A M-sequence geomagnetic polarity time scale that steadies spreading rates globally and incorporates cyclostratigraphy constraints

Geomagnetic polarity time scales (GPTSs) are constructed from magnetic anomaly lineations of the Earth’s mid-ocean ridge system by interpolating between a few absolute radiometric dates while assuming nearly constant spreading rates over time. An optimal GPTS should therefore minimize the global variation of spreading rates. In current practice, however, the variation of spreading rates is minimized only for a few spreading centers; at all other locations worldwide, the spreading rates vary rather erratically. Moreover, GPTSs constructed on the basis of oceanic magnetic lineations do not directly incorporate chron durations from cyclostratigraphy. Cyclostratigraphic studies estimate the duration of a chron by matching sediment cycles with established orbital periodicities in sequences that have a measured magnetic stratigraphy. Including information on these chron durations would improve the accuracy of the GPTS. We employ a Monte Carlo method to construct a GPTS that overcomes these limitations. This procedure is designed to generate a large sample of GPTSs that simultaneously match absolute age constraints, minimize the global variation of spreading rates, and agree with chron durations from cyclostratigraphy. The sampled GPTSs fit these constraints within their respective uncertainties (e.g., uncertainties on chron durations from cyclostratigraphy). The objective is not only to obtain a “best” time scale but also to determine its uncertainty given all the information available. The mean of all the sampled GPTSs gives the final time scale, and the sample variance measures the uncertainty of the time scale. We apply the method to construct an improved version of the GPTS for the M-sequence lineations (Late Jurassic-Early Cretaceous, ~160-120 Ma). The resulting GPTS minimizes the variation in spreading rates in a global data set of magnetic lineations from the Western Pacific, North Atlantic, and Indian Ocean NW of Australia. The time scale also accounts for the duration of five magnetic chrons (M0r through M3r) established from cyclostratigraphic studies of Lower Cretaceous sequences in the Italian Alps and Apennines. The M-sequence GPTS we obtain can be easily updated by repeating the Monte Carlo sampling with additional data that may become available in the future.