Orbital Chronology Research Articles

Drivers of phytoplankton community structure change with ecosystem ontogeny during the Quaternary

Freshwater species are particularly sensitive to climate fluctuations, but little is known of their response to the large-scale environmental change that took place during the Quaternary. This is partly due to the scarcity of continuously preserved freshwater sedimentary records with orbital chronology. We use a 1.363 Ma high-resolution fossil record of planktonic diatoms from ancient Lake Ohrid to evaluate the role of global and regional versus local-scale environmental change in driving temporal community dynamics. By using a Bayesian joint species distribution model, we found that communities were mostly driven by the local-scale environment. Its effects decreased over time, becoming less important than global and regional environment at the onset of the penultimate glacial, 0.183 Ma. Global and regional control over the environment became important with successive deepening of the lake at around 1.0 Ma, and its influence remained persistent until the present. Our high-resolution data demonstrate the critical role of lake depth and its thermal dynamics in determining phytoplankton response to environmental change by influencing lake mixing, nutrient and light availability.With this study we demonstrate the relative impact of various environmental factors and their scale-dependant effect on the phytoplankton communities during the Quaternary, emphasizing the importance of not only considering climate fluctuations in driving their structure and temporal dynamics but also the local environment.

A late Pliocene to early Pleistocene (3.3–2.1 Ma) orbital chronology for the Qaidam Basin paleolake (NE Tibetan Plateau) based on the SG-1b drillcore record

A late Pliocene to early Pleistocene (3.3–2.1 Ma) orbital chronology for the Qaidam Basin paleolake (NE Tibetan Plateau) based on the SG-1b drillcore record

Cyclostratigraphic analysis of magnetic records for orbital chronology of the Lower Cretaceous Xiagou Formation in Linze, northwestern China

In this study, we report a tuned astronomical time scale for a sedimentary formation in Early Cretaceous – the lower Linze Section located in the Zhangye Basin of the central Hexi Corridor, northwestern China. Based on detailed field investigations, we identified this sediment as the Xiagou Formation, which is widely distributed in the region. A thick, prominent limestone layer crops out in the middle portion of the Linze Section, and can be correlated to a similar limestone layer in the Liupanshan Group, which has been dated via high-resolution magnetostratigraphy. This correlation places an age constraint of around 114Ma for the top of the underlying strata. A total of 1245 samples were collected from the 474-m-thick lower Linze Section, for magnetic susceptibility measurements and cyclostratigraphic analysis. Spectral and continuous wavelet analyses were applied to the low-frequency magnetic susceptibility series and the percentage frequency-dependent magnetic susceptibility series, as well as to specific subsections exhibiting visible rhythmic bedding. A hierarchy of sedimentary cycles was identified, with wavelengths of 21.5–37.0m, 7.5–9.5m, 2.1–4.4m, and 1.4–1.7m. The ratios between these lengths correspond well to the ratios between the durations of Earth's orbital long-eccentricity, short-eccentricity, obliquity, and precession cycles. We interpret these sedimentary cycles to reflect the effects of orbital forcing on sedimentary processes. Using band-pass filtering to specifically extract eccentricity cycles, we established a tuned astronomical time scale for the Xiagou Formation in the lower Linze Section, with a total duration of about 6.1Myr (114.1Ma to 120.2Ma). Based on this timescale, the sedimentation rate of the Xiagou Formation is estimated to be in the range of 62–105m/Myr, with most intervals yielding rates in the 70–80m/Myr range.

