Toward an optimal geomagnetic field intensity determination technique

Abstract

Paleointensity determinations based on double heating techniques (in‐field/zero‐field cooling, zero‐field/in‐field cooling, and two in‐field steps with opposite laboratory fields) are generally considered to be functionally interchangeable producing equally reliable paleointensity estimates. To investigate this premise, we have developed a simple mathematical model. We find that both the zero‐field first and in‐field first methods have a strong angular dependence on the laboratory field (parallel, orthogonal, and anti‐parallel) while the two in‐field steps method is independent of the direction of the laboratory‐produced field. Contrary to common practice, each method yields quite different outcomes if the condition of reciprocity of blocking and unblocking temperatures is not met, even with marginal (10%) tails of partial thermoremanence. Our calculations suggest that the zero field first method with the laboratory‐produced field anti‐parallel to the natural remanence (NRM) is the most robust paleointensity determination technique when the intensity of the lab‐induced field is smaller than ancient field. However, the zero field first method with the laboratory‐field parallel to the NRM is the optimum approach when the intensity of the lab‐induced field is larger than the ancient field. By far the best approach, however, is to alternatethe infield‐zerofield (IZ) steps with zerofield‐infield (ZI) steps.