Paleointensity

Paleointensity is the ancient strength of the Earth's magnetic field, as recorded by minerals in rocks and by fired archaeological materials. It differs from information about the field's direction or from dating signals and aims to quantify the field’s magnitude at the time the magnetization was acquired.

Estimating paleointensity relies on laboratory experiments that compare the natural remanent magnetization (NRM) of a sample with the remanence acquired under known magnetic fields. The most widely used approach is the Thellier-Thellier protocol, which involves progressive heating and cooling to remove and re-record TRM while applying controlled laboratory fields. Variants and improvements (such as partial TRM checks, anisotropy corrections, and IZZI or multisample methods) are used to enhance reliability and quantify uncertainties.

Materials suitable for paleointensity studies include volcanic rocks, baked archeological clays and ceramics, and lake or marine sediments. In volcanic and archeomagnetic materials, TRM is common and can preserve a record of the ancient field. Sedimentary records can also reflect field strength, but are more susceptible to post-depositional alteration, requiring careful screening and correction.

Units and interpretation: paleointensity results are expressed in microteslas (µT). The present-day field varies by location, roughly 25–65 µT, while paleointensity reconstructions span broad time intervals and document field variations, excursions, and reversals.

Limitations and uncertainties arise from alteration during heating, magnetic anisotropy, cooling-rate effects, and multiple magnetization components. Robust paleointensity estimates rely on multiple specimens, rigorous protocols, and cross-validation with independent records.