Obliquity Bands Research Articles

High-resolution cyclostratigraphic analysis from magnetic susceptibility in a Lower Kimmeridgian (Upper Jurassic) marl–limestone succession (La Méouge, Vocontian Basin, France)

High-resolution magnetic susceptibility (MS) analysis was carried out on a Lower Kimmeridgian alternating marl–limestone succession of pelagic origin that crops out at La Méouge (Vocontian Basin, southeastern France). The aim of the study was to characterize the strong, dm-scale sedimentary cyclicity of the succession at a very high resolution, and to analyze the cycles for evidence of astronomical forcing. From marl to limestone, MS varies progressively and closely tracks the highest frequency cyclicity corresponding to the basic marl–limestone couplets. Long-term wavelength cycling modulates the high-frequency cyclicity (couplets), and appears to be controlled by clay content. Spectral analysis of the MS record reveals the presence of the complete suite of orbital frequencies in the precession, obliquity, and eccentricity (95–128 ka and 405 ka) bands with very high amplitude of the precession index cycles originating from dm-scale couplets. 405 ka-eccentricity cycles are very pronounced in the MS maxima of the marl members of the couplets, suggesting eccentricity-driven detrital input to the basin. 405 ka-orbital tuning of the MS maxima further sharpens all of the orbital frequencies present in the succession. These results are similar to those of previous studies at La Méouge that used carbonate content observed in field. Our results are also in accordance with cyclostratigraphic studies in Spain and Canada that report dominant precession index forcing. By contrast, in the Kimmeridge Clay (Dorset, UK), obliquity forcing dominates cyclic sedimentation, with weaker influence from the precession index. Ammonite zone duration estimates are made by counting the interpreted precession cycles, and provide an ultra-high resolution assessment of geologic time. In sum, this study demonstrates the power of the MS as a proxy in characterizing the high-resolution cyclostratigraphy of Mesozoic sections, particularly in alternating marl–limestone successions, and for high-resolution correlation and astronomical calibration of the geologic time scale.

Coherent obliquity band and heterogeneous precession band responses in early Pleistocene tropical sea surface temperatures

The nature of the connection between high‐ and low‐latitude climates during the early Pleistocene “41 kyr world” has important implications for our understanding of the feedbacks involved in translating insolation changes into global climate states. Here we focus on the tropical marine record, presenting alkenone‐derived sea surface temperature (SST) and productivity records from the eastern equatorial Atlantic, eastern equatorial Pacific, the Arabian Sea, and the South China Sea for a time interval covering the heart of the 41 kyr world (1.2–2.0 Ma). All four SST records are dominated by variance in the obliquity band, suggesting that high‐latitude dynamics and low‐latitude climate were tightly coupled in the 41 kyr world, despite smaller ice volume variability during this interval as compared to the late Pleistocene. At the 41 kyr period, SST varied coherently and nearly synchronously between the four study regions, suggesting a tropic‐wide feedback to high‐latitude processes. Productivity variations at our equatorial Atlantic and Pacific sites were also coherent in the obliquity band, implying tropical trade wind variability at this frequency during the early Pleistocene. In contrast, we observe heterogeneous SST and productivity responses in the precession band between each of the tropical locations. Local atmospheric circulation patterns, rather than a globally coordinated response to precessional insolation forcing, apparently determined SSTs and productivity in the tropics at precessional frequencies during the early Pleistocene.

Paleohydrography of the Kuroshio-Kuroshio Extension off Sanriku Coast based on fossil diatoms

The high-resolution analyses of fossil diatom assemblages in four cores recovered from the area off Sanriku Coast revealed the most complicated hydrographic history in this area. The fossil diatom assemblages changed in responding to the expansion of the warm-water eddies detached from the Kuroshio Extension and of the southward Oyashio Intrusion. Diatom abundances are higher in the cores from near shore than in the area off shore. The diatom assemblages were affected more seri- ously by the environmental changes in the cores from near shore. The diatom assemblages were also changed by the fluctua- tions at the intervals of ~40-kyr corresponding to the obliquity (tilt) band variations among the Earth's orbital parameters and ~60-kyr intervals suggesting changes in the sea level due to glacial/interglacial phases, with secondary and smaller fluctuatio ns at ~20-kyr intervals corresponding to the precessional changes. Diatom assemblages in the southern margin of the Perturbed Area have been controlled mainly by the Kuroshio-Kuroshio Extension, but in the northern margin they changed in responding to the mixing of warm-water eddies and Oyashio Intrusion.

