Records of geomagnetic secular variation since 1200 AD and the potential for chronological control of lake sediments in northern Alberta
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abstract
We assess whether absolutely aged declination changes can be used to date recent, massive lake sediments at the sub-century scale. Three issues limiting such studies are addressed: recovery of undisturbed recent lake sediments, assessment of the reliability of the geomagnetic record, and the comparison of lake-sediment records with historic records. A 793-year long geomagnetic chronology was obtained from the annually laminated sediment of Rainbow Lake A. Sediment disturbance was minimized by freezing the sediment in situ and maintaining this state for geomagnetic analyses. Geomagnetic chronologies ca. 600 years long from the massive sediments of Fariya Lake and Ninisith Lake were collected in removable plastic tubes using a piston-corer. Sediment disturbance was minimized by ex situ draining of water from the plastic tubes. The reliability of the geomagnetic record was assessed using a measure of internal coherence with an absolute cutoff of 15°. By further separating the samples at coherence values of 5° and 7.5°, it was possible to identify zones of disturbance and reduced compaction. The most reliable samples had a high magnetic intensity due to high inputs of magnetic minerals, sediment compression, and inputs of stable magnetic minerals. Neither freezing nor dewatering of the sediments appeared to decrease reliability of the geomagnetic record. The declination chronology from the annually laminated sediments was significantly correlated with a 230-year long historical record from Churchill, Manitoba, and with the massive lake sediment chronology from Fariya Lake. Geomagnetic dating was not possible, however, because of the large amplitude in the annually laminated sediment declination record, the large number of years of sediment in many specimens, and the lack of reliability for samples that contain few years of sediment per specimen.
