Changes in grain size and magnetic fabric at Blake–Bahama Outer Ridge during the late Pleistocene (marine isotope stages 8–10)

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.