Archaeologic and geologic records of paleomagnetic secular variation during the late Holocene in Israel

Yael Segal (MSc thesis)

Partners:

Haim Barbe, Israel Antiquities Authority

Anick Chauvin, Rennes1 University

Abstract

We recorded the paleo-geomagnetic field (intensity and direction) during the last four millennia in Israel. The data for the paleo field directions were collected from the Ze’elim Formation in the alluvial terrace of the Zeelim Creek (hereafter - ZT) at the Dead Sea shore and several archeological sites in Israel. Successive sedimentation of fine laminas in most of the sediments at the ZT site can provide continuous paleomagnetic record. They show a high accumulation rate, ranging from 3 to 13 mm/yr, which enables high-resolution sample collection. Dense ¹⁴C dating, and earthquakes chronology were investigated in a previous study at the site.  I obtained 310 oriented samples from about 6.5m of the 9m of exposed sections. The sediment ages are estimated to the 17^th century BC till the 13^th century AD. Stability tests were performed on the samples to explore the properties of the measured NRM, estimate the accuracy of the results, and define the relationships between the magnetization and the post depositional deformation in ZT section. Alternative field demagnetization reveals stable, single-component vectors in most of the samples. The Fisher mean direction of 176 horizons is 355°/43° (Dec./Inc.), R= 0.97, α₉₅=1.88° and к=31. I observe four types of direction variations with time in the ZT record. The first is rapid directional fluctuation, with a shift of up to several tens of degrees from one horizon to the other, interpreted as noise. The second is gradual directional change of a few tens of degrees within several centuries interpreted as a non-dipole field effect. The third type is slow westerly shifting of about 0.01 °/yr between 17^th century BC and 4^th century AD, which is interpreted as a change in the dipole field. The fourth is an antiphase variation of the Inc to the Dec. The record from ZT is compared to data collected from eighteen archeological objects originating from ten archeological sites. The objects were heat sources such as ceramics production, lime production, fireplaces and thermal baths. These installations were heated to above the Curie temperature and acquired the magnetic field while cooling. I sampled sites from the 8^th century BC till the 19^th century AD. The mean direction sampled is 001°/50° (Dec./Inc.), R= 0.979, α₉₅= 5.2°, к= 45.6, which is identical to the present direction (003°/48°). There is a good agreement between the paleomagnetic secular variations (PSV) obtained in ZT and the results from the archeological sites. There is also correlation between the PSV results from ZT and paleomagnetic results from other regions in most of the section.  The results of this study suggest that the paleomagnetic direction curves (inclination and declination) obtained in Ze’elim sediments can be used as a paleomagnetic field reference to estimate ages of archeological sites and geological units.

For the measurements of geomagnetic absolute paleointensity I sampled nine archeological objects from five archeological sites. Their ages range from the 1^st to the 19^th century AD. The paleointensity measurements were performed using the original double-heating Thellier method. Testing of partial thermal remanent magnetization (pTRM) and corrections for the anisotropy of thermo remanent magnetization were performed. The behavior was ideal in most samples during the Thellier experiment. The results show a trend of intensity decrease during the last 13 centuries. The paleointensity results show good correlation with measurements from other regions and with the global dipole moment curves.  At the ZT site I obtained relative paleointensity of the field, which correlates well with the intensity measured from Syria and with the record obtained from the archeological sites in Israel.

Yael and a pile of copper slag in an archaeological site from the 8th century in the Arava Valley, near Elat.   back to top