Orbital chronology of the Pliensbachian – Toarcian transition from the Central High Atlas Basin (Morocco)

The Pliensbachian-Toarcian transition has been studied in depth for the major environmental changes and the marine invertebrate biodiversity crisis observed at that time. Despite a large number of studies performed, the time frames provided for this interval show large discrepancies from author to author. Major environmental changes occurring at that time impacted the sedimentation pattern and sedimentation rates, making uncertain the construction of time scales. Here, we provide a new astrochronological framework based on an expanded hemipelagic section from the Central High Atlas Basin (Morocco). delta C-13 and CaCO3 measurements performed on the studied section allowed the construction of an orbital time scale based on the 405-kyr eccentricity and the obliquity cycles. This orbital time scale is then compared to the Peniche section (GSSP of the Toarcian Stage) in order to limit the effects of eventual condensation and erosion events on the construction of the orbital time scale. The duration of the early Toarcian Polymorphum Zone is then assessed at 0.9-1.0 myr, while the interval from the base of the Toarcian Stage to the FO of C. superbus is assessed at 0.51 myr. This long duration of the Polymorphum Zone highlights the fact that numerous sections in Europe are affected by long-term condensation and hiati around the Pliensbachian Toarcian transition. Finally, we also explore the potential of the Central High Atlas basin to provide a refined time scale for the late Pliensbachian. Preliminary data lead to a duration assessment of the NJ5b calcareous nannofossil zone of at least 2.07 myr, and to a mean duration of the Emaciatum ammonite zone of 2.05 myr. These durations are similar to 1 myr longer than the durations proposed in the Geological Time Scale 2012, which illustrates the potential of the Central High Atlas Basin for calibrating the Pliensbachian times.

Astronomical calibration of the Danian stage (Early Paleocene) revisited: Settling chronologies of sedimentary records across the Atlantic and Pacific Oceans

Astronomical tuning of sedimentary records to precise orbital solutions has led to unprecedented resolution in the geological time scale. However, the construction of a consistent astronomical time scale for the Paleocene is controversial due to uncertainties in the recognition of the exact number of 405-kyr eccentricity cycles and accurate correlation between key records. Here, we present a new Danian integrated stratigraphic framework using the land-based Zumaia and Sopelana hemipelagic sections from the Basque Basin and deep-sea records drilled during Ocean Drilling Program (ODP) Legs 198 (Shatsky Rise, North Pacific) and 208 (Walvis Ridge, South Atlantic) that solves previous discrepancies. The new coherent stratigraphy utilises composite images from ODP cores, a new whole-rock δ13C isotope record at Zumaia and new magnetostratigraphic data from Sopelana.We consistently observe 11 405-kyr eccentricity cycles in all studied Danian successions. We achieve a robust correlation of bioevents and stable isotope events between all studied sections at the ∼100-kyr short-eccentricity level, a prerequisite for paleoclimatic interpretations. Comparison with and subsequent tuning of the records to the latest orbital solution La2011 provides astronomically calibrated ages of 66.022±0.040 Ma and 61.607±0.040 Ma for the Cretaceous–Paleogene (K–Pg) and Danian–Selandian (D–S) boundaries respectively. Low sedimentation rates appear common in all records in the mid-Danian interval, including conspicuous condensed intervals in the oceanic records that in the past have hampered the proper identification of cycles. The comprehensive interbasinal approach applied here reveals pitfalls in time scale construction, filtering techniques in particular, and indicates that some caution and scrutiny has to be applied when building orbital chronologies. Finally, the Zumaia section, already hosting the Selandian Global Boundary Stratotype Section and Point (GSSP), could serve as the global Danian unit stratotype in the future.

An optimized multi-proxy, multi-site Antarctic ice and gas orbital chronology (AICC2012): 120–800 ka

