Wet-bulk densities, mean atomic weights, effective porosities, grain densities, and compressional-wave velocities at DSDP Leg 58 Holes

The effects of water saturation and open pore space on the seismic velocities of crystalline rocks are extremely important when comparing laboratory data to in situ geophysical observations (e.g., Dortman and Magid, 1969; Nur and Simmons, 1969; Christensen and Salisbury, 1975). The existence of fractured rocks, flow breccias and drained pillows in oceanic crustal layer 2a, for instance, may appreciably reduce seismic velocities in that layer (Hyndman, 1976). Laboratory data assessing the influence of porosity and water saturation on seismic velocities of oceanic crustal rocks would certainly aid interpretation of marine geophysical data. Igneous rocks recovered during Leg 58 of the Deep Sea Drilling Project, in the Shikoku Basin and Daito Basin in the North Philippine Sea, are extremely vesicular, as evidenced by shipboard measurements of porosities, which range from 0 to 30 per cent (see reports on Sites 442, 443, 444, and 446, this volume). Samples with this range of porosities afford an excellent opportunity to examine the influence of porosity and water saturation on seismic velocities of oceanic basalts. This paper presents compressional-wave velocities to confining pressures of 1.5 kbars for water-saturated and air-dried basalt samples from the North Philippine Sea. Samples used in this study are from sites 442, 443 and 444 in the Shikoku Basin and Site 446 in the Daito Basin. Excellent negative correlation between porosity and compressional-wave velocity demonstrates that waterfilled pore space can significantly reduce compressionalwave velocities in porous basalts. Velocities measured in air-dried samples indicate that the velocity difference between dry samples and saturated samples is small for porosities exceeding 10 per cent, and very large for lower porosities.

Calcium carbonate stratigraphy of ODP Site 177-1090

A total of 776 sediment samples were measured for percent CaCO3 using a coulometer. These data are compared with percent blue reflectance (450-550 nm) measured with the Oregon State University split-core analysis track. In previous studies percent blue reflectance has been an excellent proxy for percent CaCO3 and in this study shows many of the main depositional trends (i.e., a 100-k.y. cycle, with a 55% reflectance range is evident in the upper 900 k.y., underlain by sediments exhibiting a 40-k.y. cycle with only a 30% reflectance range). Between ~21 and 5 Ma the average percent reflectance decreases from ~35% to ~8%. A similar decrease is also recorded between ~24 and 22 Ma. Percent CaCO3 trends closely match those of the percent blue spectral reflectance. This is especially well shown in the 100-k.y. cyclicity and in the interval between 24.5 and 21.5 Ma. In both intervals CaCO3 analyses are abundant. An exception occurs in the interval between 2 and 5 meters composite depth (~193 and 240 k.y.). There, percent CaCO3 and percent reflectance are out of phase. The lack of agreement is not likely to be due to a very wet core, in which water would dominate the spectral reflectance instead of sediment, or to problems with the composite depth slice. The discrepancy remains unexplained and provides clear evidence that when noninvasive measurements are used as proxies for chemical measurements they must be substantiated by the actual chemical or physical measurements.