Physical properties and elastic constants of ODP Holes 137-504B and 140-504B upper crustal rocks

Seismic velocities have been measured at confining pressures of 100 MPa and 600 MPa for sheeted dike samples recovered during Ocean Drilling Program Legs 137 and 140. The compressional- and shear-wave velocities show an increase with depth at Hole 504B, which is in sharp contrast to the atmospheric pressure velocity measurements performed as part of the shipboard analyses. Rocks exposed to different types of alteration and fracture patterns show distinct changes in their physical properties. The seismic reflectors observed on the vertical seismic profile (VSP) experiment performed during Leg 111 may have been caused by low velocity zones resulting from alteration. The amount of fracturing and hydrothermal alteration in several zones also may have contributed to the acoustic impedance contrast necessary to produce the E5 reflector. Poisson's ratios calculated from laboratory velocity measurements show several low values at depths ranging from 1600 mbsf to 2000 mbsf, which tends to follow similar trends obtained from previous oceanic refraction experiments. A comparison of physical properties between samples recovered from Hole 504B and ophiolite studies in the Bay of Islands and Oman shows a good correlation with the Bay of Islands but significant differences from the measurements performed in the Oman complex.

Geochemistry and isotopes at DSDP Site 76-533

Concentrations and d34S and d13C values were determined on SO4, HCO3, CO2, and CH4 in interstitial water and gas samples from the uppermost 400 m of sediment on the Blake Outer Ridge. These measurements provide the basis for detailed interpretation of diagenetic processes associated with anaerobic respiration of electrons generated by organic- matter decomposition. The sediments are anaerobic at very shallow depths (<1 m) below the seafloor. Sulfate reduction is confined to the uppermost 15 m of sediment and results in a significant outflux of oxidized carbon from the sediments. At the base of the sulfate reduction zone, upward-diffusing CH4 is being oxidized, apparently in conjunction with SO4 reduction. CH4 generation by CO2 reduction is the most important metabolic process below the 15-m depth. CO2 removal is more rapid than CO2 input over the depth interval from 15 to 100 m, and results in a slight decrease in HCO3 concentration accompanied by a 40 per mil positive shift in d13C. The differences among coexisting CH4, CO2, and HCO3 are consistent with kinetic fractionation between CH4 and dissolved CO2, and equilibrium fractionation between CO2 and HCO3. At depths greater than 100 m, the rate of input of CO2 (d13C = -25 per mil) exceeds by 2 times the rate of removal of CO2 by conversion to CH4 (d13C of -60 to -65 per mil). This results in an increase of dissolved HCO3 concentration while maintaining d13C of HCO3 relatively constant at +10 per mil. Non-steady-state deposition has resulted in significantly higher organic carbon contents and unusually high (70 meq/l) pore-water alkalinities below 150 m. These high alkalinities are believed to be related more to spontaneous decarboxylation reactions than to biological processes. The general decrease in HCO3 concentration with constant d13C over the depth interval of 200 to 400 m probably reflects increased precipitation of authigenic carbonate. Input-output carbon isotope-mass balance calculations, and carbonate system equilibria in conjunction with observed CO2-CH4 ratios in the gas phase, independently suggest that CH4 concentrations on the order of 100 mmol/kg are present in the pore waters of Blake Outer Ridge sediments. This quantity of CH4 is believed to be insufficient to saturate pore waters and stabilize the CH4*6H2O gas hydrate. Results of these calculations are in conflict with the physical recovery of gas hydrate from 238 m, and with the indirect evidence (seismic reflectors, sediment frothing, slightly decreasing salinity and chlorinity with depth, and pressure core barrel observations) of gas-hydrate occurrence in these sediments. Resolution of this apparent conflict would be possible if CH4 generation were restricted to relatively thin (1-10 m) depth intervals, and did not occur uniformly at all depths throughout the sediment column, or if another methanogenic process (e.g., acetate fermentation) were a major contributor of gas.

