(Table 1) Ages and depths of magnetostratigraphic boundaries in ODP Hole 132-810C

A magnetic stratigraphy study was undertaken on Hole 810C cores to provide age constraints and to characterize sedimentation on the summit of Shatsky Rise. Sediments from this hole are mostly nannofossil oozes recovered by the advanced hydraulic piston corer which gave virtually complete recovery to 136 m below seafloor. High-resolution magnetic measurements were made aboard ship with a pass-through cryogenic magnetometer, and these results were confirmed with discrete samples also measured with a cryogenic magnetometer. The sediments give clearly defined magnetic polarity zones within the upper 100 m below seafloor. It was possible to make a convincing match with the polarity reversal time scale from the surface to 76.8 m below seafloor, through the base of the Gilbert Chron near the Pliocene/Miocene boundary. Because of slow sedimentation and hiatuses punctuating the middle to upper Miocene section, the match with the time scale is less certain below this depth. For core recovered beneath 100 m below seafloor, it was mostly impossible to define clear polarity zones, except for a section of reversed polarity in Maastrichtian sediments, from 113.1 to 116.4 m below seafloor, that is tentatively correlated with Chron 31r. The magnetic stratigraphy shows that sediment accumulation was slow (about 2.5 m/m.y.) during the late Miocene, but increased to 15.0 to 18.5 m/m.y. during the late Pliocene and Pleistocene. Missing sections within the Matuyama Chron and an extended Brunhes Chron suggest that actual sediment accumulation rates were at times higher and lower than these averages. Furthermore, differences in sediment accumulation between Site 810 and nearby Site 577 suggest that the normal pelagic sedimentation regime atop Shatsky Rise is perturbed by bottom currents resulting in locally patchy sediment accumulation