Geochemical Methods Help Resolve A Long-Standing Debate In Amber Palaeontology

Ambers are fossilized plant resins that have found wide use in jewelry-making. They are also a major archive for the study of invertebrate evolution. Yet, their full potential for palaeontological research has long been restricted by uncertain age and place-of-origin relationships between different amber deposits. The ambers are now found far from where they formed and, in some cases, purely palaeontological methods are unable to resolve their exact origin.

In a recent study, we used geochemical techniques to show that while the western Ukrainian Rovno amber deposit formed at the same time as other major European deposits, it happened in a significantly different region. This and related studies strengthen the background for the study of amber-hosted fossils and open up the exciting possibility for European amber deposits to serve as a unique biogeographical testbed.

While fossils of vertebrates and shelly invertebrates are common in the rock record, softer-bodied invertebrates are very rarely preserved in sedimentary rocks, as their tissues are prone to decomposing. Plant resins can, however, trap organisms in such a way that their tissues are protected from decomposition. When the resin gets buried and hardens into amber, delicate fossils can survive in nearly pristine condition for tens of millions of years, thus providing invaluable material for palaeontology.

Central-eastern Europe is rich in amber deposits that formed during the Eocene Epoch, 53–34 million years ago. The most famous and best-studied of these is Baltic amber, which has been redeposited at a number of sites along the margin of the Baltic Sea; other actively-studied European deposits include Bitterfeld amber from eastern Germany and Rovno amber from western Ukraine.

There is a major debate about whether these European ambers formed independently in separate localities, or whether they represent a single primary amber source that was later carried into these separate localities by rivers and currents. Palaeontological evidence has so far been inconclusive — many fossil species are shared among these deposits, while others seem to be constrained to a single deposit. Since these opposite views on European amber origins have drastically different implications for what can be drawn from inter-deposit fossil fauna comparisons, resolving this debate is of high importance to amber palaeontology.

In recent years, isotope analyses — a long-time staple of geochemistry — have started to make inroads into amber studies. Both carbon and hydrogen occur naturally as two different isotopes that act very similarly in chemical reactions but have a difference in mass. The difference is enough to slightly affect the ratio of these isotopes in different environments and compounds. Crucially, analyzing both isotopes in amber can reveal aspects of their history.

A previous study conducted in our lab (1) found marked differences in the hydrogen isotope composition of Baltic and Bitterfeld ambers, building a strong case for their separate origins. To follow this up, we focused on the isotopic composition of the similarly controversial western Ukrainian Rovno amber deposit.

The carbon isotope composition of plant matter (including resins) is known to vary depending on the plant species and on environmental conditions such as water or nutrient availability. However, on longer timescales, global conditions can also play a role. For example, a study by Tappert and others (2) revealed a decreasing trend in amber carbon-13 isotope abundance over the past 50 million years, likely tied to changes in the composition of the atmosphere. This robust isotope trend can then be used as a tool to study the age of other amber deposits. In our case, we found that carbon isotopes in Rovno amber correspond best with Baltic and Bitterfeld ambers, suggesting that the major European amber deposits share a similar Eocene age.

The hydrogen isotopic composition of plant matter, on the other hand, is mostly inherited from that of rainwater, where the isotope ratios are related to local climate conditions. Rovno amber was found to be significantly depleted in the hydrogen-2 isotope compared to Baltic amber. Such a difference can most parsimoniously be explained if the two amber deposits formed in areas that experienced different climates and were thus relatively distant from each other. Whereas Rovno amber trends towards subtropical conditions, Baltic amber seems to have formed in a firmly temperate region.

Palaeogeographical reconstruction of Eocene Europe with the Baltic, Bitterfeld and Rovno amber origin areas indicated. Based on a map by Ivanov and others (3). Credit: Kaarel Mänd

These results help bring clarity to the debate over the origin of European Eocene amber deposits by providing independent support for previous palaeontological inferences that suggest the deposits should be treated as separate. The picture that is emerging as a result of these studies is one of an Eocene Europe which hosted at least three different amber-producing forest regions on opposite margins of the continent, all of which appear to have been active roughly simultaneously. This is exciting news for palaeontologists who seek to reveal the biogeographical complexity of Eocene Europe through comparative studies of amber deposits. It should also be noted amber played an important role in Classical Greek and Roman trade. Our studies could help to better define the provenance of archeological amber specimens and elucidate amber trade routes.

Geochemical methods such as isotope studies are proving to be valuable additions to the more traditional methods of palaeontology. Amber isotope studies, for one, are just getting started with breathing new life into the centuries-old field of amber research. One potential culmination of such research is an amber “palaeothermometer” based on hydrogen isotope ratios, which could assist in the reconstruction of climate conditions present during intervals of amber formation. While the challenges in the calibration of such a thermometer are many (for example, the original hydrogen isotope ratios have a slight tendency to be lost over tens of millions of years of being buried), it might nonetheless provide invaluable independent checks on palaeontology-based climate reconstructions.

These findings are described in the article entitled Distinct origins for Rovno and Baltic ambers: Evidence from carbon and hydrogen stable isotopes, recently published in the journal Palaeogeography, Palaeoclimatology, Palaeoecology. This work was conducted by Kaarel Mänd, Karlis Muehlenbachs, Alexander P. Wolfe, and Kurt O. Konhauser from the University of Alberta, and Ryan C. McKellar from the Royal Saskatchewan Museum.

References:

1. Wolfe, A. P., McKellar, R. C., Tappert, R., Sodhi, R. N. S. & Muehlenbachs, K. (2016). “Bitterfeld amber is not Baltic amber: Three geochemical tests and further constraints on the botanical affinities of succinite.” Review of Palaeobotany and Palynology, vol. 225, pp. 21–32.
2. Tappert, R., McKellar, R. C., Wolfe, A. P., Tappert, M. C., Ortega-Blanco, J. & Muehlenbachs, K. (2013). “Stable carbon isotopes of C₃ plant resins and ambers record changes in atmospheric oxygen since the Triassic.” Geochimica et Cosmochimica Acta, vol. 121, pp. 240–262.
3. Ivanov, V. D., Melnitsky, S., I. & Perkovsky, E. E. (2016). “Caddisflies from Cenozoic resins of Europe.” Paleontological Journal, vol. 50, pp. 485–493.