Published:January 01, 2013
We present palaeomagnetic results from the Oligocene through Miocene part of the Integrated Ocean Drilling Program Site U1333 (1030.996′N, 138°25.159′W), which is located in 4853 m-deep water over seafloor with an estimated crustal age of 46 Ma. Detailed magnetostratigraphic investigations are essential to provide a sound age model for the study of the palaeoclimatic and palaeo-oceanographic history of the Cenozoic of the Equatorial Pacific and to improve the database of Pacific magnetostratigraphy. Rock magnetic measurements were carried out at 1 cm resolution on 81 U-channel samples from the spliced section with the goal of extracting a high-resolution record of the magnetostratigraphy. Stepwise demagnetization of the natural remanent magnetization yielded a well-defined magnetostratigraphy over a time interval of approximately 10 Ma between the base of Chron C6n (19.722 Ma) and the middle of Chron C11r (>29.9 Ma) and identification of the Oligocene–Miocene transition at the base of Subchron C6Cn.2n. The palaeomagnetic data are characterized by shallow inclinations, and by 180° alternations in declinations downhole, reflecting magnetic polarity zones. The relatively high temporal resolution allowed for the identification of three possible excursions previously not identified on the geomagnetic polarity time scale, which were recorded in Subchrons C8n.1r and C11n.2n and in Chron C11r.
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Magnetic Methods and the Timing of Geological Processes
Magnetostratigraphy is best known as a technique that employs correlation among different stratigraphic sections using the magnetic directions defining geomagnetic polarity reversals as marker horizons. The ages of the polarity reversals provide common tie points among the sections, allowing accurate time correlation. Recently, studies of magnetic methods and the timing of geological processes have acquired a broader meaning, now referring to many types of magnetic measurements within a stratigraphic sequence. Many of these measurements provide correlation and age control not only for the older and younger boundaries of a polarity interval, but also within intervals. Thus, magnetostratigraphy no longer represents a dating tool based only on geomagnetic polarity reversals, but comprises a set of techniques that includes measurements of geomagnetic field parameters, environmental magnetism, rock-magnetic properties, radiometric dating and astronomically forced palaeoclimatic change recorded in sedimentary rocks, and key corrections to magnetic directions related to geodynamics, palaeocurrents, tectonics and diagenetic processes.