Trading obsidian in worlds old and new.
Obsidian is a silica rich volcanic glass that has been prized for millennia because it can easily be shaped into a point, thanks to its property of conchoidal fracture (shaped like a shell). Its use in tools dates from the lower Palaeolithic, 2.6-1.7 million years ago, long before the appearance of modern humans. Being easier to work than flint, easy to carry and high in value, it was probably the first trade item in history. Since it has only limited sources, wherever a felsic volcano has erupted and the lava has rapidly cooled to a glass, it is widely used to trace early exchange networks.
Peoples who lived near a source became rich from it, and one of the earliest known towns, Catal Huyuk (7500-5600 BCE) in modern Turkey was built upon the resulting wealth. Obsidian was also passed around in the Americas soon after man's currently recognised arrival. Some of these prehistoric exchange networks are now emerging from the fogs of time, with a little help from Archaeology's friends in the Earth sciences.
Geochemical fingerprinting of volcanic rocks for both major and trace elements is a standard tool of Earth science. Back in the 1960's, Colin Renfrew (Britain's most influential prehistorian of the last century) realised that this could be used to trace distribution routes, and be of particular importance in prehistoric studies. The abundance of obsidian artefacts in excavations worldwide set Renfrew onto his idea in the first place. It is plentiful, easily traced and can be analysed non-destructively. This idea had been applied worldwide by many archaeologists, and this method is now a standard tool of the discipline.
The analysis is done using a LAICPMS (explained in the first part of this series at http://tinyurl.com/c9nba7f) for trace elements and X-ray fluorescence for the major elements. This last methods bombards the sample with radiation to excite the electrons within. These electrons absorb the energy of the rays and jump to a higher energy level. When they fall back to their default state each element gives off radiation of a characteristic wavelength. A spectrometer then measures and collates these, and delivers a printout of the samples relative elemental composition.
Meaningful data from the Paleolithic is more scarce, as the world was less densely populated, peoples nomadic and artefact finds more isolated. The Neolithic era starting in 11000 BCE saw the first towns appear, including Jericho and Catal Huyuk. It was a time of early agriculture and evolving trade in the Near East, with obsidian the main long distance good. It was used for both hunting and agricultural implements, as well as luxury goods like mirrors. A bit earlier in the Americas, similar trade in this raw material for tool making was starting with the Clovis culture, spreading out from the obsidian sources in the west.
Obsidian distribution networks have been traced all over the Americas, from California and Oklahoma to the southern cone in Argentina. It allows us to trace movement patterns, and by dating artefacts using C14 from associated organic remains such as charcoal, to learn how sources us changed over time. In some periods all the obsidian used was local, at others some pieces came from far away.
In the Mediterranean there are several major sources, each with its own dispersion pattern. They include some of the Greek islands, Sardinia, Cappadocia and Anatolia (Hassan Dag being the volcano near Catal Huyuk). They can be distinguished from each other using trace elements such as yttrium, barium and zirconium. In Sardinia and Melos, the changing pattern of use of different lava flows as sources has also been traced, along with the different places each of these were traded to.
Using Sardinia as an example, three main sources were exported, called SA, SB and SC. Southern France liked using SA (95%), while SC was more common in Northern Italy (50%). The reasons for these preferences are unknown. Maybe they reflect choice at the customer end, or that they follow the routes chosen by different traders using one source and travel pattern.
In Anatolia the two main source areas had different routes and dispersion patterns, which evolved through time. Experts theorise that in all early obsidian trade, hunting parties played a big part in the physical movement of goods. The trade evolved from a limited amount in the early Paleolithic through to agricultural tools in early pre-metalworking farming communities. The routes widened from lines along the coasts and rivers to a more trellis like pattern over time, eventually covering the whole Levant (Lebanon, Syria) as far as the Euphrates river in modern Iraq.
As the technology has gotten cheaper, analysis of the entire excavated artefact assemblage from a site has become more common, allowing us to recognise the occasional piece from further away. Before, when only a few pieces per site were analysed, the sample chosen might not have been representative. Combined with stratigraphy it allows us to constrain when a source was exploited.
The use of obsidian fades during the transition from hunter gathering to agriculture as metals began to spread, and these imported artefacts gradually disappear from the archaeological record. Long distance exchange networks begin to be replaced by trade, mediated by money as societies began to differentiate out of the Neolithic into the copper and bronze age.
Image: Obsidian Clovis point, USA, circa 11000BCE, made from Black Tank obsidian sourced 50 miles away.
Credit: Dan Boone/Rvan Belknap
http://geokult.com/2011/06/26/hasan-dag-and-catal-huyuk/
http://shell.cas.usf.edu/~rtykot/PR22%20-%20AccChemRes%202002.pdf
http://shell.cas.usf.edu/~rtykot/PR91%20Tykot%202011.pdf
http://www.archatlas.org/ObsidianRoutes/ObsidianRoutes.php
https://inlportal.inl.gov/portal/server.pt?open=514&objID=1269&mode=2&featurestory=DA_164833
http://goafar.org/AFAR/Reading_files/Obsidian-The%20Metal%20of%20the%20Maya.pdf