Orbital forcing of continental climate during the Pleistocene: a complete astronomically tuned climatic record from Lake Baikal, SE Siberia

A new composite BDP-96 biogenic silica record over the entire Pleistocene was generated by splicing BDP-96-1 and BDP-96-2 drill cores from Lake Baikal, crosschecked against a similar record from a nearby BDP-98 drill core. A new astronomically tuned age model is proposed based on correlating peak biogenic silica responses with the timing of September perihelia. This target is derived from analysis of regional climate proxy responses during the Holocene, the last interglacial and around paleomagnetic reversals. By resolving virtually every precessional cycle during the Pleistocene, the new age model represents a major improvement compared with previously reported Lake Baikal timescales. The astronomically tuned ages of the Pleistocene paleomagnetic reversals are consistent with published dates. The minimal tuning approach we used (precession only) has also aligned high signal power in a narrow obliquity band, confirming the strong presence of orbital forcing. There are also strong ca 100-ka scale cycles, but these are not aligned with the orbital eccentricity. Despite the location of Lake Baikal in a continental interior that is highly sensitive to insolation forcing, the tuned biogenic silica record reveals a consistent phase difference of −32° (ca 4 ka) relative to insolation in the obliquity band. An inherent lag embedded in a continental proxy record, not driven by global ice volume, is an intriguing finding. Another new observation is that long-term changes in sedimentation rates in Lake Baikal appear to be related to the amplitude of orbital forcing; both amplitudes and sedimentation rates undergo significant changes during MIS 24-MIS 19 interval corresponding to the Middle Pleistocene Transition. With potential for linking continental and marine climato-stratigraphies, the new Baikal record serves a new benchmark correlation target in continental Eurasia, as an alternative to June 65°N insolation and ODP-correlated timescales.

Consequences of pacing the Pleistocene 100 kyr ice ages by nonlinear phase locking to Milankovitch forcing

The consequences of the hypothesis that Milankovitch forcing affects the phase (e.g., termination times) of the 100 kyr glacial cycles via a mechanism known as “nonlinear phase locking” are examined. Phase locking provides a mechanism by which Milankovitch forcing can act as the “pacemaker” of the glacial cycles. Nonlinear phase locking can determine the timing of the major deglaciations, nearly independently of the specific mechanism or model that is responsible for these cycles as long as this mechanism is suitably nonlinear. A consequence of this is that the fit of a certain model output to the observed ice volume record cannot be used as an indication that the glacial mechanism in this model is necessarily correct. Phase locking to obliquity and possibly precession variations is distinct from mechanisms relying on a linear or nonlinear amplification of the eccentricity forcing. Nonlinear phase locking may determine the phase of the glacial cycles even in the presence of noise in the climate system and can be effective at setting glacial termination times even when the precession and obliquity bands account only for a small portion of the total power of an ice volume record. Nonlinear phase locking can also result in the observed “quantization” of the glacial period into multiples of the obliquity or precession periods.

Late Pliocene monsoon linkage in the tropical South China Sea

The onset of Northern Hemisphere Glaciation (NHG) ∼2.7 Ma ago coincided with prominent climate changes in the tropical regions such as the African and the Asian monsoons. However, the relationship between tropical and sub-tropical monsoonal variations and high northern latitude ice sheet expansion as well as processes such as late Pliocene tropical sea surface temperature (SST) change is not clear. Our late Pliocene (2.5–3.3 Ma) monsoon proxy records and Mg/Ca derived SST records at Ocean Drilling Program (ODP) Site 1143 from the southern South China Sea (SCS) reveal that while tropical SST shows a stepwise decrease of 2–3 °C during this period, the East Asian monsoon gradually strengthens in response to the onset of the NHG. At the 41-kyr and 23-kyr bands, ice volume change lags tropical SST by ∼4 kyr, but leads the East Asian monsoon by ∼12–17 kyr. Our finding highlights the significant role of the tropical Pacific region in driving global climate change in the late Pliocene, which has invariable leading phase relative to the ice volume change as in the late Pleistocene. However, the East Asian monsoon shows a linear response to the onset of the NHG in the late Pliocene, with much bigger phase lagged at the 41-kyr and 23-kyr bands than in the Pleistocene, which suggests that at the obliquity and precession bands the phases of the Plio-Pleistocene East Asian monsoon variations relative to the global ice volume changes are not constant, but variable. Therefore, the East Asian monsoons are not only simply driven by northern summer insolation at the precession period but also modulated by global ice volume change in high latitudes.

