How Earth Magnetic Field May Provide New Ways of Dating Ancient Archaeological Artifacts
August 16, 2021 — Archaeologists traditionally rely on finding organic remains for radiocarbon dating to pinpoint new finds in time, but often these are not found on sites dating over 5,000 years ago. As a result, dating sites in the ancient Levant from the Holocene (the last ca. 10,000 years) can be problematic – leading archaeologists and geophysicists to expand the possibilities for assessing the age of ancient artifacts with archaeomagnetic dating.
In an article published* in the latest issue of the Proceedings of the National Academy of Sciences (PNAS), researchers from the University of California San Diego’s Scripps Institution of Oceanography and Department of Anthropology have filled in some of the regional gaps in the record of Earth’s magnetic field. To do so, they used artifacts from the Neolithic period spanning roughly 8,200 to 5,500 years ago.
“Earth’s magnetic field has changed significantly in the past with implications for related phenomena, such as deep-Earth processes and evolution of life,” said Scripps geophysicist Lisa Tauxe. “Accurate datasets of its past behavior also provide a dating tool, but until now we have had little evidence of changes in the magnetic field during the Neolithic and earlier periods in the Levant.”
Tauxe collaborated with UC San Diego archaeologist Thomas E. Levy and researchers from Italy, Jordan, and Israel to use data from Neolithic ceramics and flint from the Faynan region of Jordan to extend what is known about changes in the Earth’s magnetic field during that period.
In their paper, the researchers present new, high-quality constraints on the strength of the ancient Earth’s magnetic field (archaeointensity) for 129 specimens from four archaeological sites in Jordan spanning from 7,752 to 5,069 BCE – the oldest record for the Levant, modern-day Turkey, and ancient Mesopotamia. Most of the artifact data came from excavations carried out over the past 20 years by Levy and co-author Mohammad Najjar, a Jordanian archaeologist.
“The Neolithic covered in this research was a period of major changes in human history, notably in the transition from foraging to farming and sedentary societies,” explained Levy, director of the Qualcomm Institute’s Center for Cyber-Archaeology and Sustainability (CCAS), and co-director of the Scripps Center for Marine Archaeology (SCMA). “The Faynan copper-ore district in southern Jordan provides a unique 'landscape' spanning settlements that represent all the phases of the Neolithic.”
Levy added: “Archaeologically this project is remarkable because it suggests that archaeointensity can be applied to constrain the ages of one of the hearth areas of the Neolithic revolution in the Middle East, a time period before widespread use of ceramics, which are traditionally used for this method.”
Tauxe noted: “Our results suggest that one of the weakest magnetic fields of the last 10,000 years in the Levant occurred around 7,600 BCE, when it was about half of today’s magnetic field. The field recovered its strength at a relatively rapid rate during the next 600 years, then weakened gradually until 5,200 BCE.” Those regional readings are consistent with global geomagnetic models.
“The new research from Jordan contributes to lengthening what researchers refer to as the Levantine Archaeomagnetic Curve,” noted Tel Aviv University archaeology professor Erez Ben-Yosef, a senior author of the paper and UC San Diego alumnus (MA ’08, PhD ’10). “The research pushes archaeointensity dating back to the Neolithic period and adds approximately 4,000 years to record.”
The PNAS paper focuses on the peak of the middle Pre-Pottery Neolithic B period ca. 9300–8300 before present (BP) at Ghweir I, a village site near a perennial spring. The archaeologists also investigated finds at other Pre-Pottery sites including Wadi Fidan and Tel Tifdan – when inhabitants started to use raw copper ore to make beads and pigments that were traded throughout the southern Levant region.
The researchers also investigated finds at Wadi Fidan from the Pottery Neolithic period (ca. 8300 – 6500 BP) when societies began to experiment with pottery production, a key element in the storage of foodstuffs for the first time.
“The archaeointensity new data from these artifacts is helping us to fill in gaps and refine the resolution of the archaeomagnetic curve, improving our understanding of past changes in the Earth’s magnetic field for the Levantine region,” observed Scripps’ Tauxe.
The team also investigated a promising material on which to measure archaeointensity: flint. While pottery and several other clay-based materials are widely investigated in archaeomagnetic studies, the use of burnt chert (flint) is far less common, even though flint was the most common raw material for toolmaking in the Paleolithic, Neolithic, and even in the Bronze Age. In those periods, fine-grained siliceous rocks were fired to improve their flaking properties: heating helped to propagate fractures in the material, making it easier to produce stone tools.
“We retrieved burnt flint from our excavation of a room dedicated to the production of flint tools such as awls and borers used in the manufacture of beads,” said archaeologist Levy. “Professor Tauxe and Anita Di Chiara (a visiting scholar in the Scripps Lab) were able to analyze the flint material and obtain archaeointensity data – the first time it’s been shown to work well on flint, not just ceramic specimens.”
The success using burnt flint could greatly enhance use of the material in future archaeomagnetic research, especially for prehistoric periods from the first use of fire to the invention of pottery.
“The Faynan Neolithic landscape provided an ideal outdoor laboratory for testing the viability of the archaeointensity method,” added Tauxe. “It enabled us to produce some of the earliest data in the Levant.”
To compare the new archaeointensity results from Jordan, the researchers also performed radiocarbon dating on the artifacts – yielding dates from 9,750 and 7,000 years ago. Little archaeomagnetic data exists for the period when villages became a new settlement type, plants and animals were domesticated, and the Mediterranean diet arose.
With the latest results, Tauxe, Levy, and Ben-Yosef are convinced that archaeomagnetism could now prove valuable as a tool that could be used when radiocarbon dating is not available or sufficient. “We feel certain that our results, combined with ongoing research, will improve the use of archaeomagnetism as an independent and robust dating tool,” observed Tauxe. “It will also help enhance paleomagnetic global and regional field models.”
*Di Chiara, A, Tauxe, L, Levy, TE, Najjar, M, Florindo, F, and Ben-Yosef, E, The strength of the Earth’s magnetic field from Pre-Pottery to Pottery Neolithic, Jordan. Proceedings of the National Academy of Sciences (PNAS), August 2021.