COMPARING GRAIN-SIZE ANALYSIS METHODS IN MONGARILLO SHELL MIDDEN SEDIMENT MATRIX, PARITA BAY, PANAMA

The hunter-gatherer Monagrillo culture thrived in Pacific coastal sites of Panama throughout the mid-third to second millennia BCE. The resulting shell midden archaeological sediments collected from two coastal mounds serve as evidence of past post-depositional agents, both natural and anthropogenic. The middens likely had differing functions, with one featuring water-worn artifacts and complete shells, whereas the other includes greater amounts of cultural material and predominately-crushed shells. Human trampling at the latter site is suggested by bulk textural analysis of midden sediments. Comparison of the traditional sieve method with an opto-electronic particle-size analyzer (Camsizer) highlights the challenges when interpreting granulometric data on with widely varying particle densities, shapes, and sizes. Whereas bimodal size distribution and poor sorting characterizes both sites, the Camsizer revealed that the sorting increased slightly from 1.88 for the 92-96 cm interval to 1.68 at a depth of 28-32 cm at the northern site with potential trampling. The southern mound had a greater improvement in sorting from 2.71 at depth to 1.87 for the topmost horizon, suggesting a possibly greater impact of natural processes. The sieve method similarly shows better sorting for upper layers and mea size and skewness values are used to strengthen the case for differing post-depositional histories. Because shell fragments are less dense than the mineral grains, the two size-based methods used here provide a more accurate picture than a settling tube analysis. Comparison of both frequency and cumulative size distribution curves in vertical sections indicates differences in the relative length of exposure to potential trampling and suggests different site formation histories in Monagrillo shell middens. Application of a standardized sediment analysis approach to other regional sites, combined with taphonomic classification of shell fragments, will help determine the duration and magnitude of post-depositional agents.