Magnetic detection of anthropogenic fires at Xiaodong Rockshelter, Southwest China

Abstract

The Xiaodong Rockshelter, located on the southwest edge of Yunnan Province, is known as Southeast Asia's oldest (>43.5 ka) and northernmost Hoabinhian technocomplex site. The rockshelter preserves a rich record of animals, plants, and lithic artifacts excavated from sediments with a thickness of 4.6 m. New dating reported here indicates that the stratigraphic sequence spans from 65 ka to 15 ka. Several layers in the sedimentary sequence show evidence of fire, representative of the earliest evidence of fire by Hoabinhian population in a tropical-subtropical area. Here, we use magnetic methods coupled with mineral analysis to differentiate natural material from anthropogenically fired sediment. Archaeological fire events are characterized by higher magnetic concentrations and coarser magnetic grains compared to natural sediments. Significant magnetic enhancements were caused by the transformation of paramagnetic iron-bearing silicates into ferrimagnetic, spherical-shaped magnetite with increasing temperatures. Notably, a pronounced magnetic enhancement was observed between 1.8 and 2.5 m, spanning between 42 and 34 ka, indicating intense and concentrated heating, with estimated firing temperatures reaching ca. 400 °C. Additionally, three thin layers exhibiting magnetic enhancement were detected at depths of 3.65 m, 4.45 m, and 4.55 m, dating to ca. 55.6 ka, 62.3 ka and 64.8 ka respectively. This suggests three short-term fired ash deposits with minimal vertical magnetic enhancement, indicative of fire temperatures at ca. 350 °C. The magnetic method proves effective in detecting anthropogenic fire in archaeological sediments and potentially estimating ancient fire temperatures.