EMD analysis of solar insolation

A new time series analysis technique, Empirical Mode Decomposition (EMD), which has been successfully applied to nonlinear and nonstationary data, is used to examine paleoclimate cycles in the Pleistocene (1 Ma bp–20 Ka bp). The purpose of this study is to improve knowledge of the climatic significance of solar insolation. The results show that the eccentricity band signal is much larger than previously estimated, having an amplitude of about 1% of solar irradiance which is comparable to the amplitude of the precession and obliquity band signals. This finding implies the need to reconsider the role of solar radiation on the formation and maintenance of quaternary ice sheet cycles.

Seasonality intensification and long-term winter cooling as a part of the Late Pliocene climate development

Abstract A mutual climatic range method is applied to the Mediterranean marine pollen record of Semaforo (Vrica section, Calabria, Italy) covering the period from ∼2.46 Ma to ∼2.11 Ma. The method yields detailed information on summer, annual and winter temperatures and on precipitation during the nine obliquity and precession-controlled ‘glacial’ periods (marine isotope stages 96 to 80) and eight ‘interglacial’ periods (marine isotope stages 95 to 81) characterising this time interval. The reconstruction reveals higher temperatures of at least 2.8 °C in mean annual and 2.2 °C in winter temperatures, and 500 mm in precipitation during the ‘interglacials’ as compared to the present-day climate in the study area. During the ‘glacials’, temperatures are generally lower as compared to the present-day climate in the region, but precipitation is equivalent. Along the consecutive ‘interglacials’, a trend toward a reduction in annual and winter temperatures by more than 2.3 °C, and toward a higher seasonality is observed. Along the consecutive ‘glacials’, a trend toward a strong reduction in all temperature parameters of at least 1.6 °C is reconstructed. Climatic amplitudes of ‘interglacial–glacial’ transitions increase from the older to the younger cycles for summer and annual temperatures. The cross-spectral analyses suggest obliquity related warm/humid–cold/dry ‘interglacial–glacial’ cycles which are superimposed by precession related warm/dry– cold/humid cycles. A time displacement in the development of temperatures and precipitation is indicated for the obliquity band by temperatures generally leading precipitation change at ∼4 kyr, and on the precession band of ∼9.6 kyr in maximum.

Forcing mechanism of the Pleistocene east Asian monsoon variations in a phase perspective

The deep sea records from the ODP Sites 1143 and 1144 in the northern and southern South China Sea (SCS), including foraminiferal δ18O and δ13C, Opal% and pollen percentage, reveal that the variations of the east Asian monsoon have been closely correlated with the variations of the Earth’s orbital parameters (eccentricity, obliquity and precession) and the global ice volume on orbital scale. All the monsoonal proxies show strong 100 ka, 41 ka and 23 ka cycles. Although G. ruber δ13C of Site 1143 is coherent with the ETP (ETP= normalized (eccentricity + obliquity-precession) at eccentricity, obliquity and precession bands, most of the coherent relationship focuses on the precession band, and the other monsoonal proxies are coherent with the ETP only at the precession band, which indicate that precession dominates the Pleistocene tropical climate changes. The phase relationship of the monsoonal proxies with the foraminiferal δ18O implies that the global ice volume changes have played a significant role in modulating the east Asian monsoon climate, at least dominating the winter monsoon. This forcing mechanism of the east Asian monsoon is apparently different from that of the Indian ocean monsoon. The variations of the east Asian monsoon at the precession band, at least that of the winter monsoon, have been controlled not only by the sensible heating but also by the latent heating of the surface water in the South China Sea.

