High-Resolution Record of Geomagnetic Secular Variation from Late Pleistocene Lake Lisan Sediments (Paleo Dead Sea)

Partners:

Hagai Ron, Institute of Earth Sciences, Hebrew University, Jerusalem

Michael O. McWilliams, Department of Geophysics, Stanford University, California

Mordechai Stein, The Geological Survey, Jerusalem

Publications:

1. EPSL 161:145-160 (1998) (pdf file)

2. Marco, S., Ron, H., McWilliams, M. O., and Stein, M., 1999. The locking in of remanence in Late Pleistocene sediments of Lake Lisan (palaeo Dead Sea). In: Tarling, D. H. and Turner, P. (Editors), Palaeomagnetism and Diagenesis in Sediments. Geological Society, London, Special Publications, London, 151: 47-52

Abstract

We recorded geomagnetic secular variation in Lake Lisan sediments (paleo Dead Sea). More than 1500 oriented samples were collected from a 27.3-m section of alternating aragonite and detritus laminae in the Dead Sea basin ranging in age from 67 to 32 ka. Measurements were done with a 2G cryogenic magnetometer. The natural remanent magnetization (NRM) is carried by titanomagnetite in the detrital laminae whereas the aragonite is diamagnetic. The NRM is very stable and was acquired several hundred years after deposition. The mean direction of 878 horizons is D=005°; I=45° (a95=1°; k=22)

We observed three modes of directional geomagnetic variation as a function of time (see figure): very rapid inter-sample changes, slow variation in mean direction, and inclination shallowing of about 1°/m. The overall rate of change in direction is 0.57±0.57°/yr, not significantly different from zero. For about 83% of the record the rate of change is less than 1°/yr and comparable to historical values. High rates of change are observed more frequently in the Lisan than in historical records, and peak rates are up to 10 times faster. A smoothed curve resulting in a maximum rate of change of 0.66°/yr and a mean 0.10±0.10°/yr may be a more realistic representation of the field behavior.

No reverse NRMs were observed, but geomagnetic field excursions may be present where the VGPs deviate by more than 40° from the geographic north at about 52 and 41 ka; the latter may represent the Laschamp event.

Inclinations (right) and declinations (left) in sites PZ3 (up to 1400 cm) and PZ1 (1445-2730 cm). Each point represents one sample except for the seventeen 12-sample horizons that are represented by their Fisher means. The geocentric axial dipole (GAD) direction is represented by solid lines. A linear regression through all the inclinations shows a trend of upward increase, Inc.=29+H , where H is the height in meters (R=0.618). U-series ages are shown at right. Inset. Results of a reproducibility test. Two sampling sets show similar declination pattern.

The smoothed directions with corresponding a95 angles (gray bars). Each point in the smoothed curve is the Fisher mean of six consecutive samples. The averaged groups are moved by single-sample steps.