Preliminary results of paleomagnetism, rock magnetism and AMS in a soot-layered speleothem from Cueva Mayor (Atapuerca, Spain)

Speleothem are excellent recorders of the Earth’s magnetic field and climate variation. The nature and origin of the magnetic minerals trapped into the calcite laminae are controlled by climate and environmental processes acting on the surface soils and inside the cave.

In this work, we analysed samples from a stalagmite from Cueva Mayor in the archaeological site of Atapuerca (Burgos, Spain). Cueva Mayor site hosts a very important record of Pleistocene human occupation. Finding speleothems that record signs of human activity is unusual. However, in the karst system of the Sierra de Atapuerca, different works on speleothems revealed a significant human fossil record. The stalagmite studied has a small size, the sampled section measures 10 cm from base to top and is not oriented. It shows a calcite laminae alternation with darker micritic and/or ash-rich laminae, composed of aggregates of soot/smoke in the last 2.7 cm to the top. These black soot laminae are interpreted as derived from anthropogenic fires. The remaining part the stalagmite is a sequence of whitish and brownish laminae. A high detrital fraction is inferred from the brown layers. Available U-Th age data on a nearby stalagmite indicate that they grew during the last 14 kyr approximately.

In order to characterize the magnetic properties in a stalagmite of special interest considering its record of human activity, we have carried out experiments on paleomagnetism, rock magnetism, and anisotropy of magnetic susceptibility. We obtained paleomagnetic directions for most samples of calcite laminae with high detrital content and laminae with soot by alternating field demagnetization. Isothermal remanent magnetization acquisition curves of and hysteresis cycles show the presence of low coercivity ferromagnetic minerals in the soot-bearing samples, while the magnetization intensity in the rest of the samples is too weak to show clear results. The thermomagnetic curves reveal magnetite in both brownish-white and soot-containing samples. Other very low Curie temperature magnetic phases also appear in the soot samples. Finally, AMS shows a triaxial magnetic fabric with magnetic foliation pseudo-parallel to the calcite lamellae and horizontal lineation.

Acknowledgments: This work was funded by the Agencia Estatal de Investigación (España) (PID2019-105796GB-100), the postdoctoral program María Zambrano 2021 (España), the Junta de Castilla y León (España) (project BU037P23) and the Fundação para a Ciência e a Tecnologia (Portugal) (PTDC/CTA-GEO/0125/2021).