Part of the global DOC versus AOU (dissolved organic carbon/apparent oxygen utilization) data compilation, Southern Ocean

Recent evidence that dissolved organic carbon (DOC) is a significant component of the organic carbon flux below the photic layer of the ocean (1), together with verification of high respiration rates in the dark ocean (2), suggests that the downward flux of DOC may play a major role in supporting respiration there. Here we show, on the basis of examination of the relation between DOC and apparent oxygen utilization (AOU), that the DOC flux supports ~10% of the respiration in the dark ocean. The contribution of DOC to pelagic respiration below the surface mixed layer can be inferred from the relation between DOC and apparent oxygen utilization (AOU, µM O2), a variable quantifying the cumulative oxygen consumption since a water parcel was last in contact with the atmosphere. However, assessments of DOC/AOU relations have been limited to specific regions of the ocean (3, 4) and have not considered the global ocean. We assembled a large data set (N = 9824) of concurrent DOC and AOU observations collected in cruises conducted throughout the world's oceans (fig. S1, table S1) to examine the relative contribution of DOC to AOU and, therefore, respiration in the dark ocean. AOU increased from an average (±SE) 96.3 ± 2.0 µM at the base of the surface mixed layer (100 m) to 165.5 ± 4.3 µM at the bottom of the main thermocline (1000 m), with a parallel decline in the average DOC from 53.5 ± 0.2 to 43.4 ± 0.3 µM C (Fig. 1). In contrast, there is no significant decline in DOC with increasing depth beyond 1000 m depth (Fig. 1), indicating that DOC exported with overturning circulation plays a minor role in supporting respiration in the ocean interior (5). Assuming a molar respiratory quotient of 0.69, the decline in DOC accounts for 19.6 ± 0.4% of the AOU within the top 1000 m (Fig. 1). This estimate represents, however, an upper limit, because the correlation between DOC and AOU is partly due to mixing of DOC-rich warm surface waters with DOC-poor cold thermocline waters (6). Removal of this effect by regressing DOC against AOU and water temperature indicates that DOC supports only 8.4 ± 0.3% of the respiration in the mesopelagic waters.

Compressional and shear wave velocities and elastic constants at DSDP Leg 59 Holes

Prior to the Deep Sea Drilling Project the composition of the oceanic crust could only be inferred from seismic-refraction and gravity data and the recovery of a wide variety of dredged rocks. Through the success of the Deep Sea Drilling Project, it is now clear that the top of oceanic Layer 2 usually consists of basalt. Several laboratory studies (e.g., Fox et al., 1972; Christensen and Shaw, 1970; Hyndman and Drury, 1976) have demonstrated that the seismic velocities of oceanic basalt are similar to velocities reported from refraction studies of Layer 2 and that the variability in Layer 2 velocities has many causes, the most important being fracturing and sea-floor alteration produced by the interaction of basalt and sea water (Christensen and Salisbury, 1973). To date, most reported measurements of velocities in oceanic basalts are from samples obtained from the main ocean basins. With the exception of an earlier study of velocities and related elastic properties of a suite of rocks from DSDP Sites 292, 293, 294, and 296 located in the Philippine Sea (Christensen et al., 1975; Fountain et al., 1975), elastic properties have not been determined for oceanic rocks from marginal basins. In this chapter compressional- and shear-wave velocities and elastic constants are reported at elevated confining pressures for basalt and volcanic breccias from Holes 447A, 448, and 448A.

(Table 1) Compressional-wave and shear-wave velocities and elastic constants from DSDP Legs 69 and 70

Shear-wave and compressional-wave velocities of 26 basalt samples collected at Site 504 during Deep Sea Drilling Project Legs 69 and 70 were measured at elevated confining pressures. The young basalts have higher velocities than average DSDP basalts, because of their lack of alteration. Measurements of sample porosity are combined with laboratory and in situ velocity measurements to yield estimates of total crustal porosity: 13% at the top of Layer 2, and very low porosity below a depth of 2.0 km.