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40% of the total input of deep water to the world's oceans. Here we use a sedimentary record from the giant piston core MD97-2114 collected on the northern flank of the Chatham Rise located at 1935 m water depth, east of New Zealand, to investigate DWBC variability during the Pleistocene epoch when the period of glacial cycles changed progressively from a 41 kyr to 100 kyr rhythm. Magnetic grain-size may be directly related to orbitally forced fluctuations in the strength of the upper circumpolar deep water (UCDW) through its interaction with terrigenous sediments supplied from the south and west. The long-term trends in magnetic properties are characterized by two main perturbations centered at 870 ka (Marine Isotope Stage, MIS 22) 450 ka (MIS 12), which is broadly consistent with the inferred perturbation during the mid-Pleistocene climate transition based on sedimentological paleocurrent reconstruction from Ocean Drilling Program Site 1123 located at 3290 m water depth in the main core of the DWBC flow on the North Chatham Drift. This similarity suggests that both the upper and middle CDW are modulated by similar processes and fluctuations of Antarctic Bottom Water production could be directly responsible for this deep Pacific Ocean inflow variability over the past 1.2 Ma.
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doi:10.1016/S0025-3227(02)00326-2 
Copyright © 2002 Elsevier Science B.V. All rights reserved.
Changes in grain size and magnetic fabric at Blake–Bahama Outer Ridge during the late Pleistocene (marine isotope stages 8–10)
Received 7 August 2000;
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Abstract
ODP Sites 1055–1062 on the Blake-Bahama Outer Ridge (BBOR) are characterized by high sedimentation rates, which allow high-resolution analyses to reconstruct the behavior of the Deep Western Boundary Current (DWBC) as a function of depth along the BBOR. The ODP sites provide an intermediate and deep water transect from 1800 to 4760 m water depths. We focused on the time interval from 350 to 250 ka (marine isotope stages (MIS) 10.2–8.3). Grain size and magnetic properties were analyzed with centennial- to millennial-scale time resolution. Integrating the results of grain size analyses, lightness, and magnetic properties, the changes in the vertical position and the intensity of the DWBC core were inferred. During glacial periods (MIS 10.2 and 8.4–8.3) the DWBC core was located at around 2200 m water depth, whereas it deepened to 3000 m or more during warm periods (interglacial stage 9.3 and interstadial 8.5). Between these periods, the DWBC core moved to shallower depth (2500–3000 m) and its intensity increased, which generally provided coarser sediments to a broad depth range from 2100 to 4800 m. Furthermore, the magnetic properties imply that the DWBC supplied carbonate material other than terrigenous sediments to the deeper sites during MIS 9. The depositional processes at Site 1062 are characterized by sudden influxes of fine carbonate-rich sediments, which gradually decreased with time. Paleocurrent directions inferred from the direction of the great axis of the magnetic fabric are consistent with the modern topography at Sites 1055, 1056, 1058 and 1060 during the relatively warm period, from stages 9.3 to 8.5. At Site 1062, the deepest site, which exhibits a mud-wave field, the inferred flow direction is parallel to the bathymetric contours during warm periods, while the nearly cross-wave direction seemingly dominated during the other periods. It appears that topographic position may have affected the scattering of azimuth plots. Moreover, there is a possibility that the changes in flow direction correlate with climate changes.
Author Keywords: Ocean Drilling Program; Blake–Bahama Outer Ridge; late Pleistocene; grain size; anisotropy of magnetic susceptibility; Deep Western Boundary Current
Article Outline
- 1. Introduction
- 2. Methods
- 2.1. Sampling
- 2.2. Sample processing and grain size measurements
- 2.3. Weight percent of the silt fraction as a proxy for current intensity
- 2.4. Measurements and data processing of AMS
- 3. Results
- 3.1. Grain size
- 3.2. Magnetic properties
- 3.2.1. Depth and time variation of mean magnetic susceptibility and relationship with color reflectance
- 3.2.2. Relationships between mean magnetic susceptibility and corrected anisotropy degree (p′)
- 3.2.3. Temporal variations in shape parameter (T)
- 3.2.4. The azimuth of the maximum axis of the anisotropy ellipsoid (Kmax)
- 4. Discussion
- 4.1. Intensity and vertical migration of the DWBC inferred from grain size variations
- 4.2. Corrected anisotropy degree, p′: an additional proxy for current intensity?
- 4.3. Depositional processes at Site 1062 during the warm time interval from MIS 9.3 to 8.5
- 4.4. Shape parameter T and carbonate influx to the deep sites
- 4.5. Comparison of inferred paleoflow direction with modern measurements and topography
- 5. Conclusions
- Acknowledgements
- References
