Cooling Rate Correction in Paleointensity Experiments on Archeological and Geological Materials

Obtaining accurate estimates of the absolute intensity of the past geomagnetic field (paleointensity) is one of the major challenges in paleomagnetic research. Paleointensity data are typically determined by replacing the ancient thermoremanent magnetization (TRM) acquired in an unknown field with a laboratory TRM acquired under controlled conditions. A major source of uncertainty in paleointensity experiments arises from cooling rate effects, as the two TRMs are acquired under significantly different cooling conditions. Néel theory for single-domain (SD) particles predicts that ancient (slow-cooled) TRM is larger than laboratory (fast-cooled) TRM, and that the ratio between them is linearly proportional to the logarithm of the cooling rate ratios. Here, this relationship is tested for non-ideal SD materials, providing an empirical basis for the validity of cooling-rate correction experiments. Eighty-two archeological baked-clay artifacts and basalt samples were given eight TRMs under an exponential cooling process, using seven different cooling-rate constants spanning 2.5 orders of magnitude, resulting in cooling times ranging from 30 min to 1 week. These samples exhibited a range of domain state properties, including SD, vortex, strongly interacting particles, and mixtures of different populations. The results show that the ratio of slow-to fast-cooled TRMs is a linear function of the logarithm of the exponential cooling rate constants, regardless of the domain state. Cooling rate corrections, calculated for more than 2,100 archeological specimens using three different exponential cooling constants, are analyzed and provide practical guidelines for TRM effects in typical archeological materials. The results highlight that cooling rate correction should always be measured.