Astronomical timescale and palaeoclimatic implication of stacked 3.6-Myr monsoon records from the Chinese Loess Plateau

Magnetic susceptibility and grain size records from two continuous red-clay and loess–palaeosol sequences on the Chinese Loess Plateau have been generated to investigate the evolution and variability of the East Asian monsoon (EAM) during the late Pliocene and Pleistocene. Tuning the grain size records to orbital obliquity and precession yields an improved astronomical timescale for the loess–palaeosol sequence (0–2.6 Ma), and an extended age model for the upper red-clay formation (2.6–3.6 Ma). This timescale indicates older ages for a number of the magnetic polarity boundaries, consistent with lock-in depth offsets in loess sequences. Good site-to-site correlation enables generation of stacked grain size and susceptibility records spanning the last 3.6 Myr. These records indicate that monsoon evolution since the late Pliocene can be subdivided into three phases: 0–1.25, 1.25–2.72 and 2.72–3.4 Ma; each phase is characterized by unique amplitude and frequency characteristics for both summer monsoon (magnetic susceptibility) and winter monsoon (grain size). Spectral analyses of the stacked monsoon proxies indicate that characteristics of both the summer and winter monsoons are dominated mainly by variance in the eccentricity (410- and 100-kyr), obliquity (41-kyr) and precession (23- and 19-kyr) bands over the past 3.4 Myr, implying a non-linear response of the long-term EAM evolution to orbital and glacial forcing.

A Pliocene‐Pleistocene stack of 57 globally distributed benthic δ18O records

We present a 5.3‐Myr stack (the “LR04” stack) of benthic δ18O records from 57 globally distributed sites aligned by an automated graphic correlation algorithm. This is the first benthic δ18O stack composed of more than three records to extend beyond 850 ka, and we use its improved signal quality to identify 24 new marine isotope stages in the early Pliocene. We also present a new LR04 age model for the Pliocene‐Pleistocene derived from tuning the δ18O stack to a simple ice model based on 21 June insolation at 65°N. Stacked sedimentation rates provide additional age model constraints to prevent overtuning. Despite a conservative tuning strategy, the LR04 benthic stack exhibits significant coherency with insolation in the obliquity band throughout the entire 5.3 Myr and in the precession band for more than half of the record. The LR04 stack contains significantly more variance in benthic δ18O than previously published stacks of the late Pleistocene as the result of higher‐resolution records, a better alignment technique, and a greater percentage of records from the Atlantic. Finally, the relative phases of the stack's 41‐ and 23‐kyr components suggest that the precession component of δ18O from 2.7–1.6 Ma is primarily a deep‐water temperature signal and that the phase of δ18O precession response changed suddenly at 1.6 Ma.

Oligocene climate dynamics

A planktonic and benthic foraminiferal stable isotope stratigraphy of the Oligocene equatorial Pacific (Ocean Drilling Program, Site 1218) was generated at 6 kyr resolution between magnetochrons C9n and C11n.2n (∼26.4–30 Ma on a newly developed astronomically calibrated timescale). Our data allow a detailed examination of Oligocene paleoceanography, the evolution of the early cryosphere, and the influence of orbital forcing on glacioeustatic sea level variations. Spectral analysis reveals power and coherency for obliquity (40 kyr period) and eccentricity (∼110, 405 kyr) orbital bands, with an additional strong imprint of the eccentricity and 1.2 Myr obliquity amplitude cycle, driving ice sheet oscillations in the Southern Hemisphere. Planktonic and benthic foraminifera δ18O are used to constrain the magnitude and timing of major fluctuations in ice volume and global sea level change. Glacial episodes, related to obliquity and eccentricity variations, occurred at 29.16, 27.91, and 26.76 Ma, corresponding to glacioeustatic sea level fluctuations of 50–65 m. Alteration of high‐latitude temperatures and Antarctic ice volume had a significant impact on the global carbon burial and equatorial productivity, as cyclic variations are also recorded in the carbon isotope signal of planktonic and benthic foraminifera, the water column carbon isotope gradient, and estimated percent carbonate of bulk sediment. We also investigate the implications of a close correspondence between oxygen and carbon isotope events and long‐term amplitude envelope extrema in astronomical calculations during the Oligocene, and develop a new naming scheme for stable isotope events, on the basis of the 405 kyr eccentricity cycle count.

