magnetostratigraphy

Magnetostratigraphy is a geochronological and stratigraphic method that uses the record of Earth's magnetic field reversals stored in rocks to date and correlate sedimentary and volcanic sequences. Rocks acquire a remanent magnetization during formation that aligns with the ambient field at that time. Over geological times, the field has flipped polarity many times, producing a global geomagnetic polarity timescale (GPTS). By determining the polarity of successive rock intervals and matching the pattern to the GPTS, researchers can assign ages to stratigraphic sections.

In practice, magnetostratigraphy involves obtaining oriented samples, removing secondary magnetization in the laboratory, and measuring the natural remanent magnetization. Each sample is classified as normal or reversed polarity, and continuous polarity zones are delineated. The observed sequence is then correlated to the GPTS, often with anchoring points from radiometric dates or volcanic ash beds to improve precision.

Calibration frequently combines magnetostratigraphy with radiometric dating (e.g., Ar-Ar or U-Pb) and ash horizons to fix absolute ages. Additional tie-ins with biostratigraphy and astrochronology can improve resolution. The method is widely applicable to both continental and marine rocks and is particularly valuable for dating sedimentary sequences lacking fossils or volcanic layers.

Limitations include possible remagnetization or altered magnetic signals, incomplete or discontinuous records in some lithologies, diagenetic overprinting, and the non-uniqueness of correlations in sparsely sampled sections. Careful sampling, robust demagnetization procedures, and multi-proxy dating approaches are essential for reliable magnetostratigraphic results.