Regional character of geomagnetic field directional circularity: Holocene East Asia

Abstract

This study summarizes the pattern of directional paleomagnetic secular variation (PSV) in five previously-published Holocene records from East Asia. Each record is high in resolution and well-dated. We have developed equi-spaced (100-year increment) time series for the declinations and inclinations for the five sites and compared their overall directional variability. We see evidence of nine correlatable scalar features in both inclination and declination. These comparisons suggest that, to first order, these five PSV records all estimate the same regional pattern of PSV for East Asia. We specifically focus on the clockwise or counter-clockwise motion of paleomagnetic directions (termed circularity) in these PSV records. We have used two techniques to estimate the directional circularity of each PSV record. We have first calculated the incremental rate and direction of motion (clockwise or counter-clockwise) in 200-year increments for each record over the last 3000–8000 years. We have separately used a more traditional technique of looking for discernable looping (full loops down to 1/2 loops) in individual records. We estimate the loop sizes, durations, and circularity direction. We largely see the same pattern of circularity in both measurement techniques. There are eight intervals of oscillating circularity (clockwise versus counter-clockwise) in all five sites. They generally agree in timing and direction of circularity. We see evidence for 5 discernable loops alternating between clockwise and counter-clockwise in all five records. Both techniques suggest an oscillating, teeter-totter like, behavior to PSV circularity that must be due to the pattern of fluid flow/magnetic flux regeneration in the outer core. This teeter-totter behavior is unbalanced with almost twice the time spent in clockwise motion as in counter-clockwise motion. Less than 10% of the overall time series duration is spent in linear (not circulating) behavior. The loops tend to have a distribution of sizes and durations with smaller loops being shorter in duration (300–600 years) and bigger loops having longer durations (800–1500 years). One particular feature of these circularity patterns bears special note. All five PSV records show short-term (200–400 years) significant acceleration in circularity rate combined with change in circularity direction. We see evidence for three of these short intervals near 200 BCE, 400 CE, and 1300 CE. These features also are intervals of the fastest regular secular variation rates in all the records. These features are broadly analogous to geomagnetic jerks (Courtillot and LeMouel, 1976, 1984) in that they are short intervals (1–2 years) of anomalous acceleration within high-resolution historical (<200 yr) secular variation records. Gallet et al. (2003) showed evidence for similar relatively short anomalous acceleration intervals (~102 years) within PSV for the last ~3000 years, which they termed archeomagnetic jerks. We think our anomalous intervals are comparable to those of Gallet et al. (2003).