The cooling‐rate effect on microwave archeointensity estimates

Abstract

[1] New microwave (MW) paleointensity data on historical bricks from Northeast Brazil presented a bias toward higher fields when compared to previous cooling-rate corrected double-heating paleointensity estimates; the same relates to the previously reported values for pottery from Southwestern Pacific islands. A simple theoretical approach suggests that the MW bias in both collections is due to a cooling-rate effect on MW estimates. We then experimentally corrected the MW cooling-rate effect on Brazilian fragments, increasing the degree of consistency between the previous and new results (reducing discrepancies from 25% to 8%). Results indicate similar experimental behavior between microwave and thermal procedures despite the different ways in which the energy is transferred into the spin system. Finally, they allow cooling times of less than 90 s to be empirically estimated in most of these MW experiments highlighting the need for systematic cooling-rate corrections to be applied in similar MW paleointensity studies in the future.

Global archaeomagnetic data: The state of the art and future challenges

“…However, some conversion to thermal energy to heat the matrix is likely and pTRM-checks test are now integrated in the microwave technique. Recent studies have also included checks for evaluating the cooling rate effect (e.g., Poletti et al, 2013;Ertepinar et al, 2020).…”

Comparison of Thermal and Microwave Paleointensity Estimates in Specimens Displaying Non‐Ideal Behavior in Thellier‐Style Paleointensity Experiments

“…Since the MW‐Perp and the Th‐Perp data sets are not statistically distinguishable, any cooling rate effect would be relatively minimal. Further, the MW‐OT data have a faster cooling rate but give lower estimates than the Th‐OT data, which is the opposite of the expected cooling rate effect (Poletti et al, 2013).…”

Reproducibility of archaeointensity determinations with a multimethod approach on archaeological material reproductions

“…As in this case the field was applied in the direction of NRM, no anisotropy correction had to be performed. The higher mean result obtained might be ascribed to the fast cooling rate of this method (Poletti et al, 2013). One sample did not provide reliable results due to alteration during the microwave experiment.…”

“…In the microwave determinations the field is applied in the same direction as the natural remanence, thus no anisotropy correction has to be applied in this case. Poletti et al (2013) demonstrated that for a set of Brazilian ceramics the differences in cooling rate between the MW and standard heating methods can result in overestimates of MW results of up to 25%. The higher average results obtained from the MW may be due to cooling rate.…”