Abstract. An accurate and coherent chronological framework is essential for the interpretation of climatic and environmental records obtained from deep polar ice cores. Until now, one common ice core age scale had been developed based on an inverse dating method (Datice), combining glaciological modelling with absolute and stratigraphic markers between 4 ice cores covering the last 50 ka (thousands of years before present) (Lemieux-Dudon et al., 2010). In this paper, together with the companion paper of Veres et al. (2013), we present an extension of this work back to 800 ka for the NGRIP, TALDICE, EDML, Vostok and EDC ice cores using an improved version of the Datice tool. The AICC2012 (Antarctic Ice Core Chronology 2012) chronology includes numerous new gas and ice stratigraphic links as well as improved evaluation of background and associated variance scenarios. This paper concentrates on the long timescales between 120–800 ka. In this framework, new measurements of δ18Oatm over Marine Isotope Stage (MIS) 11–12 on EDC and a complete δ18Oatm record of the TALDICE ice cores permit us to derive additional orbital gas age constraints. The coherency of the different orbitally deduced ages (from δ18Oatm, δO2/N2 and air content) has been verified before implementation in AICC2012. The new chronology is now independent of other archives and shows only small differences, most of the time within the original uncertainty range calculated by Datice, when compared with the previous ice core reference age scale EDC3, the Dome F chronology, or using a comparison between speleothems and methane. For instance, the largest deviation between AICC2012 and EDC3 (5.4 ka) is obtained around MIS 12. Despite significant modifications of the chronological constraints around MIS 5, now independent of speleothem records in AICC2012, the date of Termination II is very close to the EDC3 one.

Orbital chronology of Early Eocene hyperthermals from the Contessa Road section, central Italy

High-resolution geochemical analyses of the Lower Eocene Contessa Road section (Italy) reveal orbitally controlled fluctuations in the percent concentration of calcium carbonate (wt.% CaCO 3) that include the ETM2 (Elmo) and ETM3 (“X”) hyperthermal events. Patterns of increased dissolution, negative carbon isotope excursions, and warmer global climates are intimately linked to maxima in insolation, through the global carbon cycle. Extraction of short- and long-eccentricity orbital periodicities of the wt.% CaCO 3 record provides a relative cyclochronology for the interval ranging from ∼ 52 to ∼ 55.5 Ma. The Contessa Road section is easily accessible and offers a continuous integrated stratigraphic record (stable isotopes, standard calcareous plankton biostratigraphy, magnetostratigraphy, and cyclostratigraphy), thus providing a potential type succession for the study of Early Eocene hyperthermals.

Toward an orbital chronology for the early Aptian Oceanic Anoxic Event (OAE1a, ~ 120 Ma)

The early Aptian Oceanic Anoxic Event (OAE1a, 120 Ma) represents a geologically brief time interval in the mid-Cretaceous greenhouse world that is characterized by increased organic carbon accumulation in marine sediments, sudden biotic changes, and abrupt carbon-isotope excursions indicative of significant perturbations to global carbon cycling. The brevity of these drastic environmental changes (< 10 6 year) and the typically 10 6 year temporal resolution of the available chronologies, however, represent a critical gap in our knowledge of OAE1a. We have conducted a high-resolution investigation of three widely distributed sections, including the Cismon APTICORE in Italy, Santa Rosa Canyon in northeastern Mexico, and Deep Sea Drilling Project (DSDP) Site 398 off the Iberian margin in the North Atlantic Ocean, which represent a range of depositional environments where condensed and moderately expanded OAE1a intervals are recorded. The objectives of this study are to establish orbital chronologies for these sections and to construct a common, high-resolution timescale for OAE1a. Spectral analyses of the closely-spaced (corresponding to ~ 5 to 10 kyr) measurements of calcium carbonate content of the APTICORE, magnetic susceptibility (MS) and anhysteretic remanent magnetization (ARM) of the Santa Rosa samples, and MS, ARM and ARM/IRM, where IRM is isothermal remanent magnetization, of Site 398 samples reveal statistically significant cycles. These cycles exhibit periodicity ratios and modulation patterns similar to those of the mid-Cretaceous orbital cycles, suggesting that orbital variations may have modulated depositional processes. Orbital control allows us to estimate the duration of unique, globally identifiable stages of OAE1a. Although OAE1a had a duration of ~ 1.0 to 1.3 Myr, the initial perturbation represented by the negative carbon-isotope excursion was rapid, lasting for ~ 27–44 kyr. This estimate could serve as a basis for constraining triggering mechanisms for OAE1a.

