This month, we are discussing something new for Science Notes: ice-core analysis. This technique is based on the fact that, as atmospheric particles settle on glaciers – whether through precipitation or wind – they become trapped in the outermost layer of ice. As these layers accumulate each year, they create a sequential time capsule. By extracting cores from these glaciers, the layers can be separated and the particles analysed, providing evidence for global climate and pollution fluctuations through history.
Recent research is now showing that ice-core analysis can also be used to pinpoint the timing and location of more localised events. A team from the universities of Harvard, Nottingham, and Heidelberg, and the Climate Change Institute (CCI) at the University of Maine, are currently analysing a 72m-core that was extracted from the Colle Gnifetti glacier in the Swiss Alps in August 2013. They are pursuing a range of questions related to economic and social transformations, pandemic disease, and climate change on a yearly level. One of the events they are interested in occurred in the mid-7th century AD, when there was an abrupt change in coin production in north-west Europe, with silver largely replacing gold. While a seemingly minor change, it greatly transformed the early medieval economy, increasing long-distance trade and leading to the growth of large port settlements, including London.
Despite this being a rather dramatic shift, there is little evidence in either the documentary or archaeological record of where the silver for this coinage came from. Ice-core data offers a possible answer, though: as most silver ores also contain large amounts of lead, silver mining usually leads to an increase in atmospheric lead pollution, the evidence of which is likely to have become encapsulated in glacial ice.
Since the Colle Gnifetti glacier is in the middle of Western Europe, the team thought it was more likely to provide a higher-resolution result for that region than glaciers in Greenland. Previous research by the team on the Colle Gnifetti glacier had shown that during the period of the Black Death – which wiped out nearly a third of Europe’s population between 1349 and 1353 – there was a total collapse in lead pollution, reflecting the fact that the mining and smelting industries were interrupted by the catastrophe. So they knew that the technique could be used to great effect.
In order to analyse the ice-core data correctly, a timeline needed to be established matching the layers to known periods of time. For instance, the layers closest to the surface were identified using major 20th-century Saharan dust-storm events. By then counting the layers down with a new laser-ablation technique, the Colle Gnifetti core produced a theoretical error margin of ±72 years. But the chronology was able to be refined by using three absolute chronological markers that had previously been identified. The first was the Black Death, as mentioned above; the second was the massive eruption of the Icelandic volcano Eldgjá between AD 934 and 939; and the third was based on the recent discovery that volcanic particles found in one layer of the Colle Gnifetti core are chemically all but identical to those from a Greenland ice-core, which is believed to correspond to another eruption in AD 536. Through these known layer dates, the team was able to establish a maximum error margin of ±10 years.
With the dates established, the team found that there was a significant increase in lead pollution around the mid-620s, with a pronounced peak at 640±10. Two other peaks were identified in 660±10 and 695±10. As these peaks do not correspond with an increase in bismuth – which would most likely indicate a volcanic event – it is probable that they are due to human activity, and they happen to fall right within the expected timeline for the shift to silver coinage.
As the core taken from Colle Gnifetti was located on a north-facing slope, the origins of this pollution must have been located in an area where the pollution could reach this area. Analysis of atmospheric circulation patterns, incorporating all NOAA/NASA climate data from 1979-2014, pointed to modern-day France and Britain being the most likely candidates, and it helped rule out Spain, Sardinia, and Tuscany – locations known for their silver production later in the medieval period. Within France and Britain, only one area has archaeological evidence for mid-7th-century silver mining: Melle, France.
Further peaks in later decades also point to Melle as the likely source. Lead peaks at AD 805±10 and 825±10 correspond with the known minting of the Metullo coinage issue of Charlemagne at Melle, between 793 and 814, and the later issue of his son, Emperor Louis the Pious, between 816 and 823. Additionally, a sharp decrease in lead at 850±10 lines up with the recorded sack of Melle by Vikings in 848, as well as a civil war in the region during the mid-850s, further cementing the importance of the mine for European silver production. Overall, the team was able to demonstrate convincingly that ice-core data can be used to great effect in identifying not only global climatic events and fluctuations, but more regional ones as well.
The paper in Antiquity highlighting these results can be read for free at https://doi.org/10.15184/aqy.2018.110.
This article appeared in CA 347.