M.A. Theses: Gregg Matthew Sullivan, 1993

Postdepositional Leaching of Shell in Two Northwest Coast Shell Middens

Leaching is the dissolution and removal of a chemical constituent from a sedimentary deposit. Recent research has questioned the assumption that dark, shell-less deposits found at the base of some prehistoric shell midden sites on the Northwest Coast represent periods during which shellfish were not being exploited by ancient peoples. Instead, the composition of this layer is thought to be caused by postdepositional leaching of shell by groundwater. The applicability of this hypothesis is assessed using sediment samples from two archaeological sites on the British Columbia coast, Namu and Tsawwassen, both of which possess a black, shell-less layer at their bases.

The mobility of calcium carbonate, the main component of shell, in sedimentary environments is reviewed, and the main factors in its dissolution are identified. Archaeological studies are reviewed which propose the loss of shell from archaeological deposits to explain differences in shell content. Based on this literature review, a list of the expected effects of chemical weathering and leaching of shell on the composition and texture of archaeological shell-bearing deposits is presented. Samples are analyzed for particle size distribution of shell and mineral component, carbonate and organic matter content, pH, and composition of part of the sand size fraction by point counting. At Tsawwassen the dark layer does not appear to have been affected by cultural processes, although other layers show evidence of chemical weathering of shell. While shell seems to have undergone severe chemical weathering in the dark layer at Namu, it is difficult to ascertain the extent to which leaching has removed shell content over time. In conclusion, the technical and theoretical difficulties presented by the use of leaching to explain differences among archaeological shell-bearing deposits are discussed, and alternative approaches for the future study of this phenomenon are presented.