Early response of continental Asia to Pleistocene climate cycles ( retuned orbital chronologies for Baikal, marine, and ice core records)

Assuming orbital modulation of Pleistocene climate cycles, we have generated a new time scale for the Asian geochemical limnic record in the BDP-96-2 Baikal and the KDP-01 Hovsgol cores and updated the chronologies for the global marine δ 18O and Vostok ice-gas records. The time scales were obtained by orbital tuning with the assumption of arbitrary but time invariable amplitudes and phase lags of the orbital parameters and responses. The retuned chronologies highlighted the cycles of eccentricity (100 kyr), obliquity (41 kyr), and precession (23 and 19 kyr), but the combined 70- and 30-kyr cycles became less prominent in the continental (Baikal) record though persisted in the global data (Vostok δD). The residual 70- and 30-kyr harmonics more likely result from errors in the untuned chronology for the Baikal record but are rather due to nonlinearity in the climate response. We investigated the leads and lags of orbital climate signals with a special focus on the 100-kyr cycle. The phases of precession, obliquity, and eccentricity cycles, compared among the records, showed the lead of the continent. The Baikal geochemical signal at the 100-kyr band led the global glacial and greenhouse CH 4 responses and was almost synchronous with the earliest responding polar temperature signal. The reported results characterize the continent as a system highly responsive to eccentricity variations but do not contradict alternative hypotheses for the origin of the 100-kyr cycle in the Earth’s climate history.

On the duration of the Paleocene‐Eocene thermal maximum (PETM)

The Paleocene‐Eocene thermal maximum (PETM) is one of the best known examples of a transient climate perturbation, associated with a brief, but intense, interval of global warming and a massive perturbation of the global carbon cycle from injection of isotopically light carbon into the ocean‐atmosphere system. One key to quantifying the mass of carbon released, identifying the source(s), and understanding the ultimate fate of this carbon is to develop high‐resolution age models. Two independent strategies have been employed, cycle stratigraphy and analysis of extraterrestrial helium (HeET), both of which were first tested on Ocean Drilling Program (ODP) Site 690. These two methods are in agreement for the onset of the PETM and initial recovery, or the clay layer (“main body”), but seem to differ in the final recovery phase of the event above the clay layer, where the carbonate contents rise and carbon isotope values return toward background values. Here we present a state‐of‐the‐art age model for the PETM derived from a new orbital chronology developed with cycle stratigraphic records from sites drilled during ODP Leg 208 (Walvis Ridge, Southeastern Atlantic) integrated with published records from Site 690 (Weddell Sea, Southern Ocean, ODP Leg 113). During Leg 208, five Paleocene‐Eocene (P‐E) boundary sections (Sites 1262 to 1267) were recovered in multiple holes over a depth transect of more than 2200 m at the Walvis Ridge, yielding the first stratigraphically complete P‐E deep‐sea sequence with moderate to relatively high sedimentation rates (1 to 3 cm/ka, where “a” is years). A detailed chronology was developed with nondestructive X‐ray fluorescence (XRF) core scanning records on the scale of precession cycles, with a total duration of the PETM now estimated to be ∼170 ka. The revised cycle stratigraphic record confirms original estimates for the duration of the onset and initial recovery but suggests a new duration for the final recovery that is intermediate to the previous estimates by cycle stratigraphy and HeET.

Sequence stratigraphy and orbital cyclostratigraphy of the Mooreville Chalk (Santonian–Campanian), northeastern Gulf of Mexico area, USA

Cycles from the depositional sequence level to decimeter-scale rhythmic alteration of chalk and marl beds in the Mooreville Chalk (Santonian–Campanian) in the northeastern Gulf of Mexico area are studied employing an integrated biostratigraphic, cyclostratigraphic, and sequence stratigraphic approach. Two dominant frequencies, identified from the natural gamma-ray log from the Alabama Power Selma Site Test well, site 3 hole 3, are attributed to two of the Milankovitch cycles, the long eccentricity (413 ka) and eccentricity (100 ka) cycles. Parasequences, the building blocks of systems tracts, appear to be modulated by long eccentricity cycles. Individual eccentricity cycles were counted to establish a high-resolution orbital chronology, which agrees with established biostratigraphic chronology. A high-resolution record of temporal changes in sedimentation rate is established on the basis of cyclostratigraphic chronology. Two condensed sections are identified. The lower condensed section is found to correspond to a downlap surface recognized in seismic data in the offshore Alabama and Mississippi area. This condensed section and downlap surface are interpreted to represent the maximum flooding event of a third-order depositional sequence. The maximum flooding event is also represented by a marine flooding surface in eastern Mississippi and by the Mooreville tongue in eastern Alabama. The upper condensed section is found to correspond to a faunal abundance peak recognized in data from outcrop studies, and is interpreted to represent the maximum bathometric surface that was achieved as a result of a condition of rising sea level in association with low sediment input during the deposition of the early highstand systems tract.

