The Role of Mineral and Organic Composition on the Phosphorus Content of Prehistoric Pottery (Middle Neolithic to Late Bronze Age) from NW Spain

Abstract

Phosphorus is a key element for identifying past human activity. Recently, phosphorus analyses have been extended to archaeological objects, aiming at distinguishing how depositional contexts contribute to its enrichment. In archaeological pottery, phosphorus might depend on several manufacturing and postdepositional processes (i.e., addition of organic temper, pigments, diagenetic incorporation). We analyzed by XRD, XRF, and mid-infrared (FTIR-ATR) spectroscopy 178 pots from eight NW Spain archaeological sites. These sites encompass different chronologies, contexts, and local geology. The phosphorus content was highly variable (224–27,722 mg kg−1) overall but also between archeological sites (1644 ± 487 to 13,635 ± 6623 mg kg−1) and within archaeological sites (4–36, max/min ratio). No phosphate minerals were identified by XRD nor FTIR-ATR, but correlations between phosphorus content and MIR absorbances showed maxima at 1515 and 980 cm−1, suggesting the presence of two sources: one organic (i.e., phosphorylated aromatic compounds) and another inorganic (i.e., albite and K-feldspar). Phosphorylated aromatics were most likely formed during pottery firing and were preserved due to their high resistance to temperature and oxidation. Meanwhile, albite and K-feldspar are among the P-bearing minerals with higher P concentrations. Our results suggest that P content is related to intentional and non-intentional actions taken in the pottery production process.