Abs~ract. We analy:ro five high-resolution time series spanmng the last 1.65 m.y.: benthic foraminiferal 5110 and 5'~C. percent CaC03o and estimated sea surface temperature (SS1) at Nonh Atlantic Deep Sea Drilling Project site 007 and percent CaC~ at site 609. E ach record is a multicore compo<;ite verifiC;d fo~ c?nti.nuity by splicing among multiple hole~. The~e chmauc. mdtces portray changes in northern hemtsphere tee sheet stze and in North Atlantic surface and deep circulation. By tuning obliquity and precession components in the 5 11 0 record to orbital variations we have devised a time scale (TP607) for the entire Pleist~ene that agrees in age with aU K/Ar-dated magnetic reversals to within 1.5%. The Brunhes time scale is taken from Imbrie et al. (1984], except for differences near the stage 17/16 transition (0.70 to 0.64 Ma). All indicators show a similar evolution from the ~uyama to the B.runhes chrons: orbital eccentricity and_ prece~1o.n responses mcreased in amplitude; those at orbttal obliqutty decreased. The change in dominance from Paper number 89PA00434. 0883-8305/89/89PA-00434$10.00 obliquity to eccentricity occurred over several hundred thousand years, with fa stest changes around 0.7 to 0.6 Ma. The coherent, in-phase responses of 5"0. 5 1 3C, CaC03 and SST at these rhythms indicate that northern hemisphere ice volume changes have controlled most of the North Atlantic surfaceocean and deep-ocean responses for the last 1.6 m.y. Tbe 5 1 3C, percent CaCO; and SST records at site 607 also show prominent changes at low frequencies, including a prominent long-wavelength oscillation toward glacial conditions that is centered between 0.9 and 0.6 Ma These changes appear to be associated neither with orbital forcing nor with changes in ice volume.

High‐resolution records of δ18O, δ13O, and percent CaCO3 from the late Pliocene North Atlantic (Deep Sea Drilling Project sites 607 and 609) are presented and oxygen isotope stages are formalized back to stage 116 at 2.73 Ma. From 2.8 to 1.6 Ma, the interval studied, variations in these records were dominated by the 41‐kyr component of orbital obliquity. Significant variation at the orbital frequencies of eccentricity (96‐kyr) and precession (23‐kyr) are observed in the δ18O record between 1.6 and 2.1 Ma, but not before. Prior to 2.4 Ma (stage 100), δ18O variations suggest ice sheet growth 1/4 to 1/2 as large as late Pleistocene ice volumes; however, these events are below the threshold needed to result in extensive ice‐rafting to the open North Atlantic Ocean. After 2.4 Ma, ice sheets appear to be, on average, 1/2 as large as those of the late Pleistocene. The δ18O record indicates that some glacial suppression of North Atlantic Deep Water occurred both before and after 2.4 Ma and that glacial‐interglacial transfers of 12C between the continents and oceans appear to have been larger in the late Pliocene relative to the late Pleistocene. In addition, the strong 23‐kyr power observed in δ18O between 2.75 and 2.10 Ma suggests that deep‐sea circulation (or changes in biomass) is controlled, in part, by climatic variations unrelated to ice sheets.

We developed an isotope (87Sr/86Sr, 5180) reference section for the uppermost Oligocene to lower upper Miocene (ca. 25-8 Ma) at Site 608 in the northeastern North Atlantic. This site contains the least ambiguous magnetostratigraphic record of Miocene polarity changes available, providing direct correlations to the Geomagnetic Polarity Time Scale (GPTS). We integrate biostratigraphic, magnetostratigraphic, Sr isotope, and stable isotope data to provide a reference section for Miocene isotope fluctuations. The direct correlation of isotopes and biostratigraphy to the Geomagnetic Polarity Time Scale (GPTS) provides relatively precise age estimates. We use these age estimates to evaluate the timing of first and last occurrences of planktonic foraminifera, and conclude that many of these are synchronous within a 0.5 m.y. resolution between subtropical Site 563 (33øN) and high-latitude Site 608 (43øN). In addition, we use this chronology to estimate the ages of previously established Miocene oxygen isotope Zones Mil through Mi7 and to compare the Sr isotope record at Site 608 with previously published 87Srff6Sr records. We approximate latest Oligocene to early late Miocene (25-8 Ma) Sr isotope changes with two linear regressions. The rate of increase of 87Sr/86Sr was high from the latest Oligocene (-25 Ma) to earliest middle Miocene (-15 Ma), with an estimated rate of 0.000059/m.y. Our ability to reproduce Sr isotope measurements is +0.000030 or better, yielding a stratigraphic resolution of as good as +0.5 m.y. for this interval. The rate of change was much lower from about 15 to 8 Ma (on average, 0.000013/m.y.), yielding Sr isotope stratigraphic resolution of worse than +2.3 m.y. The causes of the late Eocene to Miocene 87Srff6Sr increases are not known. We speculate that a moderate 87Sr/86Sr increase (0.000030/m.y) which occurred during the late Eocene-latest Oligocene can be explained by intermittent glaciations and deglaciations of the Antarctic continent. These pulse-like changes in the input of glacial weathering products yield what appears to be a monotonic, linear increase. The increase in the frequency of glaciations during the latest Oligocene-early Miocene can explain the higher rate of change of 87Sr/86Sr at this time. We speculate that by the middle Miocene, the development of a permanent east Antarctica ice sheet resulted in decreased input of glacial weathering products and a lower rate of 87Srff6Sr change. BACKGROUND Firm stratigraphic correlations are indispensable for solving problems in earth history. Stratigraphic correlations of Cenozoic marine strata have improved considerably during the past 35 years as a result of four advances: 1. Development of relatively refined planktonic zonations allowed correlation of pelagic sediments with a 0.5-4.0 m.y. resolution (e.g., Bolli [ 1957]; Blow [1969, 1979]; Martini [1971]; see discussions of Bolli et al. [1985], Miller and Kent [ 1987], and Berggren and Miller [ 1988]). 2. Recovery of relatively continuous deep-sea sections by the Deep Sea Drilling Projec...

An important constraint on the processes governing the geodynamo--the flow in the outer core responsible for generating Earth's magnetic field--is the duration of geomagnetic polarity reversals; that is, how long it takes for Earth's magnetic field to reverse. It is generally accepted that Earth's magnetic field strength drops to low levels during polarity reversals, and the field direction progresses through a 180 degrees change while the field is weak. The time it takes for this process to happen, however, remains uncertain, with estimates ranging from a few thousand up to 28,000 years. Here I present an analysis of the available sediment records of the four most recent polarity reversals. These records yield an average estimate of about 7,000 years for the time it takes for the directional change to occur. The variation about this mean duration is not random, but instead varies with site latitude, with shorter durations observed at low-latitude sites, and longer durations observed at mid- to high-latitude sites. Such variation of duration with site latitude is predicted by simple geometrical reversal models, in which non-dipole fields are allowed to persist while the axial dipole decays through zero and then builds in the opposite direction, and provides a constraint on numerical dynamo models.

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