Building an astrochronological polarity time scale
Dennis V. Kent,
Lamont-Doherty Earth Observatory, Columbia University & Earth and Planetary Sciences, Rutgers University,
Web Lecture on May 20, 2021 at 3:00 PM (Paris Time)
Integrated geologic time scales developed in the 1990s from the sea-floor record (1-5) are the conceptual foundations of modern geochronologies for the Late Jurassic to Recent and extensions to earlier time intervals including the nonmarine realm (6). Progress has been summarized in the ever-more comprehensive octennial (2004–2020) Geologic Time Scale volumes (7-9) that we recently heard about from Felix Gradstein. Over the ~160 Ma to Recent age-range of the oceanic record, astronomical periodicities mainly provide refinements to the geomagnetic polarity time scale (GPTS) derived from sea- floor spreading magnetic anomalies and the stochastic signature of the polarity reversal sequence linked using magnetostratigraphy to relatively few well-dated horizons for age calibration in the virtual absence of direct radiometric dating of oceanic basement. The GPTS thus provides a basis of temporal correlation independent of latitude and global climate as well as in marine and nonmarine facies with different biota and potentially different (and interesting) responses to Milankovitch climate forcing. Prior to the age of the oldest sea-floor record, construction of a usable polarity reversal framework has been more contingent on finding long continuous sedimentary sequences (marine or nonmarine), ideally recovered in scientific drill cores to avoid ambiguity in superposition and recording a reproducible magnetostratigraphy that is embedded with or correlated to climate cycles, especially the 405 kyr metronome, to produce an astrochronological polarity time scale (APTS) for testing and adjustments with high-precision radiometric dates. I will try to highlight the current status of the APTS as the integration of signals from an arrhythmic core geodynamo and rhythmic orbital motions that enables a better understanding of the underlying phenomena.
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