Susceptibility and time-dependent magnetization of basalts at DSDP Hole 76-534A

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 (beta), 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

(Table 1) Components of magnetization at DSDP Hole 77-538A

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

Inclination of the two main components of magnetization and 87Sr/86Sr ratios for dolomites at DSDP Hole 77-536

Site 536 terminated in a shallow-water dolomite of unknown age. Paleomagnetic measurements combined with strontium isotope analyses suggest that the dolomite was deposited in the Middle Jurassic to Early Cretaceous time interval. However, the assumptions required to reach this determination make these results less than conclusive

Heinrich H1 and 8200 yr B.P. climate events recorded in Hall\u27s Cave, Texas

Measurements of magnetic susceptibility (MS) have been performed on a continuous set of samples from a well-dated ∼3 m sequence of sediments collected from Hall\u27s Cave, Edwards Plateau, central Texas. Three major climatic events are represented by distinctive MS increases in the sequence: (1) the Heinrich event H1 (from 17,500 to 17,000 yr B.P.); (2) the end of full glacial conditions on the Edwards Plateau at 14,200 yr B.P.; and (3) the 8200 yr B.P. climatic event (from 8250 to 8050 yr B.P.). In addition, a minor event at 4400 yr B.P. is also well defined here and in Canada, indicating that it is a broadly regional event. These data indicate that all four events represent a shift toward milder climatic conditions accompanied by increased rainfall at this location. The general climatic trends interpreted from the MS data are consistent with independent vertebrate assemblages, floral indicators, and isotopic data recovered from the cave. © 2006 Geological Society of America