Improving archaeomagnetic interpretations by reusing magnetically oriented samples for micromorphological analysis

Here we present a novel approach that combines soil micromorphology with the analysis of magnetically oriented samples to improve the interpretation of archaeomagnetic results. The aim is to test whether irregular archaeomagnetic data can be examined with micromorphological soil analysis in a single sample methodology, allowing said archaeomagnetic data to contribute to more meaningful archaeological interpretation. Experimental work included reusing oriented magnetic samples previously treated with sodium silicate resin, which were then re-impregnated with polyester or epoxy resins to produce thin sections. Initial experimentation was conducted to understand the potential effects of sodium silicate treatment on the optical and mechanical qualities of the thin sections. When no significant optical anomalies were identified, the methodology was tested using samples gathered from hearths at level X (ca. 52 ky BP) at the Middle Palaeolithic site of El Salt, Spain. Thin section analysis was conducted on archaeological hearth samples to identify syn- and post-depositional processes influencing the magnetic data. Micromorphological analysis of magnetic samples can identify the causes of anomalous magnetic direction, attributing them to specific sedimentary features rather than sampling or measurement errors. Discrepancies in magnetic signatures are linked to the presence or absence of certain combustion layers, such as black and white layers, as well as the impact of bioturbation and mechanical deformation. This integrative approach improves the interpretive potential of archaeomagnetic studies by not only validating the magnetic data as representative of the deposit but also providing a visual understanding of the sedimentary structure creating the magnetic signature. Our findings advocate for the routine inclusion of thin section inspection in archaeomagnetic research, particularly when dealing with complex sedimentary sequences and ambiguous magnetic data.