From the Superchron to the Microchron: Magnetic Stratigraphy in Deep Sea Sediments

Article excerpt

Sediments acquire the signature of Earth's prevailing magnetic field at the time of their deposition. Because the polarity of the geomagnetic field has reversed repeatedly in the geological past, the successive polarity changes imprinted in sedimentary sequences provide the physical basis for magnetic polarity stratigraphy. This "magnetostratigraphy" can be used as a correlation and dating method. A general outline of the magnetic polarity time scale has emerged from scientific studies over the past 30 years; the ultimate goal is to extend and date the record over even older periods. Recent new methods in chronology considerably improve the time resolution of marine sediment magnetic records and provide the first opportunity to resolve fine-scale features of Earth's magnetic field.

We consider the present-day polarity field to be normal: Magnetic lines of force are directed toward the north magnetic pole, and the north-seeking pole of a compass needle points north. However, when the field has the opposite polarity, the lines of force are directed south and a compass needle points south. Until the mid 1960s, magnetic polarity time scales were calibrated using only continental volcanic rocks younger than 5 million years old. Study of marine sections became possible in the mid 1960s with the development of more sensitive magnetometers that could measure the weak magnetization of sediments. Correlation of magnetic records from various deep-sea cores and with paleomagnetic and radiometric studies of on-land lava flows followed and verified the value of sedimentary sequences as records of polarity changes in Earth's geomagnetic field. With succeeding work on much longer time series, magnetostratigraphy has become a very accurate method of elating sedimentary sequences.

The first long (pre-Pliocene) magnetic polarity time scale was proposed by geophysicists from the Lamont-Doherty Geological Observatory of Columbia University in 1968. Covering the last 80 million years, the scale was constructed from profiles of marine magnetic anomalies of the South Atlantic Ocean. A few years later, this scale was extended to the Lower Cretaceous and late Jurassic periods, with the first continuous sequence of reversals for the last 160 million years, using magnetic surveys from the Pacific Ocean In the meantime, some authors cautioned against uncritical acceptance of sediment magnetostratigraphy because the record may be complicated by several factors, such as post-depositional overprinting of the signal due to chemical changes in the sediment. The situation then greatly improved with the development of extremely sensitive (cryogenic) magnetometers, making it possible to measure large numbers of weakly magnetized samples.

During the 1970s, magnetostratigraphic studies from pelagic limestone sections of land and deep sea sediments drilled during DSDP confirmed most of the magnetic polarity intervals (or chrons) determined from profiles of marine magnetic anomalies. Magnetostratigraphic results were also used to calibrate the polarity time scale. This was achieved by cross-correlating biostratigraphic zonations deduced from paleontological studies with the magnetic polarity sequences observed in sedimentary sections and revealed from the magnetic stripes of the seafloor. This research has advanced significantly through the work of the ocean drilling programs. For example, coring on DSDP Leg 73 in the South Atlantic yielded a tight calibration between bio- and magnetic-polarity time scales for the Paleogene. Magnetostratigraphic and paleontological data are now available for most of the geological boundaries since the late Jurassic, the age of the oldest oceanic crust. Among these boundaries, the Tertiary-Cretaceous time boundary, which is important because of its signature faunal extinctions, is particularly well documented. The relationship to biostratigraphic zones is in general well established, but it is not yet possible to relate the zones to isotopic ages with the same precision. …