Stress field of basaltic rocks of ODP Hole 127-794C

We carried out an experiment to estimate in-situ stresses at ODP Hole 794C (water depth: 2809 m) from the basaltic core samples by deformation rate analysis (DRA). Site 794 is located at the northern end of the Yamato Basin and 70 km west of the eastern Japan Sea intraplate or interplate convergent zone. Stress previously applied to a rock specimen is identified in the inelastic strain behavior of the specimen under uniaxial compression by the method used. Natural remanent magnetization of the sample was also measured to get a reference for the orientation of the horizontal stresses. The vertical, maximum, and minimum horizontal in-situ stresses estimated at a depth of 582 mbsf are 36.4, 43.1, and 31.2 MPa, respectively. The average of the largest and the least horizontal stresses is nearly equal in value to the vertical stress. This suggests that the site is in the stress field of the strike slip regime at the depth, while the stress field of the reverse fault regime has been estimated from the focal mechanism solutions of the earthquakes whose hypocenters are located near or on the convergent boundary. The directions of the largest and the least horizontal stress are estimated to be northeast-southwest and in northwest-southeast, respectively, in taking account of rotation tectonics of the Japan Sea since its formation. The directions of the largest and the least horizontal stresses are opposite to those determined from the earthquakes. These discrepancies of our results with those from earthquakes may be due mainly to the fact that the site is not in the convergent zone.

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

Samples for total organic carbon (TOC) analysis were collected on WOCE Line P15S (0° to 67°S along 170°W) and from 53° to 67°S along 170°E in the western South Pacific, and on Line I8 (5°N to 43°S along 80°/90°E) in the central Indian Ocean. TOC concentrations in the upper ocean varied greatly between the regions studied. Highest surface TOC concentrations (81-85 µM C and 68-73 µM C) were observed in the warmest waters (>27°C) of the western South Pacific and central Indian Oceans, respectively. Lowest surface TOC concentrations (45-65 µM C) were recorded in the southernmost waters occupied (>50°S along 170°W and 170°E). Deep water (>1000 m) TOC concentrations were uniform across all regions analyzed, averaging between 42.3 and 43 µM C (SD: ±0.9 µM C). Mixing between TOC-rich surface waters and TOC-poor deep waters was indicated by the strong correlations between TOC and temperature (r2>0.80, north of 45°S) and TOC and density (r2>0.50, southernmost regions). TOC was inversely correlated with apparent oxygen utilization (AOU) along isopycnal surfaces north of the Polar Frontal Zone (PFZ) and at depths <500 m. The TOC:AOU molar ratios at densities of sigmaT 23-27 ranged from -0.15 to -0.34 in the South Pacific and from -0.13 to -0.31 in the Indian Ocean. These ratios indicate that TOC oxidation was responsible for 21%-47% and 18%-43% of oxygen consumption in the upper South Pacific and Indian Oceans, respectively. At greater depths, TOC did not contribute to the development of AOU. There was no evidence for significant export of dissolved and suspended organic carbon along isopycnal surfaces that ventilate near the PFZ.