F. A. Albarede

The redox state of arc mantle using Zn/Fe systematics

Many arc lavas are more oxidized than mid-ocean-ridge basalts and subduction introduces oxidized components into the mantle. As a consequence, the sub-arc mantle wedge is widely believed to be oxidized. The Fe oxidation state of sub-arc mantle is, however, difficult to determine directly, and debate persists as to whether this oxidation is intrinsic to the mantle source. Here we show that Zn/FeT (where FeT = Fe2+u2009+u2009Fe3+) is redox-sensitive and retains a memory of the valence state of Fe in primary arc basalts and their mantle sources. During melting of mantle peridotite, Fe2+ and Zn behave similarly, but because Fe3+ is more incompatible than Fe2+, melts generated in oxidized environments have low Zn/FeT. Primitive arc magmas have identical Zn/FeT to mid-ocean-ridge basalts, suggesting that primary mantle melts in arcs and ridges have similar Fe oxidation states. The constancy of Zn/FeT during early differentiation involving olivine requires that Fe3+/FeT remains low in the magma. Only after progressive fractionation does Fe3+/FeT increase and stabilize magnetite as a fractionating phase. These results suggest that subduction of oxidized crustal material may not significantly alter the redox state of the mantle wedge. Thus, the higher oxidation states of arc lavas must be in part a consequence of shallow-level differentiation processes, though such processes remain poorly understood.

World map of Nd isotopes in sea-floor ferromanganese deposits

The Nd isotopic compositions of hydrogenous ferromanganese encrustations and nodules largely reflect those of the overlying seawater, integrated over the few millions of years over which they grow. Available Nd isotope data on ocean-floor hydrogenous deposits suggest control by Nd from particulates blown from the continents and released by volcanic activity associated with subduction zones and midocean ridges. The regional Nd isotope variations display a marked similarity to the broad patterns of present-day deep-ocean circulation, suggesting that the particulates that are the sources of oceanic Nd travel along with water masses. Despite rapid (10-100 ka) perturbations during glacial periods, the long-term patterns of water-mass movements and oceanic fluxes of particulates have remained essentially stable over the last few million years.

Lead in ancient Rome's city waters

Significance Thirty years ago, Jerome Nriagu argued in a milestone paper that Roman civilization collapsed as a result of lead poisoning. Clair Patterson, the scientist who convinced governments to ban lead from gasoline, enthusiastically endorsed this idea, which nevertheless triggered a volley of publications aimed at refuting it. Although today lead is no longer seen as the prime culprit of Rome’s demise, its status in the system of water distribution by lead pipes (fistulæ) still stands as a major public health issue. By measuring Pb isotope compositions of sediments from the Tiber River and the Trajanic Harbor, the present work shows that “tap water” from ancient Rome had 100 times more lead than local spring waters. It is now universally accepted that utilization of lead for domestic purposes and water distribution presents a major health hazard. The ancient Roman world was unaware of these risks. How far the gigantic network of lead pipes used in ancient Rome compromised public health in the city is unknown. Lead isotopes in sediments from the harbor of Imperial Rome register the presence of a strong anthropogenic component during the beginning of the Common Era and the Early Middle Ages. They demonstrate that the lead pipes of the water distribution system increased Pb contents in drinking water of the capital city by up to two orders of magnitude over the natural background. The Pb isotope record shows that the discontinuities in the pollution of the Tiber by lead are intimately entwined with the major issues affecting Late Antique Rome and its water distribution system.

Copper, lead, and silver isotopes solve a major economic conundrum of Tudor and early Stuart Europe

The unrelenting economic inflation of A.D. 1515–1650 in Europe has been variously explained by the influx of silver from Mexico and the Viceroyalty of Peru (now Bolivia), the growth of the European population, and the decline of the silver market price. Silver, copper, and lead isotope analyses of A.D. 1550–1650 English coinage show a dominance of silver from Europe and Mexico, contrasting with a spectacularly small contribution from the Viceroyalty of Peru. This observation contrasts with the registration of metal production in the mines of the Spanish Americas. Hence the question: Where did Potosi silver go? This novel observation indicates that silver from Mexico was exported eastward, whereas Potosi silver flowed westward. However, aware of the Pacific route of the silver trade, scholars never agreed upon the volumes transported. Our work demonstrates that there was a Potosi-China route, and that it was largely disconnected from the Mexico-Europe routes.

A lead isotope perspective on urban development in ancient Naples

Significance A well-dated sedimentary sequence from the ancient harbor of Naples sheds new light on an old problem: could the great AD 79 Vesuvius eruption have affected the water supply of the cities around the Bay of Naples? We here show, using Pb isotopes, that this volcanic catastrophe not only destroyed the urban lead pipe water supply network, but that it took the Roman administration several decades to replace it, and that the commissioning of the new system, once built, occurred nearly instantaneously. Moreover, discontinuities in the Pb isotopic record of the harbor deposits prove a powerful tool for tracking both Naples’ urbanization and later major conflicts at the end of the Roman period and in early Byzantine times. The influence of a sophisticated water distribution system on urban development in Roman times is tested against the impact of Vesuvius volcanic activity, in particular the great eruption of AD 79, on all of the ancient cities of the Bay of Naples (Neapolis). Written accounts on urbanization outside of Rome are scarce and the archaeological record sketchy, especially during the tumultuous fifth and sixth centuries AD when Neapolis became the dominant city in the region. Here we show that isotopic ratios of lead measured on a well-dated sedimentary sequence from Neapolis’ harbor covering the first six centuries CE have recorded how the AD 79 eruption was followed by a complete overhaul of Neapolis’ water supply network. The Pb isotopic signatures of the sediments further reveal that the previously steady growth of Neapolis’ water distribution system ceased during the collapse of the fifth century AD, although vital repairs to this critical infrastructure were still carried out in the aftermath of invasions and volcanic eruptions.