Geomagnetic excursions: Knowns and unknowns

Geomagnetic excursions are short-lived episodes when Earth's magnetic field deviated into an intermediate polarity state. Many questions remain unanswered concerning the origin, frequency, amplitude, duration, and field behavior associated with excursions. I provide an overview of recent advances in excursion research, and suggest key further research to resolve major unanswered questions. Excursions are temporally associated with collapses in field intensity and have been suggested to result when the field reverses polarity in Earth's liquid outer core without accompanying reversal in the solid inner core. However, numerical dynamo simulations suggest that the inner core is too small for its electrical conductivity contrast to significantly affect dynamo processes. The cause of excursions therefore remains an open question. Detailed paleomagnetic studies and precise geochronology over the last decade have significantly improved our understanding of excursions. New excursions have been identified, poorly dated excursions have been dated precisely, unverified excursions have been discredited, and others remain to be fully validated and accepted. It is now clear that excursions are an intrinsic and frequent component of field behavior and a high quality excursion catalog is likely to grow as additional detailed studies are performed. Further work is required to: improve the global distribution of excursion records, particularly from the southern hemisphere; obtain high-resolution sedimentary excursion records with good age control from sites with sedimentation rates >10 cm/kyr; obtain volcanic excursion records coupled with high-precision geochronology; and estimate excursion duration with high chronological precision. Such efforts will facilitate hypothesis testing concerning the deep earth dynamics that generate geomagnetic excursions.