Phosphate as an indicator of occupational intensity at shell midden sites on the central coast of British Columbia

We use reagent colorimetry to measure phosphate concentrations in fine-fraction matrix from shell midden sites on the central coast of British Columbia, Canada. The results show strong correlations with fish bone densities (that is, number of identified specimens per litre of fine fraction matrix) and site area, which have been used previously to infer the intensity of site occupation in the region. Despite a long history of use in shell midden studies, elemental analysis, and phosphate analysis in particular, is not commonly applied in these contexts. Using multiple lines of evidence, we show that phosphate measurement from small amounts of matrix from bucket-auger samples or cores is an inexpensive and replicable way to assess variability in the intensity of site use, conceptualized as a function of the scale, duration, and frequency of occupational events, between locations and over time. This method should be easily extended to other shell midden sites, especially where the majority of fine matrix is of anthropogenic origin.