Response of planktonic foraminifera to glacial cycles: Mid-Pleistocene change in the southern South China Sea

A high-resolution study (∼2 ka) of planktonic foraminifers in 997 samples from ODP Site 1143 (9°21.72′N, 113°17.11′E, water depth 2772 m) in the southern South China Sea (SCS) reveals faunal response to glacial cycles in the last 2.1 Ma. Most distinctive is the contrasting variations in the abundance of two tropical–subtropical species that exhibit similar resistance to dissolution in modern oceans. The relative and absolute abundances of Globorotalia menardii are high in interglacials and low in glacials and covary with δ 18O fluctuations over the studied interval. Pulleniatina obliquiloculata, however, follow oxygen isotopes only before the Mid-Pleistocene Revolution (MPR), showing high abundances during glacials and low during interglacials after ∼850 ka. Three abundance maxima of P. obliquiloculata are observed, corresponding to three major glacial episodes, Marine Isotope Stages (MIS) 2, 12, and 16, respectively. This change in abundance patterns of P. obliquiloculata appears to be unique to the southern SCS, as in late Pleistocene records from other western Pacific regions, including the Okinawa Trough and the northern SCS, all show co-varying patterns with δ 18O as G. menardii does. Cross-spectral analyses performed between percentages of Globorotalia menardii and δ 18O records show high coherency at 41 ka obliquity and then at 100 ka eccentricity bands. We suggest that the high abundances of G. menardii during interglacials in the past 2.1 Ma reflect warmer temperatures. Percentages of Pulleniatina obliquiloculata vs. oxygen isotope records show quite large phase difference at 41 ka obliquity band before the MPR. As a contrast, there are negligible phase differences in the vicinity of 100 and 41 ka after the MPR, which means reverse trend between percentages of P. obliquiloculata and δ 18O fluctuations. The differences of P. obliquiloculata from G. menardii in abundance and orbital forcings are interpreted as resulting from a profound change in the regional upper ocean structure during the MPR time, probably relating to a final glacial cut-off of the southern SCS from the Indian water at ∼850 ka and to higher salinity during glacial times after 850 ka. We speculate that this change is related to the decreased connection between the Pacific and Indian Ocean.

The 100Ka Period Problem of Solar Radiation

AbstractWith its successful application to the nonlinear and nonstationary data, the new time series technique Empirical Mode Decomposition (EMD) is introduced in order to gain insight into the paleoclimate cycles in Pleistocene (1MaBP~20kaBP), we hope the EMD result of solar insolations records would increase our knowledge about the climate significance of solar insolations. Our result shows that eccentricity band signal is much larger than previously estimated, i.e. 1% on solar radiations, and comparable with the precession and obliquity band signals. These results should lead us to reconsider the role of solar radiations on the formation and maintenance of Quaternary ice sheets cycles.

Responses of foraminiferal isotopic variations at ODP Site 1143 in the southern South China Sea to orbital forcing

The foraminiferal δ 18 O and δ 13 C records for the past 5 Ma at ODP Site 1143 reveal the linear responses of the Plio-Pleistocene climatic changes in the southern South China Sea to orbital forcing at the obliquity and precession bands. The phase of the δ 18 O variations with the orbital forcing is opposite to that of the δ 13 C, which may be caused by the frequent El Nino events from the equatorial Pacific. The amplification of the Northern Hemisphere Ice Sheet at ~3.3 Ma probably affected the development of the 100-ka climatic cycles. Its further spreading may spur the 100-ka climatic cycle to become the dominant cycle in the late Pleistocene. The “Mid-Pleistocene Transition” event has localized influence on the isotopic variations in the southern South China Sea. The foraminiferal δ 13 C records for the past 5 Ma at Site 1143 are highly coherent with the orbital forcing at the long eccentricity band, and lead the δ 18 O records at the shorter eccentricity band, highlighting the importance of the carbon cycle in the global climate change.

