ALBERT

Description: Oxygen and carbon isotope ratios were measured in benthic foraminifers from the entire Pliocene and latest Miocene sections of Site 846, a 180-m section, at a sampling interval of 10 cm. This provides a temporal resolution of about 2500 yr. The documented continuity of the record is excellent. Using the time scale that was developed on the basis of orbital tuning of GRAPE density records, we observed a fairly constant phase relationship between delta18O and variations in the obliquity of Earth's rotational axis. A new numbering scheme for Pliocene isotope stages is proposed. This high-resolution delta18O record clarifies several interesting aspects of late Neogene climatic evolution, including a "glacial" event that may have caused the final Messinian desiccation of the Mediterranean Sea; one or more "interglacial" events that might have caused refilling of the Mediterranean; a well-resolved couplet of glacial events at about the age of the Sidujfall Subchron; interglacial extremes in the early part of the Gauss that could have resulted from either significant deglaciation on Antarctica or from warming of deep water; and a gradual ramp of increasingly extreme "glacial" events, starting at about the Kaena Subchron and culminating with delta18O stage 100 in the earliest Matuyama.

Description: Several hundred samples of bulk sediments from all the 11 sites drilled during Ocean Drilling Program (ODP) Leg 138 in the eastern equatorial Pacific Ocean have been analyzed for d13C and d18O. The d13C records provide information on the changes in the operation of the global carbon cycle during the late Neogene that were postulated by Shackleton (1985) on the basis of analogous data from Deep Sea Drilling Program (DSDP) Leg 74 in the South Atlantic. The general validity of the trend from isotopically heavier d13C values during the mid-Miocene toward lighter values today is confirmed. The data appear to be useful for high-resolution correlation among sites. The origin of the variability is enigmatic; even in the Pleistocene, there is low-frequency variability in bulk sediment d13C having an amplitude of more than 1 per mil that is not present in d13C records from foraminifers, and the concerns expressed by Broecker and Woodruff (1992, doi:10.1016/0016-7037(92)90302-Y) regarding the interpretation of the record must be taken seriously. The bulk-sediment d18O data for the past 1 m.y. show a latitudinal trend that is consistent with the present-day surface temperature gradient, with cooler water on the equator caused by upwelling. The data for the interval 1 to 4 Ma show a similar trend, but between 4 and 7 Ma, the gradient across latitude is steeper. When interpreted in terms of temperature, this implies that surface temperatures along the equator were lower during the interval of exceptionally high sediment accumulation rates of the latest Miocene and early Pliocene. This intense equatorial upwelling declined at about 4 Ma, perhaps as the connection to the Atlantic Ocean across Middle America became ineffective. In the sediment immediately above 11- to 12-Ma-old basement, the d18O values at Sites 849, 850, 851, and 852 become isotopically heavier than the values for sediment of similar age from Site 845 (which lies on 16-Ma-old basement and preserves a longer record). This may imply diagenetic exchange at a relatively low temperature (altering the d18O value toward a more positive value). Overall, the data sets are remarkably consistent. This suggests that other than close to basement, the isotopic effects of diagenetic change must be small and spatially smooth. It should be possible to retrieve long sea-surface temperature records of immense value from these data.