Downhole Logs as Paleoclimate Tools: A Case Study from Ocean Drilling Project Leg 128, Sea of Japan

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The primary goal of this proposal is to reconstruct continuous, quantitative biogenic and terrigenous deposition records from downhole logs and core data at ODP Site 798 on the Oki Ridge, Sea of Japan. We employ an innovative and efficient research strategy whereby continuous, multivariable log data are calibrated using "ground-truth" sediment lithological information obtained from cores. Site 798 was the Leg 128 target for a complete late Neogene paleoceanographic "reference" section for this climatically-important region; however, sporatic core recovery precludes a solely core-based reconstruction of these records below ca. 1 Myr. We describe separate core-based and log-based lines of research whose products will be merged to reconstruct sediment component percentage and flux records for the last 3 Myr. Discrete core sections are analyzed for biogenic opal, carbonate, organic carbon, and terrigenous percent using contiguous 30-cm "channel" samples which mimic log resolution. Ash layers provide accurate core-log depth correlations, and oxygen isotopic data are used to define glacial-interglacial stratigraphy, sedimentation rates, and phasing relationships. New computational techniques developed by Schlumberger-Doll Research are used to enhance the log resolution and sensitivity. These and other statistical techniques are used to quantitatively relate specific electrical, chemical, and physical log responses to the measured core lithological variations. The continuous percentage and flux records developed from this research provide us with an unique opportunity to: 1) Evaluate the utility of downhole logging methods to paleoclimate research, and 2) Understand the Pliocene-Pleistocene evolution of Japan Sea paleoceanography and paleoclimate within the context of other global climate records.