Quantitative estimate of the Milankovitch-forced contribution to observed Quaternary climate change

A number of records commonly described as showing control of climate change by Milankovitch insolation forcing are re-examined. The fraction of the record variance attributable to orbital changes never exceeds 20%. In no case, including a tuned core, do these forcing bands explain the overall behavior of the records. At zero order, all records are consistent with stochastic models of varying complexity with a small superimposed Milankovitch response, mainly in the obliquity band. Evidence cited to support the hypothesis that the 100 Ka glacial/interglacial cycles are controlled by the quasi-periodic insolation forcing is likely indistinguishable from chance, given the small sample size and near-integer ratios of 100 Ka to the precessional periods. At the least, the stochastic background “noise” is likely to be of importance.

A 350,000 year summer-monsoon multi-proxy stack from the Owen Ridge, Northern Arabian Sea

Five summer-monsoon proxies from the Northern Arabian Sea are combined using stacking and principal components analysis (PCA) to create two very similar multi-proxy records of summer-monsoon variability. The five individual proxies all respond to monsoon variability but are largely independent in terms of the processes that complicate their interpretation as summer-monsoon indicators (e.g. preservation, dissolution, diagenesis, sediment reworking). As such, stacking and PCA average out non-monsoon variance, yielding a more pure monsoon signal. These stacked and PCA records (hereafter summer-monsoon stack and summer-monsoon factor) allow evaluation of relative monsoon strength through time as well as the relative concentration of variance within orbital bands; these two parameters are less reliable when estimated from individual proxy records. In fact, the summer-monsoon factor (SMF) accounts for only 33% of the total variance in the five records, suggesting that relative amplitude variations in each individual proxy time series are influenced by non-monsoon processes. The summer-monsoon stack (SMS) and SMF are spectrally very similar, dominated by variance in the 41-k.y. (obliquity) and 23-k.y. (precession) bands; there is very little variance at the 100-k.y. (eccentricity) band associated with large-scale changes in global ice-volume. Indeed, equally strong monsoons occur in both glacial and interglacial intervals. Within the 23-k.y. precession cycle, monsoon maxima fall at −121° relative to precession minima (June 21 perihelion, maximum Northern Hemisphere (NH) summer insolation). This phase falls midway between δ 18O minima (−78°) and December 21 perihelion (−180°) indicating that two mechanisms exert equal influence in determining the timing of strong summer monsoons within the precession band: (1) sensible heating of the Asian Plateau which is maximized at times of ice-volume minima (−78°), and (2) latent heat export from the southern subtropical Indian Ocean which is maximized at times of December 21 perihelion (−180°). The seasonal cycle at December 21 perihelion is characterized by warm Southern Hemisphere (SH) summers followed by cold SH winters, a combination that preconditions the ocean to export latent heat during the boreal summer-monsoon season. Summer-monsoon winds transport this latent heat into Asia where it is released during precipitation, enhancing the Asian monsoon low. Within the 41-k.y. obliquity cycle, monsoon maxima are in phase with obliquity maxima. This indicates that two mechanisms, quite similar to those in the precession band, influence the timing of strong summer-monsoons in the obliquity band: (1) sensible heating of the Asian Plateau but with no ice-volume delay, and (2) latent heat export from the southern subtropical Indian Ocean which is maximized at times of obliquity maxima. Again, the seasonal cycle at obliquity maximum is characterized by warm SH summers followed by cold SH winters, ideal for maximizing latent heat export during the boreal summer monsoon.

Orbital insolation forcing of the Indian Monsoon – a motor for global climate changes?

