Regional character of geomagnetic field directional circularity:
Holocene Eastern North America
Abstract
This study summarizes paleomagnetic secular variation (PSV) in five
published Holocene records from Eastern North America. We have developed
100-year increment time series for the declinations and inclinations for
all sites and compared their directional variability. We see evidence of
ten correlatable features in both inclination and declination. We focus
on the clockwise or counter-clockwise motion of paleomagnetic directions
(termed circularity) in these PSV records. We have first calculated the
incremental rate and direction of motion (clockwise or
counter-clockwise) for each record over the last 4000-8000 years. We
have separately looked for discernable looping in individual records. We
estimate the loop sizes, durations, and circularity direction. We see
the same pattern of circularity in both measurement techniques. There
are seven intervals of oscillating circularity and looping in all five
sites. Both techniques suggest a distinctive oscillating, teeter-totter
like, behavior to PSV circularity that must be due to the pattern of
fluid flow in the outer core. This teeter-totter behavior is unbalanced
with more time spent in clockwise motion than in counter-clockwise
motion. We think the teeter-totter oscillation may be due to torsional
oscillation in the outer core fluid flow. The loops have a distribution
of sizes and durations with smaller loops being shorter in duration and
bigger loops having longer durations. All five PSV records show 5
intervals of ~102 yr significant acceleration in
circularity rate and PSV rate combined with change in circularity
direction. These features are broadly analogous to historic geomagnetic
jerks.