Isotopes tell the story of lead in ancient Rome

Abstract

At the beginning of the second century AD, Rome was not only the capital of a vast empire, it was also a city of perhaps a million inhabitants. Servicing such a population challenged the infrastructure of the day. A network of aqueducts and lead pipes distributed water from the Tiber River throughout the city. And the arrival of goods from the Mediterranean region was facilitated by the protected harbor of Portus, built around AD 112 at the mouth of the Tiber, 25 km from the city center.Inspired by a 30-year-old hypothesis—though now largely discredited—that lead poisoning was responsible for the Roman Empire’s demise, Hugo Delile, Francis Albarède (École Normale Supérieure de Lyon and Rice University), and colleagues measured lead isotope ratios of sediments in the Portus harbor.11. H. Delile et al., Proc. Natl. Acad. Sci. USA 111, 6594 (2014). https://doi.org/10.1073/pnas.1400097111 From their data, the researchers not only estimated the level of lead in ancient Roman drinking water but also found evidence of changes in the aqueduct system’s use and disuse over time.Lead in the waterSection:ChooseTop of pageABSTRACTLead in the water <<Decline and fallREFERENCESCITING ARTICLESThree of the four stable isotopes of lead are radiogenic: Lead-206, lead-207, and lead-208 are the endpoints of the decay chains of uranium-238, uranium-235, and thorium-232, respectively. The relative abundance of those isotopes, and of the entirely primordial 204Pb, in a particular mining district depends on how much U and Th were originally present in the area. As a result, Pb isotope ratios can vary considerably from location to location. If the Pb used to build Rome’s water pipes was brought in from elsewhere in Europe, the Pb that leached out of the pipes would probably differ in isotopic composition from the Pb naturally present in Tiber River water.Albarède and colleagues excavated two cores from different parts of the Portus harbor; the first 700–800 cm of each consisted of sediment deposited in the harbor over the past two millennia. Using carbon-14 dating of organic matter in the cores, they established the relationship between depth and age, accurate to within 100 years. Then they measured the Pb isotope ratios as a function of depth. As shown in the figure for one core, those ratios rose and fell in concert.Lead isotopes in sediments from Portus, the harbor that served ancient Rome. Higher isotope ratios, shaded in blue, represent higher levels of anthropogenic Pb leached into the water from Rome’s water pipes. The dates along the right edge of the figure are derived from radiocarbon dating of organic matter and are accurate to within 100 years. (Adapted from ref. 11. H. Delile et al., Proc. Natl. Acad. Sci. USA 111, 6594 (2014). https://doi.org/10.1073/pnas.1400097111.)PPT|High resolutionIn fact, all of the Pb samples could be neatly characterized as a sum of two components: a natural component, corresponding to unpolluted river water, and an anthropogenic component. When the researchers sampled several surviving first- and second-century Roman water pipes, they found that their isotopic composition matched the anthropogenic component exactly. “That was a wonderful surprise,” says Albarède.The pipes’ isotopic composition shed some light on where the Pb might have come from. Similar isotope ratios have been found in Pb deposits in the British Isles, parts of modern-day France and Germany, and southwestern Spain. Notably, the pipes couldn’t have come from southeastern Spain, near modern-day Cartagena, which had been a major mining center of the ancient world.At the times of the highest Pb pollution—the early Roman Empire and early Middle Ages—the anthropogenic component accounted for between 50% and 75% of the Pb in the harbor water. At the height of the Roman Empire, an estimated 3% of the Tiber’s flow was diverted through Rome’s water distribution system; it follows that the water in the pipes had 40–100 times more lead than the unpolluted river water. Explains Albarède, “We concluded that although lead excesses were significant, they were unlikely to have created a major health issue in antique Rome.”Decline and fallSection:ChooseTop of pageABSTRACTLead in the waterDecline and fall <<REFERENCESCITING ARTICLESOver the history of the harbor, anthropogenic Pb levels dropped, rose, and dropped again in a way that can be tentatively related to events in Roman history. The ultimate fall of the Roman Empire, as marked by the overthrow of the last emperor in AD 476, was preceded by a long period of decline, which may have included a drastic decrease in urban population and a deterioration of infrastructure. The exact trajectory of Rome’s population during that period remains uncertain. But the gradual decrease in anthropogenic Pb around AD 250 suggests that the water distribution system was falling into disuse.In the middle of the sixth century, the Byzantine Empire conquered the short-lived Gothic kingdom that had replaced the Roman Empire on the Italian peninsula. Byzantine repairs of the Roman aqueducts have been historically documented, and they’re consistent to within Albarède and colleagues’ error bars with the spike in anthropogenic Pb around AD 500. The sharp drop in anthropogenic Pb around AD 800 could be related to the sack of Rome by Arab raiders in AD 846 or to major flooding in AD 856.Geochemical analysis of more cores in more locations will help to refine the timeline and to link the data to recorded and unrecorded historical events. Albarède and colleagues are currently working on cores taken from the harbor of the ancient religious center of Ephesus, in modern-day Turkey (see Physics Today, March 2014, page 24).REFERENCESSection:ChooseTop of pageABSTRACTLead in the waterDecline and fallREFERENCES <<CITING ARTICLES1. H. Delile et al., Proc. Natl. Acad. Sci. USA 111, 6594 (2014). https://doi.org/10.1073/pnas.1400097111, Google ScholarCrossref, ISI© 2014 American Institute of Physics.