The sediments recovered during Leg 138 provide a remarkable opportunity to improve the geological time scale of the late Neogene. We have developed new time scales in the following steps. First, we constructed age models on the basis of shipboard magnetostratigraphy and biostratigraphy, using the time scale of Berggren, Kent, and Flynn (1985). Second, we refined these age models using shipboard GRAPE density measurements to provide more accurate correlation points. Third, we calibrated a time scale for the past 6 m.y. by matching the high-frequency GRAPE density variations to the orbital insolation record of Berger and Loutre (1991); we also took into account δ 18 θ records, where they were available. Fourth, we generated a new seafloor anomaly time scale using our astronomical calibration of C3A.n (t) at 5.875 Ma and an age of 9.639 Ma for C5n.ln (t) that is based on a new radiometric calibration (Baksi, 1992). Fifth, we recalibrated the records older than 6 Ma to this new scale. Finally, we reconsidered the 6-to 10-Ma interval and found that this could also be partially tuned astronomically.

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Climate variability at periods from 10 kyr to 12 kyr that originates from Milankovitch band forcing is quantified at three locations for the late Pleistocene (eastern equatorial Pacific Ocean Drilling Program site 846, eastern equatorial Atlantic ODP site 663, and northeastern Atlantic Deep Sea Drilling Project site 609). Variability at these periods is not present in the primary Milankovitch forcing, so no linear linkage to Milankovitch band variations is possible. However, these periods are equal to harmonics of precession band oscillations. The magnitude of interactions between processes that occur at different timescales can only be resolved in time series data through application of higher‐order statistics. Through such an application, we demonstrate that up to 75% of the variance in the 10‐ to 12‐kyr band in the sediment records is nonlinearly transferred from precession band (19–23 kyr) variations. Within the millennial to sub‐Milankovitch band, defined as the band of variance ranging from ∼15 to ∼2 kyr, approximately 1/3 of the variability in the records studied is consistent with a low‐frequency, Milankovitch band origin. This variability may derive from high sensitivity of the tropics to summertime insolation in both hemispheres relative to wintertime insolation. A mechanism having equatorial origin and related to low‐latitude precession variations appears consistent with the observations. Because the phase coupling between 10‐ to 12‐kyr oscillations and precession is resolved, this result has implications for development of models which seek to explain global climate variations on this timescale.

Ocean Drilling Program (ODP) Leg 138 was designed to study the late Neogene paleoceanography of the equatorial Pacific Ocean at time scales of thousands to millions of years. Crucial to this objective was the acquisition of continuous, high-resolution sedimentary records. It is well known that between successive advanced piston corer (APC) cores, portions of the sedimentary sequence often are absent, despite the fact that core recovery is often recorded as 100%. To confirm that a continuous sedimentary sequence was sampled, each of the 11 drill sites was multiple-APC-cored. At each site, continuously measured records of magnetic susceptibility, gamma-ray attenuation porosity evaluator (GRAPE), wet-bulk density, and digital color reflectance were used to monitor section recovery. These data were used to construct a composite depth section while at the site. This strategy often verified 100% recovery of the complete sedimentary sequence with two or three offset piston-cored holes.Here, these initial efforts have been extended to document the recovery of a complete sediment section and to investigate sources of error associated with sediment density measurements and changes in local sedimentation rates. At Sites 846 through 852, fine-scale correlation (on the order of centimeters) of the GRAPE records was accomplished using the inverse correlation techniques of Martinson et al. (1982). Having a common depth scale for all holes at each site facilitated comparison of highresolution data from different holes. After refining the interhole correlation, GRAPE records from adjacent holes were "stacked" to produce a less noisy estimate of sediment wet-bulk density for Sites 846 through 852. The continuity of the stacked GRAPE record is confirmed with reflectance and susceptibility records. The resulting stacked GRAPE records have a temporal resolution of less than 1000 yr for the past 5 m.y. Moreover, the stacking procedure allows for development of error estimates for measurements present in more than one hole. An important advantage provided by this framework is that one can determine the range of sedimentation variability between adjacent holes at a given site. This variability is caused by local sedimentation variability and by artifacts of the coring process. We demonstrate that the depth domain changes in sedimentation variability required to correlate among adjacent holes are larger than the changes induced by time-scale tuning procedures.

