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.

A New Zealand climatic template back to c. 3.9 Ma: ODP Site 1119, Canterbury Bight, south‐west Pacific Ocean, and its relationship to onland successions

“…This, and the often micaceous nature of the muds (Griggs et al 1983), are consistent with the gamma ray variations representing changes in the amount of detrital mica and clay present. A cyclic signature is apparent also in other log data from the site, with density and resistivity logs reflecting clearly the lower porosity of glacial clays compared with their sandy, interglacial counterparts (Carter et al 2004a). deMenocal et al (1992 described cyclicity of Milankovitch periodicity in natural gamma records from ODP Site 798 in the Sea of Japan, where, however, the terrigenous content of the high gamma clay rich layers is interpreted to result from glacial drought, erosion of nearby continental loess, and aeolian transport.…”

Sediment Depths Determined by Comparisons of GRAPE and Logging Density Data during Leg 138

“…The minimum sedimentation rate necessary to resolve precessional variations using the HLDT was estimated to be about 40 m/m.y. by deMenocal et al (1992). In cores having lower sedimentation rates, logging density records are less useful for studies of orbital scale …”

Milankovitch Cycles from Fourier Analysis of Logs from Sites 865 and 866

“…Obliquity and possibly eccentricity cycles also were observed in amplitude spectra of calcium and silica from Site 704 on Meteor Rise (Mwenifumbo and Blangy, 1991;Nobes et al, 1991). The results of an analysis of the natural gamma-ray log from Site 798 on the Oki Ridge suggested obliquity modulation of the Pliocene-Pleistocene eolian dust influx to the Sea of Japan (DeMenocal et al, 1992). Molinie and Ogg (1992) reported cycles of variable concentrations of radiolarians and clay and degree of silicification in upper Middle Jurassic to Lower Cretaceous radiolarites from spectral analysis of gamma-ray logs from Site 801.…”

“…A major limitation for using the downhole logging data is one of vertical resolution (Jarrard and Arthur, 1989;deMenocal et al, 1992); that is, the resolution and sampling interval of the tool, combined with a particular sedimentation rate, determines the minimum detectable cycle. For example, a resistivity-log having a 1-m vertical resolution (Schlumberger, 1987) requires a minimum sedimentation rate of 20 m/m.y.…”

“…Slight spectral contami- nation may exist near the Nyquist frequency because the sampling interval (0.15 m) is smaller than the vertical resolution of the logs, however, that is not within the frequency band of interest here. Although the gamma-ray spectra possess more power at higher frequencies, the gamma-ray tool also is affected by residual noise that degrades the low-frequency resolution (deMenocal et al, 1992). Whereas the response of the depositional environment to astronomical climate forcing is periodic in time, the data recorded by the logging tool is periodic in depth.…”