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

Abstract

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.

Expedition 356 summary

“…These authors correlated the gamma profile from ODP Site 1119 to oxygen isotope data to achieve a millennial-scale record of New Zealand upper slope sedimentation for the last 3.9 My. This type of wireline analysis has also revealed Pliocene-Pleistocene 40 ky-scale climate variability in the Japan Sea (deMenocal et al, 1992).…”

Neogene changes in Southern Ocean sedimentation based on mass accumulation rates at four continental margins

“…Stepwise multiple regression is an alternative multivariate statistical method that has been used in the analysis of log data [e.g., deMenocal et al , 1992] as well as to invert color reflectance data for carbonate [e.g., Balsam and Deaton , 1996; Ortiz et al , 1999; Millwood et al , 2002]. Stepwise multiple regression produces a carbonate proxy for our data with slightly improved R 2 values, but with almost identical predicted values (Figure 3).…”

Cenozoic mass accumulation rates in the equatorial Pacific based on high‐resolution mineralogy of Ocean Drilling Program Leg 199

“…Carbonate percentage has been calculated from MST bulk density [ Mayer , 1991; Herbert and Mayer , 1991; Hagelberg et al , 1995]; from light reflectance spectroscopy [ Balsam and Deaton , 1991, 1996; Schneider et al , 1995; Ortiz et al , 1999b; Balsam et al , 1999]; and from both these and downhole logs [ Handwerger and Jarrard , 2003]. Concentrations of opal, carbonate, and terrigenous materials were calculated from downhole logs [ deMenocal et al , 1992]. The Leg 199 calculations of mineralogy from physical properties by Vanden Berg and Jarrard (submitted manuscript, 2004) differ from previous studies in two significant respects.…”

“…The Leg 199 calculations of mineralogy from physical properties by Vanden Berg and Jarrard (submitted manuscript, 2004) differ from previous studies in two significant respects. First, they quantify four components, whereas most previous analyses distinguished only two components (but see deMenocal et al [1992] and Balsam and Damuth [2000]). Second, their VNIS‐based mineralogy provides an order of magnitude more ground truth data than are usually available to calibrate the conversion from physical properties to mineralogy.…”