Paleomagnetic of igneous rocks of ODP Leg 114

A paleomagnetic study was made of 12 samples of trachytic basalt from the base of ODP Hole 698A on the Northeast Georgia Rise (southwest Atlantic) and four samples of andesitic basalt and nine samples of volcanic breccia from the base of ODP Hole 703A on the Meteor Rise (southeast Atlantic). The magnetic intensities of the Hole 703A samples are anomalously low, possibly reflecting alteration effects. The mean magnetic intensity of the Hole 698A samples is high, and compatible with the model of Bleil and Petersen (1983) for the variation of magnetic intensity with age in oceanic basalts, involving progressive low-temperature oxidation of titanomagnetite to titanomaghemite for some 20 m.y. followed by inversion to intergrowths of magnetite and other Fe-Ti oxides during the subsequent 100 m.y. These results support the interpretation of the Hole 698A basalts as true oceanic basement of Late Cretaceous age rather than a younger intrusion. Well-defined stable components of magnetization were identified from AF and thermal demagnetization of the Hole 698A basalts, and less well-defined components were identified for the Hole 703A samples. Studies of the magnetic homogeneity of the Hole 698A basalts, involving harmonic analysis of the spinner magnetometer output, indicate the presence of an unevenly distributed low-coercivity component superimposed on the more homogeneous high-coercivity characteristic magnetization. The former component is believed to reside in irregularly distributed multidomain magnetite grains formed along cracks within the basalt, whilst the latter resides in more uniformly distributed finer magnetic grains. The inclination values for the high-coercivity magnetization of five Hole 698A basalt samples form an internally consistent set with a mean value of 59° ± 5°. The corresponding Late Cretaceous paleolatitude of 40° ± 5° is shallower than expected for this site but is broadly compatible with models for the opening of the South Atlantic involving pivoting of South America away from Africa since the Early Cretaceous. The polarity of the stable characteristic magnetization of the Site 698 basalts is normal. This is consistent with their emplacement during the long Campanian to Maestrichtian normal polarity Chron C33N.

Parameters of the near-bottom nepheloid layer off the northwestern Africa coast

Vertical profiles of light scattering at a right angle and turbidity profiles in seawater indicating suspended matter concentration in the near-bottom nepheloid layer (NNL) were measured simultaneously with temperature, salinity, and density profiles at the continental slope off the northwestern Africa. About 100 stations 5' apart in latitude and longitude were carried out over an ocean area of 6100 sq. km. Special features of the NNL variability in the area were analyzed. It was found that some structural parameters of the NNL (maximum transparency depth, that is the upper boundary of NNL; NNL thickness; maximum and total turbidity) correlate with ocean depth. On the average, thickness of the NNL in the area is 20-40% of the ocean depth. At most stations the NNL is fairly strong. In the shelf region NNL turbidity was influenced by the intensive near-shore upwelling. Formation of ''high-energy near-bottom layers'' in the shelf region resulted from passing of a mesoscale cyclonic eddy that caused redistribution of measured quantities within the entire water column.