CONCLUSIONS

The first set of continuous sediment records older than the latest Pleistocene along the California margin were recovered during Leg 167. The recovery of these records and their assembly into continuous time series was one of the major shipboard accomplishments of the leg. Studies during Leg 167 and immediately postcruise have set the stage for a much more comprehensive understanding of the California current system and the sedimentary environment along the California margin.

Leg 167 studies have documented the high sensitivity of the California margin to millennial-scale climate forcing. There is a strong influence of Dansgaard-Oeschger events on surface-ocean conditions that suggests strong climate links between the northeastern Pacific Ocean and the North Atlantic. There is much yet to be done, however. Only a small percentage of the total available sediment column has been studied, so we do not yet have a sufficiently long historical perspective of the millennial-scale events or their relationship to orbitally forced insolation. We have not yet developed time series on sufficient numbers of different proxies to understand the cause of these events or the physical response of the North Pacific Ocean. We have not yet intercalibrated time scales to study the evolution of the millennial-scale events in a latitudinal sense.

We now have produced oxygen isotope stratigraphy for the late Pleistocene for seven drill sites, and alternate high-resolution stratigraphies based upon other sediment components are being developed. We have discovered that the California margin is highly sensitive to climate changes on orbitally forced time scales, and we have found that there is a major change in conditions about 400 ka, around MIS 11. Prior to this time we have much smaller variation of alkenone SST, and higher burial of biogenic remains along the northern California margin. These events do not seem as strong in the south, but we lack long high-resolution time series from the southern drill sites. There is also a need for better age control in Pleistocene sections older than about 200 ka for most of the Leg 167 drill sites. More long oxygen isotope profiles are needed for further work.

Orbitally forced insolation changes invoke a strong SST response along the California margin. SST measured by the alkenone paleothermometer is highly coherent with the oxygen isotope record, being cold in glacials and much warmer in interglacials. By comparing northern and southern SST records it appears, somewhat paradoxically, that the glacial maxima were periods of weakest California current flow and the strong interglacials had strongest flow. Faunal and floral plankton assemblages vary strongly on the glacial-interglacial scale. Coastal plant communities show a glacial-interglacial variability that is most pronounced in the north, near the Cordilleran Ice Sheet.

We have identified but not really studied a series of major late Neogene oceanographic events. Opaline silica burial in the middle and upper Miocene sections of Sites 1010 and 1021 have step-like drops from high opal deposition in the middle Miocene. One major drop occurs at ~11 Ma. A second major drop occurs at about 7.5-8 Ma, equivalent in age to the end of the Monterey formation (Barron, 1998; other refs). A third drop occurs slightly younger than the end of the Miocene. We do not yet know why there is a disappearance of diatoms in the younger sections or periodic burial events. We have also identified CaCO₃ burial events at roughly 5.2, 6.8, 8.8, 10, and 12-14 Ma, coincident with but sometimes slightly offset from the opal peaks. We do not yet know the timing of these burial events with respect to each other nor the oceanographic dynamics that produced the sediment records.

A lower Pliocene interval, roughly from 5 to 4.2 Ma, is low in all biogenic components. It separates the Miocene high-opal sediments from upper Pliocene high-carbonate sediments. High CaCO₃ deposition occurred all along the entire California margin in the late Pliocene, but CaCO₃ burial dropped abruptly with the beginning of Northern Hemisphere glaciation (2.6 Ma).

The Neogene sections from Leg 167 are not well explored but indicate major periodic, poorly understood changes in oceanographic conditions since the middle Miocene. These sections represent the only detailed information we have to date on the temperate North Pacific Ocean and have the potential to provide valuable insights into global changes in Neogene paleoceanographic conditions and biogeochemical cycles.