Atmospheric carbon dioxide concentrations and climate are regulated on geological timescales by the balance between carbon input from volcanic and metamorphic outgassing and its removal by weathering feedbacks; these feedbacks involve the erosion of silicate rocks and organic-carbon-bearing rocks. The integrated effect of these processes is reflected in the calcium carbonate compensation depth, which is the oceanic depth at which calcium carbonate is dissolved. Here we present a carbonate accumulation record that covers the past 53 million years from a depth transect in the equatorial Pacific Ocean. The carbonate compensation depth tracks long-term ocean cooling, deepening from 3.0-3.5 kilometres during the early Cenozoic (approximately 55 million years ago) to 4.6 kilometres at present, consistent with an overall Cenozoic increase in weathering. We find large superimposed fluctuations in carbonate compensation depth during the middle and late Eocene. Using Earth system models, we identify changes in weathering and the mode of organic-carbon delivery as two key processes to explain these large-scale Eocene fluctuations of the carbonate compensation depth.

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Prior to Ocean Drilling Program Leg 155, the architecture of the Amazon Fan, as well as those of other modern submarine fans, had been investigated primarily by seismic reflection profiles. The acoustic facies and stratal patterns observed on these profiles provided a wealth of information that allowed deciphering fan growth patterns and the geometry of fan deposits. However, lithofacies could only be inferred from the seismic reflection data. Here we analyze grain-size data from 13 sites drilled during Leg 155, and place the results in the context of the stratigraphic units interpreted from seismic reflection data. Clay, silt, and fine sand are the dominant grain sizes of all cores retrieved from the Amazon Fan. In this mud-rich fan, sand is concentrated within the channel thalweg, at the base of channel-levee systems within the middle fan, and forms a significant fraction of the lower fan deposits drilled. All channel-levee deposits drilled are characterized by a fining-upward trend and by an overall coarsening in the downfan direction. Downfan coarsening of the levee deposits probably results from very efficient sorting associated with the channelized flow of turbidity currents together with the overall decrease in channel relief downfan. Finingupward cycles within a channel-levee system are observed in the middle fan where multiple phases of levee development are stacked upon each other. Each growth phase is marked by an abrupt coarsening that marks a channel bifurcation occurring downslope, followed by a fining-upward sequence of levee aggradation. Channel bifurcation results in the reworking of older channel deposits, and the formation of laterally widespread units. High-amplitude reflections (HARs) beneath the channel axis and high-amplitude reflection packet (HARPs) units are composed of two distinct grain-size populations. HARs and HARPs are the coarsest units that form the fan, with sizes up to 0 ø. Both units display an overall coarsening downfan. These characteristics are likely the result of multistep transport of sand downfan. Thick acoustically transparent units on seismic profiles are correlated to large mass-transport deposits, displaying grain-size characteristics similar to levee deposits with an overall finingupward and downfan-coarsening trend.

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A 767-m section of late Neogene (0-8 Ma) terrigenous sediments was cored at Ocean Drilling Program (ODP) Site 646. Continuous downhole geophysical logs, 161 laboratory measurements of core porosity and density, and 63 laboratory measurements of core velocity are used to analyze in detail the effects of porosity and mineralogy on the acoustic properties at this site. Porosity (determined from a resistivity log) agrees well with rebound-corrected laboratory measurements. Mineralogical variations (potassium feldspar, quartz plus plagioclase, calcite plus opal, and clay minerals) for the interval 206-737 mbsf were determined by matrix inversion of three logs: bound water, potassium, and uranium/ thorium ratio. These calculated mineralogical variations are similar in major features to mineral abundances from smear slides, but the wide depth spacing of smear slides and their subjective, semiquantitative mineral abundances preclude a detailed comparison. Calculated grain densities from mineralogy are consistent with laboratory measurements. A pseudodensity log from porosity and grain density is similar in character to the rebound-corrected, bulk-density measurements on cores, but about 0.1 g/cm 3 lower than core measurements in the interval 340-737 mbsf. We found from our analyses that a strong synergy exists between downhole geophysical logs and core measurements of porosity and density: (1) core recovery is best at shallow depths, and logs are more reliable at greater depths; and (2) agreement between laboratory and log measurements corroborates the different assumptions made when analyzing the two data types. At Site 646, this synergy does not extend to laboratory measurements of velocity; laboratory velocities are lower than in-situ velocities, but higher than expected when rebound is considered.Observed trends of laboratory and log porosity, density, and velocity as a function of depth at Site 646 are in reasonable agreement with empirical trends. In contrast, empirical relationships of velocity to porosity do not agree well with our data. Application of Hookean elastic equations to our data is hampered by the lack of shear wave velocities and the sensitivity of the technique to small errors in porosities. Nevertheless, this theoretical approach yields a pseudovelocity log that agrees remarkably well with observed in-situ log velocities.