Ancient Olmec tar trade revealed by combined biomarker and chemometric analysis

Resources such as tar, basalt, kaolin clay, and hematite are available in distinct areas near the ancient Olmec region of the southern Gulf Coast of Mexico. This uneven distribution of raw materials has led scholars to suggest that Olmec leaders controlled the sources of raw materials and regional trade, from which they derived economic and political power. The purpose of this study is to improve understanding of Olmec tar trade, which served as binder, sealant, and decoration. Our novel approach combines molecular archaeology based on the geochemistry of biomarkers (biodegradation-resistant molecular fossils) with chemometrics (multivariate statistics) to identify genetic relationships among crude oils, seep oils, modern applications, and archaeological artifacts excavated from ten Early and Middle Formative sites (1800-400 BCE) sites. Seven source-related biomarker ratios were calculated from peak heights on terpane and sterane mass chromatograms. We employed hierarchical cluster analysis dendrograms, which yield a simple view of genetic families of samples where cluster distance measures the degree of similarity. Our results illustrate patterns of Olmec commodity exchange and intra-regional interaction and the possibility of elite control of some tar procurement and distribution.