The Gargasian (Middle Aptian) of La Marcouline section at Cassis-La Bédoule (SE France): Stable isotope record and orbital cyclicity

Bulk rock stable isotope analysis of La Marcouline section (Cassis-La Bedoule area, SE France) revealed a general trend of decreasing δ13C and δ18O values from the bottom to the top of the section. The decrease in δ13C values reflects a global trend in Middle Aptian times, namely a return to pre-excursion values of δ13C values following a major, positive excursion in the Early Aptian, which is a reflection of the Oceanic Anoxic Event (OAE) 1a. Power spectra of the bedding rhythms suggest that precessional and long eccentricity cycles of the Milankovitch band controlled the deposition of marl-limestone bundles. This can be interpreted as the result of a forcing by insolation at low latitudes which resulted in a monsoon-controlled precipitation pattern, that in turn induced the deposition of clay-rich beds. Conversely, limestone beds were formed in periods of dryer climate. Higher sea-surface productivity during wet periods may have been caused by an increase in continental runoff and a consequent enhancement in the delivery of nutrients to epicontinental basins. An orbital cyclicity in the sedimentary patterns in La Marcouline section provides the opportunity to calibrate the duration of the well-established G. ferreolensis foraminiferal Zone with that of orbital chronology. The G. ferreolensis Zone at Cassis-La Bedoule spans 33 precessional cycles and its duration is thus estimated to be approximately 760 ka. This length of time is significantly shorter than the estimates of published timescales for this zone and thus may be an argument for proposing that the Gargasian substage is significantly shorter than its currently accepted range.

Lower Cretaceous stage durations combining radiometric data and orbital chronology: Towards a more stable relative time scale?

We propose an alternative calibration of Lower Cretaceous stage durations constrained by direct absolute dating of each stage combined with orbital chronology. Ten glauconitic horizons sampled in the Vocontian basin (SE, France) from the base of the Lower Hauterivian to Upper Albian, yielded K–Ar ages from 123.3 ± 1.7 Ma to 96.9 ± 1.4 Ma, respectively. The relative duration of each stage has been derived by cyclostratigraphy previously obtained in the south-east France and central and south Italy basins. Using the GL-O standard from the Albian–Cenomanian boundary at 95.3 Ma as the anchor point, a cyclostratigraphic age for each stage boundaries has been extrapolated and thus compared with the K–Ar ages. This shows a very well-defined linear correlation which demonstrates the robustness of the proposed durations of the Lower Cretaceous stages. The estimated durations are 5.3 ± 0.4 my, 5.1 ± 0.3 my, 6.8 ± 0.4 my and 11.6 ± 0.2 my for the Hauterivian, Barremian, Aptian and Albian stages, respectively. It also shows that glauconite minerals are powerful radiochronometric tools, when precisely stratigraphically defined and carefully selected. Moreover, the large discrepancy of the estimated Aptian duration of more than 6 my between the most recent published time scale and this study highlights the problem of the radiometric calibration of the M-0 magnetic chron. Finally, the stage durations and boundary ages proposed here bring strong constraints towards the calibration of the Lower Cretaceous time scale. Such accurate temporal calibration is required before any relationship between major biological crises and magmatic emplacement, for instance, could be further investigated.

