Breaking ground, beyond the floor assumption: An experimental approach to seaweed ash-cemented deposits in the Atacama Desert coast

On the Atacama Desert coast, a unique type of ash-cemented deposit has been reported in architectural stone-built structures from the Late Archaic Period (ca. 5500–4000 BP). Functional approaches traditionally infer these layers as domestic constructed floors that also served to seal funerary deposits. During five decades, it was assumed that coastal communities constructed these “sealing-floors” using a mortar made with kelp ashes, water, sand, and shells, without a rigorous investigation into their formational aspects. To address this gap, this paper explores the formation of seaweed ash-cemented deposits using a 14-trial experimental design. The study explores two formational techniques: (1) in-situ hearths of naturally dried-cemented; and (2) ash-mortars artificially made and applied to flat surfaces. The trials verified the seaweed combustion hypothesis advanced in the local literature and identified new aspects of kelp’s combustion qualities and the sedimentary trajectories of the resultant ash deposits. Several trials yielded results highly consistent with previously reported constructed floors on the Atacama Desert coast, demonstrating their distinctive cementation. These findings enable more nuanced interpretations of coastal contexts where these features have been reported and broaden the interpretive potential for future research.