Abstract:
Shatsky Rise, located in the Northwest Pacific, is covered with a thick cap of pelagic, carbonate-dominated sediments with a secondary aeolian component. Ocean Drilling Program Leg 132 recovered Pleistocene to Maestrictian oozes at Site 810, near the summit of the rise, using the advanced hydraulic piston corer to achieve virtually complete recovery in the upper 127 meters below the sea floor. The down-core stratigraphy of whole-core magnetic susceptibility displayed peaks with amplitudes of 30-40X10 (super -8) SI and widths of tens of centimeters. Overall, magnetic susceptibility values correlated inversely with calcium carbonate percentages. Similar variations in other deep-sea sediments have been interpreted as records of glacial-interglacial climate cycles. A study of the magnetic properties of the Pleistocene-Pliocene sediments was undertaken to understand the nature of the magnetic susceptibility cycles. The acquisition and demagnetization properties of natural and artificial magnetic remanences were examined. The results indicated that the primary magnetic mineral is titanomagnetite, but there was also a secondary, higher coercivity component, perhaps hematite or goethite. The data also suggested that the magnetic susceptibility peaks resulted from high relative variations in magnetite concentration, and that the size of the magnetic particles is relatively uniform. Scanning electron microscopy and energy dispersive X-ray analyses confirmed that titanomagnetite was the primary magnetic carrier. Magnetic susceptibility peaks from Hole 8 1 OC were correlated to those from Deep Sea Drilling Project Site 577, 11 km to the west and at a water depth that is 50 meters deeper. Peaks in one or the other of the two holes were commonly missing or cannot be correlated. In all, only 35% of the peaks in Hole 81 OC match those in Hole 577. These observations suggest that sedimentation atop Shatsky Rise has been irregular or disturbed by local erosion. Missing features in Hole 577 suggest a greater degree of erosion and perhaps stronger currents at that site. As a consequence of possible erosion at both sites, it may be difficult to obtain continuous sediment sections with adequate time control for paleoclimate studies in this locality.