Organic Phosphorus and Elemental Ratios as Indicators of Prehistoric Human Occupation

Abstract

Human occupation enriches soils with phosphorus, which may provide a useful chemical tracer for site delineation in archaeological studies. In the present study, fine scale soil profiles of inorganic, particulate organic and total phosphorus through a prehistoric Indian site and nearby control site in Cape Cod, MA, indicated a greater predictive value of particulate organic P over inorganic P and total P in determining the vertical boundaries of an anthrosol. The value of inorganic P and total P as occupation tracers was confounded by vertical migration of inorganic P species. Inorganic P mobility was found to result from diagenesis and other post-depositional alterations, including the effects of recent sea water flooding on soil concentrations of dominant exchangeable cations. Parallel measurements of organic carbon and total nitrogen confirmed organic enrichment within the anthrosol. Evaluations of elemental ratios distinguished the anthrosol from the unimpacted soil locations by a 50% higher average C:P ratio and a two-fold higher average C:N ratio of its residual organic fraction which declined exponentially with depth. Combining C, N and organic P data yielded a robust tracer for distinguishing anthropogenic organic matter enrichment from naturally occurring deposition through all soil levels examined at this prehistoric site. Profiles of organic matter C:OP and C:N ratios indicated that C enrichment occurred well below the stratigraphically defined anthrosol boundary indicating site habitation prior to the anthrosol deposition. This contention of earlier site usage was supported by comparison of depth profiles of C:OP ratios with lithic debitage density profiles. Significant correlations between C:OP ratios and lithic debitage density suggested that C enrichment relative to OP may be a useful indicator of habitation and may yield insight into intra-site variation in the intensity of land use.