A solar storm dates Viking Age trade

We used a solar particle event to anchor a sequence of radiocarbon dates in time, to date the trade routes of Scandinavia’s oldest town and the beginning of the Viking Age.
A solar storm dates Viking Age trade

Chronology, the measurement of time, is vital for our understanding of the past. It puts events on a timeline, tells us which events were synchronous or in which order they happened. It is thus the first step on our way to disentangle causes and effects. Historians use e.g. written sources, or artefacts with date stamps such as coins, for building a chronology. However, this only works for recent times and for regions where writing and calendars were established.

Imported beads from the excavations in Ribe, in stratigraphical order.
Imported beads from the excavation in Ribe, in stratigraphical order. Radiocarbon dating puts absolute dates (i.e., calendar dates) on the relative chronology (i.e., the temporal order of artefacts).

For prehistory, in contrast, scientific dating methods are used to build chronologies. One of these is radiocarbon dating, which is a well-established dating method for the past 50,000 years. Until now, however, I have only applied radiocarbon dating to questions about prehistory, especially the Stone Age. For more recent periods, such as the Early Medieval/Viking Age, however, the precision of radiocarbon dating had been too limited. This was particularly true for urban contexts, that is, early towns. In the mind of a radiocarbon dating specialist, urban sites are sites where a lot is happening fast on the same spot for a long period of time. This means that town deposits are very complex and require detailed chronologies – a level of detail that radiocarbon dating hitherto has not been able to deliver. Other dating methods such as dendrochronology (tree-ring dating) or artefact typologies (e.g., which types of pottery were common in which periods) had been used instead.

A photograph of the Viking Age layers at Ribe, exposed in a profil.
This profile at the corner of the excavation trench shows the great number of archaeological layers that had been deposited on top of each other. Co-author Søren M. Sindbæk has joined the photo to provide a scale.

This changed when I got the opportunity to work at the Centre for Urban Network Evolutions (UrbNet), Aarhus University, at a time when more detailed radiocarbon calibration curves were developed in a global effort. Calibration is the process of converting the measured radiocarbon age into an age on our calendar scale. The two are not the same because of small fluctuations in the atmospheric radiocarbon concentration. Tree rings form the basis of calibration curves, because each tree ring records the radiocarbon concentration of the year in which it grew. Until recently, calibration curves had been based on decadal samples, thus averaging over ten years of growth.

However, in 2021, Fusa Miyake and her colleagues discovered a rapid increase in radiocarbon concentration in tree rings from the year 774-775 CE. This spurred intense activity at radiocarbon dating centres around the world, and soon more events of this kind were found. The increases in atmospheric radiocarbon concentrations are caused by solar particle events and illustrate that some detail is lost when calibration curves are made from decadal samples.

Therefore, I and many colleagues around the world cut up slices of trees into individual year rings and measured their radiocarbon concentrations. This led to the first version of a calibration curve allowing for annual calibration, IntCal20. Work on annual calibration curves is ongoing, and you can be quite sure that in the moment you are reading this, a radiocarbon specialist somewhere on this planet is cutting, extracting or measuring on tree rings.

Photograph of tree rings, seen through a microscope
A slab of oak tree, seen through a microscope, with the tip of the scalpel we use for cutting the tree-rings. The bands of pores are the early wood, produced in spring and partially incorporating carbon from the previous growth season. The other, more solid, bands are the late wood, which represent the summer growth of only one year, and which we use for our calibration curve.

During my work at UrbNet, my task was to develop chronologies for urban sites. One of these sites is the town of Ribe in present-day Denmark, known as the oldest town in Scandinavia and an important Viking Age trade centre.

In Ribe, a large grant from the Carlsberg foundation gave us the chance to do a research excavation on a small plot in the centre of the town. We excavated an area of c. 87 m2 covering most of a building plot, together with parts of a street and adjacent plots. Layer upon layer of clay floors revealed where the houses once stood. We followed a systematic stratigraphic protocol and high-definition documentation methods, including 3D laser scanning of all exposed surfaces, wet sieving of all excavated soil, and systematic geoarchaeological sampling of excavation units. It was amazing to witness the huge numbers of tiny artefacts that were recovered as all the soil from the excavation was sieved.

A photograph of different artefacts from the Northern Emporium excavation in Ribe.
Different artefacts from the excavation in Ribe. The tiny object in the middle is a miniature axe made of amber. Although it broke into three pieces already in the Viking Age, it could be reassembled after we found the fragments in three different wheelbarrows of soil during wet-sieving.

Some of these artefacts had been imported from far away – we found antler and slate from Norway, pottery from the Rhine area, and glass beads from the Middle East. By radiocarbon dating the exact layers where these artefacts were found, we could put time stamps on the different trade routes. As we used the new IntCal20 calibration curve, with some additional tree-ring data measured for this study, we were able to date those layers with a precision of a few decades. Furthermore, we identified the layer in which the 775 CE Miyake event, a massive Solar Particle Event, had taken place. This was like an anchor for our entire sequence of about 140 radiocarbon dates. Now we can conclude that oversea trade routes with Norway were established long before impulses from the Islamic world initiated proto-globalised trade in Afro-Eurasia, and long before Viking raids were recorded in European historical sources. The Viking Age, defined by artefact styles and imports such as Middle Eastern beads, began in 790±10 CE in a society which had been used to maritime trade for several decades.

Photo of the excavation
At the excavation in Ribe. I wonder which treasures are hidden in these buckets full of dirt.

References

Miyake F, Nagaya K, Masuda K, Nakamura T. A signature of cosmic-ray increase in AD 774–775 from tree rings in Japan. Nature. 2012;486(7402):240-2

Philippsen B, Feveile C, Olsen J, Sindbæk SM. Single-year radiocarbon dating anchors Viking Age trade cycles in time. Nature. 2021. https://doi.org/10.1038/s41586-021-04240-5

Reimer PJ, Austin WEN, Bard E, Bayliss A, Blackwell PG, Ramsey CB, et al. The IntCal20 Northern Hemisphere radiocarbon age calibration curve (0-55 kcal BP). Radiocarbon. 2020;62(4):725-57.

 

Funding

This study was funded by the Carlsberg Foundation (Semper Ardens Grant CF16-0008 Northern Emporium) and the Danish National Research Foundation grant DNRF119 – Centre of Excellence for Urban Network Evolutions (UrbNet). Brødrene Hartmanns Fond (grant application A34514) and Grosserer P.L. Jørgensens Mindefond supported the tree-rings measurements with 50.000 DKK and 25.000 DKK, respectively.