Orbital climate forcing of δ13C excursions in the late Paleocene–early Eocene (chrons C24n–C25n)

High‐resolution stable carbon isotope records for upper Paleocene–lower Eocene sections at Ocean Drilling Program Sites 1051 and 690 and Deep Sea Drilling Project Sites 550 and 577 show numerous rapid (40–60 kyr duration) negative excursions of up to 1‰. We demonstrate that these transient decreases are the expected result of nonlinear insolation forcing of the carbon cycle in the context of a long carbon residence time. The transients occur at maxima in Earth's orbital eccentricity, which result in high‐amplitude variations in insolation due to forcing by climatic precession. The construction of accurate orbital chronologies for geologic sections older than ∼35 Ma relies on identifying a high‐fidelity recorder of variations in Earth's orbital eccentricity. We use the carbon isotope records as such a recorder, establishing a robust orbitally tuned chronology for latest Paleocene–earliest Eocene events. Moreover, the transient decreases provide a means of precise correlation among the four sites that is independent of magnetostratigraphic and biostratigraphic data at the &lt;105‐year scale. While the eccentricity‐controlled transient decreases bear some resemblance to the much larger‐amplitude carbon isotope excursion (CIE) that marks the Paleocene/Eocene boundary, the latter event is found to occur near a minimum in the ∼400‐kyr eccentricity cycle. Thus the CIE occurred during a time of minimal variability in insolation, the dominant mechanism for forcing climate change on 104‐year scales. We argue that this is inconsistent with mechanisms that rely on a threshold climate event to trigger the Paleocene/Eocene thermal maximum since any threshold would more likely be crossed during a period of high‐amplitude climate variations.

New chronology for the late Paleocene thermal maximum and its environmental implications

The late Paleocene thermal maximum (LPTM) is associated with a brief, but intense, interval of global warming and a massive perturbation of the global carbon cycle. We have developed a new orbital chronology for Ocean Drilling Program (ODP) Site 690 (Weddell Sea, Southern Ocean) by using spectral analysis of high-resolution geochemical records. The LPTM interval spans 11 precessional cycles yielding a duration of 210 to 220 k.y. The δ 13 C anomaly associated with the LPTM has a magnitude of about −2.5‰ to −3‰; we show that about −2‰ of the excursion occurs within two steps that each were less than 1000 yr in duration. The remainder developed through a series of steps over ∼52 k.y. The timing of these steps is consistent with a series of nearly catastrophic releases of methane from gas hydrates, punctuated by intervals of relative equilibria between hydrate dissociation and carbon burial. Further, we are able to correlate the records between ODP Sites 690 and 1051 (western North Atlantic) on the scale of 21 k.y. cycles, which demonstrates that the details of the δ 13 C excursion are recognizable between distant sites. Comparison of cycle records at Sites 690 and 1051 suggests that sediment representing the interval ∼30 k.y. just prior to and at the onset of the LPTM are missing in the latter location. This unconformity probably resulted from slope failure accompanying methane hydrate dissociation within 10 k.y. of the start of the LPTM.

Toward a composite orbital chronology for the Late Cretaceous and Early Palaeocene GPTS

Drill sites recovered from the South Atlantic by the Deep Sea Drilling Project provide excellent magnetostratigraphies for the time–interval of polarity chrons C33r through C28n (83 to 62.5 Ma: Cande and Kent time–scale). Cyclic patterns of carbonate sedimentation, with a mean repeat time of ca . 20 ka, can be correlated between sites. The cycles show the full hierarchy of eccentricity amplitude modulations that would be expected of precessional orbital forcing. A significant modulation component exists at ca . 400 ka, which in all likelihood matches the modern 404 ka repeat time for eccentricity. This modulation pattern may provide the ‘tuning fork’ for tying cyclical sedimentation to the most stable astronomical eccentricity period. A ca . 2.5 Ma modulation also emerges that seems surprisingly similar to the modern long–wavelength eccentricity modulation of precessional amplitude, which should average 2.425 Ma. Results of ‘cyclochronology’ of polarity chrons C29n to C31n suggest that the Cande and Kent time–scale will need to be revised to allow for a more gentle change in South Atlantic sea–floor spreading than modelled by those authors.

