Cyclostratigraphy

Another feature of magnetostratigraphic correlations that must be kept constantly in mind is that the isochroneity of the reversal boundaries is actually somewhat fuzzy because transitions-the interval in which the Earth has essentially no magnetic field-require at least 5 Kyr. Furthermore, the imprinting of a new polarity regime on strata can be delayed by thousands of years in certain environments. Bioturbation disorders the acquired polarity of seafloor sediments, and the fixation of detrital geomagnetic orientation in open-ocean marine deposits normally does not take place until the material is buried to ca. 40 cm. The time required for this depth of burial varies and is, of course, much longer in slowly deposited sediments. The age of a microfossil specimen is, therefore, synchronous not with the remanent magnetization in the horizon in which it occurs, but with that of a horizon ca. 40 cm below. Thus, the apparent microfossil “date” of a paleomagnetic horizon is always somewhat younger in deep-sea

Cyclostratigraphy is the analysis of rhythmic featuresin the stratigraphic record according to astronomical cycles and, in particular, the Earth-orbital cycles with periodicities between 0.1 and 1.0 Myr that support an orbital-forcing time scale (OFT). Precision in OFT dating does not decrease with increasing age because the cyclic effects are stratigraphic features that are directly observable in the rocks. The accuracy of OFT dates, which depends on the extrapolation of orbital and rotational cycles into the past, is also well controlled because the cycles are independent and can be crosschecked against one another, and because changes in the astronomical periodicities over time can be calculated with great confidence. Other appellations for this relatively new discipline include “cosmostratigraphy,” “orbital stratigraphy,” and astrochronology.