Using light to describe the ancient world

Archive for September, 2011

#Archive: The origin of Baltic Amber

Alexander Wolfe and colleagues used Fourier-transform mid-infrared spectroscopy to determine the botanical origin of Baltic amber. Image is from Wikimedia commons

The value of amber has been recognised throughout human history. Amber beads were found with Teti, a pharaoh who reigned during the sixth dynasty of Egyptin ~2340 BC. Amber remained a prized possession into the 18th Egyptian dynasty as large beads were among the chattels in Tutankhamen’s tomb (Gestoso Singer 2008; Serpico and White 2000). The trade of amber flowed to Northern Africa down the ‘Amber Road’, where it was collected from the shoreline of the Baltic Sea. Such was the importance of amber that knights of the medieval Teutonic Order occupied the southern coast of the Baltic Sea in order to control the Amber Road (Heinze, 2003). The value of amber outlasted the Teutonic Knights, and in the early 1700s an entire room of the Catherine Palace was decorated in panels of amber from the coast of theBaltic Sea (the amber was later stolen during World War II). Baltic amber is still a valuable commodity, not least of all for the scientific secrets it holds.

In 2009 Alexander Wolfe and colleagues sought the botanical origin of Baltic amber. Amber is hardened tree resin and can be chemically complex. Wolfe and colleagues describe amber as being “…polymerized from a broad range of isoprenoid compounds originally produced by plant secondary metabolism. These compounds include primarily terpenoids, carboxylic (resin) acids and associated alcohols…” Further, “…the great diversity of organic compounds present in modern and fossil resins…are of considerable use in establishing relationships between amber and source trees…” Hence, the chemistry of amber is a ‘fingerprint’ for the tree that produced it. In this way, it should be possible to reconstruct the forest that was responsible for the Baltic amber.

Earlier studies had linked the production of Baltic amber to trees in one of two families, Araucariaceae or Pinaceae. Araucariaceae is mostly found in the Southern Hemisphere, and resin from the araucarian Agathis australis was a significant trade good in New Zealand during the 19th and 20th centuries. Pinaceae are mostly found in the Northern Hemisphere and are typified by pine trees. Spruce and other trees in family Pinaceae occur around theBaltic Sea today. Both tree families are viable candidates for the vast quantities of Baltic amber that has been recovered, but Wolfe and colleagues observed that “…neither group fully satisfies the range of geochemical and phytogeographical criteria…” In response, the authors decided to study the amber with vibrational spectroscopy.

Baltic amber was produced by a family of trees with a single descendent that today lives only in Japan. Image above is of Koyamaki (Sciadopitys verticillata), from Wikimedia commons.

Fourier-transform mid-infrared (FTIR) spectra were collected from modern conifer resins: Five species of Araucariaceae, 17 species of Cupressaceae, 26 species of Pinaceae and Sciadopitys verticillata, the only living representative of Sciadopityaceae. A set of modern and fossil resins were also analysed in tandem, to gauge whether the chemistry of extant trees could be matched to ancient amber. Finally, specimens of Baltic amber fromGermany,Latvia,Poland,Russia and southernSweden were analysed. From Wolfe and colleagues, “…[s]amples were first examined with a binocular microscope and crushed to fragments of less than 500 mm prior to mounting directly on NaCl stages…”

The FTIR spectra of the modern and fossil pairs were indistinguishable, meaning that the identity of fossil resins could be predicted from modern specimens. Key diagnostic regions were subsequently identified in the resin and amber spectra. These regions were to attributed functional groups within the specimens (e.g. OH stretching/asymmetric CH stretching of terminal alkene): the amber and resin specimens could be distinguished from variations in these six regions. Wolfe and colleagues analysed these diagnostic spectral regions with hierarchical clustering. In essence, spectra that were most similar clustered together, producing a ‘tree’ that described the relatedness of the samples to one another (the same method is used in cladistics). Surprisingly, Baltic amber clustered most closely with Sciadopitys verticillata and not species of Araucariaceae or Pinaceae, strongly suggesting that Baltic amber was produced by family Sciadopityaceae. The only living member of Sciadopityaceae is the Koyamaki which is endemic toJapan. In the words of Wolfe and colleagues, “…[o]ur conclusions challenge hypotheses advocating members of either of the families Araucariaceae or Pinaceae as the primary amber-producing trees and correlate favourably with the progressive demise of subtropical forest biomes from northernEurope as palaeotemperatures cooled following the Eocene climate optimum…”

Gestoso Singer G. 2008. Amber in the ancient Near East. I-Medjat 2: 17-19. http://independent.academia.edu/GracielaGestosoSinger/Papers/180348/Amber_in_the_Ancient_Near_East

Heinze KG. 2003. Baltic sagas: events and personalities that changed the world! Virtualbookworm.com, 340 pp.

Serpico M, White R. 2000. Resins, amber and bitumen. In Nicholson PT, Shaw I (eds.). Ancient Egyptian materials and technology.CambridgeUniversityPress, p 430-475.

Wolfe AP, Tappert R, Muehlenbachs K, Boudreau M, McKellar RC, Basinger JF, Garrett A. 2009. A new proposal concerning the botanical origin of Baltic amber. Proceedings of the Royal Society B: Biological Sciences 276: 3403-3412.

Images sourced from Wikimedia commons:

http://upload.wikimedia.org/wikipedia/commons/2/2b/Sciadopitys_verticillata3.jpg

http://upload.wikimedia.org/wikipedia/commons/d/d7/Insects_in_baltic_amber.jpg

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