Leg 76 of the Deep Sea Drilling Project achieved two major scientific objectives. The first objective was met at Site 533, where on the Blake Outer Ridge, gas hydrates were identified by geophysical, geochemical, and geological studies. Gas-hydrate decomposition produced a volu-metric expansion of 20: I of gas volume to pore-fluid volume; this expansion exceeded by about a factor of four the volume of gas that could be released from solution in pore water under similar conditions. The gas hydrate includes methane, ethane, propane, and isobutane but apparently excluded normal butane and higher molecular weight hydrocarbons as predicted from gas hydrate crystallography. For the first time, marine gas hydrates were tested with a pressure core barrel. The second objective was achieved when coring at Site 534 in the Blake-Bahama Basin sampled the oldest oceanic sediments yet recovered. The sequence of oceanic basement and overlying sediments documents the geologic history of the early stages of the opening of the North Atlantic Ocean in detail. The oldest oceanic sediments are red claystones and laminated green and brown claystones of middle Cal-lovian age. This finding supports the interpretation that the beginning of the modern North Atlantic occurred in the early Callo-vian (~155 m.y. B.P.), as much as 20 m.y. later in time than often previously thought.
At DSDP Sites 536 and 538 in the southeastern Gulf of Mexico, rocks from near the Cretaceous/Tertiary boundary were recovered. The Danian/Maestrichtian samples from Site 536 were stepwise demagnetized in alternating fields up to 400 Oe. Two distinct components of magnetization could be identified, neither of which displays a reversal pattern compatible with the age of the samples. Samples from the Danian section at Site 538 were subjected to a combination of alternating field and thermal demagnetization. Although several reversals are observed, a unique correlation with the magnetic time scale is not possible until the paleontology of the section is better defined.
Basalts from Hole 534A are among the oldest recovered from the ocean bottom, dating from the opening of the Atlantic 155 Ma. Upon exposure to a 1-Oe field for one week, these basalts acquire a viscous remanent magnetization (VRM), which ranges from 4 to 223% of their natural remanent magnetization (NRM). A magnetic field of similar magnitude is observed in the paleomagnetic lab of the Glomar Challenger, and it is therefore doubtful if accurate measurements of magnetic moment in such rocks can be made on board unless the paleomagnetic area is magnetically shielded. No correlation is observed between the Konigsberger ratio (ß), which is usually less than 3, and the ability to acquire a VRM. The VRM shows both a log t dependence and a Richter aftereffect. Both of these, but especially the log t dependence, will cause the susceptibility measurements (made by applying a magnetic field for a very short time) to be minimum values. The susceptibility and derived Q should therefore be used cautiously for magnetic anomaly interpretation, because they can cause the importance of the induced magnetization to be underestimated.
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