Residue analysis links sandstone abraders to shell fishhook production on San Nicolas Island, California

Excavations at the upper component of the Tule Creek site (CA-SNI-25), dating between approximately 600-350 cal BP, yielded numerous well-preserved sandstone abraders referred to as saws. Many of these tools show heavy use-wear and abundant white residue still adhering to the surface. We used X-ray diffraction (XRD) analysis to characterize the residue from two of the abraders, which identified the mineral phases calcite and aragonite (both CaCO3), albite (NaAlSi3O8), and quartz (SiO2). A scanning electron microscope (SEM) equipped for Energy Dispersive X-Ray (EDS) analysis identified the elements C, Ca, S, Na, and Al in the samples, confirming the XRD results. Albite, quartz, and calcite in the scrapings are consistent with the mineralogy of sandstone, though the presence of calcium carbonate in the form of calcite and aragonite suggests marine shell is also present in the residue samples. XRD and SEM analysis of a modern red abalone (Haliotis rufescens) shell indicates that the inner-layer (nacre) consists mostly of aragonite phase calcium carbonate, whereas the outer layer (epidermis) is made up mostly of calcite phase. SEM images revealed that calcite and aragonite from the archaeological residues display similar morphologies as the material from a modern abalone sample, and a greater presence of aragonite over calcite suggests the abraders were primarily used to work the inner layer of the abalone shell. These results provide a functional linkage between sandstone saws and shell fishhook production at CA-SNI-25.