Quaternary stable oxygen isotope record of planktonic foraminifera from ODP Site 806 on the Ontong Java Plateau

We present an oxygen isotope record that can serve as a new standard for Quaternary chronology in the western Pacific. The record is not entirely without problems. A coring gap misses Isotope Stage 19, which contains the Brunhes/Matuyama boundary. We bridged the gap by importing the corresponding portion of the record from adjacent Hole 805C. By counting the cycles related to obliquity, within the completed delta18O record, we date the Brunhes/Matuyama boundary at 792 ka (+/-10 k.y.), in excellent agreement with previous estimates based on tuning to astronomical signals and with recent radiometric determinations. Thus, the conventional age of 730 ka for this boundary should be abandoned. The major feature of the record is the appearance of a strong component in the band centered on 100 k.y., near 900 ka. At this time, the obliquity-dominated fluctuations give way to eccentricity-dominated fluctuations (whereas precession-related signals remain insignificant throughout). It is not necessary to call for a sudden event at the position of the main change; instead, superposition of long cycles is sufficient cause. Nevertheless, the break in the character of the cyclicity of the record is very clear; we put it at 918 ka, at the entrance to glacial Isotope Stage 22 (mid-Pleistocene climate revolution [MPR]). The change in response of the climate system to astronomic forcing apparently is not accompanied by distinct changes in the trends of sedimentation rates. Sand content increases and carbonate decreases, on the whole, after the mid-Pleistocene climate shift. We suggest that this is a result of increased winnowing during glacial periods.