Previous investigations of the response of Plio-Pleistocene climatic records to long-term, orbitally induced changes in radiation have considered a linear response of climate. While the second-order statistics of power spectra and cross spectra provide necessary information on linear processes, insight into the nonlinear characteristics of Pliocene and Pleistocene climate is not provided by these statistical quantities. Second-order statistics do not contain the phase information necessary to investigate nonlinear, phase-coupled processes. Such information is provided by higher-order statistical quantities. In particular, bispectral analysis indicates that nontinear couplings are present in the climatic (radiative) forcing at the Milankovitch frequencies. Through a linear transfer, this forcing produces similar nonlinear couptings in deep-sea sedimentary oxygen isotope records (ODP site 677 and DSDP site 607) from 1.0 to 0 Ma during the late Neogene. This analysis suggests that during the late Pleistocene, the dominance of the 100,000 year cycle in the climate record is consistent with a linear, resonant response to eccentricity forcing. In the period from 2.6 to 1.0 Ma, a change in the nature of the climatic response to orbital forcing is indicated, as phase couptings present in the isotopic time series are not similar to the phase couplings present in the insolation forcing. Third-order moments (skewness and asymmetry) are used to quantify the shape of the climatic response. From 2.6 Ma to present, an increase in the asymmetry (sawtoothness) of the oxygen isotopic records is accompanied by a corresponding decrease in the skewness (peakedness) of the records. This indicates an evolution in the nature of the phase coupling within the climate system. These results may provide important constraints useful in development of models of paleoclimate.

The imprint of orbital variations on the geological record of climatic variability is well documented, especially for the Plio‐Pleistocene. There is considerable interest in developing very high resolution timescales through the Cenozoic and into the Mesozoic by tuning geological records to the assumed astronomical forcing. Since the precession signal is so highly amplitude modulated, it is widely believed that high coherence between record and assumed forcing in the precession band is an indication that the timescale is probably correct, because coherence is supposed to provide a measure of the degree of common amplitude modulation. We show that this is misleading; even a sinusoidal variation that has been “tuned” to an insolation record shows highly significant coherence at the 23‐kyr and 19‐kyr precession frequencies. Coherence is a good indication that the tuning has generated a consistent phase relationship, but complex demodulation is a better tool for assessing the relationship between amplitude modulation in the data and in the hypothetical forcing.

High-resolution, continuous records of GRAPE wet bulk density (a carbonate proxy) from Ocean Drilling Program Leg 138 provide one the opportunity for a detailed study of eastern equatorial Pacific Ocean carbonate sedimentation during the last 6 m.y. The transect of sites drilled spans both latitude and longitude in the eastern equatorial Pacific from 90° to 110°W and from 5°S to 10°N. Two modes of variability are resolved through the use of Empirical Orthogonal Function (EOF) analysis. In the presence of large tectonic and climatic boundary condition changes over the last 6 m.y., the dominant mode of spatial variability in carbonate sedimentation is remarkably constant. The first mode accounts for over 50% of the variance in the data, and is consistent with forcing by equatorial divergence. This mode characterizes both carbonate concentration and carbonate mass accumulation rate time series. Variability in the first mode is highly coherent with insolation, indicating a strong linear relationship between equatorial Pacific carbonate sedimentation and Milankovitch variability. Frequency domain analysis indicates that the coupling to equatorial divergence in carbonate sedimentation is strongest in the precession band (19-23 k.y.) and weakest though present at lower frequencies. The second mode of variability has a consistent spatial pattern of east-west asymmetry over the past 4 m.y. only; prior to 4 Ma, a different mode of spatial variability may have been present, possibly suggesting influence by closure of the Isthmus of Panama or other tectonic changes. The second mode of variability may indicate influence by CaCO 3 dissolution. The second mode of variability is not highly coherent with insolation. Comparison of the modes of carbonate variability to a 4 m.y. record of benthic δ 18 θ indicates that although overall correlation between carbonate and δ 18 θ is low, both modes of variability in carbonate sedimentation are coherent with δ 18 θ changes at some frequencies. The first mode of carbonate variability is coherent with Sites 846/849 δ 18 θ at the dominant insolation periods, and the second mode is coherent at 100 k.y. during the last 2 m.y. The coherence between carbonate sedimentation and δ 18 θ in both EOF modes suggests that multiple uncorrelated modes of variability operated within the climate system during the late Neogene.

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