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We address two key issues relevant to using downhole logs to obtain quantitative paleoclimate data: Tool resolution and the origin of log responses. The vertical resolution of a given logging device sets an upper limit on its temporal resolution, so we present theoretical estimates for the minimum sedimentation rates required to resolve bedding cycles at the primary orbital periodicities using the standard suite of Schlumberger logging tools. We also investigate the sedimentologic origin of the 5- to 6-m bedding cycles within the logging data collected at Hole 798B on the Oki Ridge. Contiguous 30-cm channel samples were taken from Cores 128-798B-13H, -14H, and -15H to mimic tool resolution, and these were analyzed for biogenic opal, carbonate, terrigenous percent, and bulk mineralogy by X-ray diffraction. The 5- to 6-m cycles apparent within the log data are equivalent to -40 k.y. based on the average sedimentation rate (-13 cm/k.y.) of the 1.0- to 2.8-Ma sequence. The core data demonstrate that the log variations reflect periodic changes in opal and terrigenous sedimentation. Much of the log variance is concentrated at the 41-k.y. obliquity band, and there is a close correspondence of the log data with the Pliocene-Pleistocene marine δ 18 θ record. Quartz is highly correlated with terrigenous fraction variations, and the bulk mineralogical assemblage is similar to that of Chinese loess, a probable upwind source of eolian dust. The Chinese loess deposits indicate a linkage between glacial climate and Asian aridity, so the periodic increases in terrigenous concentration may reflect the downwind propagation of this signal. Alternatively, the logs could be reflecting variations in biogenic opal production resulting from glacial-interglacial changes in surface productivity.

Paleoclimatic Applications of Downhole Logs: Pliocene-Pleistocene Results from Hole 798B, Sea of Japan