While it lasts, the ancient ice of Earth contains a record of the atmosphere, containing tiny bubbles of air, as well as whatever was in the air, including pollen and volcanic ash. Ice also preserves traces of human activity, AKA pollution.
This spring an international team reports on a careful study of the chemistry of 423 meters of ice cores from Greenland, which they date to run from 1200 BCE to 1200 CE, some 2500 years .
The measurement found fluctuating traces of lead and copper apparently dating to historical times. Given the location of Greenland, these metals presumably reflect emissions into the atmosphere from Europe. The new study corroborate earlier isotopic analyses, showing that prevailing winds would transport emissions from ancient European activity to Greenland, while other putative sources would have little representation in Greenland ice. The study also compared the temporal patterns to lead traces in peat bogs, and found them to match.
The lead particulates are hypothesized to be from silver lead mining activity, in which high temperature smelting used lead as part of the process of recovering silver. The process would have emitted lead into the air at major mining sites, such as Spain. These would blow to Greenland, leaving a record of the mining activities in the ice.
While the record is variable and uncertain, there are periods of sustained high levels of lead in the record. These were found to correllate with overall economic activity and presumably the intensity of mining.
The researchers report swings in mining that match recorded wars in Spain, which they argue reflects the disruption and intensification of silver mining due to political developments.
“The repeated pattern of dips in production coinciding with the outbreak of wars primarily affecting the Iberian Peninsula, and then recovery again after the end of each war, suggests that warfare caused major interruptions to lead–silver production during the middle and late Roman Republic.” (p. 2)
The sustained peace of Pax Romana is detectable in a long period of high lead levels in the Greenland ice, which ends at the time of a great plague. This period probably reflects emissions from a number of sites in Northern Europe besides Spain. This peak drops off around 9CE, “coincident with Roman abandonment of territory to the east of the Rhine, including the Sauerland mines, after three legions were annihilated in the Teutoburg forest.” (p. 3)
The activity remained low until circa 800CE, a time when mining was intensified in medieval France and Britain. There is an earlier spike that is attributed to Phoenician activity, though there is no detailed record of mining in that period.
Finally, they find that these fluctuations in lead pollution tracked the metal content of Roman currency. Drops in the lead in Greenland correspond to periods of lower silver content in coins, presumably due to low production from silver mines.
“The fluctuations in lead–silver mining and smelting indicated by the Greenland lead pollution record and estimated lead emissions were directly reflected in the fineness and metallurgy of Rome’s silver coinage, the denarius…” (p. 3)
This is a pretty cool study, and the detailed dating of the records is pretty impressive. Cross referencing peat bogs, volcanic activity, and historical records is a good idea, though I worry that the study is combining multiple imperfect measurements, hoping the combination is better rather than worse.
A key of the study is the simulation of the atmospheric deposition, and I worry that it is logically circular. This kind of simulation is tricky under any circumstance, and projecting thousands of years into the past is problematic. The researchers are well aware of this, and are reasonably careful about how they use the simulations. They look at general circulation patterns, not short term events, projecting from the last century of detailed data reports.
There are two questions that nag me about though. First, there is an assumption that the general circulation is basically unchanged over the last 3000 years. I’m not sure that is true, but it may well be true for the effects of interest in this study (i.e., wind blown particles transported from Europe to Greenland). I simply don’t know. If there was a big enough change in wind patterns during the period, this could show up as a drop in lead that has nothing to do with mining activity.
The study makes a number of claims about the sensitivity of the Greenland ice record to sources in different geographical locations (Figure 1 in the paper). Mostly, this is a matter of distance, which is straightforward. But they also find some substantial differences in locations not that far apart (Spain versus Britain versus Germany), from which they infer proportionate responsibility for the lead pollution.
Essentially, they work backward from the measured lead to various potential sources where ancient mining was known to have occurred. Granting these calculations are mainly correct, they are mainly looking at known sources from the historical and archaeological record. If there are other sources, they would be difficult to detect in this methodology, and they would be misestimating the contributions of the known sources. If there are other mines unaccounted for, the whole result could be way off.
Future work may be able to extract detailed isotope composition from the deposits, which might reveal the sources more directly. I fear that that will be difficult to achieve from these ice cores.
These simulations are aligned with known history, but that is (literally) post hoc reasoning. Basically, they are able to find explanatory events for major changes in the lead pollution, including at least one surprise (i.e., the outsized effects of the Antonine plague, among sever plagues and wars). Yet there are plenty of other events that do not show up in the lead pollution record. I would note that the history of the metallic content of Roman coins is complicated, and not necessarily tied to the production of silver as the key factor.
In short, the perceived correlation between the ice core records and historical events is self-confirming. Who knows how many alternative events might be chosen that do not match the ice cores as well?
In any case, the study doesn’t actually reveal any new findings. It shows that ancient industrial activity may well be reflected in Greenland ice, and that pollution seems to be correlated with the overall mining activity in Europe and the Mediterranean (but not China or other more distant locals). In fact, it would be surprising if this were not true.
What the study does seem to prove is that methods have improved to be able to at least provide detailed support for this kind of historical hypothesis, even if it isn’t really able to confirm or deny detailed hypotheses.
- Joseph R. McConnell, Andrew I. Wilson, Andreas Stohl, Monica M. Arienzo, Nathan J. Chellman, Sabine Eckhardt, Elisabeth M. Thompson, A. Mark Pollard, and Jørgen Peder Steffensen, Lead pollution recorded in Greenland ice indicates European emissions tracked plagues, wars, and imperial expansion during antiquity. Proceedings of the National Academy of Sciences, 2018. http://www.pnas.org/content/early/2018/05/08/1721818115.abstract