Sourcing archaeological asphaltum (bitumen) from the California Channel Islands to submarine seeps

Abstract

Asphaltum, often referred to as bitumen, is a naturally occurring form of petroleum that was used by ancient cultures for thousands of years. Asphaltum deposits are found throughout the world and occur both on land and submerged under water. Ethnohistoric accounts of native Californians suggest that asphaltum from terrestrial seeps was shaped by hand into cakes and traded throughout Southern California and was the only grade of asphaltum used to manufacture plank-canoes. While there are no terrestrial seeps on the California Channel Islands, drift asphaltum exuded from submarine seeps can frequently be found washed up on the shore. It remains unclear to what extent prehistoric island populations relied on this drift asphaltum and whether or not they acquired terrestrial asphaltum through trade. This study combines gas chromatography/mass spectrometry (GC/MS) and liquid chromatography coupled with carbon isotopic analysis in an effort to identify the sources of six archaeological bituminous mixtures from San Nicolas and San Miguel islands. We compare the archaeological asphaltum to four modern samples collected from marine tarballs and a mainland terrestrial seep. Further, we compare our results to a USGS chemometric database to determine if our archaeological samples match extant sources. Our results show that prehistoric peoples on the Channel Islands utilized drift asphaltum from submarine seeps in a variety of technological applications throughout the Holocene. The methods used in our study are globally applicable and can be used to address a variety of broad anthropological questions.