the necessary level to allow diatoms to dominate. Although we cannot yet state with any certainty what these conditions would be, a slight average warming of the North Pacific should cause a net loss of fresh water to the North American ice cap and weaken the stability of the surface mixed layer somewhat. In addition, relatively small increases in storm frequency or intensity could provide mixing events to support additional CaCO3 production. Slight increases in northerly winds along the coast may have also contributed. These different hypotheses will be investigated in more detail during follow-up studies.
The change in plankton community would also explain why the CaCO3 and Corg time series are significantly different, either because the Corg time series is a mixed signal of terrestrial and marine sources or because the production of CaCO3 is not directly linked to primary productivity, but to the plankton community that is the producer. We argue against the strong influence of terrestrial Corg, because all of our time series have similar signals despite being located as much as 600 km apart.