Sonostratigraphy of tropical Indian Ocean giant piston cores: toward a rapid and high-resolution tool for tracking dissolution cycles in Pleistocene carbonate sediments

During the seymama expedition of the French R/V Marion Dufresne in the equatorial Indian Ocean, we retrieved giant piston cores (30–53 m long) as part of a high resolution palaeo-oceanographic and stratigraphic study of Pliocene-Pleistocene pelagic carbonates. Major changes in the compressional wave (P wave) velocity profiles recorded in these cores appear to be correlatable from the Madingley Rise (western equatorial Indian Ocean) to the southeast of the Maldives archipelago (central equatorial Indian Ocean), about 1700 km away, thus emphasizing the stratigraphic potential of acoustic records in uncemented pelagic carbonates. As expected in deep-sea carbonate deposits, changes in P-wave velocity parallel past changes in coarse fraction content (> 63 μm). Changes in grain size appear to be mainly controlled by carbonate dissolution, as evidenced by a strong relationship between sand content and a foraminifer preservation index. Thus, in uncemented pelagic carbonates, P-wave velocities provide quick and easy to obtain qualitative information on carbonate dissolution pulses. As diagenesis takes place, however, compaction and cementation change the dynamic rigidity (μ) of the sediments and may conceal the original grain size signal. Due to the strong positive relationship between P-wave velocity and coarse fraction content in uncemented pelagic carbonates, P-wave velocity profiles can be tied to a precise chronologic framework by correlating them to the composite grain size index curve (CGSI) established by Bassinot et al. for the tropical Indian Ocean [1,2]. This composite curve has been constructed by stacking the normalized coarse fraction records from ODP Site 722 (Owen Ridge, Arabian Sea [3]) and ODP Site 758 (Ninetyeast Ridge, central equatorial Indian Ocean [4]). In these two sites, detailed δ 18O records provide the basis for precise inter-site correlations. They ensure the accuracy of the stacking procedure, which tends to reduce most of the local grain size signals and enhances the regional signal related to carbonate dissolution pulses [1,2]. A detailed chronostratigraphy of CGSI curve was developed by correlating the δ 18O records of Sites 722 and 758 to the orbital chronology recently developed from ODP Site 677 [5]. The CGSI may be used as a reference curve for developing a sonostratigraphy in the tropical Indian Ocean.

Towards a high-resolution, global, deep-sea chronology for the last 750,000 years

Variations in the ratio of 18O/ 16O as measured in shells of marine calcareous microfossils are primarily dominated by changes in global ice volume; hence these variations provide a set of global time lines in deep-sea sediments. It is likely that the timing of major changes in oxygen isotope values is strongly influenced, if not controlled, by variations in the geometry of the Earth's orbit. Since the variation of orbital parameters can be accurately calculated, the opportunity exists for transforming this orbital chronology into a geological chronology. Through careful correlation of oxygen isotope records in a set of deep-sea cores from the sub-Antarctic, South Atlantic and equatorial Pacific, we have assembled a composite isotopic section spanning the last 750,000 years with an average sedimentation rate of 2.3 cm/1000 years. A new chronology for this time period was developed by adjusting the ages of the oxygen isotope stage boundaries in this composite section so as to extend the consistent phase relationships that exist between variations in oxygen isotope ratios and changes in obliquity and precession during the last 300,000 years to the entire 750,000-year record. Previously identified difficulties in phase locking precession with the filtered isotopic signal between 365,000 and 465,000 years B.P. have been resolved with the recognition that precessional variations have an average period of 19,000 years and not 23,000 years during this interval. Since this new age model yields the best match between variations in obliquity and precession and their corresponding frequency components in the oxygen isotope record, we believe that it presents the most accurate chronology yet developed for deep-sea sediments. With this new age model providing the time control, power spectral analyses of South Atlantic and sub-Antarctic chemical and biotic indices show that there is a strong tendency for variance to be concentrated at frequencies corresponding to periods of ∼ 100,000, 41,000 and 23,000 years.