Both modern and ancient Indian summer monsoons are driven by transequatorial pressure differences, directly coupled with the insolation difference between the Northern and Southern subtropical Hemispheres. A high-resolution record of upwelling and dust flux from the western Arabian Sea resembles an insolation-based Indian Summer Monsoon Index. This index and the observed monsoonal climate variations share major elements on the orbital obliquity and precessional band with the Specmap marine oxygen isotope record, representing global ice volume. The long-term evolution of the index mirrors almost exactly the insolation changes at 65°N, showing that the forcing of low latitude climate variability has a structure similar to that of the insolation forcing in the high northern latitudes. Moreover, insolation forcing in the low latitudes directly controls atmospheric processes in the African, Indian and Asian Monsoon, being responsible for a huge amount of transequatorial water vapour and therefore latent heat transport. Millennial-scale variability of the monsoonal climate is concentrated at periodicities near 1100, 1450, 1750 and 2300 years. These cycles are not strictly periodic, but occur in bands, with specific activity phases and amplitude increases during warm stages and interstadials for the 1100- and 1750-yr cycles, whereas the 1450-yr cycle dominates the cold intervals.

Astronomically tuned Plio–Pleistocene benthic δ18O record from South China Sea and Atlantic–Pacific comparison

Based on benthic foraminiferal δ18O from ODP Site 1143, a 5-Myr astronomical timescale for the West Pacific Plio–Pleistocene was established using an automatic orbital tuning method. The tuned Brunhes/Matuyama paleomagnetic polarity reversal age agrees well with the previously published age of 0.78 Ma. The tuned ages for several planktonic foraminifer bio-events also agree well with published dates, and new ages for some other bio-events in the South China Sea were also estimated. The benthic δ18O from Site 1143 is highly coherent with the Earth’s orbit (ETP) both at the obliquity and precession bands for the last 5 Myr, and at the eccentricity band for the last 2 Myr. In general, the 41-kyr cycle was dominant through the Plio–Pleistocene although the 23-kyr cycle was also very strong. The 100-kyr cycle became dominant only during the last 1 Myr. A comparison of the benthic δ18O between the Atlantic (ODP 659) and the East and West Pacific (846 and 1143) reveals that the Atlantic–Pacific benthic oxygen isotope difference ratio (Δδ18OAtl-Pac) displays an increasing trend in three time intervals: 3.6–2.7 Ma, 2.7–2.1 Ma and 1.5–0.25 Ma. Each of the intervals begins with a rapid negative shift in Δδ18OAtl-Pac, followed by a long period with an increasing trend, corresponding to the growth of the Northern Hemisphere ice sheet. This means that all three intervals of ice sheet growth in the Northern Hemisphere were accompanied at the beginning by a rapid relative warming of deep water in the Atlantic as compared to that of the Pacific, followed by its gradual relative cooling. This general trend, superimposed on the frequent fluctuations with glacial cycles, should yield insights into the processes leading to the boreal glaciation. Cross-spectral analyses of the Δδ18OAtl-Pac with the Earth’s orbit suggests that after the initiation of Northern Hemisphere glaciation at about 2.5 Ma, obliquity rather than precession had become the dominant force controlling the vertical structure or thermohaline circulation in the paleo-ocean.

Astronomical age models for Pleistocene drift sediments from the western North Atlantic (ODP Sites 1055–1063)

Ten ODP sites drilled in a depth transect (2164–4775 m water depth) during Leg 172 recovered high-deposition rate (>20 cm/kyr) sedimentary sections from sediment drifts in the western North Atlantic. For each site an age model covering the past 0.8–0.9 Ma has been developed. The time scales have a resolution of 10–20 kyr and are derived by tuning variations of estimated carbonate content to the orbital parameters precession and obliquity. Based on the similarity in the signature of proxy records and the spectral character of the time series, the sites are divided into two groups: precession cycles are better developed in carbonate records from a group of shallow sites (2164–2975 m water depth, Sites 1055–1058) while the deeper sites (2995–4775 m water depth, Sites 1060–1063) are characterized by higher spectral density in the obliquity band. The resulting time scales show excellent coherence with other dated carbonate and isotope records from low latitudes. Besides the typical Milankovitch cyclicity significant variance of the resulting carbonate time series is concentrated at millennial-scale changes with periods of about 12, 6, 4, 2.5, and 1.5 kyr. Comparisons of carbonate records from the Blake Bahama Outer Ridge and the Bermuda Rise reveal a remarkable similarity in the time and frequency domain indicating a basin-wide uniform sedimentation pattern during the last 0.9 Ma.