CHEMOSTRATIGRAPHY, DIAGENESIS, AND A PALEO-ULTISOL IN PLEISTOCENE ASH (POZZOLANA) NEAR ROME, ITALY: IMPLICATIONS FOR THE FORMULATION OF ANCIENT ROMAN CONCRETE

To determine the geological basis for the durable concretes of Imperial Rome (~2000-1700 B.P.), we are pursuing Hay's initial findings that highlight the importance of the alteration of Roman ashes and their pozzolanic properties. Detailed geochemistry and mineralogy clarify complex microstratigraphy and allow diagnosis of geochemical alteration. Pozz. Rosse (~457ka) at Castèl di Leva quarry has Zr/TiO₂ ~625 in the 5.5m thick ignimbrite (pyroclastic flow), whereas the overlying 1m thick co-ignimbritic ash cloud deposit has an upward-increasing trend from 625-650. Later airfall ashes have lower values ~540. Overlying Pozz. Nere (~407ka) is similar, but with higher Zr/TiO₂ ~675 in 3m of ignimbrite, ~650 in 0.6m of ash cloud deposit, and ~500 in 1.2m of later air falls. Tenuta di Capannacce quarry has similar trends. From geochemistry and petrography, we infer that at Castèl di Leva, a 2.5-3.0m thick paleo-ultisol developed on the surface of the Pozz. Rosse ash cloud deposit, prior to burial by later air falls. The paleosol carries diagnostic ultisolic properties including strong leaching, no calcareous horizon, and low base cation saturation. Pedogenesis is indicated by Al enrichment, base cation depletion, and indicators of hydrolysis (e.g. Ba/Sr). Micromorphology shows root traces, translocated clay, and crystal weathering consistent with strong paleosol development. At Capannacce, either no paleosol developed or it was eroded by overlying Conglomerato Giallo. Leucite dissolution is complete near the top of the paleosol, confirmed by dramatic trends in K₂O/SiO₂ and Ba/Rb. Deeper in the ignimbrite, leucite is intact but has decomposition rinds of opal & clay (w/ K-clinoptilolite?). In the air fall ashes, leucite is intact but clinopyroxene is etched and dissolved, reflecting paleo-waters unrelated to the paleosol. Silica enrichment below the paleosol is consistent with modern silica-saturated water in regional aquifers. This paleo-ultisol likely was critical to the formation of some of the "alteration facies" of Pozz. Rosse proposed by Hay and Jackson. Understanding paleosols and other alteration mechanisms in Roman ashes will shed light on the raw materials available to ancient Romans, and add important evidence to Mediterranean paleoclimatology and paleohydrology.