The Intensity of the Ancient Geomagnetic Field: A Review and Analysis

Latest Pleistocene and Holocene Geomagnetic Paleointensity on Hawaii

Geomagnetic paleointensity determinations from radiocarbon-dated lava flows on the island of Hawaii provide an estimate of broad trends in paleointensity for Holocene time and offer a glimpse of intensity variations near the end of the last glacial period. When the data from Hawaii are compared with others worldwide, the intensity of the geomagnetic field seems to have been reduced from the Holocene average by about 35 percent between 45,000 and 10,000 years ago. A long-term reduction of this magnitude is compatible with reported increases in the production rate of cosmogenic nuclides during the same interval.

Changes in Atmospheric Carbon-14 Attributed to a Variable Sun

The (14)C production rate in the upper atmosphere changes with time because the galactic cosmic-ray flux responsible for (14)C production is modulated by the changes in solar wind magnetic properties. The resulting changes in the atmospheric (14)C level are recorded in tree rings and are used to calculate past (14)C production rates from a carbon reservoir model that describes terrestrial carbon exchange between the atmosphere, ocean, and biosphere. These past (14)C production rate changes are compared with (14)C production rates determined from 20th-century neutron flux measurements, and a theory relating (14)C production and solar variability, as given by geomagnetic Aa indices and sunspot numbers, is developed. This theory takes into account long-term solar changes that were previously neglected. The 860-year (14)C record indicates three episodes when sunspots apparently were absent: A.D. 1654 to 1714 (Maunder minimum), 1416 to 1534 (Spörer minimum), and 1282 to 1342 (Wolf minimum). A less precisely defined minimum occurred near A.D. 1040. The part of this record after A.D. 1645 correlates well with the basic features of the historical record of sunspot numbers. The magnitude of the calculated (14)C production rates points to a further increase in cosmic-ray flux when sunspots are absent. This flux was greatest during the Spörer minimum. A record of approximate sunspot numbers and Aa indices for the current millennium is also presented.

East Pacific Rise Crest: A Near-Bottom Geophysical Profile

A deep-towed magnetometer profile made across the East Pacific Rise crest shows many anomalies with about 1000-gamma amplitudes and 500-meter wavelengths and has larger amplitude changes corresponding to magnetic field reversals. This profile across contacts between normal and reversely magnetized crustal blocks is interpreted to place an upper limit of 4700 years on the time required for field reversals and an upper limit of 280 meters on the width of the intrusion center at the rise crest. This intrusion center may occasionally shift several kilometers laterally with respect to the rise axis. The magnetometer records are compatible with the hypothesis that the magnetic field has undergone many fluctuations of short period and small intensity in the past 2 million years. Sediment accumulation increases from less than 2 meters at the rise crest axis to about 20 meters at the western end and 10 meters at the eastern end of the profile. This increase in accutmiulation appears to be the result of ocean-floor spreading.