Astronomical age models for Pleistocene drift sediments from the western North Atlantic (ODP Sites 1055–1063)
Introduction
The astronomical theory of climate first formulated by Milutin Milankovitch, 1920, Milankovitch, 1941 provided not only deep insights into the mechanisms of global climatic change but also opened the way for the creation of accurate high-resolution geochronologies. After Hays et al. (1976) had shown that variations in the earth’s orbital geometry are responsible for climatic changes which can be documented in sedimentary records, time scales have been developed by correlating paleoclimatic indicators to astronomical forcing functions (e.g. Johnson, 1982, Imbrie et al., 1984, Martinson et al., 1987). Meanwhile, these chronologies have been extended and modified in some cases (Shackleton et al., 1990, Tiedemann et al., 1994, Bassinot et al., 1994, Berger et al., 1995). The astronomical dating technique is called ‘orbital tuning’ and has a theoretical accuracy of a few thousand yr (Martinson et al., 1987). Most of the new age models were created by the correlation of isotopic records (18O–16O in tests of fossil foraminifera) with orbital target curves (e.g. Imbrie et al., 1984, Shackleton et al., 1990). However, if isotopic data were lacking, other climate proxies like bulk density, magnetic susceptibility, or carbonate content have been successfully used (e.g. Shackleton et al., 1995, Bickert et al., 1997, Tiedemann and Franz, 1997, Ortiz et al., 1999). As tuning targets several different forcing functions such as insolation, ice volume models, precession or obliquity have been used. Martinson et al. (1987) tested four different tuning approaches with a stacked isotope data set and demonstrated that the chronology produced is insensitive to the specific tuning technique used.
In this study we present orbitally (precession and obliquity) tuned age models for the Pleistocene (0–0.9 Ma) of ten ODP sites in the western North Atlantic based on carbonate content records. The 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 and provide an archive to study millennial-scale climate changes as well as variations in the depth distribution of Atlantic water masses (Keigwin et al., 1998). The age models developed here are an important prerequisite for detailed comparisons of proxy records from the different locations. In this paper we use them to compare suborbital (<19 kyr) climatic variability at the Blake Bahama Outer Ridge and at the Bermuda Rise.
Section snippets
Materials, data and initial dating
Fig. 1 shows the locations of nine ODP sites drilled during Leg 172 on the Carolina Slope (Site 1055), the Blake Bahama Outer Ridge (Sites 1056 to 1062) and the Bermuda Rise (Site 1063). Multiple holes were drilled at each site so that the continuity of recovery was confirmed by the development of composite depth sections (depth in meters composite depth, mcd) and splices (Shipboard Scientific Party, 1998a). At Site 1062, eight holes were drilled on either flank and into the crest of a mud wave
Astronomical calibration
Spectral analyses of the initial carbonate time series (based on the initial age models) reveal significant power spectral density close to Milankovitch periods of 100, 41, and 19–23 kyr (Fig. 5) confirming that sedimentation changes in the western North Atlantic are linked to variations in earth orbital parameters. This is an important precondition for using the orbital time series of precession and obliquity as target curves to improve the initial age scales. The slight deviation of the
Final time series
All orbital tuned and phase-adjusted carbonate time series are shown in Fig. 8. For glacial episodes average carbonate content is consistently lower at the deeper sites, as expected from both higher dissolution and dilution with depth. Interglacial intervals display a different pattern with frequent carbonate ‘spikes’ at the deeper sites. This is especially evident for MIS 9 and 11. The high carbonate peaks (>90%) observed for MIS 15 at Site 1062 (east and west) do not occur at the other Blake
An application: the detection of sub-Milankovitch climatic variability
The deep water Sites 1061 (4050 m water depth) and 1063 (4570 m water depth) were chosen to study millennial-scale climatic variability in the western North Atlantic. Both sites show very high and variable sedimentation rates (23.4±13.0 cm/kyr at Site 1061 and 19.2±12.0 at Site 1063) over the last 0.9 Ma and allow comparison of the sub-Milankovitch spectral pattern from the Blake Outer Ridge (Site 1061) to that from the Bermuda Rise (Site 1063). The sample interval of 2–4 cm for the carbonate
Acknowledgements
We acknowledge the work done by the crew of the JOIDES Resolution for the acquisition of the cores. This research used samples and data provided by the Ocean Drilling Program (ODP). The ODP is sponsored by the U.S. National Science Foundation (NSF) and participating countries under management of Joint Oceanographic Institutions (JOI), Inc. We thank F.J. Hilgen and one anonymous reviewer for their constructive criticism that significantly improved the paper.
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