Description: Geochemical logging is a routine part of the Ocean Drilling Program, yet the reliability of ODP geochemical logs has rarely been evaluated quantitatively. On ODP Leg 117, geochemical logs were obtained at Sites 723, 728, and 731. We report here an evaluation of ODP geochemical log quality based on high-resolution sampling and X-ray fluorescence measurement of 398 core samples from the three sites. At these sites we lacked the complete suite of high-quality logs needed for accurate log-based estimation of elemental percentages; only calcium and silicon logs had magnitudes similar to those from XRF. However, relative variations of log-based elemental abundances could be determined. Our comparisons of the XRF analyses with the character of variations in geochemical logs indicates that the reliability of ODP geochemical logs varies substantially, within short intervals and particularly between sites. In general, the geochemical logs are capable of detecting changes in formation geochemistry that are larger than the following thresholds: 2% for Ca, 2%-6% for Si, 0.5%-1% for K, 0.1% for Ti, 0.5% for Fe, and 0.4% for Al. All sulfur variations observed in the XRF data, as well as many of the iron variations, were below the resolving power of the geochemical logging tools. These precisions are generally similar to those determined at the Conoco test well by Chapman et al. (1987), in spite of the very different ODP logging conditions.

Description: Geochemical well logs were obtained in sediment at Sites 805 and 806 of Ocean Drilling Program Leg 130. Corrections have been applied to the logs to account for variations in bore-hole size, drilling-fluid composition, and drill-pipe attenuation. Oxide and calcium carbonate weight percentages were calculated from the logs and are compared with available carbonate measurements from core. Log-derived CaCO3 values from Hole 805C, though lower than shipboard CaCO3 core measurements, show a similar variation in character. In Hole 806B, log and shipboard CaCO3 sample values have a similar mean and variance. The CaCO3 high-resolution sample values from Hole 806B, however, have a similar mean but a lower variance than the CaCO3 logs. It is probable that discrepancies result from gamma-ray spectrometry tool malfunctions experienced throughout the logging operations of this leg as well as from chlorine-induced interference in the calcium yield. Because the amplitude of CaCO3 variations was extremely low (standard deviation of 1.38 on high-resolution samples), the tool, which has a sensitivity of roughly 2% for Ca, probably would not have been able to detect these variations, even if the tool had been working properly.

Description: We used well logs, in some cases combined with shipboard physical properties measurements to make more complete profiles and to correlate between sites on the Ontong Java Plateau. By comparing sediment bulk density, velocity, and resistivity logs from adjacent holes at the same site, we showed that even subtle features of the well logs are reproducible and are caused by variations in sedimentation. With only minor amounts of biostratigraphic information, we could readily correlate these sedimentary features across the entire top of the Ontong Java Plateau, demonstrating that for most of the Neogene the top of the plateau is a single sedimentary province. We found it more difficult, but still possible, to correlate in detail sites from the top of the plateau to those drilled on the flanks. The pattern of sedimentation rate variation down the flank of the plateau cannot be interpreted as simply controlled by dissolution. Site 805, in particular, oscillates between accumulating sediment at roughly the same rate as cores on top of the Ontong Java Plateau, and accumulating sediment as slowly as Site 803, 200 m deeper in the water column. These oscillations do not match earlier reconstructions of central Pacific carbonate compensation depth variations.

Description: The Multitracers Experiment studied a transect of water column, sediment trap, and sediment data taken across the California Current to develop quantitative methods for hindcasting paleoproductivity. The experiment used three sediment trap moorings located 120 km, 270 km, and 630 km from shore at the Oregon/California border in North America. We report here about the sedimentation and burial of particulate organic carbon (Corg) and CaCO3. In order to observe how the integrated CaCO3 and Corg burial across the transect has changed since the last glacial maximum, we have correlated core from the three sites using time scales constrained by both radiocarbon and oxygen isotopes. By comparing surface sediments to a two-and-a-half year sediment trap record, we have also defined the modern preservation rates for many of the labile sedimentary materials. Our analysis of the Corg data indicates that significant amounts (20-40%) of the total Corg being buried today in surface sediments is terrestrial. At the last glacial maximum, the terrestrial Corg fraction within 300 km of the coast was about twice as large. Such large fluxes of terrestrial Corg obscure the marine Corg record, which can be interpreted as productivity. When we corrected for the terrestrial organic matter, we found that the mass accumulation rate of marine Corg roughly doubled from the glacial maximum to the present. Because preservation rates of organic carbon are high in the high sedimentation rate cores, corrections for degradation are straightforward and we can be confident that organic carbon rain rate (new productivity) also doubled. As confirmation, the highest burial fluxes of other biogenic components (opal and Ba) also occur in the Holocene. Productivity off Oregon has thus increased dramatically since the last glacial maximum. CaCO3 fluxes also changed radically through the deglaciation; however, they are linked not to CaCO3 production but rather to changes in deepwater carbonate chemistry between 18 Ka and now.